Ferrite and Pearlite
Alpha iron, strictly speaking, refers only to the body-centered cubic (bcc) form of pure iron that is stable below 912 ºC (1674 ºF) while ferrite is a solid solution of one or more elements in bcc iron. Often these terms are used as synonyms, which is not correct. Ferrite may precipitate from austenite in acicular form (Widmanstätten ferrite) under certain cooling conditions. Sheet steels are fully ferritic and there are fully ferritic silicon electrical steels and stainless steels. Ferrite is a very soft, ductile phase, although it looses its toughness below some critical temperature.
If the isothermal transformation temperature is close to the lower critical temperature, and the steel is hypoeutectoid (< 0.8% C), ferrite will precipitate first before pearlite is formed. This ferrite is called proeutectoid ferrite. The amount of proeutectoid ferrite decreases as the carbon content increases and as the isothermal transformation temperature is decreased towards the “nose” of the TTT diagram. Figure 1 shows proeutectoid ferrite and martensite in 8620 alloy steel (0.2%C-0.8%Mn-0.25% Si-0.55%Ni-0.5%Cr-0.2%Mo) austenitized at 927 ºC, isothermally transformed at 677 ºC for 1 minute, and then water quenched. The specimens were etched with 2% nital, 4% picral and 10% SMB. The holding time was short enough so that only ferrite formed isothermally before the specimen was water quenched (which transformed the remaining austenite to martensite). Figure 1 shows that nital revealed the martensite well and left the ferrite white while revealing ferrite-ferrite grain boundaries. 10% SMB revealed the martensite (m) in similar manner, slightly colored some of the ferrite (f) grains, and also revealed the ferrite grain boundaries. However, 4% picral revealed nothing (the slight relief between the ferrite and martensite, due to hardness differences can be seen, as the aperture diaphragm was stopped down).