A new optimized post weld heat treatment (PWHT) of 347 type stainless steels has been developed and laboratory tested up to 1562°F (850°C) to address the needs of the petrochemical industry in stretching the upper operating temperature limits beyond 950°F (510°C). Experimentation focused on 347H, 347LN, and 16Crl 1Ni2.5MoNb. 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.
A new and optimized heat treatment of 347 type stainless steels has been developed to address the needs of the petrochemical industry in stretching the upper temperature limits for 347 type stainless steels beyond 950°F (510°C) for high pressure piping and equipment.
Typically, for operating temperatures above 900°F (482°C) a 1650°F (899°C) stabilization treatment is conducted after fabrication. At elevated temperatures, above 900°F (482°C), the classical stabilization treatment compromises the high temperature service weld and heat affected zone (HAZ) integrity through sigma phase embrittlement, reduces impact properties, reduces elevated temperature creep properties; and increases susceptibility to reheat cracking. Various mitigation techniques are traditionally employed to alleviate these effects; yet, fabricator's continuing experiences show that susceptibility to cracking cannot be eliminated.
Use of 347 type stainless steels in high temperature operating environments has been limited due to incidents of reheat cracking during PWHT and stress relaxation cracking after long-term elevated temperature service.
The new "multi-step PWHT" (patent pending) consists of stress relieving, solution annealing, followed by a stabilizing stress relief, and air cooling. The optimized process eliminates weld sigma phase embrittlement and HAZ low ductility properties associated with classical stabilization heat treatments. An improved microstructure and substantial resistance to elevated temperature reheat cracking were obtained. The multi-step PWHT provides high immunity to sigma phase embrittlement, reheat cracking, and stress relaxation cracking, while retaining good mechanical and corrosion resistant properties.
BACKGROUND
The rapid progress in elevated temperature petrochemical technology and the demand for higher process efficiency has created opportunities to use stainless steels beyond the traditional operating limits of 950°F (510°C).
The purpose of this experimental study was to test and evaluate a theoretically optimized PWHT for austenitic stainless steels (SS) in order to reduce the probability of reheat cracking and stress relaxation cracking while still retaining excellent mechanical and corrosion resistance properties. Materials examined included 347H, 347H low nitrogen (347HLN) and 16CrllNi2.5MoNb stainless steel. The authors developed the 16Crl 1Ni2.5MoNb for its excellent corrosion and creep resistance properties 1.
It is common practice that many users do not perform any heat treatment of these stainless steel materials since they are inherently corrosion resistant in many environment
