Effects of P92 Overlay Welding on Microstructure and Mechanical Properties of P91 Steel Weld Joints

Literature Overview

The study by Li Yong, Fang Yiming, Wang Wanli, Zhan Xianqiang, Wu Yue, and Tang Wenming, published in Journal of Materials Heat Treatment (2026), investigates the effects of P92 overlay welding on the microstructure and mechanical properties of P91 steel weld joints. The research was conducted by the Datang Boiler and Pressure Vessel Inspection Center Co., Ltd., the East China Power Testing Research Institute of China Datang Corporation, and Hefei University of Technology.

P91 and P92 steels are advanced low-alloy steels widely used in supercritical and ultra-supercritical power plant components, including boiler tubes, headers, and pressure vessels. P91 steel (9Cr-1Mo-V-Nb) is designed for service temperatures up to 625°C, while P92 steel (9Cr-1Mo-V-Nb-W) incorporates additional tungsten to extend the service temperature range to 650°C. The microalloying elements (V, Nb, W) form fine precipitates (MX carbonitrides, L12-type γ' phases) that provide exceptional strength and creep resistance at elevated temperatures.

The challenge addressed in this study is the welding of P92 overlay layers onto P91 steel substrates. This scenario arises in the repair and upgrade of existing P91 components, where a P92 overlay is applied to enhance the high-temperature performance of the component. The welding of these dissimilar advanced steels introduces unique metallurgical challenges due to the differences in chemical composition, thermal properties, and transformation behavior.

Material Characteristics and Weldability Challenges

Chemical Composition Comparison

Element	P91 Steel	P92 Steel	Typical P92 Filler
C (wt%)	0.08–0.12	0.08–0.12	0.08–0.12
Cr (wt%)	8.5–9.5	8.5–9.5	8.5–9.5
Mo (wt%)	0.85–1.05	0.85–1.05	0.85–1.05
W (wt%)	—	1.8–2.2	1.8–2.2
V (wt%)	0.13–0.18	0.13–0.18	0.13–0.18
Nb (wt%)	0.06–0.10	0.06–0.10	0.06–0.10
Mn (wt%)	0.30–0.60	0.30–0.60	0.30–0.60

The key difference between P91 and P92 is the addition of 1.8–2.2 wt% tungsten in P92. The tungsten contributes to solid solution strengthening and stabilizes the MX carbonitrides (VN, NbN, Mo₂C) against coarsening at elevated temperatures. This results in higher strength and creep resistance for P92 compared to P91 at temperatures above 600°C.

Weldability Challenges

The welding of P91 and P92 steels presents several challenges:

High hardenability: The high carbon equivalent (CE ≈ 0.55–0.65) of both steels makes them susceptible to cold cracking (hydrogen-induced cracking) during welding.
Temper embrittlement: The P91/P92 weld metal is susceptible to temper embrittlement in the temperature range of 370–570°C, which can severely reduce the ductility and toughness of the weld.
Precipitate evolution: The fine precipitates (MX carbonitrides, γ' phases) that provide strength to the P92 steel can coarsen or dissolve during the welding thermal cycle, leading to a loss of strength in the heat-affected zone (HAZ).
Dissimilar joint effects: The difference in thermal expansion and thermal conductivity between P91 and P92 (due to the tungsten content) can introduce additional residual stresses at the interface.

Welding Process and Parameters

The overlay welding was performed using a submerged arc welding (SAW) process with a flux-cored wire of matching P92 composition. The welding parameters were selected to minimize the risk of cold cracking and to achieve a favorable microstructure in the weld metal and HAZ.

Parameter	Value
Welding process	Submerged arc welding (SAW)
Filler metal	P92 flux-c