Low temperature sensitization of type 304 stainless steel pipe weld heat affected zone
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INTRODUCTION
A major corrosion problem in boiling water reactors (BWR's) is intergranular stress corrosion cracking (IGSCC) in the weld heat affected zone (HAZ) of Type 304 stainless steel pipes in recirculation systems and, to a lesser extent, in core spray systems. IGSCC first occurred in the weld HAZ of 10- and 25-era-diameter piping, and, eventually, in large-diameter (e.g., 61-cm) primary coolant lines. The slower development of IGSCC in the weld HAZ of large diameter pipe is attributed to the lower level of residual stresses that result from welding. For BWR pipe cracking, as for stress corrosion cracking in general, three conditions are necessary: a sensitized microstructure, a critical level of stress (or stress intensity), and an aggressive environment. 2 Combating stress corrosion invariably depends on alleviating one or more of these conditions. Approaches that have been investigated for reducing the cracking of BWR piping include improved weld design to reduce residual stresses from welding, improved water chemistry control to reduce the impurity content to acceptably low levels, and use of alternative alloys that are inherently immune to stress corrosion in BWR service environments. 3 A sensitized microstructure is obtained when a stainless steel is held in a critical temperature range (usually 550 ~ to 800 ~ long enough to allow the grain boundary chemistry to change. In s~ainless steels containing relatively high levels of carbon (0.04 to 0.07 pet), this change can be the precipitation of chromium-rich carbides at the grain boundaries. 45~ Chromium carbide precipitation is accompanied by chromium depletion adjacent to the grain boundaries:
CHARLES G SCHMIDT. Material,, Scientist. ROBERT D. CALIGIURI, Program Manager. LAWRENCE E EISELSTEIN, Metallurgist, and SHARON S WING, Cheinp, t, are with SRI lnlernallomd. Menlo Park. CA 94025 DANIEL CUBICCIOTTI, Project Managel, is with Electric Power Research Institute, Palo Alto, CA 94304. Manuscript 'mbmitted August 4, 1986 METALLURGICAL TRANSACTIONS A
this, in turn, can inhibit passivation which accelerates attack from aggressive environments near the grain boundaries. Concurrent with chromium carbide precipitation, grain boundary segregation of impurities such as sulfur and phosphorus can occur which might also enhance the susceptibility of grain boundaries to chemical attack. 7'~'9 It has been suggested "~'~ that Type 304 stainless steel may be sensitized at relatively low temperatures (350 ~ to 500 ~ if carbide nuclei are first formed in the normal sensitization temperature range (typically 55(1~ to 800 ~ but dependent on alloy composition and cokl work). The chromium depletion accompanying the formation of the nuclei is not sufficient to cause severe sensitization: however, once the nuclei are formed, they are able to grow at temperatures below the normal sensitization range and thus increase the degree of sensitization. The combination of the nucleating heat treatment and the subsequent prolonged low temperature heat treatment has been termed low
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