NANOCRYSTALLINE S304H AUSTENITIC STAINLESS STEEL PROCESSED BY MULTIPLE FORGING

A Super304H austenitic stainless steel, Fe - 0.1%C - 0.12%N - 0.1c/oSi - 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 size of about 10 pm was used as the starting material. The multiple forging was carried out by means of multi-pass compressions at room temperature. The hardness of 6 GPa and the yield strength of 1430 MPa were achieved after total strain of 4. The strengthening during the multiple forging resulted from the formation of almost equiaxed nanocrystalline structure with an average grain size of about 30 nm. The softening behaviour of the nanocrystalline samples was studied by means of isochronal annealing at temperatures of 500 to 700 degrees C for 30 min. The structural mechanisms responsible for the grain refinement during the large strain cold working and those operating upon the subsequent annealing and their effect on the mechanical properties are considered.