Effect of lamellarizing process on the microstructure and mechanical properties of marine 10Ni5CrMoV steel

In this work, multi-stage heat treatment involving quenching (Q), lamellarizing (L), and tempering (T) is applied in marine 10Ni5CrMoV steel to study the microstructure and mechanical properties by multi-scale characterizations, kinetics of reverse austenite transformation, strain hardening behavior, and toughening mechanism are further investigated. The specimens treated by lamellarizing process possess low yield strength but high toughness, especially cryogenic toughness. Introducing lamellarizing process leads to the film-like reversed austenite develops at martensite block and lath boundaries, which refines the martensite structure, and possess the lower equivalent grain size. Kinetic analysis of austenite reversion based on JMAK model shows that the isothermal transformation is dominated by the growth of reversed austenite, and there is a peak temperature (750°C) which makes the transformation of reversed austenite reach the maximum. The strain hardening behavior based on the modified Crussad-Jaoul analysis is indicated that reversed austenite obtained by lamellarizing process reduce the proportion of martensite, whereas produce a significant effect to hinder the propagation of cracks via martensitic transformation during the deformation, which are responsible for the QLT specimens exhibit high machinability and low yield strength. The ductile-brittle transition temperature of QLT specimens is decreased from -116°C to -130°C due to the low equivalent grain size and reversed austenite, which increase the cleavage force required for the propagation of cracks, and absorb the energy of external load, respectively. This work provides an idea to improve the cryogenic toughness of marine 10Ni5CrMoV steel, and lays a theoretical foundation for the industrial application and comprehensive performance improvement.