AUSTENITE STABILIZATION FROM DIRECT CEMENTITE CONVERSION IN LOW-ALLOYSTEELS

Transformation induced elasticity (TRIP) effects associated with auste nite dispersions in low alloy Fe-Mn-Si steels can be enhanced by auste nite stabilisation. Austenite which forms during conventional intercri tical annealing does not possess the required stability in order to ex hibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite con version which occurs under paraequilibrium conditions, i.e with partit ion of carbon but with no partition of substitutional alloying element s. In this way the austenite inherits the manganese content of cementi te and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn- enriched cementite dispersion in ferrite is formed. In the second step , intercritical annealing, austenite nucleates on the cementite partic les, which are consumed to form austenite. It was experimentally deter mined that this austenite has been enriched in manganese and carbon an d, therefore, is stabilised. The conversion reaction is followed by th e conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of th e two-step annealing treatment is a significant austenite stabilisatio n relative to simple intercritical annealing, indicating a potential f or enhanced TRIP effects in this class of steels.