HETEROGENEOUS AUSTENITE GRAIN GROWTH IN AN ASTM A213 T91 STEEL: COUPLED INFLUENCE OF THE INITIAL METALLURGICAL STATE AND THE HEATING RATE TO AUSTENITE

摘要

The coupled influence of the initial metallurgical state and the heating rate to austenite on the occurrence of heterogeneous grain growth during austenitization of an ASTM A213 – T91 steel has been studied using a high speed, high resolution dilatometer. To that aim, two-step thermal cycles were designed, with a first step of heat treatment at 780 ºC -the usual tempering temperature- and a second step of austenitization “in situ” at different heating rates. In the first step, different starting metallurgical conditions were obtained by “tempering” for increasing times up to 6 hours. In the second step, austenitization was performed at 1, 30 and 50 ºC/s. A heterogeneous austenite grain size distribution was observed after 30 min. austenite holding time at 1050 ºC when austenitization was performed at 30 and 50 ºC/s heating rate for the as-received material. For the “tempered” materials, heterogeneous grain growth at 1050 ºC was observed up to 3 hours of tempering time when the heating rate to austenite was 30 ºC/s; for times longer than the mentioned value, a homogeneous austenite grain size distribution was obtained. When the heating rate to austenite was 50 ºC/s only heterogeneous grain growth was observed up to 6 hours of tempering time.