KINETICS OF AUSTENITE TO MARTENSITE TRANSFORMATIONS IN IRON-NICKEL-CARBONE ALLOYS DURING THEIR COOLING OR TENSILE TESTING

Two austenitic Fe-Ni-C alloys, one having 23.9 wt.% Ni, 0.39 wt.% C, a nd a martensit start point of 228 K, the other with 21.8 wt.% Ni, 0.48 wt.% C, and an M(S) of 233 K, were tested to determine the kinetics o f their austenite to martensite transformation both during cooling, at a rate of 5 K.min(-1), and during tensile testing at strain rates fro m 8.3x10(-5) to 8.3x10(-3) s(-1) and at temperatures from 233 to 323 K . Both alloys displayed variations in the critical plastic strain epsi lon(c) for the onset of serrated yielding. Metallography confirmed tha t serrated yielding can occur in austenite before any strain-induced m artensite is formed. Although strain-induced martensite formed at all of the examined strain rates and temperatures, inflections on the true stress versus true strain, or sigma-epsilon diagrams, were observed o nly at low test temperatures. The effect of transformation-induced pla sticity, or TRIP, assessed by the maxima of uniform plastic strain eps ilon(u), (epsilon(u) = 0.8), was detected only in tests run at 253, 27 3 anti 293 K. Kinetic diagrams for the formation of strain-induced mar tensite, in dependence on epsilon, were compiled for all the test temp eratures and strain rates, and were compared with kinetic diagrams of heat release gained by differential scanning calorimetry for the auste nite to martensite transformation. Diagrams of the amounts of heat rel eased, Delta Q(A-->M), in the martensite formation process, against th e plastic strain magnitude in the presence of similar proportions of m artensite, were found to yield linear relationship.