Zhuojin Pipe Fitting Co., LtdHigh-Temperature Wear Resistance of Plasma Surfaced Stellite 6 Alloy
Literature Overview
This paper, published in Special Casting and Nonferrous Alloys (Volume 43, Issue 6, 2023, pages 815–820) by Guan Meng, Yong Xingping, Tian Yun, Zou Youxu, Lin Xuejian, and Huang Hongjun from Shenyang Blower Group Nuclear Pump Co., Ltd. and the School of Materials Science and Engineering at Shenyang University of Technology, investigates the microstructure and high-temperature wear resistance of Stellite 6 alloy deposited on 304H stainless steel using plasma arc surfacing. The study examines the effect of different temperatures on wear performance through systematic wear testing at 250, 350, 450, and 500 degrees Celsius.
Core Technical Findings
The plasma surfacing deposit exhibits good bonding with the substrate, with the Stellite 6 surfacing layer consisting primarily of columnar crystals and the fusion zone exhibiting a planar crystal region. The surfacing layer is composed mainly of gamma-cobalt solid solution with M7C3-type composite carbides formed by Cr, Ni, W, and C elements, along with WC. The hardness increases from the substrate through the fusion zone into the surfacing layer, stabilizing at values above 460 HV at a distance of 0.2 mm from the fusion line.
The following table summarizes the microstructural and mechanical characteristics:
| Zone | Microstructure | Hardness |
|---|---|---|
| Substrate (304H stainless steel) | Austenitic | Baseline (lower) |
| Fusion zone | Planar crystal region | Transition zone |
| Surfacing layer (0–0.2 mm from fusion line) | Transition microstructure | Increasing |
| Surfacing layer (>0.2 mm from fusion line) | Columnar crystals | Stable, >460 HV |
Wear Performance Analysis
The wear testing was conducted at elevated temperatures (250, 350, 450, and 500 degrees Celsius) against Stellite 21 counterfaces. The results show that both the friction coefficient and wear mass loss increase with increasing temperature. The friction surface roughness remains within the range of 3.9 to 8.6 micrometers across all test temperatures. The high-temperature wear mechanisms include abrasive wear, adhesive wear, and oxidative wear, with the wear severity increasing with temperature but without significant changes in the dominant wear mechanism.
The following table summarizes the wear testing conditions and observed trends:
| Test Temperature | Friction Coefficient Trend | Wear Mass Loss Trend | Surface Roughness |
|---|---|---|---|
| 250 °C | Baseline | Baseline | 3.9–8.6 μm |
| 350 °C | Increased | Increased | 3.9–8.6 μm |
| 450 °C | Further increased | Further increased | 3.9–8.6 μm |
| 500 °C | Highest | Highest | 3.9–8.6 μm |
Technical Interpretation
The columnar crystal structure of the Stellite 6 surfacing layer is characteristic of directional solidification under plasma arc conditions. The planar crystal region in the fusion zone indicates rapid solidification influenced by the thermal gradient between the hot surfacing layer and the cooler substrate. This microstructural gradient contributes to good metallurgical bonding between the deposit and the substrate, which is critical for the long-term durability of the surfacing layer under thermal and mechanical loading.
The M7C3-type composite carbides formed by Cr, Ni, W, and C elements are the primary wear-resistant phases in the Stellite 6 alloy. These carbides provide high hardness and good thermal stability, making them effective in resisting wear at elevated temperatures. The presence of WC further enhances the hardness and wear resistance of the deposit. The gamma-cobalt solid solution matrix provides toughness and thermal stability, creating a balanced microstructure for high-temperature service.
High-Temperature Wear Mechanism Discussion
At elevated temperatures, the wear behavior of Stellite 6 surfacing is governed by the interaction of multiple mechanisms. Abrasive wear occurs when hard particles or surface asperities on the counterface plow through the deposit surface, with the M7C3 carbides providing primary resistance. Adhesive wear occurs through micro-welding at asperity contacts, with material transfer between the Stellite 6 deposit and the Stellite 21 counterface. Oxidative wear becomes increasingly significant at higher temperatures as the oxide scale formation rate increases, potentially leading to accelerated material loss through oxide spalling.
The observation that the wear mechanism does not significantly change with temperature, but rather the severity increases, indicates that the Stellite 6 alloy maintains its fundamental wear resistance characteristics across the tested temperature range. This is an important finding for engineering applications where the operating temperature may vary during service.
Engineering Practice Implications
The plasma arc surfacing of Stellite 6 on 304H stainless steel is particularly relevant for nuclear pump applications, where components are subjected to elevated temperatures, corrosive coolant environments, and mechanical wear. The good bonding between the Stellite 6 deposit and the 304H substrate ensures the durability of the surfacing layer under thermal cycling conditions. The stable hardness above 460 HV provides reliable wear protection across the tested temperature range.
For quality control of plasma surfaced Stellite 6 deposits, the following inspections are recommended:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface quality | No cracks, porosity, or undercut |
| Hardness testing | Hardness verification | >460 HV at 0.2 mm from fusion line |
| Microstructural examination | Phase verification | Expected carbides present |
| Bond strength testing | Interface integrity | Adequate adhesion to substrate |
| High-temperature wear testing | Performance validation | Wear rate within specification |
Study Insights and Reflections
This research provides valuable data on the high-temperature wear performance of plasma surfaced Stellite 6 alloy, which is directly applicable to nuclear pump component design and maintenance. The finding that wear severity increases with temperature without a change in dominant mechanism suggests that Stellite 6 surfacing can be reliably applied across a range of operating temperatures, with appropriate wear rate expectations set for each temperature level.
The columnar crystal microstructure of the surfacing layer and the planar crystal fusion zone are characteristic of the rapid solidification conditions in plasma arc surfacing. This microstructure provides good mechanical properties but may be susceptible to cracking under thermal cycling if the thermal expansion mismatch between the deposit and substrate is significant. Future work should investigate the fatigue and thermal cycling performance of the surfacing layer, as well as the long-term stability of the carbide phases under sustained high-temperature exposure. The findings contribute to the development of reliable surfacing solutions for demanding nuclear and power generation applications where high-temperature wear resistance is critical.