Any Effect of Processing History on Precipitation Hardening of Metastable Austenitic Stainless Steels

The strain induced martensite formation is known to be sensitive to the stress state [I]. The amount of martensite formed varies if the same amount of load is applied in different states of stress. For example, martensite formed is maximum in tension, minimum in compression and somewhere in between, in shear. Martensite could also originate due to elastic stress, or in absence of any mechanical energy, solely, by change in temperature (thermal martensite). Thus, it is the aim of this project to understand the different kinds of martensite that originate due to different processing paths and then, to understand how the precipitation behaviour is affected by the process of arriving at the martensite. Basically, it is to understand how the dislocation substructure varies under various stress states and thermal states, and how it affects the kinetics and type of precipitation in metastable austenitic stainless steels. This work is carried out with a high alloy, metastable and precipitation hardenable stainless steel called Sandvik NanoflexTM. Formation of strain induced martensite in semi austenitic metastable stainless steels is strongly a function of strain rate. It is also a function of stress state: in this study a planar shear state and an axial tensile state of stress are compared. The morphology of martensite formed in tension is different from that in shear. Predominantly sheared samples show rippled structures with ridges and valleys where as predominantly tensile deformation creates samples with planar laths separated by crevasse-like boundaries. The morphological difference in the martensite formed under different stress-states, creates different shape of precipitates in semi austenitic metastable stainless steels. The predominantly sheared samples show roundish precipitates where as predominantly tensile deformed samples show precipitates with a core. Difference in the dislocation substructure is thought to be the root cause of such morphological differences in the martensite and precipitates formed through different stress states.