Effect of Temperature on Microstructure and Properties of Stellite 6 and Stellite 21 Cladding Layers

Literature Overview

The 2024 study by Yu Shiqing and colleagues from Shenyang University of Technology and Shenyang Blower Group Nuclear Pump Co., Ltd., published in Heat Processing Technology, investigates the influence of substrate preheating temperature on the microstructure and mechanical properties of Stellite 6 and Stellite 21 cladding layers. This research is particularly relevant to the nuclear pump industry, where wear-resistant cladding of impellers, diffusers, and other rotating components is essential for extending service life in slurry and erosion service. The study was supported by the Liaoning Provincial Department of Education General Program (LJKZ0122), reflecting the academic and industrial significance of this research.

Background and Technical Significance

Stellite alloys are cobalt-chromium-tungsten-based casting alloys renowned for their exceptional wear resistance, corrosion resistance, and high-temperature strength. Stellite 6 (Co-Cr-W with 5% Mo) and Stellite 21 (Co-Cr-W with 10% Mo) are among the most widely used variants in industrial applications. The key difference between these two alloys lies in the molybdenum content, which influences the precipitation hardening response and high-temperature performance.

The cladding of Stellite alloys onto steel substrates is commonly performed using gas tungsten arc welding (GTAW), plasma transferred arc (PTA), or laser cladding. The substrate preheating temperature is a critical process parameter that affects:

• Dilution rate between the cladding material and substrate
• Solidification microstructure and grain morphology
• Phase composition and carbide precipitation
• Residual stress distribution
• Mechanical properties including hardness, tensile strength, and fatigue resistance

Experimental Methodology and Key Findings

The study examined the effect of substrate preheating temperatures ranging from room temperature to 400°C on the microstructure and properties of Stellite 6 and Stellite 21 cladding layers deposited by GTAW. The following key findings were reported:

Microstructural Evolution

The microstructure of the cladding layers was characterized by metallographic examination, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The primary phases identified were:

• γ-Co solid solution matrix: The base phase providing ductility and toughness.
• M7C3 carbides: Primary hard phase providing wear resistance, appearing as blocky or skeletal morphologies.
• M6C carbides: Secondary phase that may form during solidification or post-weld heat treatment.
• δ-ferrite: Observed in some conditions, particularly at higher preheating temperatures, which can adversely affect toughness.

The preheating temperature significantly influenced the solidification mode and grain morphology. At lower preheating temperatures (room temperature to 150°C), the solidification rate was higher, producing finer grain structures with more equiaxed grains. As the preheating temperature increased to 300-400°C, the solidification rate decreased, leading to coarser columnar grains and increased dilution due to greater substrate heat contribution.

Mechanical Property Response

The hardness, tensile strength, and wear resistance of the cladding layers were evaluated as a function of preheating temperature:

The results demonstrate that increasing the preheating temperature leads to a progressive decrease in hardness, primarily due to increased dilution and coarser microstructure. Stellite 21 consistently exhibits slightly higher hardness than Stellite 6 under comparable conditions, attributed to the higher molybdenum content promoting more effective precipitation hardening.

Metallurgical Analysis and Mechanisms

The study provides detailed metallurgical analysis of the microstructural evolution mechanisms:

1. Dilution control: Higher preheating temperatures increase the amount of substrate material melted and incorporated into the weld pool, leading to higher dilution rates. This is quantified through spectroscopic analysis of the clad layer composition, showing increased Fe content and decreased Co, Cr, and W concentrations.
2. Carbide precipitation: The M7C3 carbides, which are the primary wear-resistant phase, are affected by both solidification conditions and post-solidification diffusion. At higher preheating temperatures, the slower cooling rates allow for more complete carbide precipitation but may also lead to coarser carbide distributions.
3. Grain boundary effects: The grain boundary network plays a critical role in determining the fracture behavior and corrosion resistance of the cladding layer. Columnar grains with long vertical extent may provide pathways for crack propagation, while equiaxed grains offer better toughness.
4. Residual stress distribution: The preheating temperature influences the thermal gradient and consequently the residual stress state. Higher preheating temperatures reduce thermal gradients but may increase the risk of creep deformation in the substrate.

Engineering Implications and Recommendations

The findings of this study have direct implications for the optimization of Stellite cladding processes in nuclear pump manufacturing:

• Optimal preheating temperature: For most applications, a preheating temperature of 150-250°C provides an acceptable balance between dilution control and crack prevention. Temperatures above 300°C should be avoided unless specifically required for crack-sensitive substrates.
• Material selection: Stellite 21 offers marginally superior wear resistance compared to Stellite 6, but the difference may not justify the additional cost for all applications. The selection should be based on the specific service conditions and wear mechanisms.
• Process parameter optimization: The study recommends using lower travel speeds and higher current densities to compensate for the reduced dilution at lower preheating temperatures, maintaining adequate deposition rates while controlling heat input.
• Post-weld heat treatment: A stress relief anneal at 800-850°C for 1-2 hours is recommended to reduce residual stresses without significantly affecting the microstructure or hardness of the Stellite layer.

Study Insights and Reflections

This research provides valuable quantitative data on the influence of preheating temperature on Stellite cladding quality, filling an important gap in the technical literature. The key insight is that the preheating temperature is not merely a process parameter for crack prevention but a critical variable that directly affects the metallurgical quality and service performance of the cladding layer. For engineers involved in nuclear pump cladding, the study underscores the importance of systematic parameter optimization and the need for procedure qualification that includes dilution verification and mechanical property testing. The findings also highlight the potential for further optimization through advanced process control techniques, such as real-time temperature monitoring and adaptive parameter adjustment, to achieve consistent cladding quality across production volumes.