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

The stability of retained austenite is the most important parameter control ling the transformation plasticity effects in multiphase low alloy TRIP ste els. 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 tensio n test technique (SS-TV-TT) has been employed, which allowed to link the ap pearance of yield points in the stress-strain curve with the mechanically-i nduced martensitic transformation of the retained austenite. The results in dicated that the M-s(sigma) temperature varies with BIT temperature and tim e. Higher austenite stability is associated with a BIT temperature of 400 d egrees C rather than 375 degrees C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growi ng bainite is lowered at short BIT times. This stability drop is due to car bide 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 agre ement with model predictions within the range of anticipated carbon enrichm ent of the retained austenite and measured austenite particle size.