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Materials Technology I

EMTL 250
Module 2

Manufacturing and Automation Revised: 2017

but there are limits based upon the chemical composition).e. 3. plastic deformation can alter material properties such as strength. In addition to alloying. Learning Objectives After you complete this module. Introduction Once an alloy (chemical composition) has been chosen. its potential mechanical properties are fixed (i. alter their shape and alter the mechanical properties. Complete the exercise questions found throughout the module. can alter the mechanical properties so that a whole range of material strength and ductility can be obtained from any given alloy grade. which enables plastic deformation to increase the strength of a material. the “big picture”). The use of plastic deformation. In order to illustrate how plastic deformation can affect these properties. Discuss the processes of hot and cold working. 2.e.DEFORMATION THEORY Altering Material Properties through Plastic Deformation Rationale Why is it important for you to learn this material? Manufacturing processes use plastic deformation to manipulate materials. you will be asked to: 1. 5.. we need to understand what is occurring on a microscopic scale and how it relates to the macroscopic properties (i. Performance Evaluation To show you have mastered the material. 4. toughness and ductility on both the atomic level (micro) as well as the macro level. Describe the basic processes involved in modern steelmaking. Explain the response of different lattice structures to plastic deformation. there is a range of properties that may be obtained. Discuss what is occurring on the microscopic scale. Complete Assignments in this module. 1 . however. you will be able to: 1. Learning Outcome When you complete this module you will be able to: Explain the positive and negative effects of plastic deformation on the mechanical properties of alloys. 2. Explain the difference between elastic and plastic deformation..

Discussion One Engineered materials selection is based upon matching material properties with service conditions. 2. When stress is applied to a part. the material is deformed or distorted (strained). 3. which enables plastic deformation to increase the strength of a material. 2. Completed Exercise One. both will still strain the same amount. Discuss what occurs on the microscopic scale. however. larger parts will be able to handle larger loads more easily. For example. Explain the difference between elastic and plastic deformation. a 100 cm long spring will stretch ten times as much as a 10 cm long spring under the same stress. Engineers can alter the mechanical properties of materials in three primary ways: 1. Plastic deformation . will allow the part to handle twice the amount of force. Thermal treatment . which will be discussed in Module 3 When a force or load is applied. which was discussed in Module 1 2. Doubling the size (cross-sectional area). Read Discussion One. Change in Length Strain  Original Length Strain is used to compensate for the different size (length) of parts. the focus of Module 2 3. the stress being applied will remain the and cold working. Compare with Exercise One answers at the end of this module. the part has stress being applied to it. 2 . Grade selection – alloying. Stress is used to compensate for the different size (cross-sectional area) of parts. However. Load Stress  Original Area Obviously. OBJECTIVE ONE When you complete this section you will be able to: 1.heat treating. Learning Activities Complete each of the following Learning Activities: 1.

Force (stress) is directly proportional to the amount of (strain) stretch. Other stresses such as bending. picking up a load with a crane will cause tensile stress on the cable and bending force in the crane boom. but the bonds do not break. In this region. Figure 1: Elastic Deformation between Atoms. tension and shear. For example. Source: SAIT 2012 If we look at the atomic scale and examine what happens to the lattice structure. will cause thermal stress. however. Stress is usually caused by the application of force on a part.g. This can be seen when a material flexes. the atoms must either: 1. This occurs when the elastic limit (or yield stress) of a material is reached and the material can no longer return to its’ original shape. stretching an elastic band and then allowing it to return to its original shape). The effect can be seen in the load-elongation curve shown in Figure 7  where the linear portion shows the elastic deformation region. If a plate is heated on the top surface rapidly. thermal expansion will occur and cause bending (heat warpage). a diving springboard is designed to elastically flex a long way without plastic deformation. Elastic Deformation occurs when a load (or stress) is applied to a material and it is able to return to its original shape once the load is removed (e. are simply combinations of compression. 3 . etc. the atoms will go back to their original locations (see Figure 1). Application of heat to a part. we can see that an elastic load will stretch the atomic bonds between adjacent metal atoms.Stress may either be in compression. when the load is doubled the elongation (stretch) will also double. Break the bonds between them to produce a fracture (brittle materials). tension or shear and the resulting strain will also be in the same direction. Once the load is released. Plastic deformation is defined as a permanent change in shape when metal is worked. or 2. When the applied stress exceeds the elastic limit (yield strength) of the material.. The atomic planes within the crystal must slide over one another to produce a permanent shift of atom positions. called slip (see Figure 2). torsion.

g. when adjacent planes of atoms slide past one another. Therefore.missing atoms in the lattice structure (see Figure 3). the atoms can easily reform new bonds with their neighbors during slip. the slip between the atomic planes is permanent. The amount of stress to cause the first slip is called the “elastic limit” or “Yield Strength”. A higher number of lattice defects will require more stress to cause slip (higher yield strength) and reduces the ability of the material to plastically deform (lower percentage elongation). 2. The elastic strain disappears after the stress is removed. Ductility is the ability of a material to plastically deform without failure. so slip usually occurs. Anything that causes atomic bonds to be pre-stretched will increase the strength. When discussing plastic deformation.solute atoms located at the interstitial sites (see Figure 4). which are ductile. whereas a brittle material will fail with little or no permanent change in shape (e. Plastic deformation in metals occurs by slip. Slip causes the metal to plastically deform. Plastic deformation has occurred. 4 .. In metals. These lattice defects include: 1. materials are characterized as being ductile or brittle. It will also reduce the amount of ductility (stretch) left in the atomic bonds. Source: SAIT 2012 The lattice structure in crystals is never perfect. Vacancies . Percent elongation and percent reduction in area are two measures of ductility in the tensile test. Figure 2 shows a 20% increase in length after slip occurs. A ductile material will exhibit plastic flow or necking prior to failure. however. < 2 percent elongation). Interstitial atoms . the stress required to cause slip is lower than the stress for complete fracture. Figure 2: Slip occurs along the space lattice when the stress applied exceeds the elastic limit (Yield Strength). They contain lattice defects which cause stretched atomic bonds.

Kenny. because they act like a wrinkle in the lattice structure. 4. SAIT 2012 Dislocations allow slip to occur at much lower applied stress. Dislocation . The dislocation is the stretched opening or line of missing atoms (going into page). Source: B.solute atoms replacing solvent atoms in the lattice structure. The dislocation line moves to the right as the atoms slide to the left. Atom 1 moves to the left.line defect or missing line of atoms (see Figure 5). atoms sequentially slide along the slip plane. is much 5 . Kenny. The force required. Source: B. Substitutional atoms . SAIT 2012 Figure 4: Alloying creates Interstitial and Substitutional Lattice Defects making it more difficult for slip to occur. SAIT 2012 Figure 5: During plastic deformation. Imagine the force required to pull a rug across the floor if the rug is lying flat. 3. Figure 3: Vacancy Lattice Defect. Kenny. or stress. followed by atom 2 and so on. Source: B. Now imagine pulling the same rug when it contains a series of wrinkles (see Figure 6).

Figure 7: A Load-Elongation Curve showing Elastic and Work-Hardening Regions. interstitial elements and other dislocations. When this occurs. Kenny. Source: SAIT 2012 As slip occurs. Slip will be more difficult going in the vertical direction since atoms are no longer lined up in a perfect crystal structure. Source: B. Slip along a wrinkle (dislocation) can be easily moved. SAIT 2012 6 . thus the material’s resistance to a load becomes higher (the yield strength of the material will increase). This will result in the yield strength being higher. Figure 6: A high force is required to move a rug all at once. the dislocation movement can be impeded by lattice defects such as substitutional atoms. lower. an increased force is required to move the dislocation. so that more force is required to move the rug. These lattice defects act like hooks in the “wrinkled rug”.

Elastically straining a metal (below the yield strength). This results in work hardening (also known as strain strengthening).) will cause the metal to increase in strength and hardness. so there is a limit to the amount of work hardening that a material can undergo before it becomes brittle and fractures (cracks) occur. Strain is calculated from elongation divided by the original length. Figure 7 illustrates the portion of the load-elongation curve where work hardening It is rarely used in engineering since it is extremely difficult to measure accurately. more dislocations are created in all directions. Stress is calculated from Load divided by the original area.2% plastic deformation). Work hardening is a common method used to increase the strength of metals.strain curve. Yield strength= Yield load/ Cross Sectional Area UTS= Ultimate Tensile Strength= Maximum load Cross sectional Area During cold working. which effectively increases the UTS. etc. cold working (such as bending.The shape of the curve in figure 7 is identical to a stress . A negative effect of work hardening is that the material loses ). however. rolling. drawing. The strength of the material increases as the increased number of dislocations impede each other (http://www. 7 . It should be pointed out that cold rolling would reduce the cross-sectional area of the part. Work hardening only occurs above the yield as the metal is worked and causes the yield strength to increase. The UTS is the maximum stress to cause failure. The strength of the material is usually measured by the yield strength or by the Ultimate Tensile Strength (UTS). will not harden the metal since no permanent work is done. The elastic limit is similar to the yield strength except it is defined as when the first atomic bonds break and reform during slip. it is sometimes used when the yield strength is meant. The yield strength is the amount of stress to cause permanent change in shape (usually set at 0.

plastic deformation is the result of: a. 11. 9. the yield strength of metals often increases. False 2. 4. True b. Always in tension. 10. Plastic deformation is defined as . Dislocations allow slip to occur more easily: a. 8. Molecules slipping past each other b. True b. Elastic strain 3. What is slip and why does it occur in metals? 5. As a result of plastic deformation. c. Elastic deformation will allow metals to return to their original shape. 8 . 7. d. When atomic bonds break and reform. Name the lattice defects. Plastic deformation occurs: a. Distortion of the crystal structure d. In metals. a. Materials which undergo some plastic deformation are ______ . ______materials show only elastic deformation prior to fracture. Atomic planes moving past each other c. If the part returns to its original shape when the load is released. This is called ______ . which impede dislocation movement and help increase strength. False 6. Exercise One 1. Below the elastic limit b. Describe the effect of work hardening on strength and ductility.

have high strength but low ductility (so crack easily when cold worked). Explain the response of different lattice structures to plastic deformation. 2. Less stress is required to cause slip. are relatively weak with excellent ductility since there are many close-pack easy slip directions. Many close-packed planes. Walking over rough ground is more difficult than over smooth ground with no holes. such as titanium. have high strength with moderate ductility since they have no close-packing atom planes. A combination of close-packed (slip) plane and direction is referred to as a slip system. such as iron. and from one grain to another. 3. will occur more easily when the crystals have: 1. 2. The grains elongate in the direction of rolling but the volume remains the same. The micrographs in figure 8 show how the shape of the grains is affected by the amount of work. 3. there will be lower force required for slip to occur. toughness and ductility. Atoms are “close packed” within a plane when the spacing between atoms is as tight as possible. Read Discussion Two. Compare Exercise Two answers at the end of this module. Discussion Two Metals are made up of many crystals (or grains). HCP crystals cannot deform in the vertical direction.OBJECTIVE TWO When you complete this section you will be able to: 1. During plastic deformation of metal.  BCC metals. The shearing force is applied parallel to one of the preferred slip directions. cold working occurs. The more slip systems that a lattice structure has.  HCP metals.  FCC metals. Slip within a grain. As slip (deformation) moves from one grain to the next. Many close-packed directions (easy slip directions). such as aluminum. Discuss the processes of hot and cold working. The ease with which slip can occur determines the properties such as strength. the direction of slip changes. The planes of atoms within each of these grains have a different orientation. the more likely slip is to occur. Learning Activities Complete each of the following Learning Activities: 1. 2. With a small distance between atom planes. Complete Exercise Two. 9 .

Unfortunately. Stretched atomic bonds also lead to increased corrosion rates since the atoms more easily react with the environment. Kenny. When the plate thickness is reduced by 50%. When the surface is machined away the part will bend. The part may also distort during service or machining. the surface will be in compression while the center is in tension. SAIT 2012 10 . When atomic bonds are stretched there will be an increase in hardness and strength. the length of the grains will double. Kenny. there is a great deal of internal stress (residual stresses from dislocations stretching the atomic bonds). but the grain volume does not change. there will also be a reduction in ductility and toughness. 2012 Figure 8: Cold rolling elongates the grains in the Rolling Direction. Since the grains at the surface are worked more than the center. SAIT Polytechnic. Source B. Source: B. which can lead to cracking. During cold working.

If there’s too much cold work then all the ductility will be used up and cracks will be created. Figure 9 shows three types of annealing: recovery. Recovery anneal (or stress relief anneal) is a low temperature anneal (slightly less than 1/2 of the melting point in Kelvin). This allows much of the space lattice to be stress free. toughness and ductility). a refined (smaller) grain size is achieved compared to the original casting. dramatically improves ductility with a corresponding loss of strength and hardness. They’re all similar as they use controlled heating and cooling to restore ductility. The internal stresses caused by the lattice distortion (dislocations) are relieved and new grains are created (nucleated) within the old distorted grain. SAIT 2012 Annealing refers to many different types of heat treatments. Source: B. Once cracks occur they cannot be healed. Some thermal treatments such as recrystallization anneal may be performed to also refine the grain size. hardness.During cold work. Kenny. recrystallization. In this way. For recrystallization to occur.g. the plastic deformation occurs at a temperature well below the recrystallization temperature and the atoms cannot move around easily to make perfect crystals. rolling) increases the strength (both UTS and Yield). which allows some movement of dislocations into tighter bunches. the metal must have: 11 . and grain growth. This relief of internal stress:  Increases the ductility without significantly changing the yield strength  Makes the part more dimensionally stable for machining  Is less prone to stress corrosion cracking (SCC)  Improves fatigue resistance Recrystallization anneal (or process anneal) uses temperature slightly higher than 1/2 MP (melting point). Figure 8: Cold work (e. It also shows effect of temperature on some the mechanical properties (strength. Elimination of internal stress (no stretch between atoms). 50% cold work means the cross sectional area of the plate has been reduced by ½ during rolling.

less time is required since the atoms are more mobile. At higher temperatures. Above 1/2 melting point (Kelvin). A second anneal only grows the grains since there is no internal stress (energy) to cause grains to form. the atoms can move from their stressed positions to form new grains (perfect unstressed crystals).  Sufficient cold work (minimum 10 percent). Grain growth occurs when the metal is heated very high or for very long.  Sufficient temperature. A few grains grow and occupy the space of their neighboring grains – while others get fat as they consume their neighbours. Higher temperatures allow atom movement to occur more easily. 2012 Grain growth anneal may be used when the metal is required to be as soft as possible or when high temperature creep strength is necessary. Figure 9: The effect of temperature on the mechanical properties. There dramatic changes at ½ MP (K).  Sufficient time. which reduce the amount of high-energy grain boundaries. Source: B. Grain growth is usually not desirable for most applications since both the strength and 12 . Kenny. SAIT.

g. go to http://www. For more information on grain growth anneal. as some metals can only be hot worked.  Easy to form (e. 13 .. rolling). Source B.  No annealing step during processing (  Slightly more corrosion resistant (especially for stress corrosion cracking). aluminium) hot work at lower temperatures and do not form heavy oxides. Kenny. HCP metals).. due to low ductility Advantages of hot working compared to cold working:  Ductility remains high. (e.g.toughness are Hot working occurs above the recrystallization temperature. One of the disadvantages of hot worked steel is that a heavy oxide scale (mill scale) forms as the surface burns. SAIT Polytechnic. Figure 10 illustrates the steps that occur during hot working. Other alloys (e.g. which means that spontaneous recrystallization of the grains occurs during the working process.. 2012 Figure 10: The Hot Working (rolling) Process Produces Grains Smaller than the Original Advantages of cold working (cold finished) compared to hot working:  Strength is increased  Close dimensional tolerances (no thermal shrinkage)  Good surface finish (no oxide scale on steel)  Chips form more easily during machining (free machining). small motors and rollers).

Face centered cubic at low temperature (below 0ºC) d. Normalizing d. Shorter 7. Quenching b. Longer b. Stress analysis b. The ___ process is used to completely remove work hardening. 14 . Cold finished (cold worked) steel is: a. The grain size becomes smaller c. a. Describe the mechanical property characteristics commonly associated with the three common lattice structures. Immersed in dry ice to stabilize structure b. Recrystallization c. a. Elongated in the forming direction b. An advantage of cold work over the cheaper hot work is that: a. Give two reasons why heavily cold worked metals are often recrystallization annealed. Exercise Two 1. ___ time at the recrystallization temperature makes the grains larger. Annealing c. Equiax and randomly oriented d. Peening 2. Internal stress improves corrosion resistance 5. Carburizing 6. Columnar and oriented in similar directions c. The metal remains softer and is easier to deform b. After cold working the grains are: a. Machining d. Quenched from high temperature into cold water c. Internal stresses caused by cold working can be removed by: a. Cubic 3. Heavy oxides do not form d. 8. Deformed below the recrystallization temperature 4.

will dissolve and contaminate the molten steel at this point. and limestone (flux to help remove impurities) in the blast furnace. 2. Since limestone (calcium carbonate) is basic.OBJECTIVE THREE When you complete this objective you will be able to: Describe the basic processes involved in modern steelmaking. Complete Exercise Three. In the LMF:  Alloy additions are made  Dissolved gases can be removed (H. especially oxygen. Some cast irons may be produced directly in this way but generally cast irons require further refining in a cupola furnace. Compare Exercise Three answers to the answers at the end of this module.O. The basic oxygen furnace (BOF) is the common type of steel making furnace used to change pig iron into steel by adding oxygen and limestone (basic compound). Learning Activities Complete each of the following Learning Activities: 1. Pig iron is of little use due to the high impurity content and very high carbon content senses so brittle. A hot blast of air gives good contact between the coke.” which lowers the carbon content to make steel. The molten steel is transferred to a ladle metallurgy facility (LMF) which provides final purification and fine tuning of both the composition and the temperature of the molten metal. Steelmaking Pig iron is produced when iron ore (iron oxide) is reduced with coke (coal which has been heated to remove extra hydrogen). Supersonic velocity oxygen is blown over the surface of the molten pig iron and the carbon is “burned. Read Discussion Three.N) by vacuum degassing The molten steel is solidified by either continuous or ingot casting. 15 . 3. it reacts and removes acidic impurities (S and P). Gases. Discussion Three Figure 11 shows a flow chart outlining a typical sequence for reducing iron ore into pig iron and then subsequently oxidizing the carbon to low levels to create steel. limestone and iron ore.

ingots or billets) for further processing (e. Kenny. During continuous casting:  Molten metal is poured into the top of a water cooled ring mold. shrinkage and gas porosity. 16 . as illustrated in Figure 12. This process solidifies molten metal into a long continuous strand. Source B.  Cooling is controlled so that the surface of the bar has solidified before the metal exits the bottom of the mold. 2006 Figure 11: Iron Ore to Steelmaking Continuous casting (Con-cast) reduces some of the problems associated with ingot casting (non-homogeneous composition.  Further cooling occurs by water sprays. SAIT. hot rolling). etc..  The solidified metal is cut to length (slabs.g.).

so carbon monoxide or oxygen bubbles do not evolve during solidification (steel appears dead). molten metal may be poured into a mold or other container to produce an ingot casting.)  Shrinkage (thermal) To reduce the gas porosity. 2012 Figure 12: Continuous Caster (con-cast) Instead of continuous casting. the molten metal may be treated to remove dissolved oxygen. two types of porosity can occur:  Gas (release of dissolved carbon monoxide. “semi-killed” or “rimmed”). Semi- killed steel is partially deoxidized. etc. Figure 13 shows the three basic types of cast steel (“killed”. Al2O3). nitrogen. Silicon and aluminum have a high affinity for oxygen and will react to form solid oxides (SiO2. SAIT. Kenny. Large ingots may also be cast. which produces killed steels. Killed steels have been fully deoxidized by the addition of aluminum or ferrosilicon to the molten metal. oxygen. Since the molten metal is confined by the mold. Source B. Rimmed steels are not degassed. These solid oxides usually float to the surface of the molten metal however they may 17 .

Kenny.. so it lies still in the mold during freezing). Rimmed steels undergo no or very little deoxidization before solidification. The characteristics of rimmed steel:  Molten steel bubbles vigorously in the mold during freezing  Gross porosity and blowholes  Low carbon rim at the ingot surface (carbon segregates to the center)  Little shrinkage.  High shrinkage (piping) at the top of the ingot. Semi-killed steels have been partially deoxidized by addition of either aluminum or ferrosilicon.g. Killed steels:  Are common for high strength and alloy steels (pressure vessels). Source B.  Have little porosity (no bubbles form. The soft. which will be cropped off. Hot rolling can usually weld the gas pores shut. such as can stock). The gas holes may fuse together from the high heat and pressure during hot- rolling. Most grades of structural steels are semi-killed. ductile rim provides easier rolling. so gases which were dissolved will form porosity (bubbles) during freezing. The solubility of gas in liquid steel is twenty times that in solid steel. These steels are used for low strength applications. 2012 Figure 13: Types of Cast Steel Ingots 18 . which will be heavily hot-worked (e. SAIT. be present as inclusions evenly dispersed throughout the steel ingot. thin sheet. The characteristics of semi-killed steels fall in between killed and rimmed steels.

the inclusion content can be maintained at very low levels. These routes (amount of hot or cold working. 2012 Figure 14: The Electric Arc Furnace shown in the upright position where scrap is melted. The furnace is then tilted to the left to pour the refined steel into the ladle for subsequent casting. Once the metal has solidified. SAIT. Figure 14 shows the furnace and the upright position. which melts the steel rapidly. Since oxygen is excluded from the molten steel surface. such as stainless steels. cooling rates.) affect the microstructure and mechanical properties of the final product. but instead melt scrap in large electric arc furnaces (Figure 14). the furnace is tilted to the right to skim off the slag and impurities that float to the top of the molten steel. Kenny. Using scrap as the starting material reduces costs and mills can be setup near their customers. 19 . High quality steels. Source B. it is then homogenize annealed to minimize coring and segregation. are often produced in electric arc furnaces since the furnace can be sealed and argon used as an inert cover gas. etc. It is then worked to shape. heating times. Prior to pouring the steel. Figure 15 illustrates the possible processing routes. Many steelmakers do not make steel from pig iron.

Semi-killed steels are . An electric arc furnace is used to .com/watch?feature=endscreen&v=9l7JqonyoKA&NR=1 http://www. 3. Molten steel is solidified using either .youtube. Figure 15: Steel Production Routes For further information see the following videos: Steelmaking Exercise Three 1. Pig iron is of little practical use because . 5. (1944) Continuous Casting http://www. Killed steels are . 20 .

) 11. 4. c. stretching an elastic band. which makes it easier to move.g. When atomic bonds break and reform 8. 9. Cold working is a plastic deformation process which results in a permanent change in shape of the material. (The forces required to cause planes of atoms to slip past each other is much less than the forces required causing the material to fracture. Lattice defects include interstitial atoms. Plastic deformation results in a permanent change in shape caused by planes of atoms moving past each other. Heavy oxides do not form. On the microscopic scale this is seen as elongated grains. a. Slip occurs in metals because the required forces to cause slip are much lower than those required to cause fracture. Elongated in the forming direction. Ductile (Materials showing some plastic deformation or necking before failure. d. b.) Note: (a)) and (c) are incorrect answers.) 10. Work hardening increases strength and decreases ductility. The first stage of recrystallization annealing reduces internal residual stresses caused by the distortion of the atomic bonds. 7. Atomic planes moving past each other 3. Annealing. substitutional atoms and dislocations. True (Dislocations are like a wrinkle in an area rug.) 2. Elastic deformation occurs below the yield limit of the material and when the load is removed the material returns to its original shape (e. Deformed below the recrystallization temperature Deformation above the recrystallization temperature is called hot working where plastic deformation of the grains is accompanied by spontaneous recrystallization of the grains. 3. The volume and density of the grains does not change. 21 .) 6. a. Brittle (Materials showing little or no plastic deformation before failure. Slip is the sliding of adjacent planes of atoms over one another when a load is applied. d. True. 2. since metals do not contain molecules and elastic deformation also causes distortion.. 4. b. 5. a. Work hardening or strain hardening Exercise Two Answers 1.Exercise One Answers 1.

8. They have more porosity then killed steels due to the formation of gas bubbles during solidification. 22 . which suppresses the formation of oxygen bubbles in the mold. 5. the metal can be worked further towards the final shape. Heavily cold worked materials are recrystallized (process annealed). HCP is strong due to the ability to impede dislocation movement (lack of slip systems so slip does not occur easily) and more brittle than either FCC or BCC metals. 7. FCC is weak due to favourable conditions for slip (dislocations move easily within the lattice) and more ductile than BCC or HCP metals. Exercise Three Answers 1. Some grains will cannibalize their neighbours and become fat. BCC is strong due to the ability to impede dislocation movement (lack of close packed planes for easy slip to occur). to restore ductility and to refine the grain size. a. and less ductile when compared with FCC structures.) 4. Longer The longer the material is left at the elevated temperature. Killed steels are fully deoxidized. Pig iron has high amounts of carbon and impurities so it is very brittle. Aluminum and /or silicon added ties up the oxygen in the steel (This may result in higher inclusion (oxides) content in the solidified steel. Recrystallization Recrystallization restores ductility by allowing nucleation of new grains to occur. In this way ductility is increased and. The steel surface reacts with the surrounding air and forms scale (oxides) at the temperatures used for hot working. the more atoms will move into their ideal lattice locations. The strength and toughness is higher for small-grained metals compared to large grain size castings. 5. Semi-killed steels are only partially deoxidized and therefore the formation of some gas is allowed in the mold. 3. 6. 2. An electric arc furnace is used to melt scrap or used to make high quality steels. Ingot or continuous casting. b.

4. 2. Why do we not require a recrystallization anneal after the hot working process? 5. What forms on the steel surface during hot working? 6. Briefly explain the difference in behavior between a brittle material and a ductile material as each undergoes plastic deformation. State four advantages of cold-rolling compared to the cheaper hot-rolling. 3. 23 .Assignment 1. Ductility is measured by or . Sketch and label a typical Stress-Strain curve and indicate the work hardening region.