It has been shown that the sensitization of Type 304 stainless steel occurs at temperatures well below the normal isothermal temperature range for sensitization. A prerequisite for this low temperature sensitization (LTS) is the presence of chromium carbide nuclei along grain boundaries. This paper shows that the thermal exposure during welding can nucleate the grain boundary carbides necessary for subsequent LTS. Quantitative transmission electron microscopy studies show that no new carbides nucleate during an LTS heat treatment at 400 C. However, carbides that were nucleated at the time of welding grow during the LTS heat treatment. These findings are consistent with the proposed nucleation and growth LTS model. Using an accelerated test for intergranular stress corrosion cracking (IGSCC) in high temperature, high purity, 8 ppm oxygen water, it has been found that the rate of LTS in a Type 304 stainless steel weld heat affected zone obeys an Arrhenius temperature dependence which predicts that LTS enhanced susceptibility to IGSCC may occur in less than 10 years at 300 C. Similar IGSCC tests in water containing reduced dissolved oxygen indicate that reducing the oxygen level to 0.2 ppm would greatly reduce the probability of IGSCC, even in low temperature sensitized Type 304 stainless steel. The value of LTS as an improved screening test for the presence of carbides and for the qualification of mill annealed stainless steel is noted.

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The variation in stress corrosion susceptibility and cracking morphology of sensitized Type 304 stainless steel in high purity water is reported as a function of combinations of oxygen (0.01 to 8 ppm) and temperature (75 to 288 C). The results are discussed both in terms of the fundamental parameters involved in stress corrosion cracking (SCC), and the relevance of the data vis a vis the prevention of cracking in operaitonal BWR's.

The apparatus and technique employed in slow strain-rate testing of stainless steels in high temperature high purity water are described and discussed. Typical results from work in progress on the influence of heat treatment and surface treatment on the intergranular stress corrosion cracking of Types 304 and 308 stainless steel and Alloy XM-19 are described. The correspondence between strain-rate tests varies from alloy to alloy. The minimum strain rate required to cause intergranular cracking varied significantly with the alloy and with the surface treatment. It is suggested that reduction of area and observation of the fracture mode provide better measures of environmental interactions than measures of tensile parameters obtained from the test records.

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