Decomposition Process of Martensite-Austenite Phase in a Heat-Affected Zone of a High Strength Steel

Abstract

Abstract A systematic study has been made of the decomposition of the martensite-austenite phase (M-A phase) formed in the heat-affected zone (HAZ) of a 800 MPa type of high strength steel. A heat-treatment to improve the toughness of HAZ was also proposed as a result of this study.Electron metallography indicated that the M-A phase decomposed by the precipitation of carbides even at temperatures of the order of 300°C. The activation energies for the decompositions of martensite and austenite have been determined to be 71.6 and 13.8 kJ/mol, respectively. The decomposition products of the M-A phase appear to be tempered martensite and hypereutectoid upper bainite.A drastic improvement in the ductility and toughness of the HAZ was obtained by annealing at temperatures ranging from 175 to 300°C, which can be attributed to the decomposition of martensite. A lower annealing temperature is preferable in order to retain the presence of the austenite constituent which improves the toughness.