How does post-weld heat treatment of welded stainless steel pipe deal with corrosion?

Update:07 Sep
Abstract:Post-weld heat treatment (PWHT) of welded stainless steel pipe primarily addresses the issue of sensitization, which can make stainless steel more sus
Post-weld heat treatment (PWHT) of welded stainless steel pipe primarily addresses the issue of sensitization, which can make stainless steel more susceptible to corrosion, particularly in certain aggressive environments. Sensitization is a condition that occurs when the chromium in stainless steel is depleted near the grain boundaries, leaving these areas vulnerable to corrosion. PWHT is a method used to reverse sensitization and restore the corrosion resistance of stainless steel. Here's how PWHT helps with corrosion prevention:
Restores Chromium Distribution: Stainless steel derives its corrosion resistance from the chromium content in the alloy. When stainless steel is welded, especially in the heat-affected zone (HAZ), the high temperatures can cause the chromium near the grain boundaries to combine with carbon, forming chromium carbides. This depletion of chromium reduces the protective oxide layer on the surface of the steel, making it more susceptible to corrosion. PWHT, typically conducted at a temperature range of 1,900°F to 2,200°F (1,040°C to 1,205°C), allows the chromium carbides to dissolve back into the steel matrix, effectively restoring the even distribution of chromium along the grain boundaries.

Austenitic stainless steel welded pipe/tube EN10217-7 1.4301/1.4307/ 1.4404 / 1.4571 OD 50.8mm
Eliminates Residual Stresses: Welding induces residual stresses in the welded joint and the surrounding material. These residual stresses can create localized areas of increased susceptibility to corrosion. PWHT helps relieve these residual stresses, reducing the risk of stress corrosion cracking and other corrosion-related issues.
Enhances Microstructure Stability: The high temperatures used during PWHT promote the formation of a more stable microstructure in the stainless steel, which is less prone to corrosion and sensitization.
Improves Mechanical Properties: In addition to its corrosion resistance benefits, PWHT can improve the mechanical properties of the welded joint, making it more reliable in service. This includes increased tensile strength and ductility.
The decision to perform PWHT depends on several factors, including the stainless steel grade, the welding method used, the intended service conditions, and relevant industry standards or specifications. Overheating during PWHT or using improper parameters can have adverse effects, so it should be carried out with careful consideration and adherence to recommended procedures.
PWHT of welded stainless steel pipes is a crucial step in preventing corrosion by reversing sensitization, relieving residual stresses, and enhancing the microstructure of the material. Properly executed PWHT can significantly contribute to the long-term corrosion resistance and structural integrity of stainless steel pipes, especially in demanding or corrosive environments.