Stellite Alloy Cladding of Supercritical Main Steam Inlet Nozzles

Literature Overview

This study by Chen Lijuan, Sun Zhongmin, Zhang Liyan, and Li Quan from Harbin Turbine Co., Ltd., published in 2007 in the field of turbine technology, investigates the application of Stellite alloy cladding on supercritical main steam inlet nozzles. Supercritical steam turbines operate at pressures exceeding 22.1 MPa and temperatures above 566 °C, creating extreme erosion and corrosion conditions at steam inlet passages where high-velocity steam carries particulate matter. The study represents a critical engineering solution for extending the service life of turbine components in coal-fired power generation plants.

Core Technical Approach

Stellite alloys, particularly Stellite 6 (UNS 31000) and Stellite 21 (UNS 31021), are cobalt-chromium-tungsten-molybdenum based alloys that maintain their mechanical properties at elevated temperatures and exhibit excellent resistance to erosion-corrosion. The cladding of supercritical main steam inlet nozzles requires careful consideration of the operating environment: steam temperatures of 566-623 °C, pressures of 25-30 MPa, and potential exposure to calcium, sodium, and vanadium deposits from coal combustion.

The typical cladding process parameters for this application are:

Metallurgical Considerations

The dilution between the Stellite alloy and the Cr-Mo steel base material creates a transition zone with unique microstructural characteristics. The dilution rate in SAW overlay typically ranges from 20% to 40%, depending on the number of passes and the wire diameter used. At 30% dilution, the overlay retains sufficient cobalt and chromium content to provide the required erosion resistance, but the microstructure shifts from a fully austenitic structure to a mixed austenite-ferrite structure.

The hardness profile through the overlay thickness is critical for performance. The outer layers should maintain 35-45 HRC hardness, while the transition zone may show 30-35 HRC. This gradient provides a combination of surface hardness for erosion resistance and toughness at the interface for crack resistance.

Erosion-Corrosion Performance

The erosion resistance of Stellite alloy cladding is evaluated through erosion test coupons and field performance data. The erosion rate for Stellite 6 cladding in supercritical steam conditions is typically 0.01-0.05 mm/year, compared to 0.5-2.0 mm/year for unprotected Cr-Mo steel. This represents a 50-100 fold improvement in erosion resistance, justifying the cost of the cladding operation.

The key factors influencing erosion resistance include:

1. Carbide morphology: Fine, dispersed carbides (WC, Mo2C, Cr7C3) provide superior erosion resistance compared to coarse, interconnected carbide networks.
2. Matrix hardness: The cobalt-based matrix must maintain sufficient hardness at operating temperature to resist plastic deformation under particle impact.
3. Thermal stability: The alloy must not undergo excessive phase transformation during thermal cycling, which would degrade the protective microstructure.

Defect Analysis and Inspection

Engineering Practice and Standards Compliance

The fabrication of clad supercritical turbine nozzles must comply with ASME Section VIII Division 1, with additional requirements from the project specification and the turbine manufacturer's quality plan. The weld procedure qualification must follow ASME Section IX, with impact testing of the weld metal and HAZ at the minimum service temperature. For supercritical applications, the impact energy requirement at -20 °C is typically 47 J for the base material and 27 J for the overlay dilution zone.

A critical engineering consideration is the post-weld machining of the nozzle bore. The overlay layer must be machined to achieve the required aerodynamic profile, and the residual stress state after machining must be evaluated. Stress relief treatment after machining is often required to prevent distortion during subsequent thermal cycling.

Study Insights and Reflections

The most important insight from this study is that Stellite alloy cladding of supercritical turbine nozzles is a multi-parameter optimization problem where overlay thickness, dilution rate, heat input, and post-weld treatment must be balanced against each other. Increasing overlay thickness improves erosion resistance but increases the risk of cracking and cost. Reducing dilution improves overlay properties but requires more passes and longer fabrication time. The engineering solution lies in selecting the minimum overlay thickness that meets the erosion life requirement while maintaining acceptable dilution and processability. This work demonstrates that surface engineering in power generation is not merely about applying a hard overlay but about creating a metallurgically coherent system that performs reliably under extreme thermal-mechanical loading.