Superhardening in medium-carbon steel

Abstract

The ‘superhardening’ effect gives rise to the enhancement of steel hardenability at relatively low cost; when applied to certain medium-carbon steels it is possible to produce the performance obtained only from steels of much higher alloy content. To obtain superhardening, three essential conditions must be satisfied simultaneously; (a) the base composition must be such that a threshold level of hardenability is exceeded; (b) the melt must be superheated, preferably into the range 1680°–1700°C; and (c) the deoxidant addition (Al or Ti) must be higher than that needed purely for deoxidation alone. The necessary conditions identified in the laboratory were tested by making steels in afull-scale steelplant. The same factors were found to be of importance. In the steelworks a number of full-sized ingots were produced according to the superhardening procedure; these were rolled into bar and the bar tested. The variation in performance throughout the ingot was found to be no wider than would be expected with most medium-alloy steels. The heat-treatment properties were such that superhardening steels can be used either to produce high strength in relatively small sections or medium strengths in very thick sections.