Stainless steels are widely used as structural materials for chemical plants, nuclear power plants, etc, because of the superior general corrosion resistance, mechanical properties, heat resistance, and weldability 1) . However, it is well known that austenitic stainless steels are susceptible to stress corrosion cracking(SCC) in certain environments under tensile stress. SCC induced by chlorides, such as sea salt particles, for example, is observed on various components in chemical plants 2),

more »

... ) , and also on piping of a nuclear power plant 4) built in the coastal area. This type of SCC is called as external SCC(ESCC) or atmospheric SCC since the cracking starts from out side of the equipment in air 2), 3) . ESCC manifests itself as inter-granular cracking or trans-granular cracking depending on relative humidity, material condition, such as sensitized or not, and temperature. Inter-granular SCC(IGSCC) was usually observed in sensitized parts of stainless steel components at around ambient temperature. On the other hand, trans-granular SCC(TGSCC) observed regardless of the material conditions at relatively high temperature 323 K. For the environmental condition, a certain degree of relative humidity(RH) is necessary to moisten the chlorides adhered on the stainless steel surface. The RH for ESCC easy to occur(RHp) is dependent on the type of chlorides. Thus the RHp of NaCl, for example, is 60 and that of MgCl 2 is 30 according to Shoji et al . 5) Another important factor of ESCC is the residual tensile stress derived from the welding or cold working processes. Hence, ESCC was usually observed at the weld joints or geometrical disconti-nuities. To prevent premature failures of equipment caused by ESCC of stainless steels, it is necessary to clarify the stress-ESCC failure time relationships after considering the probabilistic aspect of ESCC phenomena, and the environmental conditions which lead to occurrence of ESCC in the stainless steel equipment. Hence, the purposes of this study are to obtain the stress-ESCC failure time relationship for SUS304L stainless steel, and also to make a statistical analysis on ESCC crack size distribution. Austenitic stainless steel(SS) plates, SUS304L(304L), SUS316L(316L) and SUS304 (304), were examined in this study, since these SS are widely used as the structural materials for chemical plants and also nuclear power plants. Table 1 shows chemical composition of the test materials. Tensile specimens with a gage section of 2.0 mm thick, 5.0 mm wide, and 30 mm long were machined from the plate materials. After machining, the specimen was polished by emery papers to 600, degreased with acetone, rinsed with de-ionized water, and then attached to a loading apparatus that uses a spring to apply stress. Some specimens from 304L SS were aged at 923 K to simulate the heat history during welding process. Stress-ESCC failure tests were conducted on 304L and 316L SS by a constant load method using a spring for load-Zairyo-to-Kankyo, Chlorides induced stress corrosion cracking(ESCC) behavior was examined for SUS304L, SUS316L and SUS304 stainless steels(SS) after depositing synthetic sea water simulating sea salt particles at a temperature range from 333 K to 353 K with relative humidity of 35 . A stress-ESCC failure time relationship was formulated for SUS304L SS as Alog(t f ) B, where : applied stress(MPa), t f : time to failure(h), A 23.7T 7020, B 43.7T 11600, T : absolute temperature(K). Even an incipient micro-crack was not observed on the specimen surface of the SS at the applied stress level of 0.25 y ( y : 0.2 proof stress), although SCC initiated at the applied stress level of 0.5 y . Thus the threshold stress should be between 0.5 y and 0.25 y for these SS, and the threshold stress would rise beyond these values if the stress concentration be considered at the bottom and circumference of a pit. A statistical analysis suggested that the ESCC crack length and crack depth conformed to the double exponential distribution. Table 1 Chemical composition of specimen.