Stel Casting Handbook - Sample Pages

8/13/2019 Stel Casting Handbook - Sample Pages

1/5

1 2 Part1:General InformationIntroduction

Steel is the most versatile engineering material available today.Steel can be easily welded and processed and plays a vital role inmaintaining the high standard of living enjoyed by the industrializednations of the world.The v ersatility of steel can be easily recognized by its applicationswhich range from high strength structural applications to excellentcorrosion resistance in aggressive fluids.The differences between steel castings and its wrought counterparts are principally in the method of production. In the case ofwrough t steel cast bars, slabs and ingots are mechanically worked toproduce sheet, bar, tube and other product forms. However, steelcastings are produced in the final product form without any intermediate mechanical working. Steel castings are near net shape products.Although cast steel accounts for only 10% of the total foundryindustry sales, steel castings are used for vitally important components in the mining, railroad, truck, construction, military, and oiland gas industries. The total capacity of the steel casting industry inthe United States is approxim ately 1.6 m illion tons with a sales value

of approximately two billion dollars (US). Steel castings are specified for applications which require weldability, abrasion resistance,

high strength, low and high temperature service and corrosion resistance.There are approximately 300 steel foundries in North America.Due to the diversity of market requirements such as size, tolerances,chemistry, volume, etc., a single foundry cannot serve all of themarket and each com pany w ill tend to specialize in a portion of thetotal market. Some of the specialized areas are: railroad, construction equipment, truck and mining industries. high alloy stainless steel used in corrosion and hea t resistantapplications or low volum e prototype and service parts.The balance of this chapter indicates chapters which will containdetailed information regarding the casting processes, applicationsfor steel castings and sugg estions regard ing the use of steel castings.

What Is a Steel Casting?A steel casting is the product formed by pouring molten steel intoa moldcavity.The liquid steel cools and solidifies in the mold cavityand is then removed for cleaning. Heat treating may be required tomeet desired p roperties.This process provides the near net shape and m echanical properties required by a purchaser to meet his specifications.

MELTING P A T T E R N M A K I N G

MOLDING

POURING MOLDC LOSI N G COREMAKING

SHAKEOUT

RISER CUTOFFaG TE REMOV L

INITI LHE T TRE TMENT

CLEANINGaF I N I S H I N GF I N A L

HEAT TREATMENT

INSPECTIONaS H I P P I N GFig. 1-1 Simplified flow diagram of the basic operations forproducing a steel casting

Casting ProcessThe manufacture of steel castings is discussed in this handbook,however, a simplified flow diagram (Figures 1-1 and 1-2) is shownto give an overview of the process steps.

Pattern EquipmentChapter 11discusses this important step in the casting process indetail, however, the following points summ arize some of the im portant aspects to be considered.

The dimensional accuracy of a steel casting is dependent on thetype and quality of the pattern. The foundry should b e responsiblefor determining the type of pattern w hich will produce the highestquality part on their equipment.Types of Patterns

Pattern equipment can be made of wood, metal, wax, plastic,foam, etc. The selection depends on the quantity and size of parts,molding m ethod, casting tolerances required, and cost.W o o d p a t t e r n s are less costly than other materials, but aresuited only for prototypes and limited production. They can easilybe converted to plastic if production requirements increase.M e t a l p a t t e r n s are the most costly, but they are required forhigh volume production. The m aterial can be aluminum, iron, brass,bronze, etc. Cast aluminum is the most comm only used m aterial formedium volume requirements. Metal patterns are also required forboth cores and molds produced by the shell process.W a x p a t t e r n s are used in the investment, or lost wax, moldingprocess. These processes use dies to form the patterns. The waxpatterns are coated with a ceramic slurry to form a shell. The wax isthen removed by heat.Pattern costs will vary considerably depending on material, volume required and tolerance requirements. Consultation with thefoundry during the design stage will ensure that the type of patternequipment necessary will be obtained at the lowest cost.F o a m p a t t e r n s may be used in the ceramic or bonded sandmo lding system s. In the ceramic system s they may be substitutes forwax and in the bonded systems inexpensive substitutes for otherstronger materials.

Steel Castings Handbook, 6th Edition (#06820G)

Editor(s): Malcolm Blair, Thomas L. Stevens

Copyright 1995 ASM Internationa

All rights rese

www.asminternationa


2/5

Introduction to Steel Castings 1 3

MECHANICAL DRAWING OFDESIRED PART

COPE PATTERN PLATE DRAG PATTERN PLATE

^ ^ COPE AFTER RAMMING WITHCOPE READY FOR SAND SAND AND REMOVINGPATTERN, SPRUE a RISERS

DRAG READY FOR SAND DRAG AFTERREMOVING PATTERN

DRAG WITH CORE COPE AND DRAGSET IN PLACE ASSEMBLED READY FOR POURING

CASTING AS REMOVED CASTINGS READYFROM MOLD FOR SHIPM ENTFig.1-2 Typical steps involved in making a casting from a greensand mold

Molding ProcessesThe Molding Processes are discussed in detail in Chapter 13.

G r e e n s a n d m o l d i n g is the most widely used system and dueto mechanization in many green sand foundries, the least expensiveprocess. Water and clay in the sand allows molds to be producedwith a high degree of hardness and an accurate mold cavity.S h e l l m o l d i n g uses resin bonded sand and a heated pattern toproduce a fused sand mold with excellent detail and dimensionalaccuracy. Energy and material costs are higher than green sand. Thisprocess is not suitable for larger castings.C h e m i c a l l y b o n d e d m o l d i n g uses sand and various chem icals or gases to form a dry hard mold. Dimensional accuracy isgood, and the process is suited to all sizes of parts; however, sandreclamation costs are high, and the process is more expensive thangreen sand.V a c u u m m o l d i n g uses dry unbonded sand. The mold relies onthe vacuum for its hardness; the vacuum must be maintained duringpouring and cooling. All sizes of parts can be made by this process;accuracy and surface appearance are good.I n v e s t m e n t c a s t i n g (also called Lost Wax ) uses a wax orfoam pattern formed by a very precise metal mold. Several patternsare fixed to a tree and then dipped into a ceramic slurry. Successive dipping and drying produces a thick shell of ceramic whichbecomes the mold. The wax or foam is removed by heat prior topouring. This process is limited to smaller castings and is generallynot competitive unless some machining can be eliminated.Expendable pattern casting is also called Lost Foam orFull Mold process and uses a pattern of polystyrene which can becut from stock or formed in a metal die, depending on volumerequirements. The patterns are coated with a ceramic wash. Thepattern and polystyrene gating system are embedded in dry sand,and when p oured, the polystyrene m elts and evaporates. Cores arenot required, and capital requirements are low. Dry sand is easilyreclaimed compared to other processes. This process produces aclean, tightly toleranced casting. This process is currently in thedevelopment phase for steel castings.G r a p h i t e m o l d i n g utilizes graphite molds wh ich are semi-permanent. Dimensional control and surface appearance are excellent.This is a highly specialized process suited mainly to parts likerailroad wheels.P e r m a n e n t m o l d castings poured into molybdenum molds areseverely limited in size.C e n t r i f u g a l m o l d i n g produces parts from molten metalpoured into rotating molds. Rotation of the mold causes the metal tobe held to the inside diamete r of the mold. It is ideally suited for pipeand symmetrical configurations.C e r a m i c m o l d i n g employs a mixture of refractory materials,hydrolyzed ethyl silicate, and a catalyst which is poured over apattern. The ceram ic shell is stripped before fully setting, then fired,and assembled for pouring. Th is process produces excellent surfaceappearance and accuracy and is particularly adapted to turbineblades and manifolds.Melt ing and Pouring Operations

Melting and Pouring are summarized here and are detailed inChapter 14.Several types of melting furnaces are used in the production ofsteel castings.E l e c t ri c a r c f u r n a c e s (EAF) are responsible for the production of the majority (84%) of steel castings. These units are composed of a steel shell, refractory lining, and a refractory lined roof




All rights rese

www.asminternationa


3/5

1 4 Part1 :General Informationwith three openings for graphite electrodes. Melting is accomplishedbythe heat from the electric arc.The EAF is themost flexible unit for melting steel in thatthecharge materialcan bevariedand thesteelcan berefined in thefurnace before tapping.Electr ic induction furnaces are the most common unitforsmaller production quantities.Thefurnace consistsof asteel shellwitha refractory lining surrounded byacopper coil. Heat is generatedbyan electric currentinthe coil.Finishing and Heat Treating

Finishing and H eat Treating are summarized here and are detailedin Chapter 15.Whenacasting has cooled,it isshaken outofthe mold. Beforeitcanbeshipped, itmustbefinished orcleaned. The first stepis anabrasive blast which cleans thesurface ofall residueofthe mold.Then the extraneous metal of the gating system and fins are removedby torch cutting, sawing or grinding. Welding of discontinuitiesis acommon practiceinthe steel casting industry.Heat treatment processesmay beusedtoenhance the propertiesof specific alloys. The scale formed on casting surfaces during heattreatment is removed by abrasive blasting.Steel castingscan bestraightenedbypressingifwarpage occurs

during processing. This operation ensures dimensional accuracyofthe finished part.P r e m a c h i n i n g orrough m achininghas become very commonin steel foundries. When the customer requires very precise locationof finish stockor reduced finish stock thefoundry canperform arough machining operation to provide the desired characteristics.Many foundries can also supply parts in the finish machinedcondition.Pr oc e ss c ap ab i l i ty ndt o l e r a n c e s are coveredin Chapter 16 and aredependentonmany factors.Asmentioned earlier,pattern quality, mold material, pre-machining, straightening,etc.,can all affect tolerances. The abilitytocontrol the casting processisof extreme importanceto today's foundryman, and thesupplierofchoice can document his programsforthe buyer.

W hyUse a Steel Cast ing?The wide range of m etallurgical and m echanical properties availableinsteel castingsisdiscussedinChapters 17 through 24. Manyof the alloys requiredforsevere applications cannot be wrought andmust be cast.All carbon and low alloy steels are readily weldable (Chapter 25).Higher alloy grades such asm anganese and stainless steelsareroutinely welded using appropriate techniques.Steel castingsarereadily he at treatedbynormalizing, annealing,quench and tempering, localized or differential hardening,etc.,dependingonthe mechanical properties required (Chapter 24).Information oncast high alloy steelsispresentedinChapter 20.The corrosion resistance of CF type high alloy cast steels is comparable to wrought 300 series material but the CF grades have aslightly different composition and contain ferrite for improvedweldability. Nickel-base alloys are discussed in Chapter 21.Thehigh alloy and nickel-base alloy castings are used in chemicalprocessing plants and corrosive environments.Wear Resistance of cast steels is reviewed inChapter 19. It variesfroma relatively soft medium carbon steelof.25 percent carbontothe extremely hard work-ha rdening manganese grades and thehigh chrome irons. Cast steels exhibit superior toughness and impactresistance com pared to other m aterials.Heat resistant alloys (Chapter 22) areused at temperatures inexcessof 1200 F 649C). These m aterials areusually alloysofiron, chromium, and nickel.

High chrome irons6Austenitic Stnls 2High manganese6Martensitic Stnls 1HA heat resistant 4

F i g .1-3 Total Steel Castings Production (1993)

W e i g h t ndS i z e R a n g e . Steel castings are produced fromouncestoover 100 tons andinquantities from one to thousands perorder. Each casting will exhibit the chemistry and mechanical properties specified by the buyer.Steel castingscan bejoined to other product forms (wrought,forged, etc.)toproduc e cost-effective fabrications.Who Uses Steel Castings?

Chapter 2 covers the industries who are major users of steelcastings and the applications that requiretheunique propertiesofcast steel. The generic term steel coversawide range of grades ofmaterials, however, for simplicity two alloy groups arenormallyconsidered: Carbon and Low Alloy (C&L A) and High Alloy. HighAlloy grades include stainless steels and nickel-base alloys.Austenitic Manganese steelsand all other non-stainless steelsareusually included in the C&LAgroup.The total steel casting productionin theU.S. excluding high chrom ium ironsisdominatedby theC&LA grades (Figure 1-3). The C &LA and H igh Alloy group marketsare shown in Figures 1 -4 and 1-5.S ome of the larger users are:Railroad industry w hich uses 50% ofthe total productionofsteelcastings. These parts areused in severe applications such ascouplers, draft gears, side frames, bolsters, and wheels.Construction m achinery manufacturers use about 15%of the steelcastings produced each year. The applications here are asvariedasthe equipment produced. Parts range from endcapsonhydrauliccylinders for a small backhoe to transmission housings on largeearth-moving machinery.Valvesandfittingsofcast steel accountforabout 5%ofproduction and areused for the drilling, recovery, transportation, andrefining of natural gas and crude oil both on land and offshore. Theseparts varyinsize fromafew pou ndstomany tons. Applicationsarevery severe requiring performance incorrosive liquidsatboth subzero and elevated temperatures. Operating pressures can reach manythousandsofp.s.i.invalves and blowout preventers.

Heavy trucksboth on and off-highwayuse about5%of industry production. Some applications include: axle housings, suspension brackets, wheels, brake parts, axle spindles, differential hous ings,andfifth wheels.Mining industry makes extensiveuse of steel castings for theirextreme requirements of toughness and abrasion resistance bothforore recovery equipment andcrushing m ills. Both high manganeseand other alloy steels are widely used, and these materials can onlybe produced as castings.Numerous other industriesuse steel castings. These range fromfood processing and electronicstooil and gas, defense and pulp andpaper industries.Infact, castings touch every aspect of our lives.




All rights rese

www.asminternationa


4/5

Introduction to Steel Castings 1 5

Cast Steel Compared to OtherCast MetalsNon-ferrous alloys have excellent machinability, and most arelighter than the iron family, but they do not have the strength andtoughness required for many severe applications.Gray Iron is produced more than any other iron alloy. It offersexcellent castability and machinability, but it is essentially a brittlematerial. It is the least expensiv e of the iron alloys, and the dom inantuser is the automotive industry.Ductile Iron, also referred to as nodular iron (or S.G. iron inEurope), is a cast iron which does have some ductility. Applicationsfor this material have grown dramatically in the past 30 years, andit has supplanted malleable iron in most cases. Ductile iron has beenused for steel applications where weldability and toughness are notrequired.

Rolling mill1 ValvesConstruction { / 7o /o19Ordnance2

Mining19

Oil field2

Others10Motorvehicles6

Railroad34Fig. 1-4 Carbon and Low Alloy Steel Markets (1993)

Steel Cast ings Compared toOther ProcessesWeldments are produced from ingots which are rolled into common shapes such as bar, plate, I beam s, tube, etc.Advantages:Mill steel is less expensive than cast or forged parts. There maynot be a die or pattern cost, and there are not any size limitations.Disadvantages:If extensive welding is required, the cost increases. Large orcomplicated parts require expensive fixtures. Rolled or forged materials are anisotropic and exhibit lower ductility when impact istransverse to the direction of working.Forgings are produced in dies by hammering or pressing to thedesired shape. Simple shapes with high volume requirements aregood candidates for the forging process, as long as the required partdoes not have a composition that cannot be mechanically w orked.More detailed information on the advantages of steel castings canbe found in Chapter 3.

Paper3/0 FoodPumps \ 616

Furnaces16Valves24

Ordnance3Others31

Fig.1-5 Hig h Alloy Markets (1993)

Buying and Specifying SteelCast ingsBuying and specifying steel castings are discussed in Chapters 4and 5. When a steel casting is required to perform a specific engineering function and the specifications have b een determined, thenext step is to select a supplier for that part. The Steel Founders'Society of America publishes a directory of all foundries in theUnited States, Canada, and Mexico. This directory lists personnel,capacity, special services, size of parts produced, etc.Several potential sources should be visited by the quality, engineering, and purchasing departments to audit each facility to determine several qualified sou rces. A detailed inquiry can be sentt each

qualified company. This inquiry should include current engineeringdrawings showing: Machined surfaces Material specification (preferably a standard specifying body likeASTM)

Special requirements such as: Hardness

Dimensional tolerances Sound ness (radiography, N.D.T.) Special inspection and tests certification

In addition, the inquiry should show: Quantity and delivery required Weight (if available) Painting, if required Name and phone number of person to contact regarding questions

on the inquiry Shipping destination Machining, if required

Treat your foundry supplier as a valued source, not only forcastings, but also for advice on: Molding methods Pattern options Metal specifications Design assistance for lowest cost




All rights rese

www.asminternationa


5/5

* < . .




All rights rese

www.asminternationa

Documents

Stel Casting Handbook - Sample Pages