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Repair welding Cr-Mo steels without PWHT

Case Study

Controlled deposition repair procedures, which can be implemented without the need for Post Weld Heat Treatment (PWHT), save time and money, and are now permitted within National Board Inspection Code and ASME codes.

TWI, in collaboration with The Edison Welding Institute (EWI) in the USA, has completed a Group Sponsored Project (GSP) to develop repair procedures for 1.25Cr-0.5Mo and 2.25Cr-1Mo steels that can be used without performing a post-weld heat treatment (PWHT).

This project produced the best results seen in achieving a high degree of grain refinement and tempering to reduce hardness. The procedures the project team developed, together with the recent changes to the NBIC code, mean that repair welding of Cr-Mo steels is now a practical time- and money-saving option for electrical utility, petrochemical, and other companies.

The majority of codes that govern the weld repair of Cr-Mo steel components in high temperature pressure vessels and pipes require PWHT of the weld repairs to temper the weldment and reduce the residual welding stresses. Such PWHTs are extremely expensive and time consuming because of the often long hold times at the stress relief temperature and the slow ramp up and ramp down rates that are necessary. In addition, for large Cr-Mo steel components such as pressure vessels and turbine rotor casings, local PWHTs are extremely difficult to perform.

As part of the program, a detailed SMAW welding procedure was developed, for all welding positions, that provides excellent weldment properties in the as-welded condition for both the 1.25Cr-0.5Mo and 2.25Cr-1Mo steels. This procedure is supported by detailed welding instructions, a welder training document, and instructions for welding of the welder qualification test assembly.

A large amount of mechanical property characterisation was also developed along with performance data for as-welded repairs to both Cr-Mo steels. The performance data includes:

  • measurement of the residual stresses in the as-welded repairs and their decay with long time exposure at 1000 degrees F (538°C) and 800 degrees F (427°C)
  • creep rupture testing of as-welded repairs made in creep-service 1.25Cr-0.5Mo and 2.25Cr-1Mo steels, and creep rupture testing of the 1.25Cr-0.5Mo repair in the PWHT condition to provide a direct comparison between the as-welded and PWHT conditions
  • measurement of the susceptibility of the as-welded repairs to hydrogen attack
  • decay of weldment hardness with long time exposure at 1000 degrees F (538°C) and 800 degrees F (427°C).

There are situations, even during new construction, where the capability to weld and justify welding on a fully heat treated vessel, and not having to PWHT after that welding would be of great value. These procedures have subsequently been applied to CrMoV steels in a follow-on project.