松岡 三郎

低炭素オーステナイト系ステンレス鋼SUS316の加工硬化材における0.2%耐力とビッカース硬さの関係

低炭素オーステナイ ト系ステ ンレス鋼SUS316 の 加工硬化材における0.2% 耐 力 とビッカース硬 さの関係* 松 岡 三 郎*1 Relationship between 0.2% Proof Stress and Vickers Hardness of Work-hardened Low Carbon Austenitic Stainless Steel, 316SS Saburo MATSUOKA *2 *2 Material Information Technology Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukubashi, Ibaraki, 305-0047 Japan Stress corrosion cracking (SCC) occurs in shrouds and piping made of low carbon austenitic stainless steels at nuclear power plants. A work-hardened layer is considered to be one of the probable causes for this occurrence. The maximum Vickers hardness measured at the workhardened layer is 400 HV. It is important to determine the yield strength and tensile strength of the work-hardened layer in the investigation on the causes of SCC. However, the tensile specimen cannot be obtained since the thickness of the work-hardened layer is as mall as several hundred .tm, therefore, it is useful if we can estimate these strengths from its Vickers hardness. Consequently, we investigated the relationships between Vickers hardness versus yield strength and tensile strength using the results obtained on various steels in a series of Fatigue Data Sheets published by the National Institute for Materials Science and results newly obtained on a parent material and rolled materials (reduction of area: 10~50%, maximum hardness : 350 HV) for a low carbon stainless steel. The results showed that (1) the relationship between the 0.2% proof stress and the Vickers hardness can be described by a single straight line regardless of strength, structure, and rolling ratio, however, (2) the tensile strength is not correlated with the Vickers hardness, and the austenitic stainless steel in particular shows characteristics different from those of other steels.

低炭素オーステナイト系ステンレス鋼SUS316の加工硬化材のナノ-メゾ-マクロ強度解析

Stress corrosion cracking (SCC) occurs in shrouds and piping of low carbon 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 microstructural characteristics of work-hardened layer at the surface of shrouds or piping, the strengthen analysis of low carbon austenitic stainless steel, 316SS, 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 c e l , coarse slip spacing, l c s l , and austenitic grain size, d y . Referring 10d c e l and 10l c s l , Vickers hardness, HV, corresponding to macro strength was expressed as Hv=Hv* b a s + Hv* s o l + Hv* d i s + Hv* c e l + Hv* c s l . Hv* b a s (= 100) is the base hardness, Hv* s o l is the solid solution strengthening hardness, Hv* d i s is the dislocation strengthening hardness in the dislocation cell, and Hv* c e l and Hv* c s l are the fine grain strengthening hardness due to the dislocation cell and coarse slip. Hv* s o l was about 50, independently of RA. Hv* d i s was zero at RA 30%. Hv* c e l and Hv* c s l 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%.