Low Temperature Sensitization of Welded Type 304 Stainless Steel

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.

Progress report on the results of testing advanced conceptual design metal barrier materials under relevant environmental conditions for a tuff repository

“…Results of this analysis are illustrated in Development of low temperature sensitization can be retarded in ways other than reduction of the peak surface temperature of the container. The mechanism for development of LTS is discussed by Povich (1978) and requires a highly cold-worked structure for nucleation of subcritical carbide precipitates that are formed in the usual sensitization temperature range (above 500°C) but are not discernible by any of the ASTM tests. These subcritical precipitates then grow when the specimen is exposed to temperatures below 500°C for prolonged periods.…”

Low-Temperature Sensitization Behavior of Base, Heat-Affected Zone, and Weld Pool in AISI 304LN

“…[1][2][3][4][5] Chromium depletion in stainless steels in this temperature range progresses through both the nucleation of fresh carbides and the growth of existing carbides, if any. However, Povich et al [6,7] showed that AISI 304 at or below 500°C may get sensitized. It has also been reported that sensitization at £500°C occurs through the growth of pre-existing carbide and that fresh chromium carbide nucleation was ruled out unless specimens are coldworked to certain critical limits.…”

“…The pre-existing tiny carbide particles were observed to grow in size when failed components were investigated; this was accompanied by severe chromium depletion from adjacent grain boundaries. [6,7] Extensive intergranular stress corrosion cracking (IGSCC) failures of the recirculation pipelines in boiling water reactors were reported due to LTS. [6][7][8][9] The AISI 304L and 316L, the candidate materials for nuclear waste storage, were reported to fail from heat-affected zone (HAZ) locations by LTS-enhanced IGSCC.…”

“…[6,7] Extensive intergranular stress corrosion cracking (IGSCC) failures of the recirculation pipelines in boiling water reactors were reported due to LTS. [6][7][8][9] The AISI 304L and 316L, the candidate materials for nuclear waste storage, were reported to fail from heat-affected zone (HAZ) locations by LTS-enhanced IGSCC. The canister surfaces, which encountered a temperature range of 100°C to 300°C from the radioactive decay of radionuclides, were found to experience significant chromium depletion near the boundaries.…”

“…[8] This was concluded based on the authors' investigation, which showed that, for T £ 500°C, the number of carbide particles remained unchanged, even though their size increased significantly. [7] The sensitization below the classic sensitization temperature range is termed lowtemperature sensitization (LTS); this is extremely relevant to the several industrial processes using stainless steels that operate at above ambient temperatures. A nuclear power plant is one such example; in nuclear power plants, stainless steel pipes are being used at an operating temperature of~300°C.…”