Effect of P92 Hardfacing on Microstructure and Mechanical Properties of P91 Steel Welded Joints

Literature Overview

This research paper by Li Yong, Fang Yiming, Wang Wanli, Zhan Xianqiang, Wu Yue, and Tang Wenming, published in Journal of Thermal Analysis and Calorimetry of Materials (2026, Vol. 47, No. 2), addresses a critical engineering challenge in high-temperature power plant equipment: the susceptibility of P91 steel welded joints to Type IV cracking and softening in the heat-affected zone (HAZ). The authors from Datang Boiler and Pressure Vessel Inspection Center, China Datang Corporation Science and Technology Research Institute, and Hefei University of Technology propose an innovative solution involving P92 steel hardfacing on P91 steel before welding.

Core Technical Content

Engineering Problem Statement

P91 steel (9Cr-1Mo-V-Nb) is widely used in ultra-supercritical power plant boiler components operating at temperatures up to 650°C. However, welded joints of P91 steel are plagued by two major issues:

1. Fine-grained HAZ (FGHAZ) softening: The region just beyond the welding thermally affected zone experiences grain refinement due to partial austenitization, resulting in reduced hardness and strength. This softening zone is particularly susceptible to creep rupture under long-term high-temperature service.
2. Type IV cracking: Intergranular cracking occurs in the prior austenite grain boundaries of the FGHAZ under creep conditions. This cracking is driven by the combination of reduced grain boundary cohesion, carbide precipitation at grain boundaries, and differential creep rates between grains and grain boundaries.

The FGHAZ and intercritical HAZ (ICHAZ) typically exhibit hardness values 10-30% lower than the base metal, creating stress concentration zones that initiate creep damage.

Proposed Solution: P92 Hardfacing Strategy

The innovative approach involves:

1. Hardfacing: Deposit a 6-8 mm thick P92 steel layer on the P91 steel weld preparation surface
2. Intermediate heat treatment: Normalize at 1060°C followed by temper at 750°C
3. Welding: Perform conventional P91 welding on the P92 hardfacing layer

The key insight is that the P92 hardfacing layer shifts the HAZ location into the P92 material, which has superior creep resistance and hardness retention compared to P91.

Material Properties Comparison

Microstructural Analysis Results

The study reveals significant improvements in HAZ microstructure with P92 hardfacing:

The enhanced Nb and W content in P92 steel promotes the formation of fine MX-type carbides (Nb(C,N), W(C,N)) that pin grain boundaries and resist coarsening during heat treatment and service. This results in better hardness retention in the HAZ and reduced susceptibility to Type IV cracking.

Mechanical Properties Comparison

All mechanical properties with P92 hardfacing meet or exceed relevant standard requirements (ASME B31.1, ASTM A335, NB/T 47017) and show consistent improvement over conventional P91 welded joints.

Process Analysis and Engineering Implementation

P92 Hardfacing Process Parameters

Intermediate Heat Treatment

The intermediate normalizing and tempering treatment serves multiple purposes:

1. Homogenize composition: Reduce compositional segregation from multi-pass hardfacing
2. Stabilize microstructure: Convert as-welded martensite to tempered martensite with fine carbides
3. Relieve residual stresses: Reduce stress concentration at the hardfacing/base metal interface
4. Establish baseline properties: Ensure the hardfacing layer has the intended P92 properties before subsequent welding

Welding Process Considerations

After hardfacing and heat treatment, the conventional P91 welding process is applied. Key considerations include:

• Welding consumable selection: Use P91-matched consumables (e.g., ER91S-G, E91S-V)
• Heat input control: Limit heat input to 1.0-1.5 kJ/mm to minimize HAZ softening
• Interpass temperature: Maintain below 250°C to prevent excessive grain growth
• Post-weld heat treatment: Standard P91 PWHT at 760°C for 2-4 hours
• Non-destructive testing: Full RT and UT inspection of weld and HAZ

Quality Control and Inspection

Critical Inspection Points

1. Hardfacing layer thickness: Verify 6-8 mm thickness by ultrasonic measurement
2. Hardfacing/base metal interface: Inspect for lack of fusion, cracks, or inclusions
3. HAZ hardness profile: Map hardness across the entire HAZ to identify soft zones
4. Weld metal properties: Verify mechanical properties meet P91 requirements
5. Creep rupture testing: Long-term qualification testing for critical applications

Acceptance Criteria

Key Questions and Reflections

Several important considerations emerge from this innovative approach:

1. Cost-benefit analysis: The P92 hardfacing approach adds significant material and processing costs. The economic justification depends on the value of extended service life and reduced maintenance costs for critical power plant components.
2. Scalability: While demonstrated on test coupons, the approach needs validation on full-scale components with complex geometries. Large-diameter pipes, headers, and thick-walled components may present additional challenges.
3. Long-term creep performance: The study demonstrates improved room temperature and short-term high-temperature properties. Long-term creep rupture testing (10,000-100,000 hours) is essential for qualification in power plant service.
4. Repair procedures: If damage occurs during fabrication or service, repair procedures for the P92 hardfacing layer need to be established and qualified.
5. Code acceptance: The approach requires approval from relevant codes and standards organizations (ASME, NB/T) for use in pressure equipment design and fabrication.

Study Insights and Implications

This study presents a novel and elegant solution to a long-standing problem in high-temperature power plant engineering. By leveraging the superior creep resistance of P92 steel through hardfacing, the approach effectively eliminates the soft zone and Type IV crack susceptibility in P91 welded joints without requiring changes to the base material or welding consumables.