Diffusional growth limitation and hardenability

It has long been recognized that the effects of alloying elements on the hardenability of steels is related directly to their effects on the nucleation and growth kinetics of proeutectoid and eutectoid austenite decomposition products. In the present paper, theoretical and experimental studies of proeutectoid-ferrite and pearlite growth are reviewed for systems of the form Fe-C-X, where X is a substitutional alloying element such as Co, Cr, Mn, Mo, Ni, Si, and so forth. Of principle interest is the limitation which an alloying element X imposes on the corresponding diffusional growth kinetics. Although the agreement between theory and experiment is reasonable for most systems, there remain areas of considerable controversy (e.g., the role of interface diffusion, whether or not local equilibrium is maintained at interfaces and solute segregation to interfaces). This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee. Note added in proof: Boswellet al. have initiated a theoretical treatment of the “impurity drag” effect of carbide formers on proeutectoid ferrite growth.