Optimized Heat Treatment of 347 Type Stainless Steel Alloys for Elevated Temperature Service to Minimize Cracking

摘要

A new optimized post weld heat treatment (PWHT) of 347 type stainless steels has been developed and laboratory tested up to 1562 deg F (850 deg C) to address the needs of the petrochemical industry in stretching the upper operating temperature limits beyond 950 deg F (510 deg C). Experimentation focused on 347H, 347LN, and 16Cr11Ni2.5MoNb. The multi-step PWHT consists of stress relief and solution anneal, followed by a stabilizing stress relief. An optimized microstructure and substantial resistance to elevated temperature cracking was confirmed using a thermo-mechanical test simulator. Limitations associated with classical stabilization heat treatments, including sigma phase embrittlement and low ductility properties are eliminated. This PWHT provides high immunity to sigma phase embrittlement, reheat cracking, and stress relief cracking while retaining superior mechanical and corrosion resistance properties. Test results, mechanisms, and issues concerning alloy and nitrogen additions are discussed.