Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M-s(sigma) temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M-s(sigma) temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400 degrees C rather than 375 degrees C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.