Effect of Low-Temperature Sensitization on Intergranular Stress Corrosion Cracking Behavior of Austenitic Stainless Steels in Simulated Boiling Water Reactor Environment

Intergranular stress corrosion cracking (IGSCC) is a generic form of cracking observed for austenitic stainless steels in boiling water reactors (BWR). Sensitization has been a basic reason for IGSCC and it occurs as a result of the formation of chromium carbides (Cr₂₃C₆) at grain boundaries near weldments. Low-carbon stainless steels are used to prevent sensitization and reduce susceptibility to IGSCC. Type 304L (UNS S30403) stainless steel does not sensitize during welding, but prolonged exposure to BWR conditions are found to promote low-temperature sensitization (LTS). Addition of N to Type 304L stainless steel is known to enhance not only the alloy strength, but also its sensitization resistance. Molybdenum-containing Type 316LN (UNS S31653) stainless steel has been demonstrated to be resistant to sensitization, LTS, and IGSCC and is currently used in BWR. Type 304LN stainless steel has not been used for such applications so far. The effect of nitrogen addition to Type 304L stainless steel on its sensitization and LTS behavior has been studied. The IGSCC behavior of low-temperature-sensitized Type 304L and Type 304LN stainless steels at 288°C at the strain rate of 1 × 10⁻⁶ s⁻¹ using the slow strain rate technique (SSRT) has been compared. The effects of heat treatment on stress-elongation curves and fracture modes are discussed. Beneficial effects of nitrogen up to 0.16 wt% on sensitization, LTS, and IGSCC in a BWR simulated environment are demonstrated.