Effect of Annealing Treatment on Microstructure and Properties of P91 Steel Pipe TIG Welded Joints

Literature Overview

The study published in Electric Welding Machine (2014, Vol. 44, No. 6) by Li Binpo, Liu Peng, Zhao Baozhong, and Xu Wentao from Shandong Electric Power Engineering Consulting Institute and Shandong Jianzhu University investigates the effects of post-weld annealing treatment on the microstructure and mechanical properties of P91 heat-resistant steel pipe TIG welded joints. This research, supported by the Shandong Province Outstanding Young and Middle-aged Scientist Research Award Fund (BS2011CL027), addresses a critical materials engineering challenge in supercritical power plant applications, where P91 steel is widely used for high-temperature piping and pressure components. The study is classified under TG441.8 (heat treatment of welded joints), reflecting its focus on post-weld thermal processing.

P91 Steel and Welding Challenges

P91 steel (equivalent to ASTM A335 P91) is a 9Cr-1Mo-V-Nb heat-resistant ferritic steel designed for service temperatures up to 620°C in supercritical and ultra-supercritical power plant applications. Its excellent high-temperature creep strength and oxidation resistance make it the material of choice for high-temperature piping, but its welding presents significant metallurgical challenges. The high hardenability of P91 steel results in the formation of hard, brittle martensitic microstructures in the heat-affected zone (HAZ) during welding, which can lead to hydrogen-assisted cracking and poor toughness. Post-weld heat treatment (PWHT) is therefore mandatory for P91 welded joints to achieve the required mechanical properties and long-term service reliability.

The TIG welding process, while providing excellent weld quality and precise heat input control, still produces a severe thermal cycle in the HAZ due to the high thermal gradient inherent in arc welding. The resulting microstructural evolution in the HAZ is primarily governed by the peak temperature and cooling rate, which determine the phase transformation pathway and final microstructure.

Annealing Treatment and Microstructural Evolution

The authors investigated the effects of annealing at 750°C and 770°C on the TIG welded joint microstructure and hardness distribution. The results reveal significant microstructural improvements following annealing treatment:

The transformation of coarse lenticular martensite to finer lath martensite in the HAZ is metallurgically significant. Lenticular martensite, which forms at higher cooling rates, consists of interlocking plates with high dislocation density and retained austenite films at the plate boundaries. This microstructure is hard and brittle, with poor toughness and susceptibility to hydrogen cracking. The annealing treatment allows the martensite to partially decompose through tempering, and the retained austenite can transform to more stable phases, resulting in a finer and more ductile lath martensite structure.

The reduction in residual austenite quantity after annealing is also important. Residual austenite in P91 weldments can be problematic because it is metastable and can transform during service, leading to dimensional instability and potential cracking. The annealing temperature of 770°C appears to be more effective in reducing residual austenite compared to 750°C, which is consistent with the thermodynamic expectation that higher temperatures favor the transformation of austenite to ferrite and carbides.

Hardness Distribution and Mechanical Properties

The hardness distribution results demonstrate the effectiveness of annealing treatment in achieving uniform and acceptable mechanical properties across the welded joint:

The hardness variation across the joint becomes significantly more uniform after annealing. The HAZ hardness decreases from the as-welded range of 350-400 HV to 270-300 HV at 750°C, and to 280-310 HV at 770°C. The weld metal hardness similarly decreases to 240-270 HV (750°C) and 250-280 HV (770°C). These values are within the typical specification range for P91 welded joints, which generally requires hardness not exceeding 350 HV.

The observation that 770°C annealing results in slightly higher hardness than 750°C annealing is metallurgically interesting. This may be attributed to differences in carbide precipitation kinetics, where the higher temperature promotes more complete precipitation of fine carbides (such as M23C6 and MX-type carbides) that provide precipitation hardening. However, the difference is small, and both temperatures achieve acceptable hardness levels.

The authors note that the hardness distribution changes are closely related to the microstructural changes in the weld metal and HAZ, which is a fundamental metallurgical principle. The hardness reduction is primarily due to martensite tempering, carbide coarsening, and the transformation of residual austenite.

Engineering Practice Considerations

For P91 steel pipe welding in power plant applications, the post-weld annealing treatment is not merely optional but is a mandatory requirement specified in welding procedure specifications (WPS) and relevant standards such as ASME B31.1, ASME B31.3, and API 530. The typical PWHT temperature for P91 steel is in the range of 750-780°C, with a holding time proportional to the thickness of the component. The study's investigation of 750°C and 770°C annealing temperatures falls within this standard range.

From a practical standpoint, the choice between 750°C and 770°C annealing should consider several factors. The higher temperature of 770°C provides slightly better residual austenite elimination but may also promote grain growth and excessive carbide coarsening, which can reduce creep strength at long-term service temperatures. The lower temperature of 750°C provides adequate stress relief and martensite tempering with less risk of over-tempering. For supercritical power plant applications where long-term creep performance is critical, the lower annealing temperature may be preferred.

The presence of ferrite in the weld metal after annealing is an important observation. Ferrite formation in P91 welds can occur due to the dilution of the base metal into the weld pool, which alters the weld metal composition. While a small amount of ferrite is generally acceptable and can even be beneficial for crack resistance, excessive ferrite can reduce creep strength and oxidation resistance. The welding filler metal selection should be carefully considered to minimize unwanted phase formation in the weld metal.

Study Insights and Implications

This study provides valuable experimental data on the effects of annealing temperature on P91 welded joint microstructure and properties. The key finding that annealing effectively transforms coarse lenticular martensite to finer lath martensite and reduces residual austenite is well-established metallurgically but is important to confirm for specific welding procedures and materials. The hardness uniformity achieved after annealing is critical for ensuring that no local over-hardening exists that could initiate cracking under service loading.

For engineering practice, this study reinforces the importance of proper PWHT for P91 welded joints and provides guidance on the selection of annealing temperature. The slight advantage of 770°C in terms of residual austenite reduction should be weighed against the potential for over-tempering and grain growth. Future work should investigate the long-term creep properties and oxidation resistance of the annealed joints at service temperatures, as these are the critical performance metrics for P91 components in power plant applications. The study also highlights the need for careful control of welding parameters to minimize HAZ width and reduce the volume of severely affected material, as this directly impacts the effectiveness of the subsequent annealing treatment.