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CASTING INFORMATION

This page provides an overview of the information that is available to the designers and buyers of steel casting. The information contained in these articles is believed to accurate; however, the information should be considered recommendations. The designer and buyer should always work with the foundry personnel to determine specific specifications. Ultimately, the designer is responsible for the final product design.

Table Of Contents

Introduction and reference material

• Overview of the casting process

The Overview of the casting process highlights the advantages of the castings process as opposed other methods of manufacturing metal parts. In addition, a general overview of the casting process is included.

Information for the casting designer

• Framework of designing

One of the basic principles of the system approach in engineering design is to define the system boundaries in such a way that conflicting requirements can be recognized and resolved.

• Impact of alloy physical characteristics

Recognizing the impact of physical alloy characteristics and the mechanical characteristics of each alloy at the outset will help the design engineer avoid many of the pitfalls in assuming that all alloys can be treated alike.

Information for buyers of castings.

• Article 1. How to order steel castings

the basic requirements needed to work with your supplier.

• Article 2. An outline for purchasing steel castings

This article discusses how to obtain the optimum value from the purchase of a steel casting by a cooperative effort on the part of the buyer and of the seller from the early stages of the conception of design through the manufacturing process.

• Article 3. Specifications in used the steel casting industry

The purpose of this article is to review the role of specifications as a means of communication between all parties concerned in the purchase of steel castings.

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Overview of the Casting Process This section provides information on the casting process for those who may not be familiar with it. This overview is not intended to be the definitive word on the casting process; but rather, a broad picture of the advantages of castings and the steps involved in making a casting.

1. Advantages of the Casting Process A casting may be defined as a "metal object obtained by allowing molten metal to solidify in a mould", the shape of the object being determined by the shape of the mould cavity.

Certain advantages are inherent in the metal casting process. These often form the basis for choosing casting over other shaping processes such as machining, forging, welding, stamping, rolling, extruding, etc. Some of the reasons for the success of the casting process are:

• The most intricate of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.

• Because of their physical properties, some metals can only be cast to shape since they cannot be hot-worked into bars, rods, plates, or other shapes from ingot form as a preliminary to other processing.

• Construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.

• Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly. For example, in the automotive industry hundreds of thousands of cast engine blocks and transmission cases are produced each year.

• Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housing, valves, and hydroelectric plant parts weighing up to 200 tons illustrate this advantage of the casting process.

• Some engineering properties are obtained more favorably in cast metals. Examples are:

o More uniform properties from a directional standpoint; i.e., cast metals exhibit the same properties regardless of which direction is selected for the test piece relative to the original casting. This is not generally true for wrought metals.

o Strength and lightness in certain light metal alloys, which can be produced only as castings.

o Good bearing qualities are obtained in casting metals.

• A decided economic advantage may exist as a result of any one or a combination of points mentioned above. The price and sale factor is a dominant one which continually weighs the advantages and limitations of process used in a competitive of enterprise.

There are many more advantages to the metal-casting process; of course it is also true that conditions may exist where the casting process must give way to other methods of manufacture, when other processes may be more efficient. For example, machining procedures smooth surfaces and dimensional accuracy not obtainable in any other way; forging aids in developing the ultimate of fiber strength and toughness in steel; welding provides a convenient method of joining or fabricating wrought or cast products into more complex structures; and stamping produces lightweight sheet metal parts. Thus the engineer may select from a number of metal processing methods that one or combination, which is most suited to the needs of his work.

2. Basic Steps in Making Sand Casting

• Obtaining the casting geometry • Patternmaking • Coremaking • Moulding • Melting and pouring • Cleaning • Other procedures may be performed before delivery

The various steps in the production of castings are briefly summarized for the benefit of those who may be unfamiliar with foundries and the casting process.

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Obtaining the casting geometry

The traditional method of obtaining the casting geometry is by sending blueprint drawings to the foundry. This is usually done during the request for quotation process. However, more and more customers and foundries are exchanging part geometry via the exchange of computer aided design files.

Patternmaking

The pattern is a physical model of the casting used to make the mould.

The mould is made by packing some readily formed aggregate material, such as moulding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mould cavity, which is ultimately filled with metal to become the casting.

If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.

Coremaking

Cores are forms, usually made of sand, which are placed into a mould cavity to form the interior surfaces of castings. Thus the void space between the core and mould-cavity surface is what eventually becomes the casting.

Moulding

Moulding consists of all operations necessary to prepare a mould for receiving molten metal. Moulding usually involves placing a moulding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mould cavity, setting the cores in the mould cavity and finishing and closing the mould.

Melting and Pouring

The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the moulds are filled.

Cleaning

Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. The casting is separated from the mould and transported to the cleaning department. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Castings may be upgraded by welding or other procedures. Inspection of the casting for defects and general quality is performed.

Other Processes

Before shipment, further processing such as heat-treatment, surface treatment, additional inspection, or machining may be performed as required by the customer's specifications.

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Conceptual Framework for Designing Metal Castings Author: Mr. Mike Gwyn; Pelton Casteel, Inc.

One of the basic principles of the "system approach" in engineering design is to define the system boundaries in such a way that conflicting requirements can be recognized and resolved.

1. Understanding the differing mind-sets (An overview) Well-designed castings are known for being functional and cost efficient. Yet those directly involved in designing and producing castings know that the principles behind well designed castings are difficult to pin down. " Rules of thumb " abound that attempt to define fillets, radii, changes of casting section, minimum section thickness, tolerance capability, etc. Yet, there are regularly casting designs that seem to violate the " rules " successfully. These designs typically have combinations of geometry that should not work, but that do.

On the other hand, design engineers observing this attempt to take latitudes with geometry that seem well founded only find that their design is either not consistently castable or is castable, but at a price that is too high.

This has been a mystery for many, many years that has frustrated design engineers, aggravated foundrymen who attempt to produce troublesome designs, and caused other forms of metal products to be designed when a casting would be the best product -- if properly designed.

First, we must look at the view point of each engineer:

Design Engineer

Design Engineers typically consider functional mechanical elements, loads, function environment, failure modes, mechanical and physical properties, fabricated shapes, automated secondary operations and cosmetics.

Foundry Engineer

The metalcasters, patternmakers, and die engineers see fluid flow, heat transfer and solidification patterns in the mould, including hot spots as the metal changes from the liquid phase to the solid phase; they see possibilities for infinite variability in casting shape. They also see foundry tooling (patterns, dies, and/or core boxes) that are critical to dimensional accuracy and consistency. They see surfaces to be machined; other surfaces that must be consistent dimensionally for machining fixturing and targeting. They see possibilities for specific alloys and heat treatments that are needed for the casting's mechanical and physical properties. They see the need for pleasant casting as-cast cosmetics.

Finally, when the design geometry and the alloy's castability are in conflict with each other, the metalcaster must consider " thermal trickery, " which is the use of chills, insulating, exothermic materials and other heat transfer gimmicks to set up necessary solidification patterns in the casting which are not possible from the casting geometry itself.

Conclusions

Based on these widely differing viewpoints, it would be surprising to find good casting designs to be obvious and trivial. In fact, cost-effective casting design is a technically demanding task for the design engineer.

One of the basic principles of the "system approach" in engineering design is to define the system boundaries in such a way that conflicting requirements can be recognized and resolved. This is the principle that we are applying here. As our conceptual framework is explained, it will become apparent that geometry holds the key to resolving the design conflict identified within properly defined system boundaries.

2. Elements of Conceptual Framework Physical and Mechanical Characteristics

Four important physical characteristics affect the castability and performance of any given casting alloy. These are:

• Fluid life • Solidification Shrinkage • Slag and/or dross formation tendency • Pouring temperatures

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Each of these characteristics vary widely among alloys and be significantly different among similar alloys. Differences among these four physical characteristics significantly affect the geometry of well designed castings.

It is also important to understand two important mechanical characteristics affecting the stiffness of any give casting design:

• Modulus of elasticity • Section Modulus

The former is a function of the stiffness of the alloy itself and the latter is a function of stiffness from the casting's geometry. These two mechanical characteristics are also at the heart of well designed geometry.

Using these physical and mechanical characteristics

Recognize that the above six characteristics affect important variables in designing, producing and using metal castings. These variables include:

• Casting method • Design of casting sections • Design of junctions between casting sections • Internal integrity required • Dimensional tolerances and extent of near-net shape requirements • Cosmetic appearance

Casting geometry as a tool

Casting geometry is the most powerful tool available to improve the following:

• Castability of the alloy • Mechanical stiffness of the casting

Carefully planned geometry can offset alloy problems in fluid life, solidification shrinkage, pouring temperature and slag/dross formation tendency. Section modulus from geometry has the power to offset problems with lower modulus of elasticity.

What to avoid

In developing a sound conceptual framework for casting design, it is important to avoid reliance on some traditional concepts and tools such as:

• Rules of thumb

• General "Do's and Don'ts" typical of casting design handbooks

• Simple, orthogonal shape thinking; such as building blocks from mill shapes like plates, bars, tubes, I-beams, other kinds of extrusions of constant cross section, etc.

The above shapes limit metalcasting's power of infinite shape variability. Casting geometry can be so much more free-flowing than orthogonal, extruded, and rotated shapes.

Knowledge and Understanding

Just as important as avoiding the above traditional tendencies in design engineering is to know and understand the nature of molten metal and use it to your advantage.

• Embrace the idea of infinitely variable shape.

• Use free-hand sketches for a conceptual designing. Move mass around. Take mass out where it is not needed; put it where it is necessary. Use variability of section modulus over length.

Use the ability to vary section modulus over section length to design for uniform stress.

Systems approach style to design thinking

Develop a "systems approach" style to design thinking. Such an approach encompasses everything, from the original need for a mechanical or structural element, to molten metal flowing into a shape, to the rough casting right out of the mould or die, through casting finishing requirements, secondary processing in the foundry, secondary processing at a subcontractor and/or the customer's plant, testing, assembly, and final use and abuse of the product which contains the casting. The only way to resolve conflicting requirements without a "Rube Goldberg" result is to conceive the system needs at the outset. When applied, the "system approach" style of thinking results in truly cost-effective, simple, elegant metalcasting design.

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Alloy Characteristics Affecting Casting Design Recognizing the impact of physical alloy characteristics and mechanical characteristics at the outset will help the design engineer avoid many of the pitfalls in making the assumption that any one alloy can be treated like any other alloy.

1. Metallurgical Characteristics Fluid Life

A molten metal's fluid life is more than its ability to fill the mould cavity. The fluid life also determines how easily and how long the metal flows through narrow channels to form thin sections, and how readily it conforms to fine surface detail.

The temperature of the molten metal is not the only factor that affects the alloy's fluid life. For a give alloy, fluid life does not increase with superheat (excess temperature above the alloy's liquidus temperature). However, fluid life of every alloy does not necessarily increase equally with temperature. In other words, fluid life of a molten metal alloy is also dependent on chemical, metallurgical and surface tension factors.

Fluid life will affect the design characteristics of a casting. By understanding the nature of an alloy's fluid life, the designer will recognize several important design criteria. Some of these are:

• minimum section thickness that can be attained

• the maximum length of a thin section

• the fineness of cosmetic detail that is possible

It is also essential to understand that moderate or even poor fluid life does not limit the cost-effectiveness of design. Knowing that an alloy has limited fluid life tells the designer that the casting should feature:

• softer shapes

• finer detail in the bottom portion of the mould

• more taper leading to thin sections

• larger lettering

• etc.

Some casting processes feature moulds that are very dry or hot. These moulding process minimize the effects of convection, a mode of heat of heat transfer which reduces fluid life.

Solidification Shrinkage

There are three distinct stages of shrinkage as molten metal alloys solidify:

• liquid shrinkage

• liquid-to-solid shrinkage

• solid shrinkage

Liquid Shrinkage is the contraction of the liquid before solidification begins. While important to metalcasters, it is not an important design consideration.

Liquid-to-Solid Shrinkage is the shrinkage of the metal as it goes from the liquid's disconnected atoms and molecules to the formation of crystals of atoms and chemical compounds, the building blocks of solid metal. The amount of solidification shrinkage varies a great deal from alloy to alloy. Figure 1 provides a guide to the liquid-to-solid shrinkage of the most common ferrous and nonferrous alloys. As shown, shrinkage can vary from very little to high shrinkage volumes. Alloys can be further classified into three groups based on their solidification range:

• directional

• eutectic

• equiaxed

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Failure to recognize the impact of liquid-to-solid (solidification) shrinkage is one of the worst errors that "rule of thumb" design handbooks make.

Liquid-to-Solid shrinkage is an extremely important consideration for the design engineer. In some alloys, disregard for this type of shrinkage results in voids in the casting. Both the design and foundry engineer have the tools to combat this problem, but the designer has the most cost-effective tool, that is geometry.

Figure 1. Volume changes in iron-carbon alloys.

Geometry can be found that meets structural needs and solidification shrinkage needs. For some alloys, finding that geometry can be very simple. For other alloys, finding that geometry is the real essence of good casting design. Should that geometry not be found for difficult alloys, the foundry engineer must resort to "thermal trickery" to create fluid flow and heat transfer patterns that the geometry fails to provide.

"Thermal trickery" is creative stuff, a major weapon in the expert foundry engineer's arsenal, but it is expensive. Eliminating thermal trickery with good design makes castings that cost less to produce and cost less to process and assemble.

Solid Shrinkage (often called patternmaker's shrink) occurs after the metal has completely solidified and is cooling to ambient temperature. Solid shrinkage changes the dimension of the casting from those in the mould to those dictated by the rate of solid shrinkage for the alloy.

In other words, as the solid casting shrinks away from the mould walls, it assumes final dimensions that must be predicted by the patternmaker. This variability of patternmaker's shrink is a very important design consideration.

This uncertainty about patternmaker's shrink is why foundrymen normally recommend producing a first article (sometimes called a sample casting) to establish what dimensions really are before going into production. There is high risk in assuming that the solid shrinkage predictions built into patterns/dies and coreboxes will result in final dimensions that are "close enough" to prediction to fit within allowable tolerance.

Despite all the good planning, the nature of patternmaker's shrink is unpredictable enough and important enough that adjustments will probably be necessary on the pattern to achieve the final production dimensions and tolerances.

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Slag / Dross Formation

According to the dictionary, slag and dross are synonyms meaning: "refuse from melting of metals ". Obviously, no one wants "refuse" in castings.

Among foundrymen, slag and dross have slightly different meanings. Slag is usually is associated with the higher melting point metals (ferrous metals) and is composed of liquid nonmetallic compounds (usually fluxed refractories), products of alloying and products of oxidation in air. Dross, on the other hand, usually is associated with lower melting point metals (non-ferrous alloys) and often means the nonmetallic compounds produced primarily by the molten metal reacting with air.

Some molten metal alloys are much more sensitive to slag/dross formation than others. Castings made from these alloys are much more prone to contain nonmetallic inclusions. There are casting processes, quality control techniques and design considerations that can dramatically reduce the likelihood of nonmetallic inclusions in casting. Design geometry guidelines to minimize the possibility of nonmetallic inclusions affecting the surface quality of castings ...

Pouring Temperature

Metal castings are produced in moulds that must withstand the extremely high temperature of liquid metals. Interestingly, there really are not many choices of refractors to do the job. As a result, high molten metal temperatures are very important to casting geometry as well as what casting process should be used.

The following is a summary of common foundry alloys and their pouring temperatures:

Table 1. Pouring Temperature Chart

Alloy °F °C

Solder ~450 ~230 Tin ~600 ~300 Lead ~650 ~345 Zinc Alloys 650-850 345- 455 Aluminum Alloys 1150-1350 620- 735 Magnesium Alloys 1150-1350 620- 735 Copper-base Alloys 1650-2150 900-1180 Cast Irons; Gray, Ductile 2450-2700 1340-1480 Monel (70 Ni, 30 Cu) 2500-2800 1370-1540 Nickel-based Superalloys 2600-2800 1430-1540 High Alloy Steels 2700-2900 1480-1600 High Alloy Irons 2800-3000 1540-1650 Carbon & Low Alloy Steels 2850-3100 1565-1700 Titanium Alloys 3100-3300 1700-1820 Zirconium Alloys 3350-3450 1845-1900

For practical purposes, sand and ceramic materials with their refractory limits of 3,000 - 3,330°F (1650-1820°C) are the most common mould materials used today.

As the temperature of the molten metal alloy increases, design consideration must be given to heat transfer problems and thermal abuse of the mould itself.

Metal moulds, such as those used in diecasting and permanent moulding, also have temperature limitations. In fact, most of the alloys on the list are beyond the refractory capability of metal moulds (except for special thin geometry designs, alloys from the copper-base group and up require sand or ceramic moulds).

Carbon and low alloy steels approach the limit of sand and ceramic refractories and titanium and zirconium alloys go beyond it, creating special situations. So, it is easy to see the abuse that sand and ceramic moulds are subjected to when pouring temperatures approach the refractory limits. The same holds true for lower temperature molten metal alloys that approach the refractory limit of plaster or metal moulds.

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Again there are design considerations that will compensate for thermal abuse and hot-spot problems in the mould. These are covered in more detail later in this section.

2. Mechanical Characteristics Modulus of Elasticity

The measure of stiffness of a metal itself (without regard to material shape) is known as the modulus of elasticity or Young's Modulus. In the case of metals, it is a function of metallurgy, and it is a mechanical property of the metal alloy.

Although this is a parameter discussed at length in engineering books on material science, it is a common measurement in foundries; in most foundries, the modulus of elasticity is a parameter measured virtually every day. The modulus of elasticity is similar to the elastic (straight-line) portion of the stress/strain diagram created whenever a test bar is pulled on a tensile test machine.

Another important design engineering fact about modulus of elasticity is that it is independent of metal shape, that is, casting geometry.

Section Modulus

Another measure of stiffness is section modulus, which is stiffness from shape or geometry; unlike modulus of elasticity, it has nothing to do with the material.

Actually section modulus is an aspect of moment of inertia which is a function of a shape's cross-sectional area in combination with its height.

Two important conclusions can be drawn from the mathematics:

• The only factors in the equations are shape!

• The final equation gives the engineer clues about how the shape of the I-beam could be varied to maximize moment of inertia and therefore the section modulus while minimizing the amount of material in the beam.

Stiffness from geometry of section modulus is a very powerful engineering tool. The knowledge of section modulus enables the engineer to create metal shapes that are much stiffer than the material itself could ever be.

The most significant observation that can be made about stiffness from geometry is that there is no other method besides metal casting that can offer so much geometry in the design and manufacture of component shapes.

Another significant observation is that design stress in a structural part is directly related to section modulus. In fact, it is a direct, inverse relationship in which increasing section modulus decreases stress.

We now see an important synergism between modulus of elasticity and section modulus. Modulus of elasticity determines how much stress a metal can safety carry before it begins to deform permanently and section modulus enables the engineer to use geometry to keep the stress within safe bounds. As we have learned, creative use of section modulus enables relatively weaker metals to do the work of stronger ones.

The development of engineering computer hardware and software for making and analyzing solid models has enabled a quantum leap in the use of section modulus to increase the stiffness of structural components and reduce the stress within them. In fact, these tools are making the power of metalcasting geometry much more accessible to design engineers because they enhance so significantly the ability to visualize in three dimensions.

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Ordering Steel Castings

The ordering of steel castings takes a certain amount of time and energy to qualify a potential supplier. To get the best value from the steel casting also requires a cooperative effort on the part of the buyer and the seller from the early stages of the design through the manufacturing process. Good planning ahead of time will pay dividends for both you and your supplier.

1. How to Order Steel Castings As with any manufacturing process, in order to produce a part, it is necessary to know:

• Design - What is the part?

• Material - What should the part be made of?

• Testing - How should the part be tested before delivery?

2. Inquiry and Ordering All pertinent information must be stated on both the inquiry and order including:

• Casting shape--either by drawing or pattern. Drawing should include dimensional tolerances, indications of surfaces to be machined, datum points for locating.

• Material specification and grade (e.g. ASTM A27-87 Grade 60-30 Class I).

• Number of Parts

• Supplementary Requirements (e.g. ASTM A781-87a S2 Radiographic Examination).

o Test Methods (e.g. ASTM E94)

o Acceptance Criteria (e.g. ASTM E186 severity level 2 or MSS-SP-54)

• Any other information that might contribute to the production and use of the part.

I. Design To achieve the most efficient production and the highest quality product, the part should be designed to take advantage of the flexibility of the casting process.

The foundry must have either the designer's drawings or pattern equipment and know the length of the run (number of parts to be made).

To take advantage of the casting process, the foundry should also know which surfaces are to be machined and where datum points are located. The acceptable dimensional tolerances must be indicated when a drawing is provided.

Tolerances are normally decided by agreement between the foundry and customer. SFSA Handbook Supplement 3 represents a common starting point for such agreements. It is not a specification and care should be taken to reach agreement on what tolerances is required.

Close cooperation between the customer's design engineers and the foundry's engineer is essential to optimize the casting design.

Minimum Section Thickness

The rigidity of a section often governs the minimum thickness to which a section can be designed. There are cases, however, when a very thin section will suffice, depending upon strength and rigidity calculations, and when castability becomes the governing factor. In these cases it is necessary that a limit of minimum section thickness per length be adopted in order that liquid metal will completely fill the mould cavity in these thinner sections.

Molten steel cools rapidly as it enters a mould. In a thin section, close to the gate, which delivers the hot metal, the mould will fill readily. At a distance from the gate, the metal may be too cold to fill the same thin section. A minimum thickness of 0.25 in (6 mm) is suggested for design use when conventional steel casting techniques are employed. Wall thickness of 0.060 in (1.5 mm) are common for investment castings and sections tapering down to 0.030 in (0.76 mm) can readily be achieved.

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Draft

Draft is the amount of taper or the angle, which must be allowed, on all vertical faces of a pattern to permit its removal from the sand mould without tearing the mould walls. Draft should be added to the design dimensions while maintaining minimum metal thickness.

Regardless of the type of pattern equipment used, draft must be considered in all casting designs. (Draft can be eliminated by the use of cores; however, this adds significant costs.) In cases where the amount of draft may affect the subsequent use of the casting, the drawing should specify whether this draft is to be added to or subtracted from the casting dimensions as given.

The necessary amount of draft depends upon the size of the casting, the method of production, and whether moulding is by hand or machine. Machine moulding will require a minimum amount of draft. Interior surfaces in green sand moulding usually requires more draft than exterior surfaces. The amount of draft recommended under normal conditions is about 3/16 in. per ft. (approximately 1.5 degrees), and this allowance would normally be added to design dimensions.

Parting Line

Parting in one plane facilitates the production of the pattern as well as the production of the mould.

Patterns with straight parting lines, that is, with parting lines in one plane, can be produced more easily and at lower cost than those with irregular parting lines.

Casting shapes which are symmetrical about one center line or plane readily suggest the parting line. Such casting design simplifies moulding and coring, and should be used wherever possible. They should always be made as " split patterns " which require a minimum of handwork in the mould, improve casting finish, and reduce costs.

Cores

Number of Cores

A core is a separate piece (often made from moulding sand) placed inside the mould to create openings and cavities which cannot be made by the pattern alone. Every attempt should be made by the designed to eliminate or reduce the number of cores needed for a particular design to reduce the final cost of the casting.

The minimum diameter of a core which can be successfully used in steel castings is dependent upon three factors:

• The thickness of the metal section surrounding the core

• The length of the core

• The special precautions and procedures used by the foundry.

The adverse thermal conditions to which the core is subjected increase in severity as the metal thickness surrounding the core increases and the core diameter decreases. These increasing amounts of heat from the heavy section must be dissipated through the core. As the severity of the thermal conditions increases, the cleaning of the castings and core removal becomes much more difficult and expensive.

The thickness of the metal section surrounding the core, and the length of the core, both affect the bending stresses induced in the core by buoyancy forces and, therefore, the ability of the foundry to obtain the tolerances required. If the size of the core is large enough, rods can often be used to strengthen the core. Naturally, as the metal thickness and the core length increase, the amount of reinforcement required to resist the bending stresses also increases. Therefore, the minimum diameter core must also increase to accommodate the extra reinforcing.

Core Removal

The cost of removing cores from casting cavities may become prohibitive when the areas to be cleaned are inaccessible. The casting design should provide for openings sufficiently large to permit ready access for the removal of the core.

Internal Soundness - Directional Solidification

Steel castings begin to solidify at the mould wall, forming a continuously thickening envelope as heat is dissipated through the mould-metal interface. The volumetric contraction that occurs within a solidifying cast member must be compensated by liquid feed metal from an adjoining heavier section, or from a riser which serves as a feed metal reservoir and which is placed adjacent to, or on top of, the heavier section.

The lack of sufficient feed metal to compensate for volumetric contraction at the time of solidification is the cause of shrinkage cavities. They are found in sections which, owing to design, must be fed through thinner

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sections. The thinner sections solidify too quickly to permit liquid feed metal to pass from the riser to the thicker sections.

Design for Machining

In the final analysis the foundry casting engineer is responsible for giving the designer a cast product that is capable of being transformed by machining to meet the specific requirements intended for the function of the part. To accomplish this goal a close relationship must be maintained between the customer's engineering and purchasing staff and the casting producer. Jointly, and with a cooperative approach, the following points must be considered:

• The casting process, its advantages and its limitations.

• Machining stock allowance to assure clean up on all machined surfaces.

• Design in relation to clamping and fixturing devices to be used during machining.

• Selection of material specification and heat treatment.

• Quantity of parts to be produced.

Layout

It is imperative that every casting design, when first produced, be checked to determine whether all machining requirements called for on the drawings may be attained. This may be best accomplished by having a complete layout of the sample casting to make sure that adequate stock allowance for machining exists on all surfaces requiring machining. For many designs of simple configuration that can be measured with a simple rule, a complete layout of the casting may not be necessary. In other cases, where the machining dimensions are more complicated, it may be advisable that the casting be checked more completely, calling for target points and the scribing of lines to indicate all machined surfaces. Additional guidelines for casting design are given in the Steel Castings Handbook and Supplements 1, 3 and 4 of the Handbook.

II. Material The material to be used to produce the part must be identified in the order. Material for steel castings is generally ordered to ASTM requirements, although other specifications may be used. This section contains a summary of the scope, chemical composition requirements and mechanical property requirements of these materials or product specifications. Many requirements are common to several specifications and are given in: ASTM A781/A781M-89 and/or ASTM A703/703M-89

A781/ CASTINGS, STEEL AND ALLOY, COMMON REQUIREMENTS, FOR A781M-89 GENERAL INDUSTRIAL USE

This specification covers a group of requirements that are mandatory requirements of the following steel casting specifications issued by the American Society of Testing and Materials: A27, A128, A148, A297, A447, A486, A494, A560, A743, A744 and A747.

If the product specification specifies different requirements the product specification shall prevail.

This specification also covers a group of supplementary requirements, some of which may be applied to the above specifications as required. These are provided for use when additional testing or inspection is desired and applies only when specified individually in the order by the purchaser.

A703/ STEEL CASTINGS, GENERAL REQUIREMENTS, FOR PRESSURE-A703M-89 CONTAINING PARTS

This specification covers a group of common requirements which, unless otherwise specified in an individual specification, shall apply to steel castings for pressure-containing parts under each of the following ASTM specifications: A216, A217, A351, A352, A389, and A487.

This specification also covers a group of supplementary requirements which may be applied to the above specifications as indicated therein. These are provided for use when additional testing or inspection is desired and applies only when specified individually by the purchaser in the order.

III. Testing Testing ensures that the material meets the requirements of the specification; consequently, testing is mandatory. More frequent testing or other tests may be imposed, by use of supplementary requirements of product specifications or general requirement specifications.

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In addition to specifying test methods, acceptance criteria must be agreed upon. The more testing and tighter the acceptance criteria--the more expensive the product will be--without necessarily increasing quality or serviceability. Hence, the extent of testing and acceptance criteria should be based on the design and service requirements.

The mechanical properties are verified by the use of test bars cast either separately or attached to the castings.

The mechanical properties obtained represent the quality of the steel, but do not necessarily represent the properties of the castings themselves, which are affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size and shape. In particular, the hardenability of some grades may restrict the maximum size at which the required mechanical properties are obtainable.

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Buyer: Purchasing Steel castings

To get the best value from a steel casting requires a cooperative effort by both the buyer and the seller from the early stages of the design through the manufacturing process. Good planning ahead of time will pay dividends for both you and your supplier.

1. Purchasing Policy The responsibility for buying steel castings lies primarily with the purchasing department. However, contributions toward the evaluation, selection, and monitoring of vendors are being made more frequently by various departments. These may include product design, manufacturing, quality control, inspection, and general management. This trend towards broad input clearly indicates the need for a defined purchasing policy that defines the responsibilities of the people involved and the extent of their authority. The objectives of the steel casting buyer still remain:

• acceptable costs,

• quality,

• reliability,

• and delivery,

These goals can be readily attained when all parts of an effective purchasing program are put into action.

Each steel casting is specifically designed and manufactured to perform a given function under predetermined operating conditions. Due to the varied requirements of each user, the steel casting industry has staffed itself with qualified personnel to provide the user with expertise in the selection of the specifications, casting design, foundry techniques, and finishing processes.

2. Quotation Procedure The purpose of requesting a quotation for a steel casting is basically to determine the lowest purchased casting cost. The buyer then must weigh all of the provisions of the quotation including exceptions taken to drawings, specifications, and processing requirements, as well as vendor experience, tooling requirements, tolerances, finish allowances, and delivery. Such factors as reduced machine work, better tolerances, and reliability are particularly important to determine the lowest end cost of the casting.

To avoid misunderstandings, reduce costs, and expedite the processing of quotations the following information should be included in a request for a quotation:

Material and inspection requirements.

ASTM or other nationally recognized specifications should be used whenever possible to identify the material and inspection requirements. See Specifications in Steel Castings

Acceptable casting weight.

Actual weight information is preferred. Estimates should be provided in the absence of actual weight information.

Drawing.

Machine drawings are preferred over casting drawings. Target points should be included in the drawing. Drawings are required regardless of the existence of patterns.

Pattern.

If patterns and core boxes are available, the request for a quotation should indicate the type and condition of the equipment.

Delivery.

Present and anticipated need should be included in quotation requests together with required delivery schedules.

Beyond these basics, there are levels of buyer requirements that could include vendor liabilities, which affect the casting cost drastically. These could include receiving inspection acceptance and back charge policy, casting return policy, expediting procedures, and sophisticated controls not normally associated with the standard

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inquiry. A complete understanding of these areas is best developed by an open relationship between the buyer and the foundry representative, and the professional attitudes and experiences that both can provide during the quotation evaluation phase.

3. Selection of Casting Supplier Every service a steel casting buyer may need is available in the steel foundry marketplace. The question then becomes "where and who?" The Steel Founders' Society of America publishes a directory of its members and a detailed listing of all steel foundries in the United States, Canada, and Mexico providing the buyer with pertinent information relative to plant personnel, capacities, special services, types of steel produced, and relative size of castings. With the Directory as a starting point, an initial list of potentially acceptable suppliers can be established. The refinement of this list may require collaboration between the buyer's engineering, manufacturing, and quality control departments depending upon the scope of the company's purchasing policy.

4. Specifications Industry approved specifications as discussed in Specifying Steel Castings, provide the casting buyer with the tools necessary to establish criteria for almost any casting application. These specifications do not preclude special requirements that the buyer's technical staff members may require. Variations from standard specifications can result in misunderstandings, higher costs and disqualification of potential vendors. If exception is taken to a provision in the main body of a specification requirement (as opposed to taking exception to a supplemental requirement of a specification), the resulting casting cannot be held to compliance with that specification.

5. Patterns Pattern equipment design and the resultant costs can constitute a major source of misunderstanding between buyer and vendor. The need to construct new pattern equipment when existing equipment is available, a requirement for a full split core box in place of a half core box, pattern material, and mounted or loose patterns are but a few of the many areas of discussion that effect the cost of the equipment. Invariably, the lowest casting cost and highest casting quality evolve from the more sophisticated pattern equipment, which generates the highest pattern cost.

6. Order Placement Order placement is the most important phase in the buyer-seller relationship, not only because it is the first pure contractual agreement between buyer and seller, but also because it requires acknowledgment of schedules and specific commitments.

For purchase commitments to be fulfilled according to schedule, all relative information previously developed in the preliminary phases must be detailed in the purchase contract. The basic elements of the contract that should be clearly defined are:

• Purchase order number and date

• Pattern number and /or part number

• Pattern and machine drawing numbers with current revision notation

• Quantity required

• Material specification

• Casting price

• Delivery requirements

• Shipping instructions

If sample casting are required, the following additional requirements should be provided:

• Number of samples required

• Non-destructive examination requirements

• Dimensional examination requirements, if any

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• In-process inspection, if required

• Final machining approval, if required

• Special tests, if required

If patterns are to be constructed by the supplier, the following additional information should be provided:

• Pattern price

• Description of pattern to be supplied

• Pattern delivery

If the patterns are to be supplied by the buyer, the following additional information should be provided:

• Pattern mounting and rigging price

• Description of pattern services

• Pattern accuracy liability

• Pattern delivery

If the castings are to be supplied to special process specifications, any of the following additional requirements can be requested:

• Chemistry certification

• Mechanical test certification

• Brinell hardness ranges and test location

• Impact test, type and results required

• Ultrasonic examination including coverage requirements, specification and quality levels

• Magnetic particle or dye penetrate testing, include coverage requirements, specification, and quality level

• Radiographic examination including coverage requirements, approved shooting sketch, specification, and quality level(s)

• Special surface requirements

• Special packaging

• Preferences regarding the method of shipping.

It is extremely important that initial casting orders be complete and accurate and that verbal orders be avoided due to their propensity for generating errors through omission.

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Buyer: Specifying a Steel Casting

This section provides an overview of the role of specifications as a means of communication between all parties concerned in the purchase of steel castings.

1. Introduction One needs only a superficial knowledge of a few of the existing steel casting specifications, and of metallurgy in general, to understand that stating what one needs is not a simple matter. All requirements must be clearly and accurately stated with nothing taken for granted. This is best accomplished by the use of standards and specifications.

As you read this section, you will notice that the use of nationally recognized standards and specifications is recommended, while the use of proprietary specifications is strongly discouraged.

2. Definitions In the specification process for steel castings, there are three key words which should be understood. These are specifications, standards, and codes.

Specifications

A specification is a form of standard, which precisely states a set of requirements to be satisfied by the casting. Some of these requirements might be chemical composition, mechanical properties, repair procedures, or any other requirement that is necessary to develop the quality of the casting needed for its end use. Specifications for steel castings are sometimes expanded or limited by standards and codes.

Standard

A standard can be defined as a specification, test method, definition, or recommended practice that has been approved by a nationally recognized specification-writing body such as ASTM (American Society for Testing and Materials), ISO (international Organization of Standards), or SAE (Society of Automotive Engineers). A standard can be further be defined as a document which details properties, processes, dimensions, materials composition, relationships, or concepts. This connotation follows Webster's definition of "something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or quality." It can be seen then that there is some overlapping between specifications and standards, and for that reason the two terms are often used interchangeably where steel castings are concerned.

Code

The word " code " is a term of much broader meaning than either specification or standard and can best be described as a set of rules established by a recognized authority such as the American Society of Mechanical Engineers' (ASME), Boiler and Pressure Vessel Code or the United States of America Standards Institute's (USASI) code for pressure Piping. In adopting the rules that make up the various codes, consideration is generally based on health, safety, and environmental protection. The code-formulating bodies, in addition to writing their rules, usually adopt ASTM material specifications either in whole or in part to be come a part of the code.

3. What Can Be Specified A specification, as previously defined, is a precise statement of requirements. Therefore, any requirement can be specified or incorporated into the specification. The most common are listed and discussed in the paragraphs that follow.

It should be emphasized that ASTM specifications take into consideration all of these requirements and more, so when using an ASTM specifications, there seldom is a problem with omissions.

Composition Limits and Tolerances

Most steel casting specifications take into consideration the chemical analysis of the casting either directly by specifying the analysis, or indirectly by specifying properties that are related to the analysis, such as hardness or tensile strength, and leave the choice of composition to the foundry. But no matter how considered, the composition is important. For example, the chromium in the various stainless grades of ASTM A351 must be within prescribed limits for predictable corrosion resistance. Other elements in those same grades such as carbon, nickel, and molybdenum, must also be within limits in order to maintain a balanced microstructure necessary for

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the mechanical strength of the alloy and to insure proper corrosion resistance and performance in different environments.

There are also carbon and low alloy grades with specified chemical ranges found in some ASTM specifications such as A216, A217, A487, and several others. Most of the grades found in those specifications have been evaluated as to weldability and mechanical properties at various elevated temperatures, and approved for ASME code use by the ASME Boiler and Pressure Vessel Committee. Since any change in composition may have some effect on weldability, and on the high temperature performance of the casting, other grades whose chemistry may deviate only slightly from the approved grades are not acceptable for ASME code use.

Structural and engineering grades of high strength cast steel are covered by A148 (High Strength Steel Castings). The only chemical requirement in that specification is for sulfur and phosphorus. Other chemical requirements have been avoided because no foundry can cast all steels in the many modifications. The strength levels in A148 run from 80 ksi to 175 ksi (552-1207 MPa) tensile strength. The chemistry chosen for each grade should take into account the strength level, section sizes to be cast, the complexity of design, heat treating methods, and the end use of the casting.

Tolerances for chemical analysis are relatively new to steel casting specifications, although they have been in use for other steel products for some time. The product analysis tolerance, if one is given, merely specifies the amount by which an analysis of a sample, taken from a casting, may deviate from the specified composition range.

Dimensions, Weight and Tolerances

Variations occur in dimensions and weights of parts made by any metal-shaping process. Tolerances are the expression of the expected or acceptable variation. Dimensional tolerances should be included on any casting drawing. Quotations and acknowledgments from the foundry will often refer to a variation in weight or weight tolerance.

Properties and Performance

Steel castings, whenever possible, should be purchased to property requirements rather than to chemical analysis specifications. most of the national specification are written in terms of tensile properties and in some cases hardness values, impact values, and hardenability ranges. This permits the foundry engineer to select the alloy compositions which best satisfy mechanical property selection.

Mechanical properties of steel castings can be categorized as follows:

• Tensile properties which include tensile strength, yield strength, elongation, and reduction of area.

• Impact properties or toughness which is most often determined by the amount of energy absorbed during fracture in a Charpy V-notch impact test, involving both ductility and strength and usually expressed in US specifications as "foot pounds".

• Fatigue properties. Most fatigue testing results are expressed by plots of stress versus number of cycles. The plot is often referred to as an "S-N" curve, where S stand for stress and N for the number of cycles of stress to cause failure. The stress level at which failure does not occur regardless of the number of cycles is known as the endurance limit of the material. For steel, testing to 10 million cycles is considered sufficient insurance that the endurance limit has been reached.

• Hardness and Hardenability. Hardness and hardenability should not be confused. Hardness is the property usually specified, and is a measure of the resistance to indentation during the hardness test. Hardenability is the property that determines the depth and distribution of hardness induced by quenching. The importance of mechanical properties at a depth below the surface of the casting of a given design determines the significance which the engineer must place on hardenability. Carbon steels are less hardenable than low alloy steels and should not be used in applications requiring high hardenability.

Surface Integrity and Roughness

For many years, most casting specifications, including those issued by ASTM, contained very ambiguous wording in regard to surface inspection and integrity. For instance, castings were to be clean and "free from injurious defects." There was no definition for defect: and no basis for judgment as to what was "injurious." If the purchaser's inspector said a discontinuity was injurious, no matter how small, it had to be removed and repaired. Furthermore, the "injurious defect" had to be "completely removed to sound metal." Again, there was no definition for "sound metal" and no basis for judgment for "completely." Requirements of this type can easily be misunderstood, and misapplied; they can cause no end of grief for both the foundry and the purchaser. This problem has been rectified in the ASTM specifications.

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Unfortunately, many other specifications today still contain the same or similar ambiguous wording. Whenever requirements such as these are discovered, every attempt should be made to rewrite them in a manner similar to those found in the latest ASTM specifications. The writers of the ASTM specifications, which include both producers and users of steel castings, have replaced such ambiguous wording with requirement such as " The surface of the casting shall be examined visually and shall be free of adhering sand, scale, cracks, and hot tears. " This preferred wording goes on to say that " unacceptable visual surface discontinuities shall be removed and their removal verified by visual examination of the resultant removal verified by visual examination of the resultant cavities. " If more stringent examination of the cavity is required, the ASTM specifications allow for that option, and the purchaser may so specify in his purchase order, and then both parties will have a clear understanding of what is expected.

To help define "unacceptable visual surface discontinuities," the ASTM specifications state that "Visual Methods MSS-SP-55", which is issued by the Manufacturers Standards Society of the Valve and Fittings Industry, may be used. This standard contains photographs of various casting surfaces and defines them as acceptable or unacceptable.

ASTM A802 has a 31 piece set of surface comparators which has many more categories of surface finish from which the purchaser will specify the level of acceptance he requires.

When higher levels of surface integrity are needed, other inspection techniques such as magnetic particle or dye penetrant examination may be specified. With these methods, cracks can be revealed that would go undetected with the unaided eye.

Internal Integrity, Soundness

The soundness of a casting is most often determined by radiographic methods, although ultrasonic inspection is also used, especially on heavy sections. Pilot castings of small size or those preceding large production runs, in addition to being radiographed, are often destructively examined by sawing into slices and examining the pieces. Once the casting procedure and foundry technique have been established, the internal integrity will remain relatively unchanged.

However, when a specific internal quality level is required, it should be stated in the order. Even though a pilot casting might meet all radiographic requirements, there is no guarantee that all others will meet the same level. Some of the controlling factors, no matter how closely monitored, may change even slightly and affect the end result.

Inspection Methods and Procedures

There are three principal means of inspection in common use to detect internal and surface discontinuities in steel castings. These are:

• radiography

• magnetic particle and liquid penetrant,

• ultrasonic

Standard methods or recommended techniques for carrying out the inspection have been developed and are published in ASTM documents listed in the Testing and Inspection Specification Table. In all cases, the pertinent ASTM document should prevail over any individual company specification unless it is proven to be inadequate for the specific application.

Testing Methods and Procedures

Test results, frequency of testing, and the sampling procedures for obtaining specimens to be tested are usually specified in material specifications. The actual methods and procedure for performing the tests, such as tension, bend, hardness, and impact, are generally specified in a testing specification such as ASTM A370.

Complying with the requirements of a testing specification assures that all testing is conducted in a standard and reproducible manner.

Manufacturing and Welding Methods and Procedures

The use of " how to do it " or process specifications in the manufacturing of steel castings is to be discouraged for several reasons. There are wide variations between methods used by various foundries, yet each is capable of achieving the same end results. For a customer to decide which method should be used would seriously hamper the development of new manufacturing techniques. Such specifications might be justified in certain well-established areas, that by their very nature are not subject to further development or change, but in the field of foundry science new developments are constantly being made. As a result, there are very few outside the field who are qualified to write such a document, even if it were desirable to do so. One inherent problem with

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process specifications is that they often contain requirements which cannot be checked by the buyer. This weakens any specification.

Welding methods, likewise, should be left to the foundry and not dictated by a process specification. Upgrade welding is just as much an operation in the manufacture of steel castings as is the moulding operation or any other operation involved in casting manufacturing, and all freedom possible should be granted the foundry. However, since the weld will become a part of the casting and go into service with the casting, it is perfectly justifiable, for metallurgical reasons, to specify that the welding procedure be qualified. In fact, almost all of the ASTM casting specifications require that procedures and welders be qualified in accordance with the recommended practice described in ASTM A488. Procedures and welders qualified to ASME, Section IX, are automatically qualified to A488.

4. Overlapping, Redundant, and Contradictory Specifications All orders should reference a nationally recognized specification, preferably an ASTM specification. The requirements of ASTM cover many widely diversified applications of steel castings, including carbon, low alloy and high alloy (corrosion- and heat-resistant steels. They are prepared by knowledgeable representatives of both purchasers and producers working together to develop specifications of proven usefulness and compatibility.

These specifications are subject to review and discussion twice a year by the subcommittee on steel castings. Anyone, whether an ASTM member or not, who has a problem with an ASTM specification can write to ASTM and describe the difficulty, or meet with the subcommittee. All problems and solutions are openly discussed. Overlapping, redundant, and contradictory requirements are eliminated whenever they are discovered.

Many non-standard specifications, such as those prepared by individual company organizations, may have conflicting requirements. A common example is a Brinell hardness requirement which is not always compatible with the specified tensile strength. Since there is no absolute conversion from hardness to tensile strength and a maximum hardness. Even then, care should be taken to be sure there is a workable range between the two. Another example is specification of chemical analysis when mechanical properties might be the only requirement really needed.

Unfortunately in some cases, changes are made to existing ASTM requirements, and then incorporated into customer specifications even though they may not apply to the customer's casting needs. These changes frequently contain provisions that have been previously rejected by ASTM as impractical or unnecessary. The net result is partial duplication of specifications and some unnecessary restrictions. Thus, castings for similar end use may have requirements for two or three different quality levels.

Such multiplicity of specifications results in confusion and misunderstanding, and unnecessarily increases the cost of a casting without affecting its serviceability.

5. Specification Writing Bodies and Jurisdiction There are numerous organizations, public and private, that promulgate specifications and hence have full jurisdiction over them. By far the largest specification writing body in the United States is the American Society for Testing and Materials (ASTM) whose standards are used worldwide. Other specifications in common use for special products and under the jurisdiction of their respective trade associations are those issued by the Association of American Railroads (AAR) and the Society of Automotive Engineers (SAE). In addition, there are a number of military and other agency specifications. However, for complete details of any of the specifications, it will be necessary to refer to the complete and latest document.

One other major specification writing body whose specifications are beginning to be used in the US is the International Organization for Standards (ISO). Because of the worldwide use of the ASTM and ISO specifications and standards, an understanding of each is helpful.

6. Material Specifications Specifications and standards predominately used by the steel casting industry are those issued by ASTM and can be grouped into three general groups:

• material,

• welding, and

• testing

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To obtain complete details, the original specification should be consulted. The additional number following the ASTM designation, e.g., A216-80, indicates the year of adoption or latest version. The most recent revision of any specification should always be used.

Material specifications for structural and engineering grades of steel castings are covered by A27 and A148. Carbon and low alloy steel valves and fittings for elevated temperature service are covered by A216 and A217, while castings for low temperature service are covered by A352. Other material specifications cover castings for specific applications, such as steam turbine castings (A356) and bridge castings (A486).

Each specification has been written for a particular type of service or environment which should be described in the title and in the scope of the specification. In all ASTM specifications, the scope is stated in the first paragraph. The requirements of any specification should be compatible with the intended use of the casting. Castings should not be produced under a particular specification if the intended use of the casting is outside the scope of that specification. For instance, carbon steel valve castings intended for high temperature service should be ordered to A216 and not A27. Also, high strength structural castings should be ordered to A148 and not to A87. Ordering castings for use outside the scope of the specification may result in additional and unnecessary requirements or in the omission of requirements that are necessary for the particular application.

7. Welding Specifications Welding specifications are listed in the Testing and Inspection Specification Table.

These two specifications, ASTM A488 and Section IX of the ASME Boiler and Pressure Vessel Code, are the documents most often used for the qualification of procedures and welders.

Not all ASTM grades of steel have been adopted for pressure service under the ASME Boiler Code and referenced in Section IX. To try to qualify those grades under Section IX may result in confusion and possibly misunderstanding with the customer as to interpretation of the qualification rules. For that reason it is best to qualify to Section IX only those grades of steel actually approved for Code use and referenced in Section IX. All other grades should be qualified under the rules of ASTM A488.

There need be no duplication in qualification because A488 states that welders and procedures qualified to Section IX are automatically qualified to A488.

8. Testing Specifications Testing specifications for steel castings are included in the Testing and Inspection Specification Table.

Standard test methods for the purpose of obtaining mechanical property data, such as tension, bend, hardness, and impact, of a cast steel are specified in ASTM A370. Test coupons, specimen dimensions, and exact testing procedures are detailed for each type of mechanical test. Whenever mechanical testing is required and is not covered by the material specification, the exact type of cast coupon and type of specimen should be spelled out in the purchase order or contract to be in accordance with the provisions of ASTM A370.

If the determination of the nil-ductility transition (NDT) temperature is required, the drop-weight test method described in ASTM E208 should be used.

9. Inspection Standards The steel casting industry has numerous specifications and standards that are concerned with nondestructive testing of its products. A list is included in the Testing and Inspection Specification Table.

The chief criteria of casting quality are surface appearance and integrity, soundness of sections, and accuracy of dimensions. For this reason, most inspection standards are concerned with these properties.

Often, the designer, if he is unfamiliar with the foundry process, may specify a quality level higher than the design really requires, which serves no purpose except to increase the cost. A more favorable price and delivery can be obtained by first selecting the material specification (preferably ASTM) which meets the mechanical test requirement, and whose scope encompasses the service for which the part is intended. The necessary quality level can then be established by specifying special inspection procedures such as visual, magnetic particle, liquid penetrant, radiography, ultrasonic, and dimensional tolerance.

Surface Discontinuities

Surface discontinuities are the irregularities, imperfections, or cracks that are found on the surface of the casting. Although some are of such size that they can be seen visually, others are either not visible or go unnoticed without special inspection methods such as magnetic particle or liquid penetrant examination. For the

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examinations to be meaningful as a basis for purchase, all parties concerned must use inspection methods that are standard.

Visual Inspection Standards. Most ASTM specifications contain a requirement stating that the surface of the casting will be examined visually and free of adhering sand, scale, cracks, and hot tears. Visual Method MSS-SP-55, available from the Manufacturers Standardization Society of the Valve and Fittings Industry may be used to define acceptable surface discontinuities. This standard consists of a series of photographs which are defined as acceptable and unacceptable. Any other visual standard may also be used as long as both parties agree to it.

ASTM A802 is a 31 plate set of comparators depicting various degrees of surface discontinuities in several categories such as wrinkles, porosity, veining, etc. Under this new standard the purchaser can specify surface requirements by quoting category numbers and levels of appearance. The standard specifies nothing as being acceptable or unacceptable. The comparators are merely points of reference used in communicating a requirement.

Magnetic Particle Inspection. Magnetic particle inspection is used to detect surface discontinuities. Under ideal conditions certain discontinuities lying just below the surface can also be detected. However, this is primarily a surface inspection method and caution should prevail in attempting to ascribe other capabilities to it. Also, any conclusion with regard to depth or extent of the interior nature of the discontinuity must be based on exploration by other test methods. Magnetic particle techniques methods for dry powder and wet inspection are set forth in ASTM E109 and E138, respectively.

A set of reference photographs has been assembled by ASTM as document E125 depicting the appearance of different types of casting surface discontinuities as revealed by the dry power magnetic particle technique. Each type of discontinuity is classified in five degrees of severity, except porosity, where two examples are shown. To avoid any misunderstanding, it should be pointed out there is no correlation between degrees of the various type of discontinuities. For instance, degree 3 of type I is not equivalent to degree 3 of type II.

By prior agreement between the purchaser and the producer, these photographs may be used as standards to accept or reject castings. The acceptable degree of severity for each type of discontinuity must be spelled out in the purchase order or contract. Different types of discontinuities do not have equal effects on the serviceability of the casting and an effort should be made to assign realistic acceptance levels to each area of the casting, based upon the type and magnitude of stresses to which each area is subjected in service.

Admittedly, it is difficult to rigidly interpret magnetic particle indications on castings against a set of photographic references; consequently there is a need for close cooperation between the manufacturer and the purchaser.

One example is in the linear discontinuity of degrees 1 and 2. The degree 1 indications are approximately 1/2 in. (13mm) long, while those of degree 2 are approximately 5/8 in. (16 mm) in length. Also, the degree 2 causes the powder to cling in a wider pattern. In the interpretation of indications, however, their width is seldom considered; only the length of the indication is compared to the photographic references.

Although the separation between degree 1 and degree 2 is completely arbitrary and in no way related to service performance, there is often great concern as to whether the indication is greater than 1/2 in. (13 mm) or less than 5/8 in. (16 mm).

To overcome some of the interpretation problems, some purchasers specify acceptance standards to various degrees in E125 and then add that cracks and not tears shall not exceed 1/2, 1/8, or even 1/16 in. (13, 3, or 1.5 mm) in length. Although these dimensions are somewhat arbitrary, their being specified does eliminate much misunderstanding.

Misunderstanding can be minimized if the inspectors for both parties are least ASNT (American Society for Nondestructive Testing) Level 1 inspectors. This is a rating of the level of competence to which the individual is certified by training and completion of a prescribed number of classroom hours in inspection techniques and interpretation. The ASME Boiler and Pressure Vessel Code requires that inspectors making interpretations be certified to Level 2, which is a higher qualification than Level 1.

Additionally, orders should not state both visual and magnetic particle standards because of the possibility of overlapping or contradictory requirements. If magnetic particle examination is needed, then visual methods of inspection need not and should not be specified.

Magnetic particle inspection has probably led to more misunderstanding than any other inspection tool. It has made possible the selection of castings for critical applications by greatly assisting the upgrading effort with its outstanding ability to detect surface discontinuities. It can also, when improperly applied, increase the cost of a casting without improving its performance. Therefore, it is essential that standards of acceptance be applied with discretion.

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Liquid Penetrant Inspection. Liquid penetrant inspection is another surface discontinuity detection method. It is not generally used on the " as cast " or shot blasted surfaces because of the likelihood of obtaining false indications. The penetrant method is best suited for use on machined, ground, or very smooth " as cast " surfaces. Liquid penetrant inspection is of particular importance for austenitic alloys because they are non-magnetic and therefore their surfaces cannot be examined by magnetic particle inspection. ASTM E 165 describes the standard method for conducting this test.

A set of reference photographs for acceptance or rejection is contained in ASTM E433. It should be pointed out that there are no degrees of severity, as in E125 for the dry powder magnetic particle technique. Each of the documents must specify actual dimensions including maximum length of indications and number of indications per unit area. Also, no attempt has been made to establish the metallurgical cause of the discontinuity.

When E433 is specified, there should be a prior agreement of interpretation and acceptance to prevent subsequent misunderstanding.

Dimensional Tolerance Classes

Dimensional tolerances are permissible deviations from the nominal dimension. Deviations from the nominal, or aimed for dimension, may occur for several reasons. The primary source is the contraction of the liquid metal as it solidifies and cools in the mould.

An experienced foundryman can estimate the metal contraction that will occur on any dimension, but only trial by actual production will show precisely how the metal will behave. Tolerances for the production of a single casting, therefore, tend to be liberal. On the other hand, with castings produced in large numbers, the opportunity exists to make changes in pattern equipment and manufacturing processes to compensate for abnormal casting contraction behavior. In this situation variation will be minimized but a slight variation will still exist.

Upgrading, by grinding and gagging, coining, straightening, and other measures are available to achieve any desired tolerance level that cannot be achieved by the casing process alone. Upgrading of this type adds to the price and should be specified only where required to minimize the cost of the component.

A series of tolerance classes is a practical means of communicating the needed tolerance to the foundry and of explaining to the purchaser the ability of the foundries' process, or processes, without subsequent dimensional upgrading. The new system of tolerance classes is therefore desirable, and five tolerance classes have been suggested.

The tightest grade, or class, represents the best tolerance encountered in a recent SFSA study. It is named T3 to provide room for additional tolerance classes in the future, and for specialized processes such as investment and ceramic moulding techniques. Tolerance grades T5 and T7 represent the average, and the widest tolerance encountered, respectively. The intermediate classes T4 and T6 were selected to provide for a geometric progression from T3 to T7.

Internal Discontinuities

Radiography. There are three basic groups of reference radiographs issued by ASTM for evaluation of steel castings as seen in the Testing and Inspection Specification Table.

E446 applies to castings up to 2 in. in thickness (51 mm), E186 to 2 to 4-1/2 in. (51-114 mm) thick sections, and E280 to wall thickness of 4-1/2 in. to 12 in. (114-305 mm). Each group is available in a choice of sets based upon the source of radiation employed, such as low-voltage X-rays, iridium -192, cobalt -60, 1 to 2 MeV X-rays or 10 to 24 MeV X-rays.

A special set of reference radiographs for investment castings is available as ASTM E192.

Reference radiographs of discontinuities common to steel welding are categorized in ASTM E390. Repair welds should be inspected to the same standards employed for the original casting, i.e., E446, E186, or E280. E390 is applicable to inspection of welds used for cast-weld inspection.

Reference radiographs become standards for acceptance and rejection only after the purchaser and the producer have agreed, in the purchase order or contract, to the acceptable severity level for each individual type of discontinuity. The choice of discontinuity severity level should ideally be based upon realistic evaluation of design and stress analysis criteria under anticipated service conditions. Generally, low severity levels are specified for pressure-containing castings with high pressure rating and wall sections of 1 in. (25 mm) or less. Likewise, low severity levels are specified for machinery or dynamically loaded casting subject to high fatigue and impact stresses, and with wall sections of less than 1/2 in. (13 mm). As wall sections increase and as the fatigue and impact stresses are reduced, severity levels become somewhat relaxed. For structural castings which are not dynamically loaded, moderate severity levels are usually specified, and again, for heavier sections about 3 in. (76 mm) higher severity levels are usually called for.

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To require quality levels in excess of those justified by actual service conditions adds needlessly to the cost of the casting. Also, requiring a single across-the-board severity level for all types of discontinuities should be avoided. Some types are more detrimental than others, depending upon he nature of the stresses to which the casting is subjected in service. For instance, severity level 2 might be specified for shrinkage, and severity level 3 for gas porosity, since the latter is generally much less deleterious to tensile properties. It should also be kept in mind that the entire casting need not necessarily require radiographic inspection and that the same severity levels need not apply to all areas of the casting. This again is governed by the type of stress and the stress levels in the given casting section. Careful analysis or, at least, good judgment can affect sizable cost savings. In any case, the areas to be radiographed with the required severity level should be indicated on the casting drawing.

It should be borne in mind at all times that the severity rating is strictly arbitrary and based on little more than opinion. None of the reference radiographs are based on any kind of test data, and the severity levels are not graded to any basis of acceptability as to service performance. They only serve as a reference point in communicating the purchasers' requirements.

Consistent quality of the radiograph itself can be readily achieved if the recommendations and methods outlined in these two ASTM documents are followed: ASTM E94 is a guide for radiographic testing and E142 is a guide for controlling the reliability or quality of the radiographic images. Both are completely adequate in that internal discontinuities of any significance can thereby be detected. Except for a very few isolated cases, no deviation need be made. Reference radiographs in E242 show how such factors as radiation energy, specimen thickness, and film properties affect the radiographic images.

There is a tendency on the part of some individual company standards to specify films, unsharpness ratios, densities, and other details aimed at producing perfect films with cost being no object. It should not be forgotten that the radiographic film is a means to an end and not the end in itself. It is simply not logical to specify a technique capable of sensitivity which will show discontinuities smaller than the minimum size for rejection.

Ultrasonic. Although the ultrasonic method of inspection has not been in common use for as long as radiographic methods, it nevertheless is a valuable tool for examining heavy wall castings for internal discontinuities. The first ASTM specification for ultrasonic inspection of steel castings was issued in 1970 and is for longitudinal-beam ultrasonic inspection of heat treated carbon and low alloy steel castings. This inspection method is in general not useful for austenitic steel castings due to large grain size of these castings.

It is well recognized that ultrasonic inspection and radiography are not directly comparable. However, the technique is invaluable in detecting discontinuities in heavy sections, where radiographic methods would be considerably slower. Since no picture, in the usual sense, of the discontinuity is obtained, considerable judgment must be exercised in interpretation of results.

One approach in the examination of large heavy wall castings when ultrasonic evaluation may not be acceptable to the purchaser is to first inspect by ultrasonic, then to radiograph only those areas where a suspicious ultrasonic indication is found. Another possibility, since radiography does not reveal the depth of a discontinuity, is to follow radiography with ultrasonic in order to determine and evaluate the depth of the discontinuity.

10. Relevance of Discontinuity Acceptance Levels Surface Discontinuities

The ASTM steel casting specifications contain the requirement that the surface of the casting shall be free of visual cracks and hot tears. The meaning of this statement is quite clear and there is seldom any disagreement concerning this requirement when cracks and hot tears can be seen visually. However, when they are detected by other methods, the severity and relevance become a matter of judgment. In fact, even defining acceptance levels was once something of a problem.

To help overcome this deficiency, ASTM issued specification E125, which has become the standard for surface quality for the industry. The document consists of 37 reference photographs of surface discontinuities divided into five classes of graded severity. However, neither castings nor sections of castings were tested to determine the relationship of various degrees of discontinuity observed to the service requirements of the casting. The different degrees of severity are based on nothing more than opinion. Although the photographs show magnetic indications on steel castings to various levels of severity, the castings were never available for study.

Steel casting buyers often specify a severity level across the board and in many cases, severity level 1 is arbitrarily selected. Some buyers request wet magnetic particle inspection rather than dry powder inspection, and others specify liquid penetrant inspection. The dry power reference photographs of E125 are often employed for all three types of inspection because ASTM has not supplied reference photographs for wet magnetic particle; and the reference photographs in E433 for liquid penetrant inspection are presented in a manner different from

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and less accepted than those in E125. In fact, E433 show only examples of discontinuities and makes no attempt to classify them as to severity.

Internal Discontinuities

For a clear understanding of current radiographic standards, it is necessary to go back to the beginning of such standards. The first, " Gamma Ray Radiographic Standards for Steam Pressure Service, " was issued by the Navy's Bureau of Engineering in 1938. Personal opinion was the criterion for determining that a certain radiographic quality level was considered acceptable and another rejected. Primarily, government personnel decided, from viewing the radiographs, whether they were acceptable or rejected on the basis of the way the defect appeared to them. Some experience was available regarding valve leakage related to varying degrees of shrinkage in steam pressure service casting.

The present reference radiographs are available for the casting buyer to set his own severity levels. In other words, a buyer could select severity level 3 for gas porosity, severity level 4 for sand inclusions, severity level 2 for shrinkage, severity level 1 for linear discontinuities, and so. However, this has not been employed by most casting buyers. Most have been specifying a single severity level across the board. There is no basis provided by ASTM for concluding that severity level 2 for porosity and severity level 2 for shrinkage are related in any manner as to the ability or inability of the casting to perform the service for which it has been designed. In fact, information made available by SFSA would indicate that severity level 2 shrinkage and severity level 5 porosity is a much more comparable relationship.

To add to the confusion, it is pointed out, for example, that severity level 2 shrinkage in E446, E186 and E280 are not the same severity. This seems to cause some concern even though the committee that prepared the standards was of the opinion that severity levels could be relaxed somewhat as the section size increased.

A summation of the past and present on reference radiographs would indicate that their use has grown extensively. However, they are not based on factual test data, and the severity levels are not graded to any basis of acceptability of the casting as to its load carrying ability. They are based on the opinion that anything that is less than perfect is questionable.

11. Cost of Specifications Steel casting are specially designed and manufactured parts, and therefore, the cost of castings will depend upon the complexity of the design of the part and upon the purchasers' requirements. The cost of one casting cannot necessarily be compared to the cost of another casting similar in weight, shape, and design, because differences in quality requirements may exist. Two castings which may look alike may have different costs because the service requirements of the two are entirely different; dictating that the quality and tolerance requirements of one be of a different order than those of the other.

Steel casting costs reflect variations in material specification, tolerance limits, inspection requirements, acceptance standards, affidavits, and certification requirements. The purchasers should always rely on value analysis in the specification and buying of steel castings.

A wide range in estimated casting costs from several foundry bidders often reflects that the purchaser was not specific as to the properties and requirements desired. Specifying minimum quality requirements is necessary if castings of minimum cost are desired.

Document Maintenance

A cost area often overlooked is one of maintaining the most current editions of specification and reference standards. To produce valves, fittings, or other pressure castings, a foundry would have to have as a bare minimum, ASTM Reference Radiographs E186, E280, E446, and E99, in addition to the ASME Boiler and Pressure Vessel Code and the USASI B-31 Standards. The foundry would also have to have documents such as the ASTM standards, the American Petroleum Institute Standards, the American Welding Society Standards and the Standards published by the Manufacturers Standardization Society for the Valve and Fitting Industry.

All of these documents, costing several hundred dollars each, are considered necessary to properly process an order for parts for the construction of steam power plants, refineries, and chemical plants. There is also the necessity of maintaining files of specifications for the military and for countless other customers, some having 30 to 40 separate specifications. Some power plant contractors have specifications several hundred pages long. All of these documents must be kept up to date, since revisions are constantly being made. This is no small task and even a specification specialist cannot remember the details of every document with all of the variations in formats and requirements. As a result, a foundry doing extensive work of this nature must have a large quality control department.

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Quality Control and Quality Assurance Cost

The principal costs in this area are incurred when the castings are processed through the shop. Testing and inspection to ensure exceptional quality levels requires the careful efforts of a relatively large quality control staff along with reams of paper work. A separate book of procedures must be followed for each specification in addition to numerous special handing procedures to cover various customers' requirements.

Coincident with any extra testing and inspection is the cost of upgrading by skilled workmen, followed by further inspection and additional production time. Narrow ranges of acceptability are usually congruent with high quality levels, and a higher percentage of rejected and reworked casting is probable. Naturally, these costs must be reflected in the price of the casting.

Levels of quality which are higher than demanded by the end use are excessively costly, and add nothing to the serviceability of the casting. If the requirements of the casting are overstated, the cost of the casting will be higher than it should be. Necessary quality requirements should not be compromised in order to obtain a lower price, but it must recognized that the more requirements specified to attain higher levels of quality, the more costly the product will be.

Qualification of Facilities and Personnel

To produce castings to rigid specifications requires skilled and qualified personnel working in an adequate facility. The qualification of both facility and personnel is nothing more than being assured that the producer has the capability of supplying castings to the specified requirements.

Inspection personnel are often required to be certified as a Level 1, 2, or 3 inspector in accordance with the American Society for Nondestructive Testing Recommended Practice No. SNT-TC-1A. Radiographic facilities must be certified and licensed by city and state agencies. Welders and the procedures they use must be qualified to ASTM 488 or to Section IX of the ASME Boiler code. Although castings for ASME Boiler and Pressure Vessel Code use and castings for nuclear use may be produced without the foundry having to obtain the ASME " U " stamp or " N " stamp, the surveillance costs are extremely high when those stamps are required.

Additional costs are incurred for the approval and certification of equipment such as tensile testing machines, impact testing machines, magnetic particle inspection equipment, heat treating furnaces and temperature controllers, calibration standards and numerous other items that are used to prove conformance to the specifications.

Specified Range vs. Process Capability

The determination of an economical specification range, whether it be for chemical analysis, mechanical properties, hardness, dimensional tolerances, or any other range, requires careful study, much statistical information, and common sense. The preparation of a specification is an exacting undertaking in which buyers and producers should collaborate. Specification control can be obtained only when the normal expected value and the standard deviation are known.

The proper creation of a specification is much more time consuming than is often supposed. Averaging the results of a few tensile tests, or thumbing through the pages of handbooks and selecting average values and adopting them as specification limits is never satisfactory. When the distribution curve is normal, half the results are higher than the average and half the results are lower. If the average value is taken as the specification limit, half the results will be immediately rejected. Specification limits are never based on averages.

A specification range should be as narrow as necessary and practicable. However, if it is too small, rejections become excessive. Therefore, a balance must be maintained between the value of establishing a narrow specification range and the cost increase resulting from the more exacting quality control required in holding the process to narrow limits. On the other hand, if the range is too wide, additional processing costs in other areas may be incurred. For instance, if the chemical ranges for a low alloy heat treatable steel are too wide, the hardenability of the castings from different heats of that grade of steel might have a wide variation which will result in excessive heat treating costs when heat treating to a narrow range.

It is probably much more economical and advantageous in the long run for purchasers, producers, and engineering groups to discard private specifications and replace them with specifications created by nationally known specification writing bodies.

In the ASTM specifications, there will probably be found a closer balance between process capabilities and purchasing requirements than in any other group of specifications.