Microstructure and Properties of Hardfacing Weld Joints on NiCrMoV Steel Turbine Rotor

Literature Overview

This study by Fan Ruyi, Lu Fenggui, Liu Xia, Zhao Jian, and Qiao Shangfei (2011), published in Hot Working Technology, investigates the microstructure and properties of submerged arc hardfacing weld joints on a NiCrMoV steel turbine rotor shaft journal for a nuclear power plant high-pressure turbine. The research was conducted at the Shanghai Key Laboratory of Laser Manufacturing and Material Modification, Shanghai Jiao Tong University, in collaboration with Shanghai Electric Power Equipment Co., Ltd. The work addresses a critical maintenance and repair challenge in the nuclear power industry, where rotor shaft journals require periodic hardfacing to restore dimensional tolerances and protect against fretting and corrosion damage.

Material System and Welding Process

The substrate material is a NiCrMoV martensitic steel, which is a high-strength alloy commonly used for turbine rotors due to its excellent combination of strength, toughness, and creep resistance at elevated temperatures. The hardfacing system employs a two-layer approach:

The welding process used is submerged arc welding (SAW), which provides deep penetration, high deposition rates, and good control over the heat input. The transition layer is deposited first to bridge the metallurgical gap between the NiCrMoV base steel and the stainless steel hardfacing layer, reducing the risk of cracking and improving the overall joint integrity.

Microstructural Analysis

Electron probe microanalysis (EPMA) line scanning was employed to map the compositional gradient across the weld joint, revealing the dilution and elemental diffusion profiles from the base metal through the transition layer to the hardfacing layer. Optical microscopy was used to characterize the microstructural features of each zone, including the heat-affected zone (HAZ), transition layer, and hardfacing layer.

The microstructural evolution across the joint follows a predictable pattern:

• Base metal HAZ: The NiCrMoV steel HAZ exhibits a tempered martensite structure with varying degrees of softening depending on the peak temperature reached. The high-strength martensitic structure is partially tempered, reducing hardness but maintaining adequate toughness.
• Transition layer (NiMo1): This layer contains a martensitic structure with fine carbide precipitates. The composition is designed to be intermediate between the base metal and the hardfacing layer, ensuring good metallurgical compatibility. The transition layer acts as a buffer zone that reduces the compositional mismatch and thermal stress concentration at the interface.
• Hardfacing layer (E410NiMo15): This layer exhibits a tempered martensite structure with fine carbides, providing good wear resistance and corrosion resistance. The microstructure is uniform and free from coarse grain regions that would compromise toughness.

Mechanical and Corrosion Performance

Impact testing revealed that the transition layer significantly improves the impact toughness of the weld joint. Without the transition layer, the direct deposition of E410NiMo15 on NiCrMoV steel would result in a high dilution rate and a brittle microstructure with poor toughness. The transition layer reduces the dilution of the hardfacing layer and provides a gradual transition in mechanical properties from the base metal to the hardfacing layer.

Electrochemical corrosion testing demonstrated that the addition of the transition layer improves the corrosion resistance of the weld joint surface. The hardfacing layer exhibits a more noble corrosion potential compared to the base metal, indicating enhanced passivity. The transition layer ensures a smoother electrochemical transition, reducing the risk of galvanic corrosion at the interface.

Engineering Practice Implications

This study provides direct technical guidance for the repair and maintenance of nuclear power plant turbine rotors. The two-layer hardfacing approach with NiMo1 transition and E410NiMo15 hardfacing is a proven solution for restoring shaft journals while ensuring adequate toughness and corrosion resistance. Engineers performing such repairs should pay careful attention to the following:

• Preheat and interpass temperature: The NiCrMoV steel requires controlled preheat (typically 150-250°C) to reduce the risk of hydrogen-induced cracking in the HAZ. The interpass temperature should be maintained below 300°C to prevent excessive softening of the HAZ.
• Post-weld heat treatment: A tempering treatment is essential to relieve residual stresses and restore the toughness of the HAZ. The tempering temperature should be carefully controlled to avoid over-tempering the hardfacing layer.
• Welding sequence: The transition layer should be deposited in multiple thin passes to ensure complete fusion and uniform composition. The hardfacing layer should be deposited after the transition layer has been properly tempered or stress-relieved.

Study Insights and Reflections

This research exemplifies the importance of rational weld design in critical applications such as nuclear power equipment. The use of a transition layer is not merely a metallurgical consideration but a safety-critical design decision that ensures the long-term integrity of the rotor. The combination of EPMA line scanning with conventional mechanical and corrosion testing provides a comprehensive characterization approach that should be adopted as best practice in similar repair applications. The findings underscore that hardfacing is not simply about adding a hard surface but about creating a metallurgically sound joint that can withstand the complex loading and environmental conditions of service.