30. Malleable cast iron production, structure, properties and brands.

= Malleable cast iron is produced from white cast iron, which is made from hot liquid iron with certain chemical components. The white cast iron needs to be treated by malleablizing, such as graphitizing or oxidation and de carbonization, then its metallographic structures or chemical components will be changed, so can become into malleable cast iron. Malleable Cast Iron is the traditional material for manufacturing pipe fittings whose characteristics make it as ideal choice. It is an iron-carbon alloy which combines the outstanding properties of cast iron and steel to produce a material which can still be cast but has improved strength and ductility. It also allows the production of complex shapes combined with a thin wall section.In its cast state it is very hard and brittle and unsuitable for most engineering applications. A controlled heat treatment process (annealing) is applied to the cast material which changes the structure andreduces the carbon content. The resulting microstructure gives a material which is less hard, no longer brittle and now has good malleable and ductile properties while retaining a sufficiently high strength.

Malleable cast iron is a heat-treated iron-carbon alloy, which solidifies in the as-cast condition with a graphite-free structure, i.e. the total carbon content is present in the cementite form (Fe3C).Two groups of malleable cast iron are specified, differentiated by chemical composition, temperature and time cycles of the annealing process, the annealing atmosphere and the properties and microstructure resulting there from.

The designation according toISO 5922(1981) of malleable cast iron consists of one letter designating the type of iron, two figures designating the tensile strength and two figures designating the minimum elongation.Letters designating the type of malleable cast iron can be:W for whiteheart malleable cast iron,B for blackheart malleable cast iron,P for peariitic malleable cast iron.Blackheart and pearlitic malleable cast ironThe microstructure of blackheart malleable cast iron has a matrix essentially of ferrite. The microstructure of pearlitic malleable cast iron has a matrix, according to the grade specified, of pearlite or other transformation products of austenite.Graphite is present in the form of temper carbon nodules. The microstructure shall not contain flake graphite.

Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and low-carbon metallic matrix. Because of the way in which graphite is formed in malleable iron, however, the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates.Malleable iron and ductile iron are used for some of the applications in which ductility and toughness are important. In many cases, the choice between malleable and ductile iron is based on economy or availability rather than on properties. In certain applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings: for parts that are to be pierced, coined, or cold formed, for parts requiring maximum machinability, for parts that must retain good impact resistance at low temperatures, and for parts requiring wear resistance (martensitic malleable iron only).

31. Ductile cast iron production, structure, properties and brands.= Ductile iron, also known asductile cast iron,nodular cast iron,spheroidal graphite iron,spherulitic graphite cast iron andSG iron, is a type ofcast iron. Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the shape of thegraphite. In ductile irons, the graphite is in the form ofnodulesrather than flakes as it is ingrey iron. The sharp shape of the flakes of graphite create stress concentration points within the metal matrix and the rounded shape of the nodules less so, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. Ductile iron is a family of cast graphitic irons which possess high strength,ductility and resistance to shock. Annealed cast ductile iron can be bent, twistedor deformed without fracturing. Its strength, toughness and ductility duplicatemany grades of steel and far exceed those of standard gray irons. Yet itpossesses the advantages of design flexibility and low cost casting proceduressimilar to gray iron.The difference between ductile iron and gray iron is in the graphite formation.Ordinary gray iron is characterized by a random flake graphite pattern in themetal. In ductile iron the addition of a few hundredths of 1% of magnesium orcerium causes the graphite to form in small spheroids rather than flakes. Thesecreate fewer discontinuities in the structure of the metal and produce a stronger,more ductile iron. It is this graphite formation which accounts for the fact thatductile iron is also referred to as nodular iron. The ductile iron process wasdeveloped by The International Nickel Company, Inc. Flowserve Corporationproduces valves, pumps, and other process equipment in ASTM A395 ductile iron.

A typical chemical analysis of this material: Carbon3.3 to 3.4% Silicon2.2 to 2.8% Manganese0.1 to 0.5% Magnesium0.03 to 0.05% Phosphorus0.005 to 0.04% Sulphur 0.005 to 0.02% IronbalanceOther elements such ascopperortinmay be added to increase tensile and yield strength while simultaneously reducing ductility. Improved corrosion resistance can be achieved by replacing 15% to 30% of theironin the alloy with varying amounts ofnickel,copper, orchromium.

. Much of the annual production of ductile iron is in the form ofductile iron pipe, used for water and sewer lines. It competes withpolymericmaterials such asPVC,HDPE,LDPEandpolypropylene, which are all much lighter than steel or ductile iron, but which, being flexible, require more careful installation and protection from physical damage.Ductile iron is specifically useful in many automotive components, where strength needs surpass that of aluminium but do not necessarily require steel. Other major industrial applications include off-highway diesel trucks,Class 8 trucks, agricultural tractors, and oil well pumps.

32. Carbon alloy steel classification, characteristics, applications and brands.= Alloy steelissteelthat isalloyedwith a variety ofelementsin total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups:low-alloy steelsandhigh-alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0% Most commonly, the phrase "alloy steel" refers to low-alloy steels.any steel in the 0.35 to 1.86 percent carbon content range can be hardened using a heat-quench-temper cycle. Most commercial steels are classified into one of three groups:1. Plain carbon steels2. Low-alloy steels3. High-alloy steelsPlain Carbon SteelsThese steels usually are iron with less than 1 percent carbon, plus small amounts of manganese, phosphorus, sulphur, and silicon. The weldability and other characteristics of these steels are primarily a product of carbon content, although the alloying and residual elements do have a minor influence. It is divided into Low.Often called mild steels, low-carbon steels have less than 0.30 percent carbon and are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels.Medium.Medium-carbon steels have from 0.30 to 0.45 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.High.With 0.45 to 0.75 percent carbon, these steels can be challenging to weld. Preheating, post heating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.Very High.With up to 1.50 percent carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.

Low-alloy SteelsWhen these steels are designed for welded applications, their carbon content is usually below 0.25 percent and often below 0.15 percent. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we'll cover in a later issue.High-alloy SteelsFor the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:1. Austenitic2. Ferrite3. MartensiticMartensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high harden ability, and require both pre- and post heating when welding to prevent cracking in the heat-affected zone (HAZ).Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.Austenitic stainless steels offer excellent weld ability, but austenite isn't stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment.

33. Nature of thermal treatment, application and classification.= Heat treatingis a group ofindustrialandmetalworking processesused to alter thephysical, and sometimeschemical, properties of a material. The most common application ismetallurgical. Heat treatments are also used in the manufacture of many other materials, such asglass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques includeannealing,case hardening,precipitation strengthening,temperingandquenching. It is noteworthy that while the termheat treatmentapplies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Metallic materials consist of amicrostructureof smallcrystalscalled "grains" orcrystallites. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate ofdiffusionand the rate of cooling within the microstructure. Heat treating is often used to alter the mechanical properties of an alloy, manipulating properties such as thehardness,strength,toughness,ductility, andelasticity.There are two mechanisms that may change an alloy's properties during heat treatment. Themartensitecauses the crystals todeformintrinsically. The diffusion mechanism causes changes in the homogeneity of thealloy The crystal structure consists of atoms that are grouped in a very specific arrangement, called a lattice. In most elements, this order will rearrange itself, depending on conditions like temperature and pressure. This rearrangement, calledallotropyor polymorphism, may occur several times, at many different temperatures for a particular metal. In alloys, this rearrangement may cause an element that will not normallydissolveinto the base metal to suddenly becomesoluble, while a reversal of the allotropy will make the elements either partially or completely insoluble.Eutectoid alloysA eutectoid alloy is similar in behaviour to a eutectic alloy. Aneutecticalloy is characterized by having a single melting point. This melting point is lower than that of any of the constituents, and no change in the mixture will lower the melting point any further. When a molten eutectic alloy is cooled, all of the constituents will crystallize into their respective phases at the same temperature.

Hypo eutectoid alloysAhypoeutecticalloy has two separate melting points. Both are above the eutectic melting point for the system, but are below the melting points of any constituent forming the system. Between these two melting points, the alloy will exist as part solid and part liquid. The constituent with the lower melting point will solidify first. When completely solidified, a hypoeutectic alloy will often be in solid solution.Similarly, a hypo eutectoid alloy has two critical temperatures, called "arrests." Between these two temperatures, the alloy will exist partly as the solution and partly as a separate crystallizing phase, called the "pro eutectoid phase." These two temperatures are called the upper (A3) and lower (A1) transformation temperatures. As the solution cools from the upper transformation temperature toward an insoluble state, the excess base metal will often be forced to "crystallize-out," becoming the pro eutectoid. This will occur until the remaining concentration of solutes reaches the eutectoid level, which will then crystallize as a separate microstructure.

Hypereutectoid alloysAhypereutecticalloy also has different melting points. However, between these points, it is the constituent with the higher melting point that will be solid. Similarly, a hypereutectoid alloy has two critical temperatures. When cooling a hypereutectoid alloy from the upper transformation temperature, it will usually be the excess solutes that crystallize-out first, forming the pro eutectoid. This continues until the concentration in the remaining alloy becomes eutectoid, which then crystallizes into a separate microstructure.Hypereutectoid steel contains more than 0.77% carbon. When slowly cooling a hypereutectoid steel, the commentate will begin to crystallize first. When the remaining steel becomes eutectoid in composition, it will crystallize into pearlitic. Since commentate is much harder than pearlitic, the alloy has greater harden ability at a cost in the ductility.

34. Material heating methods and equipment. Heating environments.

= Industry relies on heating for a wide variety of processes involving a broad range of materials. Each process and material requires heating methods suitable to its properties and the desired outcome. Despite this, the literature lacks a general reference on design techniques for heating, especially for small- and medium-sized applications. Industrial Heating: Principles, Techniques, Materials, Applications, and Design fills this gap, presenting design information for both traditional and modern heating processes and auxiliary techniques.

The author leverages more than 40 years of experience into this comprehensive, authoritative guide. The book opens with fundamental topics in steady state and transient heat transfer, fluid mechanics, and aerodynamics, emphasizing analytical concepts over mathematical rigor. A discussion of fuels, their combustion, and combustion devices follows, along with waste incineration and its associated problems. The author then examines techniques related to heating, such as vacuum technology, pyrometry, protective atmosphere, and heat exchangers as well as refractory, ceramic, and metallic materials and their advantages and disadvantages. Useful appendices round out the presentation, supplying information on underlying principles such as pressure and thermal diffusivity.Induction heatingis a process which is used to bond, harden or soften metals or other conductive materials. For many modern manufacturing processes, induction heating offers an attractive combination of speed, consistency and control.The basic principles of induction heating have been understood and applied to manufacturing since the 1920s. During World War II, the technology developed rapidly to meet urgent wartime requirements for a fast, reliable process to harden metal engine parts. More recently, the focus on lean manufacturing techniques and emphasis on improved quality control have led to a rediscovery of induction technology, along with the development of precisely controlled, all solid state induction power supplies.What makes this heating method so unique? In the most common heating methods, a torch or open flame is directly applied to the metal part. But with induction heating, heat is actually "induced" within the part itself by circulating electrical currents.Induction heating relies on the unique characteristics ofradio frequency (RF) energy- that portion of the electromagnetic spectrum below infrared and microwave energy. Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the inductor itself does not get hot (watch video at upper right), and there is no product contamination. When properly set up, the process becomes very repeatable and controllable.Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a differenttemperaturefrom another body or its surroundings,heatflows so that the body and the surroundings reach the same temperature, at which point they are inthermal equilibrium. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described by thesecond law of thermodynamics.Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.

35. Steel structural transformations in heating. Critical temperature. Austenite grain.= This is a process utilized to change certain characteristics of metals and alloys in order to make them more suitable for a particular kind application Heat Treatment can greatly influence mechanical properties such as strength, hardness, ductility, toughness, and wear resitence of the alloys.

Heat Treatment of Carbon Steels and Carbon Alloy Steels:Heat Treatment on both type of the steel is done for improving mechanical properties such as tensile and yield strength. This is accomplished by altering the molecular structure of steel in order to produce more durable microstructure. The structure of steel is composed of two variables:During the alloy process elements such as carbon are introduced to the metal. These added elements interrupt the flow of the individual grains, increasing strength. Thus, control of the metal crystal structure is a key element in successful heat treating.A Metal can also exist in various phases: Ferrite, austenite and cementite. To better understand these phases, look at the Iron-Carbon Phase Diagram. The Y-axis (vertical) is a measurement of temperature while the X-Axis (Horizontal) is a measurement of the carbon content of the steel. The far left hand side of the X-axis represents the Ferrite phase of steel (low carbon content) while the far right hand. Side represents the cementite phase of steel (high carbon content), which is also known as iron carbide. The curved horizontal line that occurs just above 1333 F represents the austenite phase of steel.The following phases are involved in the transformation, occurring with iron-carbon alloys:L L represents a liquid solution of carbon in iron.Del-ferrite -ferrite is a solid solution of carbon in iron. Maximum concentration of carbon in -ferrite is 0.09% at 2719 F (1493C) temperature of the peritectic transformation. The crystal structure of -ferrite is BCC (cubic body centered).Austenite Austenite is an interstitial solid solution of carbon in -iron. Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon up to 2.06% at 2097 F (1147 C). Austenite does not exist below 1333 F (733C) and maximum carbon concentration at this temperature is 0.83%.-ferrite -ferrite is a solid solution of carbon in -iron. -ferrite has BCC crystal structure and low solubility of carbon up to 0.25% at 1333 F (733C). It exists at room temperature.Cementite Cementite is an iron carbide, intermetallic compound, having fixed composition Fe3C. Cementite is a hard and brittle substance, influencing on the properties of steels and cast irons.When ferrite (low carbon steel) is at room temperature, it has a body-centered-cubic structure, which can only absorb a low amount of carbon. Because Ferrite can only absorb a very low amount of carbon at room temperature, the un-absorbed carbon separates out of the body-centered-cubic structure to form carbides which join together to create small packets of an extremely hard crystal structure within the ferrite called cementic. However, when ferrite is heated to a temperature above the transformation line( austenite line) the body-centered-cubic structure changes to a face-centered-cubic structure, thus allowing for absorption of the carbon into the crystal structure.Once the steel enters the austenitic phase all of the cementite dissolves into austenite. If the steel is allowed to cool slowly, the carbon will separate out of the ferrite as a cubic-structure reverts from face-centered back to body-centered. The islands of cementite will reform within the ferrite, and the steel will have the same properties that it did before it was heated. However, when the steel is rapidly cooles, or quenched, in a quenching medium (such as oil, water or cold air) the carbon does not have time to exit the cubic structure of the ferrite and it becomes trapped within it. This leads to the information ofmartensitic; microstructure that produces the most sought after mechanical properties in steel fasteners.During quenching, it is impossible to cool the specimen at a uniform rate throughout. The surface will always cool more rapidly than the interior of the specimen. Therefore, the austenite will transform over a range of temperatures, yielding a possible variation of microstructure and properties depending on the position within the material.The successful heat treatment of steels to produce a predominantly martensitic micro structure throughout the cross section depends mainly on three factors:The composition of the alloy.The type and character of the quenching medium.The size and shape of the specimen.Hardenability :It is the ability of steel to transform into martensite with a particular quenching treatment. This is directly affected by the alloy composition of the steel. For every different steel alloy there is a specific relationship between its mechanical properties and its cooling rate. It is not hardness which is a resistance to indentation; rather, hardness measurements are utilized to determine the extent of a martensite transformation in the interior of the material.Tempering :It involves heating the steel to a specific temperature below that of austenite and allowing it to cool slowly. This cause the crystal structure to relax, thereby increasing the ductility and decreasing the hardness to specified levels. The specific tempering temperature will vary based on the desired result for the steel.The following example will demonstrate the effectiveness of the tempering:ASTM A193 Grade B7 , SAE J429 Grade and ASTM A574 Socket Head cap Screws are all made from alloy steels. In fact some alloy steel grades can be used to manufacture any of the three final products. Such as 4140 and 4142 alloy steel. The final mechanical properties apper in the table.AnnealingIt is the heat treating process used to soften previously cold-worked metal by allowing it to re-crystallize.The term annealing refers to a heat treatment in which a material is exposed to an elevated tempertature for an extended period and then slowly cooled. Ordinarily, annealing is carried out to, (1) Relieve stress (often introduced when cold-working the part.) ;(2) Increase softness, ductility and toughness; and (3) Produce a desired microstructure.Any annealing process consists of the three stages:Heating to the desired temperatureHolding or soaking at that temperatureSlowly cooling, usually to room temperatureTime is the important parameters in these procedures. It is used to negate the effects of cold work that is to be soften and increase the ductility of a previous strain-hardened metal.Stress relievingIt is a process that is utilized when internal residual stress develop in metal pieces in response to such thing as cold working.Failure to remove these internal stress may result in distottion and warping. A stress relief anneaing heat treatment removes these stress heating the piece to a recommended temperature, held there long enough to attain a uniform temperature, and finally cooled to a room temperature in air.NormalizingIt is an annealing heat treatment used to refine the grains and produce a more uniform and desirable size distribution. Medium and high carbon steels having microstructure containing pearlite may still be too hard to conveniently machine or plastically deform. These steels (and in fact,any steel) may be annealted to develop the spheroidite structure . Spheroidized steels have a maximum softness and ducility and are easlly machined or deformed.

36. Austenite transformation in the continuous cooling. The cooling rate and the role of diffusion.= Austenite, also known asgamma phase iron(-Fe), is a metallic, non-magneticallotrope of ironor asolid solutionofiron, with analloying element. Austenitizationmeans to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. An incomplete initial austenitization can leave undissolvedcarbidesin the matrix. For some irons, iron-based metals, and steels, the presence of carbides may occur during the austenitization step. The term commonly used for this istwo-phase austenitizationDuringheat treating, ablacksmithcauses phase changes in the iron-carbon system in order to control the material's mechanical properties, often using the annealing, quenching, and tempering processes. In this context, the colour of light, or "blackbody radiation," emitted by the work piece is an approximategauge of temperature. Temperature is often gauged by watching thecolour temperatureof the work, with the transition from a deep cherry-red to orange-red (815C (1,499F) to 871C (1,600F)) corresponding to the formation of austenite in medium and high-carbon steel. In the visible spectrum, this glow increases in brightness as temperature increases, and when cherry-red the glow is near its lowest intensity and may not be visible in ambient light. Therefore, blacksmiths usually austenize steel in low-light conditions, to help accurately judge the color of the glow.Maximum carbon solubility in austenite is 2.03% C at 1,420K (1,150C)Martensite is formed in carbon steels by the rapid cooling (quenching) ofausteniteat such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form(Fe3C). As a result, the face centered cubic austenite transforms to a highly strained body centered cubic form of ferrite that is supersaturated with carbon. The shear deformations that result produce large numbers of dislocations, which is a primary strengthening mechanism of steels. The highest hardness of a pearlitic steel is 400whereas martensite can achieve 700BrinellThe martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms) and the parent austenite becomes mechanically unstable. As the sample is quenched, an increasingly large percentage of the austenite transforms to martensite until the lower transformation temperature Mfis reached, at which time the transformation is completed.For a eutectoid steel, between 6 and 10% of austenite, called retained austenite, will remain. The percentage of retained austenite increases from insignificant for less than 0.6% C to 13% retained austenite at 0.95% C and 3047% for a 1.4% carbon steels. A very rapid quench is essential to create martensite. For a eutectoid carbon steel (0.78% C) of thin section, if the quench starting at 750C and ending at 450C takes place in 0.7 seconds (a rate of 430C/s) no pearlite will form and the steel will be martensitic with small amounts of retained austenite. For steel 0-0.6% carbon the martensite has the appearance of lath, and is called lath martensite. For steel greater than 1% carbon it will form a plate like structure called plate martensite. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows. If the cooling rate is slower than the critical cooling rate, some amount of pearlite will form, starting at the grain boundaries where it will grow into the grains at rates of cm/s until the Mstemperature is reached then the remaining austenite transforms into martensite at about half the speed of sound in steel. In certain alloy steels, martensite can also be formed by the working and hence deformation of the steel at temperature, in its austenitic form, by quenching to below Msand then working by plastic deformations to reductions of area to 40% or as little 20% of the original. The process produces dislocation densities up to 1013/cm2. The great number of dislocations, combined with precipitates that originate and pin the dislocations in place, produces a very hard steel (this property is frequently used in toughened ceramics likeyttria-stabilized zirconiaand in special steels likeTRIP steels). Thus, martensite can be thermally induced or stress induced..

37. Isothermal transformation of austenite. Formation of bainite.= It is established that kinetics of upper bainite reaction are different from those of lower bainite. The change is associated with the different nucleation and growth mechanisms, which are accompanied by a change in the morphology of upper and lower bainite. Also, it is suggestedandthat the rate-controlling factor for the formation of upper bainite is the diffusion of carbon in austenite whereas the diffusion of carbon in supersaturated ferrite is suggested as the rate controlling process for the formation of lower bainiteand Both upper and lower bainite in hypereutectoid steels nucleate predominantly at prior austenite grain boundaries. Intergranular nucleation is infrequent in hypoeutectoid steels but is a common occurrence in hypereutectoid steels Also the carbide phase associated with upper bainite is usually cementitewhereas the carbide phase that is observed in lower bainite depends on the transformation time and temperature as well as on the composition of the steelandand may be -carbide (Fe2.4C)-carbide (Fe2C), or cementite (Fe3C). It has been shown that the kinetics of the bainite transformation change in the temperature range 300350C In addition to the chemical composition of steels, plastic deformation of austenite can influence the subsequent isothermal transformation of bainite. Larn and Yangshowed that the overall transformation kinetics of bainite became slower and the final amount of bainite decreased in steels deformed in austenite region.Isothermal transformation diagrams(also known as time-temperature-transformation (TTT) diagrams) are plots oftemperatureversus time (usually on alogarithmic scale). They are generated from percentage transformation-vs logarithm of time measurements, and are useful for understanding the transformations of ansteelthat is cooledisothermally. An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics ofcrystallizationin ceramic or other materials. Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels.Isothermal transformation(IT) diagram or the C-curve is associated with mechanical properties, microconstituents/microstructures, and heat treatments in carbon steels. Diffusional transformations likeaustenitetransforming to aandferritemixture can be explained using the sigmoidal curve; For example the beginning of pearlitic transformation is represented by the pearlite start (Ps) curve. This transformation is complete at Pfcurve. Nucleation requires an incubation time. The rate of nucleation increases and the rate of microconstituent growth decreases as the temperature decreases from the liquidus temperature reaching a maximum at the bay or nose of the curve. Thereafter, the decrease in diffusion rate due to low temperature offsets the effect of increased driving force due to greater difference infree energy. As a result of the transformation, the microconstituents, form; forms at higher temperatures and bainite at lower. Austenite is slightly undercooled when quenched belowEutectoidtemperature. When given more time, stable microconstituents can form: ferrite and cementite. Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate. This transformation is complete at the pearlite finish time (Pf).However, greater undercooling by rapid quenching results in formation of martensite or bainite instead of pearlite. This is possible provided the cooling rate is such that the cooling curve intersects the martensite start temperature or the bainite start curve before intersecting the Pscurve. The martensite transformation being a diffusionless shear transformation is represented by a straight line to signify the martensite start temperature.

38. Formation process of martensite. Martensite structure and properties.= most commonly refers to a veryhardform of steel crystalline structure, but it can also refer to any crystal structure that is formed bydiffusionless transformation.It includes a class of hard minerals occurring as lath- or plate-shapedcrystalgrains. When viewed in cross section, the(lens-shaped) crystal grains are sometimes incorrectly described as (needle-shaped).Martensite is formed in carbon steels by the rapid cooling (quenching) ofausteniteat such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form(Fe3C). As a result, the face centered cubic austenite transforms to a highly strained body centered cubic form of ferrite that is supersaturated with carbon. The shear deformations that result produce large numbers of dislocations, which is a primary strengthening mechanism of steels. The highest hardness of a pearlitic steel is 400whereas martensite can achieve 700Brinell. The martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms) and the parent austenite becomes mechanically unstable. As the sample is quenched, an increasingly large percentage of the austenite transforms to martensite until the lower transformation temperature Mfis reached, at which time the transformation is completed.For a eutectoid steel, between 6 and 10% of austenite, called retained austenite, will remain. The percentage of retained austenite increases from insignificant for less than 0.6% C to 13% retained austenite at 0.95% C and 3047% for a 1.4% carbon steels. A very rapid quench is essential to create martensite. For a eutectoid carbon steel (0.78% C) of thin section, if the quench starting at 750C and ending at 450C takes place in 0.7 seconds (a rate of 430C/s) no pearlite will form and the steel will be martensitic with small amounts of retained austenite. For steel 0-0.6% carbon the martensite has the appearance of lath, and is called lath martensite. For steel greater than 1% carbon it will form a plate like structure called plate martensite. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows. If the cooling rate is slower than the critical cooling rate, some amount of pearlite will form, starting at the grain boundaries where it will grow into the grains at rates of cm/s until the Mstemperature is reached then the remaining austenite transforms into martensite at about half the speed of sound in steel. In certain alloy steels, martensite can also be formed by the working and hence deformation of the steel at temperature, in its austenitic form, by quenching to below Msand then working by plastic deformations to reductions of area to 40% or as little 20% of the original. The process produces dislocation densities up to 1013/cm2. The great number of dislocations, combined with precipitates that originate and pin the dislocations in place, produces a very hard steel (this property is frequently used in toughened ceramics likeyttria-stabilized zirconiaand in special steels likeTRIP steels). Thus, martensite can be thermally induced or stress induced. One of the differences between the two phases is that martensite has a body-centeredtetragonal(BCT) crystal structure, whereas austenite has a(FCC) structure. The transition between these two structures requires very littlethermal activationenergy because it is atransformation, which results in the subtle but rapid rearrangement of atomic positions, and has been known to occur even atcryogenictemperatures. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume.Of considerably greater importance than the volume change is the shear strain which has a magnitude of about 0.26 and which determines the shape of the plates of martensite. Martensite is not shown in the equilibriumphase diagramof the iron-carbon system because it is not an equilibrium phase. Equilibrium phases form by slow cooling rates that allow sufficient time for diffusion, whereas martensite is usually formed by very high cooling rates. Since chemical processes (the attainment of equilibrium)accelerateat higher temperature, martensite is easily destroyed by the application of heat. This process is calledtempering. In some alloys, the effect is reduced by adding elements such astungstenthat interfere withnucleation, but, more often than not, the phenomenon is allowed to proceed to relieve stresses. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. The needle-like microstructure of martensite leads to brittle behavior of the material. Too much martensite leaves steelbrittle, too little leaves itsoft.

39. Annealing types of steel, application and the resulting properties.= Annealing, inmetallurgyandmaterials science, is aheat treatmentthat alters a material to increase itsductilityand to make it more workable. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improvecold workingproperties.In the cases ofcopper,steel,silver, andbrass, this process is performed by heating the material (generally until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper, silver and brass can be cooled slowly in air, or quickly byquenchingin water, unlikeferrous metals, such as steel, which must be cooled slowly to anneal. In this fashion, the metal is softened and prepared for further worksuch as shaping, stamping, or forming.Process annealing, also calledintermediate annealing,subcritical annealing, orin-process annealing, is a heat treatment cycle that restores some of the ductility to a product during the process of cold working, so it can be worked further without breaking. Ductility is important in shaping and creating a more refined piece of work through processes such asrolling,drawing,forging,spinning,extrudingandheading. The piece is heated to a temperature typically below thetemperature, and held there long enough to relieve stresses in the metal. The piece is then furnace cooled. It can then be subjected to additional cold working. This can also be used to ensure there is reduced risk of distortion of the work piece during machining, welding, or further heat treatment cycles.The temperature range for process annealing ranges from 260 C (500 F) to 760 C (1400 F), depending on the alloy in question. Aluminium can be annealed but care must be taken whilst heating. The flame should be held at a distance to the aluminium so that it gives a generalised heating to the metal.A trick of the trade is to rub soap on to the surface of the aluminium and then heat it on the brazing hearth. It takes only a short time for the soap to turn black. The brazing torch should be turned off immediately and the aluminium allowed to cool slowly. It is now annealed and should be very soft and malleable

PHYSICAL PROPERTIES: Annealed metals are relatively soft and can be cut and shaped more easily. They bend easily when pressure is applied. As a rule they are heated and allowed to cool slowly.Hardened metals are difficult to cut and shape. They are very difficult if not impossible to bend. As a rule they are heated and cooled very quickly by quenching in clean, cold water.The Annealing ProcessFullannealingconsists of heating the work-piece to above the upper critical temperature and slow cooling, usually in the furnace. It is generally only needed for the higher alloy steels, cast irons and complex alloys. Long treatment times are necessary to produce optimum softening in the case of the higher alloy steels. Where it is considered desirable to fully austenitise a steel during a softening process, (e.g. in order to refine forged structures), but where economy is paramount, a normalising treatment is often applied instead of a full anneal. This consists of the same initial heating stage as fullannealingbut followed by removal of the work from the furnace for air-cooling. Normalising is only applicable to the lower alloy and plain carbon steels.Sub-Critical AnnealingSub-criticalannealingis, as the name implies, carried out at temperatures below the lower critical temperature. It is mainly carried out in the temperature range 630 to 700C. It reduces hardness by allowing recrystallisation of the microstructure to occur. Alternatively, if a temperature just below the lower critical temperature is used (690 to 710 C.), it is possible to produce spheroidisation of the cementite phase instead of forming the normal lammellar pearlite and ferrite structure. Spheroidising is a useful technique for softening high carbon steels to improve machinablity.Lower temperature sub-critical anneals in the temperature range 550 to 600 C. are used to stress relieve welded fabrications and to stabilise rough machined components which are to be ultimately hardened and tempered, case hardened or nitrided and whose dimensional stability is criticalannealingunder a controlled atmosphere prevents scaling, leaving components in a bright condition.

40. Steel normalization, application and resulting structures and properties.= Normalization is a type of annealing process used to relieve stress in hardenable steels after cold work and to improveductilityandtoughnessproperties. The steel is heated slightly above its upper critical temperature and held for sufficient time to allow new, smaller grains to form and high energy grain shapes to coalesce, also known as grain refinement.Normalization can also eliminate denritic segregation that may remain from the casting process. The steel is air cooled from the normalization temperature, yielding a microstructure that lends the desired toughness and ductility properties with a nominal tensile strength.Normalizing Heat Treatment process is heating a steel above the critical temperature, holding for a period of time long enough for transformation to occur, and air cooling. Normalized heat treatment establishes a more uniform carbide size and distribution which facilitates later heat treatment operations and produces a more uniform final product.Normalization is an annealing process. The objective of normalization is to intend to leave the material in a normal state, in other words with the absence of internal tensions and even distribution of carbon. For the process the high temperatures are maintained until the complete transformation of austenite with air cooling.The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, you should normalize it first to ensure the maximum desired results. Usually, low-carbon steels do not re-quire normalizing; however, if these steels are normal-ized, no harmful effects result. Castings are usually annealed, rather than normalized; however, some cast-ings require the normalizing treatment. Table 2-2 shows the approximate soaking periods for normalizing steel. Note that the soaking time varies with the thickness ofthe metal. Normalized steels are harder and stronger than an-nailed steels. In the normalized condition, steel is much tougher than in any other structural condition. Parts subjected to impact and those that require maximum toughness with resistance to external stress are usually normalized. In normalizing, the mass of metal has an influence on the cooling rate and on the resulting structure. Thin pieces cool faster and are harder after normalizing than thick ones. In annealing (furnace cooling), the hardness of the two are about the same.It is usually used as a post-treatment to forging, and pre-treatment to quenching and tempering.Induction is used in most applications of annealing and normalizing in compared to conventional ovens.Induction heating advantages:Processed in line with control of parameters in real timeMetallurgical results similar to those obtained in conventional ovensLess environmental pollutionIncrease energy efficiencyReduced processing timeAbility to control the heat, temperature accuracyAbility to heat small areas without changing the characteristics of the rest of the partCycle accurate and repetitive heatReduction of surface oxidationImproved job environmentCopper becomes hard and brittle when mechanically worked; however, it can be made soft again by annealing. The annealing temperature for copper is be-tween 700F and 900F. Copper maybe cooled rapidly or slowly since the cooling rate has no effect on the heat treatment. The one drawback experienced in annealing copper is the phenomenon called hot shortness. At about 900F, copper loses its tensile strength, and if not properly supported, it could fracture .Aluminium reacts similar to copper when heat treat-ing. It also has the characteristic of hot shortness. Number of aluminium alloys exist and each requires special heat treatment to produce their best properties.

41. Steel tempering and application. Selection of heating temperature.

= Steel Tempering is a process of heat treating, which is used to increase the toughness of alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical temperature for a certain period of time, then allowed to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures. In glass, tempering is performed by heating the glass and then quickly cooling the surface, increasing the toughness.Tempering is a heat treatment technique applied to ferrous alloys, such as steel or cast iron, to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness is usually accompanied by an increase in ductility, thereby decreasing the brittleness of the metal. Tempering is usually performed after quenching, which is rapid cooling of the metal to put it in its hardest state. Tempering is accomplished by controlled heating of the quenched work-piece to a temperature below its "lower critical temperature". This is also called the lower transformation temperature or lower arrest (A1) temperature; the temperature at which the crystalline phases of the alloy, called ferrite and commentate, begin combining to form a single-phase solid solution referred to as austenite. Heating above this temperature is avoided, so as not to destroy the very-hard, quenched microstructure, called martensite.

Precise control of time and temperature during the tempering process is critical to achieve the desired balance of physical properties. Low tempering temperatures may only relieve some of the internal stresses, decreasing brittleness while maintaining a majority of the hardness. Higher tempering temperatures tend to produce a greater reduction in the hardness, sacrificing some yield strength and tensile strength for an increase in elasticity and plasticity. However, in some low alloy steels, containing other elements like chromium and molybdenum, tempering at low temperatures may produce an increase in hardness, while at higher temperatures the hardness will decrease. Many steels with high concentrations of these alloying elements behave like precipitation hardening alloys, which produce the opposite effects under the conditions found in quenching and tempering, and are referred to as managing steels.

In carbon steels, tempering alters the size and distribution of carbides in the martensite, forming a microstructure called "tempered martensite". Tempering is also performed on normalized steels and cast irons, to increase ductility, mach inability, and impact strength. Steel is usually tempered evenly, called "through tempering," producing a nearly uniform hardness, but it is sometimes heated unevenly, referred to as "differential tempering," producing a variation in hardness.

42. Critical tempering rate. Tempering types by cooling technique and the environment.

= After the hardening treatment is applied, steel is often harder than needed and is too brittle for most practical uses. Also, severe internal stresses are set up during the rapid cooling from the hardening tempera-ture. To relieve the internal stresses and reduce brittle-ness, you should temper the steel after it is hardened. Tempering consists of heating the steel to a specific temperature (below its hardening temperature), holding it at that temperature for the required length of time, and then cooling it, usually instil air. The resultant strength, hardness, and ductility depend on the temperature to which the steel is heated during the tempering process. The purpose of tempering is to reduce the brittleness imparted by hardening and to produce definite physical properties within the steel.

Tempering always follows, never precedes, the hardening operation. Besides reducing brittleness, tempering softens the steel. That is un-avoidable, and the amount of hardness that is lost depends on the temperature that the steel is heated to during the tempering process. That is true of all steels except high-speed steel. Tempering increases the hard-ness of high-speed steel. Tempering is always conducted at temperatures be-low the low-critical point of the steel. In this respect, tempering differs from annealing, normalizing, and hardening in which the temperatures are above the upper critical point. When hardened steel is reheated, temper-ing begins at 212F and continues as the temperature increases toward the low-critical point. By selecting a definite tempering temperature, you can predetermine the resulting hardness and strength. The minimum temperature time for tempering should be 1 hour. If the part is more than 1 inch thick, increase the time by 1 hour for each additional inch of thickness. Normally, the rate of cooling from the tempering temperature has no effect on the steel. Steel parts are usually cooled in still air after being removed from the tempering furnace; however, there are a few types of steel that must be quenched from the tempering temperature to prevent brittleness. These blue brittle steels can become brittle if heated in certain temperature ranges and allowed to cool slowly. Some of the nickel chromium steels are subject to this temper brittleness. Steel may be tempered after being normalized, providing there is any hardness to temper. Annealed steel is impossible to temper. Tempering relieves quenching stresses and reduces hardness and brittleness.

Actually, the tensile strength of a hardened steel may increase as the steel is tempered up to a temperature of about 450F. Above this temperature it starts to decrease. Tempering increases softness, ductility, malleability, and impact resistance. Again, high-speed steel is an exception to the rule. High-speed steel increases in hardness on temper- ing, provided it is tempered at a high temperature (about 1550F). Remember, all steel should be removed from the quenching bath and tempered before it is complete] y cold. Failure to temper correctly results in a quick failure of the hardened part. Permanent steel magnets are made of special alloys and are heat-treated by hardening and tempering. Hardness and stability are the most important properties in permanent magnets. Magnets are tempered at the mini- mum tempering temperature of 212F by placing them in boiling water for 2 to 4 hours. Because of this low- tempering temperature, magnets are very hard. Case-hardened parts should not be tempered at too high a temperature or they may loose some of their hardness. Usually, a temperature range from 212F to 400F is high enough to relieve quenching stresses. Some metals require no tempering. The design of the part helps determine the tempering temperature.

43. Harden ability of steel. Harden ability test methods.= The hardenability of ferrous alloys, i.e. steels, is a function of the carbon content and other alloying elements and the grain size of the austenite. The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats. Substances like brine and water cool much more quickly than oil or air. Additionally, if the fluid is agitated cooling occurs even more quickly. The geometry of the part also affects the cooling rate: of two samples of equal volume, the one with higher surface area will cool faster.

The hardenability of a ferrous alloy is measured by a Jominy test: a round metal bar of standard size (indicated in the top image) is transformed to 100% austenite through heat treatment, and is then quenched on one end with room-temperature water. The cooling rate will be highest at the end being quenched, and will decrease as distance from the end increases. Subsequent to cooling a flat surface is ground on the test piece and the hardenability is then found by measuring the hardness along the bar. The farther away from the quenched end that the hardness extends, the higher the hardenability. This information is plotted on a hardenability graph.

Hardenability TestingThe rate at which austenite decomposes to form ferrite, pearlite and bainite is dependent on the composition of the steel, as well as on other factors such as the austenite grain size, and the degree of homogeneity in the distribution of the alloying elements. It is extremely difficult to predict hardenability entirely on basic principles, and reliance is placed on one of several practical tests, which allow the hardenability of any steel to be readily determined:

The Grossman test

Much of the earlier systematic work on hardenability was done by Grossman and coworkers who developed a test involving the quenching, in a particular cooling medium, of several cylindrical bars of different diameter of the steel under consideration. Transverse sections of the different bars on which hardness measurements have been made will show directly the effect of hardenability. In Fig 1, which plots this hardness data for an SAE 3140 steel (1.1-1.4% Ni, 0.55-0.75% Cr, 0.40% C) oil-quenched from 815C, it is shown that the full martensitic hardness is only obtained in the smaller sections, while for larger diameter bars the hardness drops off markedly towards the center of the bar. The softer and harder regions of the section can also be clearly resolved by etching.In the Grossman test, the transverse sections are metallographically examined to determine the particular bar, which has 50% martensite at its center. The diameter of this bar is then designated the critical diameter D0. However, this dimension is of no absolute value in expressing the hardenability as it will obviously vary if the quenching medium is changed, e.g. from water to oil. It is therefore necessary to assess quantitatively the effectiveness of the different quenching media. This is done by determining coefficients for the severity of the quench usually referred to as H-coefficients. The value for quenching in still water is set at 1, as a standard against which to compare other modes of quenching.

Using the H-coefficients, it is possible to determine in place of D0, an ideal critical diameter Di which has 50% martensite at the center of the bar when the surface is cooled at an infinitely rapid rate, i.e. when H = . Obviously, in these circumstances D0 = Di, thus providing the upper reference line in a series of graphs for different values of H. In practice, H varies between about 0.2 and 5.0, so that if a quenching experiment is carried out at an H-value of, say, 0.4, and D0 is measured, then the graph can be used to determine Di. This value will be a measure of the hardenability of given steel, which is independent of the quenching medium used.

The Jominy end quench test

While the Grossman approach to hardenability is very reliable, other less elaborate tests have been devised to provide hardenability data. Foremost amongst these is the Jominy test, in which a standardized round bar (25.4 mm diameter, 102 mm long) is heated to the austenitizing temperature, then placed on a rig in which one end of the rod is quenched by a standard jet of water. The Jominy test: A - specimen size; B - quenching rigThis results in a progressive decrease in the rate of cooling along the bar from the quenched end, the effects of which are determined by hardness measurements on flats ground 4 mm deep and parallel to the bar axis (Fig. 3). A typical hardness plot for a En 19B steel containing 1% Cr, 0.25% Mo and 0.4% C, where the upper curve represents the hardness obtained with the upper limit of composition for the steel, while the lower curve is that for the composition at the lower limit. The area between the lines is referred to as a hardenability or Jominy band.

Additional data, which is useful in conjunction with these results, is the hardness of quenched steels as a function both of carbon content and of the proportion of martensite in the structure. Therefore, the hardness for 50% martensite can be easily determined for a particular carbon content and, by inspection of the Jominy test results, the depth at which 50 % martensite is achieved can be determined.

44. Steel abatement types, the resulting structure, properties and applications.

= Steel has been used since ancient times as a paint pigment. Two major chemical forms of lead are used as colors--they are called "white lead" (a lead carbonate) and "red lead" (a lead oxide). Both types of lead provide a thick, heavy, tough coating, one that does not crack through wear or temperature variations, because it can expand and contract in unison with the base metal to which it is attached. In addition, the chemical nature of lead causes it to provide corrosion resistance as well. Because of these properties, lead paints have been and continue to be widely used for bridges and other metal structures.

The overall amount of lead that has been used can be considerable--one estimate for the Sydney Harbor Bridge in Australia is that 90 tons of red lead paint and 250 tons of battleship gray are used in a five-year painting cycle. The bridge itself contains 51,300 tons of steel. Some old structures may have a thick coating from decades of painting. Even new steel is often coated with lead paint. This may be surprising to some, because many people are under the impression that lead paint has been banned. It is true that lead has been prohibited for use as a residential interior finish. But it continues to be used for many exterior uses. Even if lead were banned today for new use on structural steel, construction workers would still face a lead hazard for the next 25-50 years, because there are so many old structures that contain it. It is estimated that 35%-40% of steel structures are coated with lead-based paint, including 90,000 bridges. Of all bridges repainted in 1985-1989, 80% of them had lead coatings. Because demolition and repair are likely to be increasingly important in the future as the Nation faces its "infrastructure problems," it is important that every construction worker be aware of this hazard.

For some Laborers, the idea that the job can involve lead exposures may be a new one. After all, lead is not a common item on the work site. It is not often present in structural steel alloys, nor used as a specialty product. It may be present as a thin film of paint, but this may seem like an unlikely possibility for causing a problem. Well, if you ask anyone who has done any renovation or demolition work on old steel structures lately, you begin to understand the problem. It is typical to use oxyacetylene torches to cut on old steel structures or to use welding equipment to weld on them. The high temperatures of the torch or welding process vaporizes the lead, so it becomes airborne and available for the worker to breathe. The purpose of this course is to explain the nature of the hazard, provide recommended work practices to allow safe work, and to describe the relevant regulations and guidelines for lead. Because lead is such an important hazard, it is covered by a very tough OSHA standard. The Lead Standard will be reviewed in this course because it addresses many of the hazards involved with lead exposure.

45. Surface strengthening of steel by the plastic deformation.

= Plastic deformation causes 1) change in grain size, 2) strain hardening, 3) increase in the dislocation density. Restoration to the state before cold-work is done by heating through two processes: recovery and re crystallization .General principles. Ability to deform plastically depends on ability of dislocations to move. Strengthening consists in hindering dislocation motion. We discuss the methods of grain-size reduction, solid-solution alloying and strain hardening. These are for single-phase metals. We discuss others when treating alloys. Ordinarily, strengthening reduces ductility. This is based on the fact that it is difficult for a dislocation to pass into another grain, especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity in slip planes. For high-angle grain boundaries, stress at end of slip plane may trigger new dislocations in adjacent grains. Small angle grain boundaries are not effective in blocking dislocations.

The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Usually, the yield strength varies with grain size d according to:

sy = s0 + ky / d1/2

Grain size can be controlled by the rate of solidification and by plastic deformation. Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working). (This is different from hot working is the shaping of materials at high temperatures where large deformation is possible.) Strain hardening (work hardening) is the reason for the elastic recovery discussed .The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one.Heating increased diffusion enhanced dislocation motion relieves internal strain energy and reduces the number of dislocation. The electrical and thermal conductivity are restored to the values existing before cold working.

Re crystallizationStrained grains of cold-worked metal are replaced, upon heating, by more regularly-spaced grains. This occurs through short-range diffusion enabled by the high temperature. Sincere crystallization occurs by diffusion, the important parameters are both temperature and time.The material becomes softer, weaker, but more ductile.Re crystallization temperature: is that at which the process is complete in one hour. It is typically 1/3 to 1/2 of the melting temperature. It falls as the %CW is increased. Below a "critical deformation", recrystallization does not occur.The growth of grain size with temperature can occur in all polycrystalline materials. It occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster at higher temperatures. The "driving force" is the reduction of energy, which is proportional to the total area. Big grains grow at the expense of the small ones.

46. Surface tempering methods: heating with the flame, the high frequency current.

= Differential heat treatment is a method used to alter the properties of various parts of a steel object differently, producing areas that are harder or softer than others. This creates greater toughness in the parts of the object where it is needed, such as the tang or spine of a sword, but produces greater hardness at the edge or other areas where greater impact resistance, wear resistance, and strength is needed. Differential heat treatment can often make certain areas harder than could be allowed if the steel was uniformly treated, or "through treated". There are several techniques used to differentially heat treat steel, but they can usually be divided into differential hardening and differential tempering methods.

During heat treating, when red-hot steel (usually between 1,500 F (820 C) and 1,600 F (870 C)) is quenched, it becomes very hard. However, it will be too hard, becoming very brittle like glass. Quenched-steel is usually heated again, slowly and evenly (usually between 400 F (204 C) and 650 F (343 C)) in a process called tempering, to soften the metal, thereby increasing the toughness. However, although this softening of the metal makes the blade less prone to breaking, it makes the edge more susceptible to damage, such as dulling, peening, or curling.Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating to harden or soften certain areas of a steel object, creating a difference in hardness between these areas. There are many techniques for creating a difference in properties, but most can be defined as either differential hardening or differential tempering.

Differential hardening consists of either two methods. It can involve heating the metal evenly to a red-hot temperature and then cooling it at different rates, turning part of the object into very hard martensite while the rest cools slower and becomes softer pearlitic. It may also consist of heating only a part of the object very quickly to red-hot and then rapidly cooling (quenching), turning only part of it into hard martensite but leaving the rest unchanged. Conversely, differential tempering methods consist of heating the object evenly to red-hot and then quenching the entire object, turning the whole thing into martensite. The object is then heated to a much lower temperature to soften it (tempering), but is only heated in a localized area, softening only a part of it. Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

47. Upper layer strengthening by chemical heat treatment, its physical foundations.

= In a salt bath with consecutive hot quenching in AS 140; for parts with maximum length < 600 mm and diameter < 300 mm; up to maximum cementation thickness: 1.2 mm.In the mono crab unit with consecutive oil quenching; for parts with maximum length < 1400 mm and diameter < 600 mm; up to maximum cementation thickness: 1.5 mm.Hardening

Oil hardening; for small parts hardening and thermal refinement; for parts with maximum length < 2300 mm and diameter < 150 mm; heating to hardening temperature without protective atmosphere.Water quenching of low carbon steel; small parts only (maximum hardening temperature=980 C).Nitriding

Gas nitriding (at 520 C); maximum hardened layer thickness 0.5-0.7 mm; for parts with maximum length < 2300 mm and diameter < 250 mm.Plasma nitriding (at 480 C 560 C according to customers requirement); maximum hardened layer thickness 0.4-0.5 mm; for parts with maximum length < 2400 mm and diameter < 600 mm.Annealing

In N2 protective atmosphere, for parts with maximum length < 2300 mm and diameter < 600 mm.A large bogie hearth furnace with gas heating; for parts with maximum dimensions < 7000x3000x1500 mm; all kinds of annealing up to 580 C available.Two bogie hearth furnaces with electric heating; for parts with maximum dimensions < 2500x1000x600 mm; all kinds of annealing up to 960 C available.Shaft furnace with electric heating; for parts with maximum length < 1700 mm and diameter < 900 mm; all kinds of annealing up to 960 C available.Batch furnace only for small parts; all kinds of annealing up to 900 C available.Alkaline Blackening

For parts with maximum dimensions < 1100x650x350 mmShot blasting

0.16 mm dia. steel shots1500 mm dia. work tableProduction of water for laboratory and industrial purposes (AQUA OSMOTIC 02) for:

chemical analyseseveryday laboratory needspreparation of solutions, diluting, etc.washing of laboratory glassreplenishment water for batteriesChemical treatments are problematic for a number of reasons, the fist being that they may not kill bed bugs, at all. Years of pesticide use has made bed bugs incredibly resistant to chemical treatments. Recent research has shown that some bed bugs can survive more than 1,000 times the amount of pesticide considered lethal just 10 years ago.

Theres no such thing as a heat-resistant bed bug, however. These pests cant survive temperatures above 122. Our treatment raises temperatures in your home up to 185 for a period of six to eight hours, ensuring that our treatment is almost always 100% effective.

Secondly, the chemicals used in pesticides may pose health threats to people and pets. The Environmental Protection Agency recognizes the following side effects of pesticide exposure: skin and eye irritation, nerve damage, disruption of the endocrine system and even cancers. Its also common for sprayed pesticides to end up in the air, soil, or water, where they pose environmental threats.

48. Steel hardening by carburizing, application, technology, structure and properties.

= Carburizing or carburising (chiefly British English) is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite, while the core remains soft and tough as a ferrite and/or pearlitic microstructure.

This manufacturing process can be characterized by the following key points: It is applied to low-carbon work pieces; work pieces are in contact with a high-carbon gas, liquid or solid; it produces a hard work piece surface; work piece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm). In some cases it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process. it is also referred to as Case Hardening, is a heat treatment process that produces a surface which is resistant to wear, while maintaining toughness and strength of the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components.Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core. This treatment is applied to low carbon steels after machining.Strong and very hard-surface parts of intricate and complex shapes can be made of relatively lower cost materials that are readily machined or formed prior to heat treatment.

Most carburizing is done by heating components in either a pit furnace, or sealed atmosphere furnace, and introducing carburizing gases at temperature. Gas carburizing allows for accurate control of both the process temperature and carburizing atmosphere (carbon potential). Carburizing is a time/temperature process; the carburizing atmosphere is introduced into the furnace for the required time to ensure the correct depth of case. The carbon potential of the gas can be lowered to permit diffusion, avoiding excess carbon in the surface layer.After carburizing, the work is either slow cooled for later quench hardening, or quenched directly into oil. Quench selection is made to achieve the optimum properties with acceptable levels of dimensional change. Hot oil quenching may be used for minimal distortion, but may be limited in application by the strength requirements for the product. Alternatively, bearing races may be press quenched to maintain their dimensional tolerances, minimizing the need for excessive post heat treatment grinding. In some cases, product is tempered, then cryogenically processed to convert retained austenite to martensite, and then retempered.Metlab has the ability to carburize and harden gears and other components that are small enough to be held in one's hand, up to 14' in diameter and 16' tall, weighing as much as 50,000 pounds. Shallow cases only 0.002 - 0.005", and deep cases, up to 0.350" have been specified and readily achieved.The press quench located in the facility allows for the dimensional control, therefore precise hardening of gears and bearings up to 16" in diameter.

49. Steel hardening by nitrogen and nirtocarburizing, technology, structure and properties.

= Steel Hardening With unalloyed steels, the nitrogen is dissolved in the iron lattice. Due to the diminishing solubility of nitrogen in iron during slow cooling, _'-Fe4N nitrides are precipitated in the outer region of the diffusion layer, some in form of needles, which are visible in the structure under the microscope. If cooling is done quickly, the nitrogen remains in super-saturated solution. With alloyed steels which contain nitride-forming elements, the formation of stable nitrides or carbonitrides takes place in the diffusion layer independent of the cooling speed. With increasing alloy content of the steel, the diffusion layer is thinner for identical nitrocarburizing parameters. However, with their higher level of nitride-forming alloying elements these steels have a greater case hardness. Fig. 3 illustrates the influence of chromium on the hardness and depth of the diffusion layer in steels with a carbon content of 0.40 - 0.45% after 90 minutes treatment at 580C (1075F). Total nitro carburizing depth shown in Fig. 4 is the distance to the point where the hardness of the nitride layer is equal to the core hardness. After a 90 minute treatment the total nitrided depth is about 1.0 mm (0.040") on unalloyed steel, but barely 0.2 mm (0.008") on a 12% Cr steel. Total nitride depth on various materials resulting from nitrocarburizing

Shows the coefficient of friction both under dry conditions and after lubrication with SAE 30 oil, measured by an Amsler machine. All samples were lapped to a roughness of R_ = 1_m after their respective surface treatments and before testing. Without lubrication the nitrocarburized QP had the lowest coefficient of friction, being less than half of that of the hard chrome or case hardened surfaces. The lowest friction level occurred when nitrocarburized QPQ is lubricated. It is 3-4 times lower than that achieved with the chrome or martensitic surfaces. These results show the direct effect of increased oxidation as it relates to friction on the surface of the nitrocarburized samples. The QPQ sample, with its extra post-oxidation step, has a much higher friction value than the QP specimen, which had part of its original oxidation in the compound layer removed by lapping. However, with this variant, due to the fine microporosity in the QPQ sample which causes the lubrication to adhere better to the surface, this option gives the lowest friction value.

If a uniform running behavior is required the QP process is appropriate. Lubrication has only a slight influence on the coefficient of friction because the oxide layer of the outer surface was removed during the polishing operation.It has been determined that, unlike with chrome surfaces, the coefficient of friction of nitrocarburized QP and QPQ treated surfaces remains constant, even at varying sliding speeds.The intermetallic stricture of the compound layer, which contains epsilon iron nitride formed during nitrocarburizing, is extremely resistant to adhesive wear and scuffing. Fig. 8 shows the scuffing loads of gears made from various materials (6). It was established by applying increasing pressure to the flank tooth until galling occurred. Austenitic steel containing 18% chromium and 8% nickel had the lowest resistance to galling, however, after nitrocarburizing its resistance was raised almost five-fold. The performance with SAE 5134 was about tripled. Even SAE 5116, which had already been carburized, more than doubled the scuffing load it could withstand through the compound layer built by the nitrocarburizing treatment. 50. Strengthen coatings, their main covering methods.= A strength coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. An example of all of these types of coating is a product label on many drinks bottles- one side has an all-over functional coating (the adhesive) and the other side has one or more decorative coatings in an appropriate pattern (the printing) to form the words and images.

Paints and lacquers are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is presumably only for the function of preventing corrosion.

Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wet ability, corrosion resistance, or wear resistance. In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property such as a magnetic response or electrical conductivity and forms an essential part of the finished product.

A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for 'non-all-over' coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes.A strength coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. An example of all of these types of coating is a product label on many drinks bottles- one side has an all-over functional coating (the adhesive) and the other side has one or more decorative coatings in an appropriate pattern (the printing) to form the words and images.

Paints and lacquers are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is presumably only for the function of preventing corrosion.Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wet ability, corrosion resistance, or wear resistance. In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property such as a magnetic response or electrical conductivity and forms an essential part of the finished product.

A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for 'non-all-over' coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes.51. Basics of steel alloyage. Effect of alloying elements on steel properties. = The carbon content of steel is between 0.002% and 2.1% by weight. Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make an alloy commonly called pig iron that is brittle and not malleable. Alloy steel is steel to which additional alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, and niobium. Additional elements may be present in steel: manganese, phosphorus, sulphur, silicon, and traces of oxygen, nitrogen, and aluminium.Effects of Alloying Elements in Steel

Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.CarbonThe basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.ManganeseManganese is added to steel to improve hot working properties and increase strength, toughness and harden ability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)ChromiumChromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving harden ability and strength.NickelNickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.MolybdenumMolybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.TitaniumThe main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.PhosphorusPhosphorus is usually added with sulphur to improve machine ability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.SulphurWhen added in small amounts sulphur improves machine ability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.SeleniumSelenium is added to improve machine ability.Niobium (Columbium)Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.NitrogenNitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming elem