Nanoscopic Strength Analysis of Work-Hardened L-Grade Austenitic Stainless Steel, 316(NG)

Abstract

Stress corrosion cracking (SCC) occurs in shrouds and piping of L-grade austenitic stainless steels at nuclear power plants. A work-hardened layer, where the transgranular SCC initiates, is considered to be one of the probable cause for this occurrence. In order to clarify the microstrucural characteristics of work-hardened layer at the surface of shrouds or piping, the strengthen analysis of L-grade austenitic stainless steel, 316(NG), rolled at the reduction in area, RA, of 10, 20, 30, 40 and 50% at room temperature were conducted on a nanoscopic scale, using an ultra-microhardness tester, TEM and SEM. TEM and SEM observation showed that the microstructural parameters are the dislocation cell size, d_cel, coarse slip spacing, l_csl, and austenitic grain size, d_γ. Referring 10d_cel and 10l_csl, Vickers hardness, Hv, corresponding to macro strength was expressed as Hv=Hv^*_bas+Hv^*_sol+Hv^*_dis+Hv^*_cel+Hv^*_csl. Hv^*_bas (=100) is the base hardness, Hv^*_sol is the solid solution strengthening hardness, Hv^*_dis is the dislocation strengthening hardness in the dislocation cell, and Hv^*_cel and Hv^*_csl are the fine grain strengthening hardness due to the dislocation cell and coarse slip. Hv^*_sol was about 50, independently of RA. Hv^*_dis was zero at RA<30% and increased at RA>30%. Hv^*_cel and Hv^*_csl increased with increasing in RA and were kept constant at about 50 and 120 at RA=20 and 30%, respectively. It was suggested from these results that all dislocations introduced by rolling might be dissipated for the creation of dislocation cells and coarse slips at RA<30% and that the microstructure contributing to the fine grain strengthening due to the dislocation cell and coarse slip might be accomplished at RA=30%. The dislocation strengthening in the dislocation cell might begin to operate at RA>30%.