Isothermal decomposition of austenite in presence of martensite in advanced high strength steels: A review

Abstract

The development of the quenching and partitioning (Q&P) process has prompted an interest in the process of isothermal transformation in presence of a pre-existing phase such as martensite. The presence of prior martensite is known to accelerate the overall kinetics of bainite formation, both in the 1-step and 2-step Q&P process. The underlying mechanisms behind this phenomenon are not fully understood. Also, the nature of the isothermal product (isothermal martensite and/or bainite) formed in presence of prior martensite seems to differ according to the thermodynamic and kinetic conditions. For certain thermodynamic conditions, depending on alloying, isothermal martensite may also form in the temperature range just above and below M s . In the event that both bainite and isothermal martensite formation are thermodynamically allowed, the competition in kinetics determines the observed transformation product. The effect of martensite on the subsequent isothermal transformation is reviewed with a focus on the nature of the transformation products and kinetics. In that context, the kinetics of the isothermal transformation in presence of other prior phases such as polygonal ferrite and bainite are also compared and discussed, together with the possible mechanisms behind the acceleration of the transformation kinetics. Guidelines for further investigation are also proposed. − Formation of both bainite and isothermal martensite is possible during isothermal heat treatment of austenite in presence of martensite. − Bainite formation kinetics is accelerated in presence of martensite, potentially due to an increase in the nucleation site density. − Opinions are differed whether bainite formed at or near the interface. Therefore the primary mechanism is not established yet. − Complexities in identifying the primary mechanism are discussed and possible research guidelines are proposed.