Microstructural features affecting tempering behaviour of 16Cr-5Ni supermartensitic steel

In industrial production processes the respect of hardness and UTS maximum values of 16Cr5Ni steel is of utmost importance and a careful control of chemical composition and thermo-mechanical treatments is a common practice. Nevertheless some scatter of properties is often observed with consequent rejection of final components. To better understand the role played by different factors, two heats of 16Cr-5Ni supermartensitic stainless steels with very close chemical compositions but different thermal behavior during tempering have been studied by means of TEM observations, X-ray diffraction measurements, dilatometry and thermo-mechanical simulations. It has been found that Ms-Mf temperature range can extend below the room temperature and the relative amount of retained austenite in as-quenched conditions plays a significant role in determining the thermal behavior. When present, the γ-phase increases the amount of reversed austenite formed in temperature and accelerates the process kinetics of martensite recovery. Moreover, increasing amounts of retained austenite after quenching lower the critical temperature for austenite destabilization and influence the optimum temperature-time combination to be adopted for controlling final mechanical properties. In the studied cases, the very close chemical composition of the heats was not a sufficient condition to guarantee the same as-quenched structure in terms of retained austenite amount. This was proven to be related to solute segregation effects during solidification of original heats.