142 8·Iron and Steel 1538 1500 1495° 8 L+Fe C T y+L (c) (Austenite) 2.11 4.3 1148° 1000 Ar Y+FegC (Ferrite) 0.77 727° 6.67 0.0218 500 FIGURE 8.1.Portion of the iron-car- a+FeC bon phase diagram.(Actually,this (Pearlite) section is known by the name Fe- Fe3C phase diagram.)Af is the highest temperature at which fer- rite can form.As before,the mass Fe 234 FeC percent of solute addition is used (Cementite) (formerly called weight percent). Composition (mass C) 8.2.Hardening Mechanisms Eutectoid Several hardening mechanisms take place.First,the a-,y-,and Steel 8-phases are solid-solution strengthened as discussed in Section 5.1.Second,pearlite involves dispersion strengthening caused by the interaction of hard and brittle cementite with the relatively soft and ductile ferrite(Section 5.4).More specifically,the a-and Fe3C phases grow in the form of thin plates or lamellae,simi- larly as in eutectic reactions and as schematically depicted in Fig- ure 8.2.However,the plates are much thinner for pearlite than in a eutectic structure,which is necessitated by the shorter dif- fusion lengths encountered at lower temperatures.In short,the primary reason why eutectoid steel (iron with 0.77 mass C) is harder than pure iron or ferrite is because of the dispersion of hard cementite in soft ferrite in the form of plate-shaped pearlite, as shown in Figure 8.2. Hypoeutectoid The above statements need some fine tuning.For hypoeutectoid Steel compositions(below 0.77%C;see Section 5.2.2)the ferrite is the primary and continuous phase which,upon cooling from the y field,nucleates and grows at the grain boundaries of austenite. In other words,the a-phase quasi-coats the grain boundaries of austenite.Below 727C,the pearlite finally precipitates in the re- maining y-phase by a eutectoid reaction.It is thus surrounded142 8 • Iron and Steel Several hardening mechanisms take place. First, the -, -, and -phases are solid-solution strengthened as discussed in Section 5.1. Second, pearlite involves dispersion strengthening caused by the interaction of hard and brittle cementite with the relatively soft and ductile ferrite (Section 5.4). More specifically, the - and Fe3C phases grow in the form of thin plates or lamellae, simi￾larly as in eutectic reactions and as schematically depicted in Fig￾ure 8.2. However, the plates are much thinner for pearlite than in a eutectic structure, which is necessitated by the shorter dif￾fusion lengths encountered at lower temperatures. In short, the primary reason why eutectoid steel (iron with 0.77 mass % C) is harder than pure iron or ferrite is because of the dispersion of hard cementite in soft ferrite in the form of plate-shaped pearlite, as shown in Figure 8.2. The above statements need some fine tuning. For hypoeutectoid compositions (below 0.77% C; see Section 5.2.2) the ferrite is the primary and continuous phase which, upon cooling from the - field, nucleates and grows at the grain boundaries of austenite. In other words, the -phase quasi-coats the grain boundaries of austenite. Below 727°C, the pearlite finally precipitates in the re￾maining -phase by a eutectoid reaction. It is thus surrounded Eutectoid Steel Hypoeutectoid Steel 8.2 • Hardening Mechanisms 1495 727 1148 1538  + L  + Fe3C L + Fe3C  + Fe3C (Pearlite)  1500 1000 500  (Ferrite)  (Austenite) 0.77 Fe 1 2 3 4 5 Fe3C (Cementite) 0.0218 6.67 4.3 L 2.11 Af Composition (mass % C) T (C) FIGURE 8.1. Portion of the iron–car￾bon phase diagram. (Actually, this section is known by the name Fe￾Fe3C phase diagram.) Af is the highest temperature at which fer￾rite can form. As before, the mass percent of solute addition is used (formerly called weight percent)