A quantitative assessment of the hardenability increase resulting from a superhardenability treatment
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INTRODUCTION
SINCE the time of the classic studies by Grossmann ~ (1942), Crafts and Lamont, 2 and Kramer, Siegel, and Brooks) it has been generally accepted that the hardenability of steel is uniquely related to its chemical composition and austenite grain size. Most of the subsequent research work in this field has therefore mainly concentrated on improving the accuracy of the existing hardenability prediction systems. 4'5'6 The rapid development of computer science led to multiple regression equations of hardenability test data 6'7'8 which accurately predicted the hardenability of certain steel grades. Observed deviations from the values predicted by such formulae led to a study 9 of the effect of steelmaking variables on the hardenability of steel. Following this study, Brown and James 9 reported that a dramatic increase in hardenability was observed after the superheating of B.S. 970 150M36 melts (similar to SAE 1340) to 1650 ~ This phenomenon, termed the superhardening effect, was observed only when the deoxidant level was high (AI ~ 0.05 pct or Ti -> 0.03 pct) and the base hardenability exceeded a certain threshold level. The hardenability increase observed was so large that the Jominy test, due to the relatively high cooling rates present even at the air-cooled end, was inadequate for assessing the hardenability. The conditions necessary for achieving superhardenability in the laboratory melts were confirmed by making twenty production casts. The initial results were verified by a group of research workers at Cambridge (Sachs, Ralph, and Slater), ~~ by conducting a detailed study of the transformation kinetics of such a production melt. This study indicated that a retardation of the ferrite growth rate is primarily responsible for the hardenability increase. It was proposed that the superheat causes a destruction of alloying element clustering in the melt, leading to a more homogeneous distribution of alloy atoms in the solidified steel. A random distribution of carbide forming elements would then more effectively retard carbon diffusion. Alternatively, it was proposed that a homogeneous distribution of austenite stabilizers could enhance the segregation of these elements to austenite grain boundaries, thus retarding austenite decomposition. The role of the aluminum was thought to be as a scavenger of nitrogen and R.J. MOSTERT is Principal Research Officer with lscor Research and Process Development, P.O. Box 450, Pretoria 0001, Republic of South Africa. G. T. VAN ROOYEN is Professor and Head, Department of Metallurgical Engineering, Pretoria University, Republic of South Africa. Manuscript submitted December 31, 1982. METALLURGICALTRANSACTIONSA
oxygen, eliminating the competitive interstitial diffusion of nitrogen and oxygen to austenite grain boundaries. A subtle modification of the austenite/ferrite interface was proposed as an alternative mechanism. Although the superhardenability treatment has an obviously large cost-saving potential, it has not yet been adopted commercially. In these times of al
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