Microstructure and Properties of NiCrMoV Steel Turbine Rotor Overlay Welded Joints

Literature Overview

This study by Fan Ruyi, Lu Fenggui, Liu Xia, Zhao Jian, and Qiao Shangfei, published in Hot Working Technology (2011, Vol. 40, No. 21, pp. 137–139), investigates the overlay welding of nuclear power plant steam turbine high-pressure rotors made of NiCrMoV steel. The research was conducted jointly by Shanghai Jiao Tong University (Key Laboratory of Laser Manufacturing and Material Modification) and Shanghai Electric Power Equipment Co., Ltd. Turbine Plant.

Core Technical Findings

The researchers selected E410NiMo15 (a martensitic stainless steel) as the overlay layer material and NiMo1 as the transition layer material for submerged arc overlay welding on the NiCrMoV steel shaft journal. The study employed EPMA line scanning for chemical composition analysis, optical microscopy for microstructure observation, impact testing for mechanical properties, and electrochemical corrosion testing for corrosion resistance evaluation.

The key finding is that the addition of a transition layer (NiMo1) is beneficial for improving both the impact toughness of the stainless steel corrosion-resistant layer and the surface corrosion resistance of the overlay welded joint.

Technical Analysis

Overlay Welded Joint Structure

The overlay welded joint consists of three distinct regions:

Role of the Transition Layer

The transition layer serves multiple critical functions:

1. Dilution management: The NiMo1 transition layer reduces the dilution of the E410NiMo15 overlay by the NiCrMoV base metal, ensuring the overlay maintains sufficient Cr content for corrosion resistance.
2. Thermal stress accommodation: The intermediate composition of the transition layer reduces the thermal expansion mismatch between the overlay and base metal, thereby reducing residual stresses.
3. Toughness improvement: The transition layer provides a ductile buffer zone that absorbs impact energy and prevents crack propagation from the overlay into the base metal.

Microstructure Evolution

The EPMA line scanning reveals a gradient in chemical composition across the overlay welded joint. At the overlay-base metal interface, there is a diffusion zone where elements interdiffuse. The microstructure transitions from the martensitic structure of the E410NiMo15 overlay through the transition layer to the tempered martensite of the NiCrMoV base metal.

Impact Toughness and Corrosion Resistance

The impact test results demonstrate that the transition layer significantly improves the Charpy V-notch impact energy of the overlay layer. Without the transition layer, the dilution from the base metal reduces the Cr content of the overlay, leading to a coarser microstructure and reduced toughness. The electrochemical corrosion tests confirm that the transition layer also improves the surface corrosion resistance by maintaining a more uniform and continuous protective oxide film.

Engineering Practice Integration

Application Context

In nuclear power plants, the high-pressure turbine rotor is a critical component that operates under extreme conditions of high temperature, high pressure, and corrosive steam environments. The shaft journal, which supports the rotor on bearings, is particularly susceptible to fretting wear and corrosion. Overlay welding provides a reliable method for surface hardening and corrosion protection without altering the dimensional tolerances of the shaft.

Process Considerations

• Welding procedure qualification: The overlay welding procedure must be qualified in accordance with applicable standards (e.g., ASME Section IX, ISO 15614) to ensure reproducibility.
• Heat input control: The heat input must be carefully controlled to minimize the heat-affected zone (HAZ) and prevent over-tempering of the NiCrMoV base metal.
• Post-weld heat treatment (PWHT): A controlled PWHT is essential to relieve residual stresses and promote the formation of a tempered martensitic structure in the overlay layer.

Quality Assurance

• Non-destructive testing: Magnetic particle testing (MT) for surface cracks, ultrasonic testing (UT) for subsurface defects, and dye penetrant testing (PT) for surface-breaking defects.
• Destructive testing: Hardness profiling, tensile testing of overlay coupons, and impact testing of witness specimens.
• Corrosion testing: Electrochemical polarization curves, immersion testing in simulated steam environments, and salt spray testing.

Study Insights and Implications

The use of a transition layer in overlay welding is a well-established practice in power generation equipment, but this study provides quantitative evidence of its effectiveness in a nuclear power plant application. The combination of E410NiMo15 and NiMo1 materials represents a carefully selected material pair that balances corrosion resistance, toughness, and weldability.

From a metallurgical perspective, the transition layer addresses the fundamental challenge of joining dissimilar materials with different thermal expansion coefficients and chemical compositions. The diffusion of elements across the interface creates a gradual transition in properties, which is essential for preventing interfacial cracking during service.

For future work, I would recommend investigating the long-term creep behavior and thermal cycling fatigue resistance of the overlay welded joint under simulated operating conditions, as these properties are critical for ensuring the reliability of nuclear power plant components over their design life.