A Comparative Analysis on Various Strategies to Produce Nano-Crystalline Austenitic Stainless Steel |
A Super304H austeniticstainless steel, Fe - 0.1 %C - 0.12%N - 0.1 %Si - 0.95%Mn - 18.4%Cr - 7.85%Ni -3.2%Cu - 0.5%Nb - 0.01%P - 0.006%S (all in mass%), with an average grain sizeof about 10 mm was used as the starting material. The multiple forging was carriedout by means of multi-pass compressions at room temperature. The hardness of 6GPa and the yield strength of 1430 MPa were achieved after total strain of 4.The strengthening during the multiple forging resulted from the formation ofalmost equiaxed nanocrystalline structure with an average grain size of about30 nm. The softening behaviour of the nanocrystalline samples was studied bymeans of isochronal annealing at temperatures of 500 to 700 °C for 30 min. The structuralmechanisms responsible for the grain refinement during the large strain coldworking and those operating upon the subsequent annealing and their effect onthe mechanical properties are considered. The development ofnanocrystalline structures in austenitic stainless steels during large straincold rolling and their tensile behavior were studied. The cold rolling to totalequivalent strains above 2 was accompanied by the evolution of nanocrystallinestructures with the transverse grain size of about 100 nm. The development ofdeformation twinning and martensitic transformation during cold workingpromoted the fast kinetics of structural changes. The development ofnanocrystalline structures resulted in significant strengthening. More than fourfoldincrease in the yield strength was achieved. The strengthening ofnanocrystalline steels after severe plastic deformation was considered as aconcurrent operation of two strengthening mechanisms, which were attributed tograin size and internal stress. The contribution of internal stresses to theyield strength is comparable with that from grain size strengthening.