Tensile deformation behavior in metastable austenitic stainless steel having ultra fine grain structure

Deformation behavior in metastable austenitic stainless steel having an ultra fine grain structure has been investigated using a 12.5%Cr-9.5%Ni-2%Mo-0.1%N steel by means of a tensile test with particular emphasis on the behavior of deformation induced martensite (α′). The ultra fine grain austenite (γ) structure was achieved by reversion from α′ to γ. The grain size was controlled in the range between 0.2 and 40 μm by selecting annealing temperatures from 950 to 1400 K. Nominal stress-strain curves of the ultra fine grain (≤0.5 μm) specimens, which were obtained by annealing at temperatures from 973 to 1023 K for 180 s, exhibit the upper and lower yield points followed by a constant stress level between them (yield point phenomena). At the plateau stage neck is locally introduced and propagated in the gage position by heterogeneous deformation, accompanied by α′ formation. The occurrence of the yield point phenomenon and the size of the neck depend on two true stresses, i.e. the work hardening stress especially by the formation of α′ (σ_R) and the concentrated stress by the reduction of area (σ_L) in the necked area. The heterogeneous deformation in the ultra fine grain specimens can be explained by the plastic instability, that is, σ_L is higher than σ_R, which arises from the fact that α′ formation is largely suppressed by the grain refinement. When σ_R is higher than σ_L as in the case of coarser grain specimens, local necking is arrested and the deformation is propagated to another position, leading to uniform deformation.