Plasma Cladding of Stellite Alloys High Temperature Tribological Properties

Literature Overview

The study by Qu Shengguan, Xiong Zhihua, Lai Fuqiang, Wang Guanghong, Li Xiaoqiang, Deng Jishao, and Li Zhiyan, published in the Tribology Transactions (Chinese and English editions) in 2016, investigates the high-temperature friction and wear characteristics of Stellite alloy coatings produced by plasma transferred arc (PTA) cladding. The research is jointly conducted by the School of Mechanical and Automotive Engineering at South China University of Technology and Huaiji Dengyun Auto Parts Co., Ltd., indicating a strong industry-academia collaboration aimed at solving real-world tribological problems in automotive and industrial applications.

Core Technical Points

Stellite alloys, particularly Stellite 6 (Co-Cr-W type) and Stellite 21 (Co-Cr-Mo type), are renowned for their exceptional hot hardness, wear resistance, and resistance to thermal fatigue. The fundamental mechanism behind their high-temperature wear resistance lies in the formation of hard carbide precipitates (Cr23C6, WC, Mo2C) dispersed within a face-centered cubic (FCC) cobalt-based solid solution matrix. When applied via PTA cladding, the rapid solidification rates inherent to the process produce a refined microstructure with finer carbide distribution compared to cast or wrought counterparts.

The study examines the tribological performance of these PTA-cladded Stellite surfaces under elevated temperature conditions, which is critical for applications such as exhaust valve seats, turbocharger components, hot forging dies, and high-temperature sliding bearings. The key findings typically revolve around the transition in wear mechanisms from adhesive wear at room temperature to oxidative wear at intermediate temperatures and potentially erosive-abrasive wear at higher temperatures.

Process Parameters and Microstructural Characteristics

Parameter	Typical Range	Effect on Microstructure
Plasma arc current	150–300 A	Higher current produces deeper dilution
Powder feed rate	0.2–0.8 kg/h	Controls layer thickness and dilution
Travel speed	100–500 mm/min	Affects cooling rate and grain size
Shielding gas flow	8–15 L/min	Prevents oxidation of molten pool
Layer thickness	0.5–3.0 mm	Multiple passes for thick coatings
Dilution rate	5–15%	Lower dilution preserves alloy chemistry

The PTA process offers several advantages over other cladding methods for Stellite alloys. The focused plasma arc provides a deep, narrow melt pool with high energy density, resulting in excellent metallurgical bonding with the substrate. The dilution rate can be controlled to remain below 15% with proper parameter selection, which is essential for maintaining the hard phase content and overall wear resistance of the coating.

High Temperature Wear Mechanism Analysis

At elevated temperatures (typically 400–800 °C), the wear behavior of PTA-cladded Stellite coatings undergoes significant transitions. At temperatures below 500 °C, the dominant wear mechanism is predominantly adhesive, where material transfer occurs at asperity contacts. As temperature increases beyond 500 °C, oxidative wear becomes increasingly significant, and a protective oxide scale (primarily Cr2O3 and Fe2O3) forms on the wear track surface. This oxide layer, when adherent and compact, actually reduces the wear rate by acting as a sacrificial layer.

The key insight from this type of research is that the high-temperature wear resistance of Stellite coatings is not solely dependent on the hardness of the matrix and carbides, but also on the ability of the alloy to form protective oxide films. Chromium content above 20 wt% is generally considered necessary for adequate oxidation resistance. The presence of tungsten and molybdenum carbides provides additional resistance to abrasive wear at elevated temperatures where the matrix softens.

Defect Analysis and Countermeasures

Defect Type	Cause	Countermeasure
Cracking in coating	High residual stress, excessive dilution	Reduce current, add intermediate layer, post-weld stress relief
Poor bonding	Contamination, inadequate cleaning	Strict surface preparation, flux removal
Porosity	Inadequate shielding, high travel speed	Increase shielding gas, reduce speed
Excessive dilution	High heat input, thin first layer	Use low current first pass, multiple thin layers
Carbide coarsening	Excessive thermal cycling	Limit number of passes, control interpass temperature

Engineering Practice Integration

In practical applications, the selection between Stellite 6 and Stellite 21 depends on the specific service environment. Stellite 6, containing tungsten carbides, offers superior resistance to abrasive and erosive wear at temperatures up to approximately 1000 °C. Stellite 21, with molybdenum carbides, provides better resistance to cavitation erosion and thermal fatigue. For automotive applications such as exhaust valve guides and seat components, Stellite 6 is commonly selected due to its proven track record in high-temperature sliding contact environments.

The industry-academia partnership evident in this study is particularly noteworthy. Huaiji Dengyun Auto Parts Co., Ltd., as a major automotive component manufacturer, provides the practical application context and failure analysis data that drives the research agenda. This collaborative model ensures that laboratory findings translate directly into manufacturing improvements.

Study Insights and Implications

This research reinforces the principle that tribological performance at elevated temperatures is a multi-mechanism phenomenon that cannot be predicted by hardness alone. Engineers designing high-temperature cladding solutions must consider the synergistic effects of matrix strength, hard phase morphology, and oxidation resistance. The PTA process, with its ability to produce coatings with controlled dilution and refined microstructure, represents an optimal manufacturing route for Stellite overlay applications where high-temperature wear resistance is the primary design requirement. The study also highlights the importance of systematic parameter optimization, as small variations in plasma current or travel speed can significantly alter the carbide distribution and, consequently, the wear performance.