Critical assessment of the strain-rate dependent work hardening behaviour of AISI 304 stainless steel

In the present work, the effect of strain rate on the micro-mechanism of plastic deformation and ensuing work hardening behaviour of AISI 304 stainless steel has been critically investigated. The defining role of the early stage of deformation (<10% strain) on subsequent work hardening behaviour which is often ignored has been discussed in detail. It was found that there is a cross-over in the work hardening plot of slow strain rate (SSR) and high strain rate (HSR) specimen at around 5% strain and therefore to understand the sequential hardening behaviour, interrupted tensile tested specimens at 3, 10, 30% strain and up to the point of fracture were examined with respect to substructure evolution using transmission electron microscope and electron backscattered diffraction. While the propensity of γ→α′ in 3% strained specimen at HSR was higher contributing to lower transition strain from stage 1 to 2 of work hardening compared to SSR, the extent of work hardening was higher in the latter case during the stage 2 due to a combined effect of extensive twinning and deformation induced martensite formation, which was limited in HSR due to occurrence of cross-slip. These differences in the work hardening mechanisms were manifested as an improvement in UTS/YS ratio from 2.66 to 2.99 in HSR and SSR specimens, respectively. Furthermore, changes in the mechanism of plastic deformation was also reflected as ~4 fold increase in dislocation density for SSR specimen compared to HSR specimen at any given strain level and nearly 27% increase in hardness at the point of fracture. A significant improvement in UTS/YS ratio and ductility for SSR specimen left its signature on the fracture surface in form of three-fold increase in the number density of finer dimples compared to HSR specimen.