CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Annealing Treatment on Microstructure and Properties of P91 Steel Pipe TIG Welded Joints
Literature Overview
This study, published in 2014 in the journal Electric Welding Machine by Li Binpo, Liu Peng, Zhao Baozhong, and Xu Wentao, investigates the influence of annealing (post-weld heat treatment, PWHT) on the microstructure and mechanical properties of TIG-welded joints in P91 steel pipes. The research was conducted by authors from Shandong Electric Power Engineering Consulting Institute Co., Ltd. (Nuclear Power Project Management Department) and Shandong Jianzhu University, and was supported by the Shandong Province Outstanding Young and Middle-aged Scientist Research Award Fund (BS2011CL027). P91 steel (9Cr-1Mo-V-Nb, equivalent to ASTM A335 P91 / EN 10216-2 14MoV36) is a high-strength, creep-resistant ferritic-martensitic steel widely used in supercritical and ultra-supercritical power plant piping systems operating at temperatures up to 625°C and pressures exceeding 25 MPa.
Core Technical Content
The study addresses a critical engineering challenge: ensuring the long-term reliability of P91 welded joints through proper post-weld heat treatment. P91 steel derives its exceptional high-temperature strength from a fine dispersion of MX-type carbonitrides (Nb(C,N), VC, TiC) and fine carbides (M23C6, M6C) precipitated within a tempered martensite matrix. The welding process disrupts this carefully engineered microstructure, creating heterogeneous zones with varying microstructures and properties that must be restored through PWHT.
The microstructural evolution in a P91 TIG weld joint can be divided into distinct zones:
| Zone | Microstructure | Typical Hardness (HV) | Key Concerns |
|---|---|---|---|
| Weld metal | Recrystallized, possibly coarse-grained martensite | 250–350 (as-welded) | Grain coarsening, brittle phases |
| Fine-grained HAZ (FGHAZ) | Recrystallized fine martensite | 300–400 (as-welded) | Maximum hardness, cracking susceptibility |
| Coarse-grained HAZ (CGHAZ) | Coarse martensite, possible retained austenite | 280–350 (as-welded) | Intergranular cracking, grain boundary embrittlement |
| Base metal | Tempered martensite with fine precipitates | 240–280 (as-received) | Reference condition |
The annealing (PWHT) process for P91 typically involves heating to 760–790°C at a controlled rate, holding for 1 hour per 25 mm of thickness (minimum 2 hours), and furnace cooling to below 300°C. This treatment achieves several objectives: tempering the as-welded martensite to reduce hardness and improve toughness, promoting the precipitation of fine MX carbonitrides and M23C6 carbides to restore creep strength, relieving residual stresses, and homogenizing the microstructure across the weld joint.
Process Parameters and Their Influence
The TIG welding parameters for P91 steel pipes are carefully selected to minimize cracking susceptibility and ensure adequate weld quality:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Shielding gas | Argon (99.99%) or Ar + 2% H₂ | H₂ reduces oxide inclusions |
| Backing gas | Argon or Ar + 5% H₂ | Prevents root oxidation |
| Current type | AC or DC- | AC provides cleaning; DC- for deeper penetration |
| Current | 80–180 A (for 10–25 mm wall thickness) | Balances penetration and heat input |
| Travel speed | 80–200 mm/min | Controls heat input per pass |
| Interpass temperature | 200–300°C | Prevents HAZ overheating |
| Preheat | 150–250°C | Reduces cooling rate, minimizes cracking |
The selection of welding consumable is equally critical. ER91GS (AWS A5.16) or equivalent low-carbon, low-sulfur, high-purity wire with controlled Nb, V, Ti, and N content is specified. The consumable chemistry must be matched to prevent deleterious phase formation such as Laves phase (Fe₂CrMo) or sigma phase (Cr₂₃C₆), which can precipitate during PWHT and severely degrade toughness.
Key Findings on Annealing Effects
The study likely demonstrates the following key effects of annealing on P91 TIG welded joints:
- Hardness reduction: As-welded hardness in the HAZ can exceed 400 HV, well above the P91 specification limit of 350 HV. After PWHT at 770°C, hardness should reduce to 240–300 HV, within the acceptable range.
- Toughness improvement: Charpy V-notch (CVN) impact energy at 20°C and 250°C increases significantly after PWHT, from potentially below 20 J (as-welded) to above 50 J (PWHT), meeting the ASME Section IX and API 934 requirements.
- Creep strength restoration: The precipitation of fine MX carbonitrides during PWHT restores the creep resistance that was lost during the high-temperature exposure of welding. Without proper PWHT, the weld metal and HAZ may exhibit creep rupture lives 50–70% lower than the base metal.
- Intergranular corrosion resistance: Proper PWHT minimizes chromium depletion at grain boundaries in the HAZ, reducing susceptibility to intergranular attack in high-temperature oxidizing environments.
Engineering Practice Implications
For nuclear power plant piping systems, where P91 is increasingly used in main steam and feedwater lines, the integrity of welded joints is paramount. The study's findings have direct relevance to:
- Welding procedure qualification: NB/T 47014 and ASME IX procedures for P91 must incorporate appropriate PWHT parameters validated by mechanical testing.
- In-service inspection: Understanding the microstructural state of PWHT'd P91 welds aids in interpreting in-service degradation mechanisms such as creep voiding, temper embrittlement, and hydrogen damage.
- Repair welding: Field repair of P91 welds requires careful control of PWHT parameters to avoid over-tempering or under-tempering of the surrounding base metal.
Key Questions and Reflections
A significant concern in P91 welding is the susceptibility to hydrogen-induced cracking (HIC) in the as-welded condition. The high cooling rates typical of TIG welding, combined with the hard martensitic microstructure, create conditions favorable for delayed cracking. While the study focuses on annealing effects, it is important to note that proper preheat (≥200°C) and interpass temperature control are essential prerequisites to prevent cracking before PWHT can be applied.
Another area of ongoing research is the optimization of PWHT parameters for thick-walled P91 components. Conventional PWHT at 770°C for extended hold times may promote Laves phase precipitation in the HAZ, particularly near the fusion boundary where local chemical composition may be slightly enriched in Mo and Cr. Alternative approaches, such as accelerated post-weld heat treatment (APWHT) or multi-step PWHT, are being investigated to minimize deleterious phase formation while achieving adequate stress relief and toughness.
Study Insights and Implications
This research reinforces the fundamental principle that for P91 steel, the welding process is incomplete without proper post-weld heat treatment. The microstructural homogeneity achieved through PWHT is essential for ensuring uniform mechanical properties across the entire weld joint, which is critical for long-term service in high-temperature, high-pressure environments. Engineers involved in nuclear and power plant piping fabrication should pay particular attention to the PWHT procedure, ensuring that heating rates, soak temperatures, hold times, and cooling rates are all within specified limits. The integration of process monitoring (thermocouple placement at critical locations), post-PWHT hardness mapping, and microstructural examination provides a comprehensive quality assurance approach that aligns with the rigorous standards governing nuclear-grade fabrication.