148 8·Iron and Steel Hardness a+y -P---Eut.Temp. b a+y+Pearlite 一P o+Pearlite FIGURE 8.5.Schematic repre- y+Bainite B sentation of a TTT diagram C- 令* Bainite for a hypoeutectoid plain M carbon steel.Ar is the highest y+Martensite temperature at which ferrite can form;see Figure 8.1.Fs M Martensite 1 is the ferrite start tempera- 1sec 1 hr 1 day ture. log time austenite eventually transforms into pearlite upon some further isothermal annealing.The transformation into pearlite is com- pleted at the pearlite finish temperature,Pr;see "b"in Figure 8.5. (c)Finally,quenching and holding the same steel to a tem- perature just below the nose yields,after crossing the Brline,only bainite,which has,in contrast to pearlite,no fixed composition. Similar TTT diagrams as in Figure 8.5 are found for hyper- eutectoid plain carbon steels.The differences are an Fe3C +y field (instead of the a +y field)and a cementite start curve,Cs (instead of the ferrite start line,Fs). The martensitic transformations for hypo-and hypereutectoid steels behave quite similar as outlined above.However,the M and Mr temperatures depend on the carbon content,as shown in Figure 8.6.Unfortunately,the M temperature cannot be clearly determined by visual inspection only.Other techniques,such as resistivity or X-ray diffraction measurements,need to be applied to obtain a reliable value.Further,the martensitic transforma- 100 T 500 (C) 400 Retained M 300 Retainedy 50 FIGURE 8.6.Schematic representa- 200 tion of the influence of carbon con- M austenite(Vol. 100 centration on the Ms and Mf tem- 8 peratures in steel and on the --一了1 amount of retained austenite(given 0.20.40.6 0.81.01.2 1.4 in volume percent). Mass carbon148 8 • Iron and Steel austenite eventually transforms into pearlite upon some further isothermal annealing. The transformation into pearlite is com￾pleted at the pearlite finish temperature, Pf; see “b” in Figure 8.5. (c) Finally, quenching and holding the same steel to a tem￾perature just below the nose yields, after crossing the Bf line, only bainite, which has, in contrast to pearlite, no fixed composition. Similar TTT diagrams as in Figure 8.5 are found for hyper￾eutectoid plain carbon steels. The differences are an Fe3C  field (instead of the   field) and a cementite start curve, Cs (instead of the ferrite start line, Fs). The martensitic transformations for hypo- and hypereutectoid steels behave quite similar as outlined above. However, the Ms and Mf temperatures depend on the carbon content, as shown in Figure 8.6. Unfortunately, the Mf temperature cannot be clearly determined by visual inspection only. Other techniques, such as resistivity or X-ray diffraction measurements, need to be applied to obtain a reliable value. Further, the martensitic transforma￾T a b c Fs Af Ps Pf Bf Bs Ms Mf  + Pearlite  + Martensite  + Bainite Bainite Martensite 1 sec 1 hr 1 day log time Hardness Eut. Temp.  +    +  + Pearlite FIGURE 8.5. Schematic repre￾sentation of a TTT diagram for a hypoeutectoid plain carbon steel. Af is the highest temperature at which ferrite can form; see Figure 8.1. Fs is the ferrite start tempera￾ture. Ms Mf 500 400 300 200 100 0.2 0.4 0.6 0.8 1.0 1.2 1.4 100 50 Retained  T (C) Mass % carbon Retained austenite (Vol. %) FIGURE 8.6. Schematic representa￾tion of the influence of carbon con￾centration on the Ms and Mf tem￾peratures in steel and on the amount of retained austenite (given in volume percent)