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* cel + Hv* cslmiddot; + 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%. © 2006 The Japan Institute of Metals.