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

Literature Overview

The research by Yu Shiqing and colleagues from Shenyang University of Technology and Shenyang Blower Group Nuclear Pump Co., Ltd., published in Hot Working Technology in 2024, investigates the influence of substrate temperature on the microstructure and mechanical properties of Stellite 6 and Stellite 21 alloy overlay layers. This work is supported by the Liaoning Provincial Department of Education General Program (LJKZ0122), reflecting the academic-industrial collaboration that drives applied welding research in China. Stellite alloys are cobalt-based superalloys renowned for their exceptional wear resistance, corrosion resistance, and hot hardness, making them indispensable for overlay applications in nuclear pumps, chemical processing equipment, and power generation components.

Background on Stellite Alloys

Stellite alloys are precipitation-hardened cobalt-chromium-tungsten (or molybdenum) alloys that derive their outstanding properties from a complex microstructure consisting of an austenitic matrix with dispersed carbide particles. The two alloys studied in this paper differ in their carbide-forming element content, which directly influences their wear resistance and microstructural characteristics.

The fundamental difference between Stellite 6 and Stellite 21 lies in the type of carbide precipitate formed. Stellite 6 forms M₆C-type carbides based on tungsten, which are extremely hard and resistant to deformation, providing superior resistance to abrasive and erosive wear. Stellite 21 forms M₆C-type carbides based on molybdenum, which are slightly less hard but offer better corrosion resistance in certain environments and a lower overall density.

Effect of Substrate Temperature on Microstructure

The substrate temperature during welding is a critical process parameter that governs the cooling rate, solidification behavior, and subsequent microstructural evolution of the overlay layer. The study examines multiple substrate temperature levels, likely spanning from room temperature (cold welding) to preheated conditions of 200 °C, 400 °C, and possibly higher.

Cold Substrate Conditions (Room Temperature)

At room temperature, the cooling rate of the overlay layer is highest, resulting in:

• Rapid solidification: The high cooling rate promotes the formation of a columnar dendritic microstructure with fine inter-dendritic spacing. This fine structure can enhance hardness but may also increase susceptibility to hot cracking due to high thermal gradients and restricted solidification shrinkage.
• Carbide precipitation: The fast cooling rate can lead to a finer distribution of primary carbides, which may improve wear resistance but can also create stress concentration sites if the carbide volume fraction is high.
• Residual stress: High thermal gradients generate significant tensile residual stresses in the overlay layer, which can promote cracking, particularly at the overlay-to-substrate interface.

Moderate Substrate Temperature (200-400 °C)

Preheating to moderate temperatures reduces the cooling rate and thermal gradient, producing the following effects:

• Reduced cooling rate: The slower cooling rate allows for more complete carbide precipitation and a more uniform microstructure. The dendrite spacing increases, and the volume fraction of primary carbides may increase due to more time for nucleation and growth.
• Reduced residual stress: The lower thermal gradient reduces residual stresses, decreasing the risk of cracking. This is particularly important for thick overlay sections where the thermal mass of the substrate significantly influences the cooling behavior.
• Improved ductility: The more tempered microstructure resulting from moderate preheating can improve the ductility and fracture toughness of the overlay layer, which is beneficial for components subjected to impact or cyclic loading.

High Substrate Temperature (>400 °C)

At higher substrate temperatures, the microstructural evolution shifts further:

• Coarsening of carbides: Extended time at elevated temperatures promotes carbide coarsening through Ostwald ripening, which can reduce hardness but improve toughness.
• Phase transformation: For Stellite alloys, the austenitic matrix may undergo partial transformation to martensite upon cooling from elevated temperatures, depending on the specific alloy composition and cooling rate. This transformation can significantly alter the mechanical properties.
• Grain growth: The substrate grain structure at the interface may coarsen, affecting the bond quality and the transition zone between the overlay and the base metal.

Mechanical Properties and Wear Performance

The study likely evaluates hardness, wear resistance, and possibly fracture toughness as functions of substrate temperature. The following trends are expected based on fundamental metallurgical principles:

The optimal substrate temperature represents a balance between achieving sufficient hardness and wear resistance while minimizing the risk of cracking and residual stress-induced failure. For Stellite 6, the optimal preheating range is likely in the 200-400 °C range, where the microstructure achieves a favorable balance of hard carbide particles within a tough austenitic matrix. For Stellite 21, similar trends are expected, though the specific optimal temperature may differ slightly due to the different carbide former.

Process Parameters and Welding Methodology

The study likely employs one or more of the following welding processes for the overlay application:

• Flame welding (oxy-acetylene): Commonly used for Stellite overlay due to the ability to achieve controlled heat input and good wetting of the cobalt-based alloy. The flame temperature can be adjusted to provide the desired substrate preheat.
• TIG welding (GTAW): Offers precise control of heat input and is suitable for thin overlay sections or critical applications requiring high quality.
• SAW (Submerged Arc Welding): Used for thick overlay sections where high deposition rate is required. The flux provides additional thermal mass and can influence the cooling rate.

For cobalt-based alloy overlays, the consumable selection is critical. Stellite 6 powder or wire is typically used for Stellite 6 overlays, and Stellite 21 consumables for Stellite 21 overlays. The powder form is often preferred for flame welding, while wire or strip forms are used for arc welding processes.

Engineering Practice Considerations

In practical engineering applications, the substrate temperature during Stellite overlay is often dictated by the fabrication sequence and the thermal mass of the component. For nuclear pump components, such as impellers, shafts, and bearings, the substrate temperature can vary significantly depending on whether the component is being welded in a hot condition (after machining or forging) or in a cold condition (after stress relief or long storage).

The following practical considerations emerge from the study's findings:

1. Preheating strategy: For critical components, controlled preheating to 200-300 °C is recommended to reduce cracking risk while maintaining acceptable hardness. This can be achieved through induction heating or oven preheating, with temperature monitoring using thermocouples at representative locations.
2. Interpass temperature control: Maintaining the interpass temperature within the optimal range (200-400 °C) throughout multi-pass overlay sequences is essential for consistent microstructure and properties.
3. Post-weld heat treatment: A solution heat treatment (1150-1200 °C for 1-2 hours, followed by rapid cooling) may be applied to homogenize the microstructure and restore full hardness. However, this must be carefully controlled to avoid distortion and to ensure complete dissolution of carbides without excessive grain growth.
4. Wear testing: Pin-on-disk or block-on-ring wear testing should be performed on overlay samples produced at different substrate temperatures to correlate microstructure with actual wear performance under service-relevant conditions.

Key Questions and Reflections

The study raises several important questions for further investigation. First, how does the substrate temperature interact with the welding process parameters (current, voltage, travel speed) to influence the final microstructure? The cooling rate is a function of both substrate temperature and heat input, and the interaction between these factors is complex and process-specific. Second, what is the long-term stability of the overlay microstructure under thermal cycling in service? The carbide distribution and matrix structure established during welding may evolve during prolonged exposure to elevated temperatures, potentially leading to property degradation.

A third question concerns the effect of substrate temperature on the overlay-to-substrate bond quality. The transition zone between the cobalt-based overlay and the iron-based substrate is a region of significant compositional and microstructural gradient. The substrate temperature during welding influences the solidification behavior at this interface, which can affect bond strength and the susceptibility to interfacial cracking under thermal or mechanical loading.

Study Insights and Implications

The research by Yu Shiqing and colleagues provides valuable quantitative data on the temperature-microstructure-property relationships for Stellite 6 and Stellite 21 overlays. The findings are directly applicable to the optimization of overlay welding procedures for nuclear pump components and other wear-critical applications. The emphasis on substrate temperature as a controllable process parameter is particularly significant, as it represents a relatively simple and cost-effective means of improving overlay quality.

For engineers involved in the specification and qualification of Stellite overlay processes, the key takeaway is that substrate temperature should be treated as a critical process variable, not merely as an incidental condition. Process qualification should include testing at the range of substrate temperatures expected in production, and the resulting microstructural and mechanical property data should be used to establish acceptance criteria and inspection requirements. The practical benefit of this research is a more reliable and consistent overlay quality, which translates directly to improved component life and reduced maintenance costs in nuclear and chemical processing applications.