Interrupted quenching and bainitising below Ms temperature of EN X37CrMoV5-1 hot-work tool steel: Bainitic transformation kinetics, microstructure and mechanical properties

A series of dilatometric experiments were performed to investigate the influence of martensite content on the kinetics of bainitic transformation during interrupted quenching and bainitising (iQ&B) treatment in EN X37CrMoV5-1 hot-work tool steel. Three iQ&B treatments were designed and conducted based on experiments to evaluate the mechanical properties and microstructure. The proposed heat treatment leads to multiphase structure providing tensile strength up to 1966 MPa, uniform elongation up to 11.33% (3.31% for quenching and tempering) and impact toughness up to 10.3 J/cm2 (6.9 J/cm2 for quenching and tempering). Obtained microstructures were favourable due to nanobainitic regions and a high amount of retained austenite (above 30 vol%) – percolating and susceptible to the TRIP effect. Prior martensite was found not only to improve the strength of the material but also to accelerate bainitic transformation. When bainitising is conducted at temperatures not above 260 °C bainitic transformation is accelerated at the very beginning. The reason for this acceleration effect, called the “left arm uplift effect”, is seen in the occurrence of “fresh” martensite-austenite interfaces with a small level of carbon enrichment, which act as favourable nucleation sites for bainitic ferrite plates.