Why Does Lag Increase With The Temperature From Which Cooling Starts ?

(New York Meeting, February, 1913.) THE transformation which steel undergoes in glow cooling, from the condition of austenite when above the transformation range into that of pearlite plus either ferrite or cementite below that range, is subject to great lag. This transformation is essentially the pearlitizing of the austenite. Indeed the hardening of steel is due to this lag, which is made so great by extremely rapid cooling that a very large part of the transformation itself is restrained, so that the rapid cooling catches the metal in an only partly transformed state, that of martensite, and under favorable conditions even in an almost wholly untransformed state, as in the cases of Maurer's austenitic manganese steel. Hadfield's austenitic manganese steel, 25 per cent. nickel steel, and other austenitic alloy steels, are austenitic even after slow cooling. It has long been known that this- lag increases (1) with the rapidity of cooling, (2) with the presence of certain retarding elements, notably manganese and nickel, and (3) with the temperature from which cooling starts. The present inquiry touches the last. of these influences, but incidentally it throws light on the first also. This influence of higher heating in increasing lag manifests itself in different ways which, though they may at first seem unrelated, are in fact but different, aspects of one and the same thing. Thus, among heatings to different high temperatures, all of them above the transformation range, the higher the temperature to which the steel is heated (1) the lower is the temperature it which the transformation occurs in cooling slowly down, (2) the shorter is the time occupied in the actual cooling from above the transformation range to the room temperature under given external conditions, whether of slow or