On the Pearlite Transformation of Ni-Cr-Mo Steel

When cooled from austenite region, bainite-transformed Ni-Cr-Mo steel contains a small quantity of stable austenite containing small quantities of martensite and called “bainitic retained austenite”, at room temperature. Through slower cooling process, most of the existing austenite is transformed into pearlite, but in this case also, a very small part of austenite is retained as it is. This is named it “pearlitic retained austenite”, hereunder. Pearlitic retained austenite is not completely transformed in the course of simple slow cooling. In order to find some process of heat treatment by which pearlitic retained austenite will be completely changed, the effect of intermediate holding in the cooling process on pearlite transformation was studied by means of S-curve testing. An X-Y recorder which records the change of sample length by differential transformer was used to trace S-curves. The effect of time of holding intermediately at the temperature just below the A1 transformation point is not simple. The stability of austenite does not always decrease according to the length of the holding time just below A1 temperature. When the intermediate holding temperature is lower, austenite becomes unstable relatively. But even in this case, the stability increases locally, if the austenite transforms to bainite partially during intermediate holding time. In short, in the austenite to pearlite transformation, there is a phenomenon which is analogous to stabilization of the austenite-martensite transformation. And it was shown that the effect of incubation period at a temperature of pearlite transformation does not proportionaly contribute to pearlite transformation at other temperature, but in some cases the austenite is rather stabilized.