Low Temperature Sensitization of Welded Type 304 Stainless Steel

Abstract

Abstract 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.