Effect of Temperature on Microstructure and Properties of Stellite 6 and Stellite 21 Hardfacing Layers on 304H Stainless Steel

Literature Overview

This study, published in Hot Working Technology (2024, Vol. 53, No. 7, pp. 61-66) by Yu Shiqing et al. from Shenyang University of Technology and Shenyang Blower Works Nuclear Pump Co., Ltd., investigates the thermal stability of cobalt-based hardfacing alloys applied to 304H stainless steel substrates via plasma hardfacing. The research was supported by the Liaoning Provincial Department of Education General Program (LJKZ0122). The work addresses a critical engineering concern: how do hardfacing deposits behave under sustained elevated temperatures in the range of 550 to 650 degrees Celsius, which is relevant for nuclear pump components and high-temperature industrial applications.

Core Technical Findings

The authors applied Stellite 6 and Stellite 21 cobalt-based alloys onto 304H stainless steel using plasma hardfacing technology and then subjected the specimens to thermal exposure at 550, 600, and 650 degrees Celsius. The microstructural characterization was conducted using optical microscopy (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), while mechanical and corrosion properties were evaluated through micro-hardness testing and electrochemical workstation measurements.

Microstructural Evolution

The hardfacing layers of both alloys exhibit a typical three-zone columnar-to-equiaxed transition morphology: planar crystals at the fusion boundary, columnar dendrites in the mid-section, and equiaxed crystals at the top surface. With increasing exposure temperature, the dendrite arm spacing shows a slight increase, indicating mild coarsening of the microstructure. Importantly, no significant elemental diffusion was observed at the fusion interface, demonstrating good metallurgical stability between the cobalt-based deposit and the 304H substrate. This finding is particularly valuable for engineers designing components that must withstand long-term thermal cycling without interfacial degradation.

Mechanical Properties

The as-welded micro-hardness values are 463 HV0.3 for Stellite 6 and 365 HV0.3 for Stellite 21. After thermal exposure, both alloys show a slight decrease in hardness, with Stellite 6 retaining its superior hardness advantage throughout the temperature range. The modest hardness reduction is attributed to the dissolution of fine carbide precipitates and slight coarsening of the gamma prime phase, which is characteristic of cobalt-chromium-tungsten alloys under prolonged thermal exposure.

Corrosion Performance

Electrochemical measurements reveal that the as-welded corrosion current densities are 1.312 microampere per square centimeter for Stellite 6 and 10.848 microampere per square centimeter for Stellite 21. After thermal exposure, both alloys show a slight increase in corrosion current density, indicating marginally accelerated corrosion kinetics. Stellite 6 consistently demonstrates superior corrosion resistance, with approximately one order of magnitude lower corrosion current density compared to Stellite 21.

Engineering Practice Integration

For nuclear pump applications, where components operate in the 550 to 650 degree Celsius range, the findings confirm that both Stellite 6 and Stellite 21 hardfacing deposits maintain acceptable hardness and corrosion resistance. However, Stellite 6 is the preferred choice when both wear resistance and corrosion resistance are required simultaneously, as evidenced by its significantly lower corrosion current density and higher hardness. The absence of interfacial elemental diffusion is a critical finding that supports the long-term reliability of plasma hardfaced nuclear pump components.

From a process control perspective, the plasma hardfacing parameters must be optimized to minimize dilution from the 304H substrate, which could compromise the cobalt-based alloy's performance. The columnar-to-equiaxed transition observed in the deposit structure suggests that the thermal gradient during solidification was sufficiently high to promote directional solidification, which is typical for plasma hardfacing with moderate heat input.

Key Questions and Reflections

A notable gap in this study is the absence of wear testing data, which would be essential for pump impeller applications where erosion-corrosion is a dominant degradation mechanism. Additionally, the study does not address cyclic thermal loading, which is more representative of actual service conditions in nuclear pumps that experience startup and shutdown transients. Future work should incorporate thermal cycling fatigue evaluation and erosion-corrosion testing to provide a more complete picture of the hardfacing performance under realistic operating conditions.

The choice of 304H stainless steel as the substrate is well-suited for nuclear applications due to its elevated carbon content, which improves high-temperature creep strength compared to standard 304 stainless steel. The metallurgical compatibility between 304H and cobalt-based alloys is an important consideration, and the observed lack of interfacial degradation is reassuring for component design engineers.

Study Insights and Implications

This study provides valuable data for the selection of hardfacing alloys for nuclear pump components operating in the 550 to 650 degree Celsius range. The key takeaway is that Stellite 6 offers superior performance in terms of both hardness and corrosion resistance, making it the recommended choice for applications where combined wear and corrosion resistance is required. The microstructural stability observed under thermal exposure supports the use of plasma hardfacing as a reliable repair and enhancement technique for nuclear-grade stainless steel components. Engineers should note that while the hardness reduction is modest, long-term service beyond the tested temperature range or exposure duration should be validated through additional testing before specification approval.