8 lron and Steel 8.1.Phases and Microconstituents A deeper understanding of the diverse properties of iron and var- ious steels is gained by inspecting the iron-carbon phase dia- gram.Actually,for the present purposes only the portion up to 6.67%C is of interest;see Figure 8.1. The various phases are known by specific names,such as the hard and brittle intermetallic phase Fe3C (6.67%C),which is called iron carbide or cementite;the FCC,non(ferro)magnetic,y- phase named austenite,and the BCC a-phase known as ferrite. Further,a high-temperature BCC phase called 8ferrite and the eutectoid phase mixture (a+Fe3C)named pearlite.Not enough. Two more microconstituents known as bainite and martensite, respectively,exist which are formed by specific heat treatments. The latter will be discussed in Section 8.3.These names came into existence either because of their properties or appearance under the microscope (such as cementite and pearlite)or to com- memorate certain scientists who devoted their lives to the study of these microconstituents (such as Sir W.C.Roberts-Austen, English Metallurgist,1843-1902;A.Martens,German Engineer, 1850-1914;and E.C.Bain,American Metallurgist). Several three-phase reactions are evident from Figure 8.1.The eutectic reaction at Fe-4.3%C lowers the melting temperature of iron to 1148C,as mentioned in Chapter 7.Further,a eutec- toid reaction(y-a+Fe3C)at 727C and a peritectic reaction at 1495C need to be emphasized.Finally,two allotropic transfor- mations during cooling from 8-ferrite to austenite and from there to ferrite take place.The a-,y-,and 6-phases consist of solid so- lutions in which the carbon is interstitially dissolved in iron.8 A deeper understanding of the diverse properties of iron and var￾ious steels is gained by inspecting the iron–carbon phase dia￾gram. Actually, for the present purposes only the portion up to 6.67% C is of interest; see Figure 8.1. The various phases are known by specific names, such as the hard and brittle intermetallic phase Fe3C (6.67% C), which is called iron carbide or cementite; the FCC, non(ferro)magnetic, - phase named austenite, and the BCC -phase known as ferrite. Further, a high-temperature BCC phase called -ferrite and the eutectoid phase mixture ( Fe3C) named pearlite. Not enough. Two more microconstituents known as bainite and martensite, respectively, exist which are formed by specific heat treatments. The latter will be discussed in Section 8.3. These names came into existence either because of their properties or appearance under the microscope (such as cementite and pearlite) or to com￾memorate certain scientists who devoted their lives to the study of these microconstituents (such as Sir W.C. Roberts–Austen, English Metallurgist, 1843–1902; A. Martens, German Engineer, 1850–1914; and E.C. Bain, American Metallurgist). Several three-phase reactions are evident from Figure 8.1. The eutectic reaction at Fe–4.3% C lowers the melting temperature of iron to 1148°C, as mentioned in Chapter 7. Further, a eutec￾toid reaction (
 Fe3C) at 727°C and a peritectic reaction at 1495°C need to be emphasized. Finally, two allotropic transfor￾mations during cooling from -ferrite to austenite and from there to ferrite take place. The -, -, and -phases consist of solid so￾lutions in which the carbon is interstitially dissolved in iron. Iron and Steel 8.1 • Phases and Microconstituents