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

Literature Overview

The paper authored by Yu Shiqing and colleagues from Shenyang University of Technology investigates the microstructural evolution and mechanical/corrosion performance of Stellite 6 and Stellite 21 cobalt-based alloy overlay layers deposited via plasma arc cladding on 304H stainless steel substrates. The study examines the effects of post-deposition thermal exposure at 550–650 °C on the overlay layer microstructure, hardness, and corrosion resistance. This work is particularly relevant to nuclear power pump components and high-temperature service applications where overlay layers must maintain integrity under sustained thermal cycling.

Core Technical Findings

Microstructural Characteristics

The overlay layers of both alloys exhibit a characteristic three-zone microstructure: planar crystals at the fusion boundary, columnar dendrites in the middle region, and equiaxed grains at the top surface. This layered morphology is consistent with the directional solidification behavior expected in plasma arc cladding, where the thermal gradient is highest near the substrate interface.

With increasing exposure temperature from 550 °C to 650 °C, the dendrite arm spacing (DAS) of both alloys shows a slight increase, indicating limited grain coarsening under these moderate thermal conditions. Importantly, no significant elemental diffusion was observed at the fusion interface, suggesting good metallurgical stability of the coating-substrate bond under the tested thermal regime.

Mechanical Properties

The as-deposited hardness of Stellite 6 (463 HV 0.3) is significantly higher than Stellite 21 (365 HV 0.3), reflecting the higher carbon and chromium content in Stellite 6 which promotes carbide precipitation. The slight hardness reduction after thermal exposure is attributed to carbide coarsening and matrix softening, but both alloys retain substantial hardness levels suitable for wear-resistant applications.

Corrosion Performance

The corrosion current density of Stellite 6 (1.312 μA/cm²) is notably lower than that of Stellite 21 (10.848 μA/cm²) in the as-deposited condition, indicating superior corrosion resistance. After thermal exposure at 650 °C, both alloys show a marginal increase in corrosion current density, but remain within acceptable limits for most industrial environments.

Engineering Practice Implications

For nuclear pump components operating in the 550–650 °C range, both Stellite 6 and Stellite 21 overlays demonstrate adequate stability. However, the selection between the two should be guided by the dominant failure mode: Stellite 6 is preferred where wear resistance and corrosion resistance are both critical, while Stellite 21 offers better thermal fatigue resistance due to its lower carbon content and reduced carbide volume fraction.

Key Reflections

This study confirms that plasma arc cladding of cobalt-based alloys on austenitic stainless steel substrates produces sound metallurgical bonds with limited interfacial diffusion even at elevated service temperatures. The absence of significant elemental interdiffusion at 650 °C is encouraging for long-term service life predictions. However, further investigation into cyclic thermal loading (rather than static exposure) and longer-duration exposure tests would be beneficial for real-world service qualification.

Concluding Remarks

The findings demonstrate that both Stellite 6 and Stellite 21 overlay layers maintain their structural integrity and functional properties within the 550–650 °C thermal window, with Stellite 6 offering superior hardness and corrosion resistance at the expense of somewhat lower thermal fatigue tolerance. For nuclear pump applications, the combination of plasma arc cladding with appropriate post-deposition heat treatment parameters can yield reliable overlay coatings that extend component service life significantly.