CHAPTER I MA TERIALS AND THEIR PROPERTIES1. 1 Metals and Non-metalsAmong numerous properties possessed by materials, their mechanical properties, in the majority of cases, are the most essential and therefore, they will be given much consideration in the book. All critical parts and elements, of which a high reliability is required, are made of metals, rather than of glass, plastics or stone. As has been given in Sec 1-l, metals are characterized by the metallic bond; where positive ions occupy the sites of the crystal lattice and are surrounded by electron gas .All non-metals have an ionic or a covalent bond. These types of bond are rigid and are due to electrostatic attraction of two ions of unlike charges. Because of the metallic bond, metals are capable of plastic deformation and self-strengthening upon plastic deformation. Therefore, if there is a defect in a material or if the shape of an element is such that there are stress concentrators, the stresses in these points may attain a great value and even cause cracking. But since the plasticity of the material is high, the metal is deformed plastically in that point, say, at the tip of a crack, undergoes strengthening, and the process of fracture comes to an arrest. This does not occur in non-metals. They are uncapable of plastic deformation and self-strengthening; therefore, fracture will occur as soon as the stresses at the tip of a defect exceed a definite value. These facts explain why metals are reliable structural materials and can not be excelled by non-metallic materials.Words and Terms:mechanical property 机械（力学）性能critical part and element 关键零部件covalent bond 共价键metalic bond crystal lattice 金属键晶格electrostatic attraction 静电吸引plastic deformation 塑性变形self-strengthening 自强化stress oncentrator 应力集中点the tip of a crack 裂纹尖端Questions: 1) What are the differences in properties between metals and non-metals?2) Why are metals capable of plastic deformation and self-strengthening?1. 2 Ferrous AlloysMore than 90 % by weight of the metallic materials used by human beings are ferrous alloys. This represents an immense family of engineering materials with a wide range of microstructures and related properties. The majority of engineering designs that require structural load support or power transmission involve ferrous alloys. As a practical matter, these alloys fall into two broad categories based on the carbon in the alloy composition. Steel generally contains between 0. 05 and 2.0 wt % carbon. The cast irons generally contain between 2.0 and 4.5 wt % carbon. Within the steel category，we shall distinguish whether or not a significant amount of alloying elements other than carbon is used . A composition of 5 wt % total non-carbon additions will serve as an arbitrary boundary between low alloy and high alloy steels. These alloy additions are chosen carefully because they invariably bring with them sharply increased materials costs. They are justified only by essential improvements in properties such as higher strength or improved corrosion resistance.Words and Terms:ferrous 铁的；含铁的corrosion resistance 耐腐蚀；抗蚀力arbitrary 特定的；武断的Questions:l) What is the difference in composition between steel and cast iron?2) How can you distinguish low alloy steels from high alloy steels?CHAPTER 2 HEA T TREA TMENT OF STEEL2. 1 Principle of Heat Treatment of SteelThe role of heat treatment in modern mechanical engineering cannot be overestimated. The changes in the properties of metals due to heat treatment are of extremely great significance.2. 1. 1 Temperature and TimeThe purpose of any heat treating process is to produce the desired changes in the structure of metal by heating to a specified temperature and by subsequent cooling.Therefore , the main factors acting in heat treatment are temperature and time , so that any processof heat treatment can be represented in temperature-time ( t-τ) coordinates .Heat treatment conditions are characterized by the following parameters: heating temperature t , i.e. the maximum temperature to which an alloy metal is heated; time of holding at the maxheating temperatureτh; heating rate νh and cooling rateνc．If heating (or cooling) is made at a constant rate, the temperature-time relationship will be described by a straight line with a respective angle of incline.With a varing heating (or cooling) rate , the actual rate should be attributed to the given temperature , more strictly , to an infinite change of temperature and time : that is the first derivative of temperature in time : νact = dt/dτ.Heat treatment may be a complex process , including multiple heating stages , interrupted or stepwise heating (cooling) , cooling to subzero temperatures , etc . Any process of heat treatment can be described by a diagram in temperature-time coordinates.Words and Terms:coordinates 坐标系heating rate 加热速度straight line 直线heating temperature 加热温度cooling rate 冷却速度first derivative 一阶导数Questions:1) What are the two main factors acting in heat treatment?2) How many stages may usually be inc luded in the heat treatment of steel?2. 1. 2 Formation of AusteniteThe transformation of pearlite into austenite can only take place at the equilibrium critical point on a very slow heating as follows from the Fe-C constitutional diagram. Under common conditions, the transformation is retarded and results in overheating, i.e. occurs at temperatures slightly higher than those indicated in the Fe-C diagram.When overheated above the critical point, pearlite transforms into austenite, the rate of transformation being dependent on the degree of overheating.The time of transformation at various temperatures (depending on the degree of overheating) shows that the transformation takes place faster (in a shorter time) at a higher temperature and occurs at a higher temperature on a quicker heating. For instance , on quick heating and holding at 780 ℃，the pearlite to austenite transformation is completed in 2 minutes and on holding at 740 ℃，in 8 minutes .The end of the transformation is characterized by the formation of austenite and the disappearance of pearlite (ferrite + cementite). This austenite is however inhomogeneous even in the volume of a single grain. In places earlier occupied by lamellae (or grains) of a pearlitic cementite , the content of carbon is greater than in places of ferritic lamellae . This is why the austenite just formed is inhomogeneous .In order to obtain homogeneous austenite , it is essential on heating not only to pass through the point of the end of pearlite to austenite transformation , but also to overheat the steel above that point and to allow a holding time to complete the diffusion processes in austenitc grains.The rate of homogenization of austenite appreciably depends on the original structure of the steel, in particular on the dispersion and particle shape of cementite. The transformations described occur more quickly when cementite particles are fine and, c therefore, have a large total surface area.Words and Terms : pearlite 珠光体constitutional diagrm 状态图inhomogeneous 不均匀的lamellae 层片critical point 临界温度overheat 过热grain 晶粒diffuse扩散Questions:1) Is there no diffusion process in the transformation from pearlite to austenite?2) Is it true that the higher the temperature, the faster the transformation from pearlite into austenite?3) How to obtain homogeneous austenite?CHAPTER 3 PRINCIPLES OF PLASTIC FORMING3. 1 Physical Metallurgy of Hot WorkingThe principles of the physical metallurgy of hot working are now well recognized. During the deformation process itself, e.g. a rolling pass, work hardening takes place but is balanced by the dynamic softening processes of recovery and recrystallization. These processes, which are thermally activated, lead to a flow stress that depends on strain rate and temperature as well as on strain. The structural changes taking place within the material result in an increase in dislocation density with strain until in austenitic steels and nickel- and copper-base alloys a critical strain （εc）is reached when the stored energy is sufficiently high to cause dynamic recrystallization . With further strain, dynamic recrystallization takes place repeatedly as the new recrystallized grains are themselves work-hardened to the critical level of stored energy. These dynamic structural changes leave the metal in an unstable state and provide the driving force for static recovery and static recrystallization to take place after the deformation pass. Static recrystallization may be followed by grain growth if the temperature is sufficiently high. In order to be able to apply these principles to commercial working processes, we require answers to two main questions: (a) how long does recrystallization take place after a deformation pass; and (b) what grain size is produced by recrystallization and grain growth? The answers determine the structure of the material entering the next and subsequent passes and hence influence the flow stress of the material and the working forces required. Eventually they determine the structure and properties of the hot worked products.Words and Terms : physical metallurgy 物理冶金work hardening 加工硬化static recovery静态回复thermally activated 热激活的hot working 热加工dynamic softening 动态软化recrystallization 再结晶dislocation density 位错密度critical strain 临界应变Questions:l) When does dynamic recrystallization take place within the material work hardened?2) What do the answers to the two questions determine?3. 1. 1 Dynamic Structural ChangeDuring the deformation of austenite at hot-working temperatures and constant strain rate, the characteristic form of stress-strain curve observed is illustrated in Fig. 3. 1. These curves are for low-alloy steels, tested in torsion, but are similar to those obtained for other steels in the austenitic condition tested in torsion, tension, or compression. After initial rapid work- hardening the curves go through a maximum associated with the occurrence of dynamic recrystallization. The peak in flow stress occurs after some low fraction of recrystallization has taken place so the strain to the peak(εp) is always greater than the critical strain for dynamic recystallization (εc ) . The relationship between the two strains is complex , but it has been suggested thatεc＝αεp( where αis a constant ) is a reasonable approximation for conditions of deformation of interest in hot working. however , the proposed values of αdiffer , being 0.83 , 0.86 , and 0.67 . It can be seen from Fig.3.1 that εp increases systematically with Zener-Hollomon parameter ( Z ) , independent of the particular combination of stain rate （ε）and temperature ( T / K ) in the relationship : Z=εexp Q def/RTWhere the activation energy Q def for this alloy steel is 314 kJ/mol. A similar value of 312kJ/mol is appropriate for a range of C-Mn steels but lower values of 270 and 286 kJ/mol have also been observed.Asεc marks a change in microstructure from one of somewhat poorly developed subgrains , produced by the action of work hardening and dynamic recovery，to one which also contains recrystallization nuclei , it is also a critical strain in terms of the static structural changes that take place after deformation . The dependence of εp，and hence of εc，on Z is shown for the low-alloy steel and a number of C-Mn steels in Fig. 3.2. It can be seen that, indicated by the Fig.3.2 ，εp generally increases with increasing Z although the curve for the data of Sakui et al. passes through a minimum at Z = 3 x 10s-1，( corrected to Q def = 312 kcal / mol ) . The curves for the data of Nakamura and Ueki, Cook, Rossard and Blain, and Hughes, and also the data of Suzuki et al. for a number of C-Mn steels were obtained from tests on material reheated to the same temperature as the testing temperature.These all show a trend for higher values of εp at higher testing temperatures.In contrast, the curves for the data of Le Bon et al. , Barraclough , and Morrision refer to tests carried out at lower temperature than the reheating temperature and these show no effect of test temperature 0n εp.In the former group of results, higher reheating/test temperatures will give larger initial grain sizes. As shown by Sah et al., Sakui et al., and Roberts et al. , increase in grain size ( d0）leads to an increase inεp and their data indicate a relationship of the form εp∝d0^ 1/2 Words and TermsStress-strain curve 应力应变曲线torsion 扭转;转矩activation energy 激活能initial grain size 原始晶粒尺寸Questions:l ) What doεc andεp mark ?2 ) What is the relationship between εc andεp ?3. 1. 2 Static Recrystallization RateAfter deformation, softening by static recovery and recrystallization take place with time at rates which depend on the prior deformation conditions and the holding temperature. These processes may be followed by studying the changes in yield or flow stress during a second deformation given after different holding times to obtain a restoration index, or recrystallization may be measured directly by metallographic examination of quenched specimens. An example of the form of recrystallization curves obtained by the latter method for low-alloy steel is shown inFig 3.3. The curves generally follow an A vrami equation of the formwhere X v is the fraction recrystallized in time t ; t F is the time for some specified fraction of recrystallization ( say 0.5 ) ; k is a constant ; and C＝-In ( 1－F ) . For the Curves shown k = 2 , which is consistent with the value observed for other steels deformed to strains <εc．With this relationship t0.05＝0 . 27t0.5 and t0.95 = 2.08 t0.5 , i.e. recrystallization proceeds over about one order of magnitude in time.The dependence on strain of the characteristic time t0.5, measured by either metallographic or restoration method, is shown for several steels in Fig. 3.4. All the curves show a steep dependence on strain for strains up to ~0. 8εp，which fits a relationship t0.5∝ε-m , where the mean value of m = 4 . This value is also given by observations on ferritic metals. The lower limit of strain to which this relationship is applicable is uncertain as the critical strain for static recrystallization has not received systematic study. The data of Norrison indicate that it is < 0.05 for low-carbon steel at 950℃whereas the observations of Djaic and Jonas indicate a value of > 0.055 for high-carbon steel at 780 ℃．It is clear whether this difference arises from thedifference in temperature or composition as the simple dependence on Z suggested by the broken line in Fig. 3.2 may be unrealistic. This deserves further study as low strains my be applied in the final passes of plate rolling and , as shown previously , these could have significant effects on the final grain size if they exceed the critical strain for static recrystallization.In the strain range of steep dependence of t0.5 on ε，Morrison observed that there was no effect of strain rate over the two orders of magnitude studied . This is somewhat surprising as interesting strain rate (or Z) increases the flow stress at any particular strain. Increasing flow stress would be expected to increase the random dislocation density and decrease the subgrain size and hence increase the stored energy．The subgrain boundaries provide the largest contribution to the stored energy and as their misorientation increases with strain, the driving force for recrystallization will increase. However, this increase would be expected to be about linear with strain so the much greater dependence of t0.5on strain must also arise from an increase in density of nucleation sites and in nucleation rate. The lack of influence of strain rate may thus reflect compensating effects on stored energy and substructure development at any strain. This contrasts with the strain rate effect observed for stainless steel.The observations of Djaic and Jonas indicate that an abrupt change takes place from strain dependence to independence at a strain ~0.8εp，as illustrated in Fig . 3. 4. This corresponds reasonably with the strain expected forεc and arises because preexisting recrystallization nuclei are always present in the deformed structure at strains greater thanεc．Static recrystallization under these conditions has been referred to as ‟metadynamic‟ to distinguish it from the 'classical ' recrystallization after lower strains when the nuclei must be formed after deformation . The restoration measurements indicate that the recrystallization kinetics may have a complex form after strains betweenεc and the onset of steady state , and direct metallographic observations of static recrystallization after stains well into steady state show that the exponent k in the A vrami equation drops to a value of ~1 . This means that t0.05 = 0.074 t0.5 and t0.95 = 4.33 t0..5, i. e. static recrystallization proceeds over about two orders of magnitude in time after strains which give dynamically recrystallized structures during deformation 。
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