8.3·Heat Treatments 147 engineering applications.Thus,a subsequent heat treatment, called tempering,needs to be applied.This causes the precipita- tion of equilibrium ferrite in which very fine cementite particles are dispersed.The result is an increase in ductility at the expense of hardness.Tempering between 450 and 600C is typical.Con- siderable skill and experience are involved when performing quenching and tempering.Because of the importance of these heat treatments,many metal shops have wall charts that provide guidance for the proper procedures which allow one to obtain specific mechanical properties. It should be noted in passing that diffusionless phase transfor- mations (i.e.,martensitic transformations)are also observed in other alloys or substances.Among them are martensitic transfor- mations in certain copper-zinc alloys,in cobalt,or many poly- morphic ceramic materials.Some alloys (such as NiTi,Cu-Al-Ni, Au-Cd,Fe-Mn-Si,Mn-Cu,Ag-Cd,or Cu-Zn-Al)which have under- gone a thermo-mechanical treatment that yields a martensitic structure possess a shape memory effect.After deformation of these alloys,the original shape can be restored by a proper heat treat- ment which returns the stress-induced martensite into the origi- nal austenite.Some materials also change their shape upon re- cooling.They are called two-way shape memory alloys in contrast to one-way alloys which change only when heated.Only those ma- terials that exert a significant force upon shape change are of commercial interest,such as Ni-Ti and the copper-based alloys. (An Italian entrepreneur exploited this effect to create a smart shirt that automatically rolls up its sleeves at elevated temperatures and that can be smoothed out by activating a hair dryer. The TTT diagrams for noneutectoid steels need to be modi- fied somewhat to allow for the austenite-containing two-phase regions (i.e.,y+a or y+Fe3C);see Figure 8.1.Let us consider, for example,a hypoeutectoid steel.To accommodate for the transformation from y to (a+y)and from there to a pearlite, etc.,an additional line has to be inserted beginning at the nose of the TTT diagram and reaching to higher temperatures.It rep- resents the ferrite start temperature F;see Figure 8.5.Let us con- sider again a few specific cases. (a)Quenching a hypoeutectoid steel from above Af (i.e.,the highest temperature at which ferrite can form)to a temperature between Ar and the eutectoid temperature results in a mixture of y and primary a;see Figures 8.1 and 8.5.Once formed,the amount of ferrite does not change any further when extending the annealing time;see "a"in Figure 8.5. (b)Austenitizing and quenching a hypoeutectoid steel to a tem- perature slightly above the nose in a TTT diagram yields rela- tively quickly a mixture of y and primary a.The remaining8.3 • Heat Treatments 147 engineering applications. Thus, a subsequent heat treatment, called tempering, needs to be applied. This causes the precipita￾tion of equilibrium ferrite in which very fine cementite particles are dispersed. The result is an increase in ductility at the expense of hardness. Tempering between 450 and 600°C is typical. Con￾siderable skill and experience are involved when performing quenching and tempering. Because of the importance of these heat treatments, many metal shops have wall charts that provide guidance for the proper procedures which allow one to obtain specific mechanical properties. It should be noted in passing that diffusionless phase transfor￾mations (i.e., martensitic transformations) are also observed in other alloys or substances. Among them are martensitic transfor￾mations in certain copper–zinc alloys, in cobalt, or many poly￾morphic ceramic materials. Some alloys (such as NiTi, Cu-Al-Ni, Au-Cd, Fe-Mn-Si, Mn-Cu, Ag-Cd, or Cu-Zn-Al) which have under￾gone a thermo-mechanical treatment that yields a martensitic structure possess a shape memory effect. After deformation of these alloys, the original shape can be restored by a proper heat treat￾ment which returns the stress-induced martensite into the origi￾nal austenite. Some materials also change their shape upon re￾cooling. They are called two-way shape memory alloys in contrast to one-way alloys which change only when heated. Only those ma￾terials that exert a significant force upon shape change are of commercial interest, such as Ni-Ti and the copper-based alloys. (An Italian entrepreneur exploited this effect to create a smart shirt that automatically rolls up its sleeves at elevated temperatures and that can be smoothed out by activating a hair dryer.) The TTT diagrams for noneutectoid steels need to be modi￾fied somewhat to allow for the austenite-containing two-phase regions (i.e.,   or  Fe3C); see Figure 8.1. Let us consider, for example, a hypoeutectoid steel. To accommodate for the transformation from  to ( ) and from there to  pearlite, etc., an additional line has to be inserted beginning at the nose of the TTT diagram and reaching to higher temperatures. It rep￾resents the ferrite start temperature Fs; see Figure 8.5. Let us con￾sider again a few specific cases. (a) Quenching a hypoeutectoid steel from above Af (i.e., the highest temperature at which ferrite can form) to a temperature between Af and the eutectoid temperature results in a mixture of  and primary ; see Figures 8.1 and 8.5. Once formed, the amount of ferrite does not change any further when extending the annealing time; see “a” in Figure 8.5. (b) Austenitizing and quenching a hypoeutectoid steel to a tem￾perature slightly above the nose in a TTT diagram yields rela￾tively quickly a mixture of  and primary . The remaining