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

Literature Overview

This study by Li Binpo, Liu Peng, Zhao Baozhong, and Xu Wentao, published in 2014 in the journal "Electric Welding Machine" (电焊机), investigates the influence of post-weld annealing treatment on the microstructural evolution and mechanical properties of P91 steel pipe TIG welded joints. P91 steel, a 9Cr-1Mo-V-Nb type modified 9Cr-1Mo steel, is extensively used in supercritical and ultra-supercritical power generation boilers, reheaters, and high-pressure piping systems operating at temperatures up to 625 °C. The research was supported by the Shandong Provincial Outstanding Young and Middle-aged Scientists Research Award Fund (BS2011CL027) and conducted jointly by Shandong Electric Power Engineering Consulting Institute and Shandong Jianzhu University. The significance of this work lies in addressing one of the most critical challenges in P91 welding: the formation of brittle phases in the heat-affected zone (HAZ) and weld metal that severely compromise long-term creep rupture strength and low-temperature toughness.

Core Technical Findings

Microstructural Characteristics Before and After Annealing

The study reveals that as-welded P91 TIG joints exhibit a martensitic microstructure in both the weld metal and HAZ, with the HAZ showing pronounced temper embrittlement susceptibility. The coarse-grained HAZ (CGHAZ) near the fusion line develops retained austenite and carbide-free zones that are particularly detrimental to creep resistance. After appropriate annealing treatment, the martensite transforms into tempered martensite with fine, uniformly dispersed M23C6 and MX-type carbides, which significantly improve creep strength and toughness.

Annealing Parameters and Their Effects

The researchers systematically varied annealing temperature and holding time to determine the optimal post-weld heat treatment (PWHT) window. Annealing temperatures between 740 °C and 780 °C were evaluated, with 760 °C identified as the optimum. Temperatures below 740 °C were insufficient for complete tempering and carbide coarsening, while temperatures above 780 °C risked over-tempering and excessive grain growth in the base metal HAZ.

The optimal PWHT parameters for P91 TIG joints, as derived from this study, are:

• Temperature: 760 ± 10 °C
• Holding time: 2 hours (adjusted based on wall thickness)
• Cooling rate: ≤ 100 °C/h in furnace, followed by air cooling below 500 °C

Welding Process Parameters

The TIG welding was performed using ER91S-F or equivalent filler wire with a multi-pass procedure. Key welding parameters included:

• Shielding gas: 100% Ar (flow rate 15–20 L/min)
• Current: 120–180 A (DC)
• Travel speed: 5–8 cm/min
• Interpass temperature: ≤ 150 °C

Engineering Practice Implications

Defect Analysis and Countermeasures

From a practical standpoint, the study highlights several critical issues encountered in P91 welding:

1. Delayed Cracking: Hydrogen-induced delayed cracking in the HAZ is a persistent concern. The annealing treatment effectively relieves residual stresses and allows hydrogen diffusion, reducing cracking susceptibility.
2. Creep Void Formation: Without proper PWHT, creep cavities nucleate at grain boundaries in the CGHAZ. The annealing process promotes carbide precipitation at grain boundaries, pinning them against creep cavity growth.
3. Temper Embrittlement: The time-temperature window for temper embrittlement (370–570 °C) must be avoided during cooling. The study confirms that controlled furnace cooling through this range is essential.

Integration with Pressure Vessel Fabrication Standards

For P91 components fabricated as pressure vessels, the PWHT requirements align with ASME VIII Div.1 (Section III of the Code) and GB/T 150. The annealing temperature of 760 °C falls within the recommended range of 730–780 °C specified in these codes. The holding time of 2 hours corresponds to approximately 1 hour per 25 mm of wall thickness, which is consistent with the minimum time requirements in ASME codes.

The study's findings are particularly relevant for nuclear power applications, where P91 components are increasingly specified for improved thermal efficiency. The Shandong Electric Power Engineering Consulting Institute's involvement underscores the practical importance of this research for nuclear and conventional power plant construction.

Key Questions and Reflections

A critical question that emerges from this work is the trade-off between achieving optimal creep strength through PWHT and maintaining adequate toughness for low-temperature service. The annealing treatment that maximizes creep rupture life at 625 °C may not necessarily optimize impact toughness at -40 °C. Engineers must carefully balance these competing requirements based on the specific service conditions.

Another important consideration is the effect of multi-pass welding on the final microstructure after annealing. Each subsequent pass subjects the previously deposited weld metal to additional thermal cycles, effectively providing partial in-situ tempering. The final annealing must account for this cumulative thermal history to ensure uniform properties throughout the weld cross-section.

The study also raises questions about the applicability of these findings to thicker sections where thermal gradients during PWHT are more severe. For P91 pipes with wall thicknesses exceeding 50 mm, the cooling rate during furnace cooling may exceed the recommended 100 °C/h, potentially leading to untempered martensite formation in the core of the section.

Study Insights and Conclusions

This research provides valuable guidance for the welding and post-weld treatment of P91 steel components in power generation applications. The identification of 760 °C as the optimal annealing temperature, combined with the comprehensive characterization of microstructural evolution, offers a solid technical foundation for process specification development. Engineers involved in P91 welding must recognize that the welding process and PWHT are inseparable aspects of the fabrication sequence; optimizing one without considering the other inevitably leads to suboptimal joint performance. The long-term reliability of P91 welded joints in supercritical service ultimately depends on achieving a tempered martensitic microstructure with fine, stable carbide dispersion, and this study demonstrates that proper annealing treatment is the most effective means of attaining this microstructural target.