Évolutions structurales d'aciers faiblement alliés au Ni-Cr-Mo au cours de chauffages et de refroidissements effectués à différentes vitesses

Mechanisms controlling the structural evolutions of Ni-Cri-Mo low alloy steels during thermal cycles performed with different rates : Part I

¹ Laboratoire de Métallurgie Structurale, UA CNRS 04 1107, Université Paris-Sud, Centre d’Orsay, Orsay, France
² Laboratoire de Structure des Matériaux Métalliques, Université Paris-Sud, Centre d’Orsay, Orsay, France

Abstract

The behaviour of the 40 NSCD 7-7-3 (AISI 300 M) low alloy steel was studied from the as - received state, which mainly consists of lower bainite and 3 % retained austenite. The different phenomena which develop during the austenite transformation were followed during the course of heatings carried out with various rates (Rl) in order to determine the operating mechanisms. According to the value of Rh, the austenite transformation takes place in one or two steps which enables to determine a critical rate separating the preponderant occurrence of diffusion or shear phenomena. Complementary dilatometric, thermomagnelic experiments and in situ or at room temperature transmission electron microscopy observations were carried out.