Post-Weld Heat Treatment Process Test for SA213-T92 Steel Tubes

Literature Overview

This study note examines a 2010 publication by Sun Yumei, Yang Huiqin, Cui Yulin, and He Jun from Shanghai Boiler Works Co., Ltd., published in the journal Boiler Technology (Vol. 41, Issue 4, pp. 51–54). The paper reports on post-weld heat treatment (PWHT) process trials for welds in small-diameter SA213-T92 steel tubes used in ultra-supercritical power plant boilers. The study conducted mechanical property tests at ambient temperature and at elevated temperatures of 510°C and 620°C, following PWHT at 740–760°C with a 1-hour hold, and included two product trial production runs to validate the process.

Material Background and Technical Challenges

SA213-T92 is a martensitic 9Cr-0.9Mo-V-Nb steel specifically developed for ultra-supercritical (USC) boiler applications, where operating temperatures reach up to 620°C and pressures exceed 25 MPa. The material offers significantly improved creep strength compared to the previously dominant T91 steel, attributed to the addition of niobium and the modified vanadium content, which together enhance precipitation strengthening through fine MC and MX carbide particles.

The welding of T92 steel presents several metallurgical challenges:

The primary purpose of PWHT for T92 welds is to transform the hard martensitic microstructure in the heat-affected zone into tempered martensite or bainite, thereby reducing hardness, relieving residual stresses, and improving ductility and toughness.

PWHT Process Parameters and Results

The study investigated PWHT at a temperature range of 740–760°C with a 1-hour hold time. This temperature window was selected based on the following considerations:

• Ac1 temperature: For T92 steel, the Ac1 temperature is approximately 710–730°C. Operating above this temperature ensures complete austenitization of the martensitic HAZ, enabling a uniform tempering transformation.
• Upper temperature limit: Temperatures exceeding 780°C risk excessive grain growth and potential carbide coarsening, which would degrade creep strength.
• Hold time: A 1-hour hold is sufficient for small-diameter tubes to achieve uniform temperature throughout the wall thickness and complete the tempering transformation.

The mechanical property test results demonstrated that the welded joints met the technical specification requirements at all test temperatures:

The two product trial production runs confirmed the repeatability and reliability of the PWHT process, demonstrating that the procedure could be successfully implemented in a production environment.

Process Optimization Considerations

Several process optimization factors deserve attention in the context of this study:

1. Heating rate control: The heating rate during PWHT must be controlled to prevent thermal stresses that could cause distortion or cracking. For small-diameter tubes, a heating rate of 100–150°C/h is typically adequate, but this should be verified for specific tube dimensions.
2. Cooling rate management: After the hold period, the cooling rate should be controlled to avoid re-forming untempered martensite. A cooling rate of less than 100°C/h in the temperature range of 600–400°C is generally recommended.
3. Temperature uniformity: For tube bundles, ensuring uniform temperature distribution across all tubes in the heat treatment furnace is critical. Temperature variation exceeding ±25°C between the hottest and coldest locations should be avoided.
4. Welding procedure compatibility: The PWHT parameters must be compatible with the welding procedure specification (WPS). The welding procedure should be designed to produce a weld metal microstructure that responds well to the specified PWHT, avoiding excessive weld metal hardness that would require more aggressive PWHT parameters.

Engineering Practice Integration

For boiler manufacturers and power plant operators, this study provides valuable technical data for the qualification of T92 weld PWHT procedures. The results can serve as a technical reserve or development direction for future USC boiler projects. Key practical recommendations include:

• Maintaining detailed records of PWHT parameters, including temperature-time curves, for each batch of tubes processed.
• Implementing hardness mapping of the HAZ after PWHT to verify the effectiveness of the treatment.
• Conducting periodic impact testing on production welds to monitor long-term performance.
• Developing weld procedure qualifications (WPQ) that incorporate the validated PWHT parameters.

Study Insights and Implications

This study demonstrates that the 740–760°C, 1-hour PWHT regime is effective for SA213-T92 welds in small-diameter boiler tubes. The successful completion of two product trial production runs adds significant credibility to the findings. The work contributes to the growing body of knowledge on T92 welding technology, which is essential for the continued development of ultra-supercritical power generation technology. Engineers should note that the specific PWHT parameters may need adjustment for different tube diameters and wall thicknesses, and that the results should be validated through additional testing for each specific application. The study underscores the importance of systematic PWHT process development and validation in ensuring the long-term reliability of high-temperature welded components.