Physical Metallurgy of Steel - Basic Principles

Abstract

Steel is primarily an alloy of carbon in iron although most commercial grades contain other alloying elements as well. It is well known that if pure iron is slowly cooled from its liquid state to room temperature it undergoes isothermal transformations at 15340C from liquid to 8 phase, and, at 13900C from 8 to y phase, and at 9100C from y to a phase (Fig.1). These phases have different crystal structures; 8 and a phases are BCC whereas y is FCC. Addition of carbon to iron signi-ficantly alters the above transformation characteristics. While in liquid state iron can dissolve considerable amount of carbon, its solubility in solid state is significantly less. This is determined by the spacing of iron atoms in the crystal lattice. FCC structure although more closely packed has larger inters-titial spacing than BCC lattice and therefore can accomm-odate relatively larger amount of carbon. For example maxi-mum solubility of carbon in a or 8 (also called ferrite) is 0.08 whereas that in y (also called austenite) it is 2.06. Carbon in excess of this limit is usually present in steel as a carbide called cementite which is a stable non equilibrium compound represented as Fe3C. Thus steel at a given temperature and pressure may therefore contain more than one phases. Equilibrium diagram provides a graphic representation of the distribution of various phases as function temperature and overall composition. If properly interpreted this also provides compositions of respective phases and their relative amount.Fig. 2 represents such a diagram for Fe-Fe3C system. This contains three horizontal lines representing three invariant transformations viz. peritectic, eutectic and cutectoid; signifying coexistence of three phases of specific composition in equilibrium at a fixed temperature. Peritectic: L (0.55) + 8 (0.08) = 7 (0.18) 14930C Eutectic: L (4.30) = y (2.06) + Fe3C(6.67) 11470C Eutectoid: y (0.80) = a (0.02) + Fe3C(6.67) 7230C Amongst these the one which takes place completely in the solid state viz. eutectoid transformation is of consi-derable importance to the heat treaters. This is because solid state diffusion is relatively slow, and hence it can be completely inhibited by quenching the steel rapidly from a temperature above 7230C giving an entirely differ-ent transformation product not indicated in the phase diagram. Development of structures ranging from equilib-rium- non equilibrium constituents (or phase) in steel products forms the very basis of heat treatment techn-ology. This lecture presents an over view of the basic principles of the evolution of various microstructures in steel and describes how it could be controlled to achieve a wide range of mechanical/physical properties the steel is known for.