CLADDING TECHNOLOGY SHANXI CO., LTDHigh Temperature Friction and Wear Properties of Plasma Cladded Stellite Alloys
Literature Overview
This study investigates the tribological behavior of Stellite alloys (primarily Stellite 6 and Stellite 21) deposited via plasma transferred arc (PTA) cladding under elevated temperature conditions. The research examines how the cladding process parameters, microstructural evolution, and high-temperature oxidation mechanisms collectively govern the friction coefficient and wear rate of the overlay layers. The work is particularly relevant for engineers designing cladding solutions for high-temperature industrial components such as hot gas path parts, valve seats, and wear plates operating above 600 degrees Celsius.
Core Technical Findings
The study demonstrates that PTA cladding produces a refined microstructure with directional columnar dendrites growing from the substrate interface, which differs significantly from the equiaxed structure obtained through conventional arc welding. The key finding is that at temperatures below 500 degrees Celsius, Stellite 21 exhibits superior wear resistance compared to Stellite 6 due to its higher cobalt content and fine carbide distribution. However, beyond 600 degrees Celsius, the advantage reverses as Stellite 6 benefits from its higher chromium content providing better oxidation resistance.
| Parameter | Stellite 6 (PTA) | Stellite 21 (PTA) | Baseline (Hardfacing) |
|---|---|---|---|
| Cr content (wt%) | 20-25 | 12-17 | 20-25 |
| Co content (wt%) | 35-45 | 45-55 | 35-45 |
| Hardness (HV) | 380-420 | 360-400 | 350-400 |
| Wear rate at 25C (mg/Nm) | 0.8-1.2 | 0.5-0.9 | 1.0-1.5 |
| Wear rate at 600C (mg/Nm) | 1.5-2.0 | 2.2-3.0 | 2.5-3.5 |
| Friction coefficient at 25C | 0.35-0.40 | 0.30-0.35 | 0.38-0.42 |
| Friction coefficient at 600C | 0.28-0.33 | 0.32-0.38 | 0.35-0.40 |
The critical insight is that the PTA process achieves a lower dilution ratio (typically 10-15%) compared to conventional surfacing methods (25-40%), preserving the alloy composition more faithfully. This lower dilution translates directly into better retention of the designed wear resistance properties at service temperature.
Microstructural Analysis and Wear Mechanisms
The PTA-deposited Stellite layers exhibit a three-zone microstructure: a heat-affected zone (HAZ) at the substrate interface, a transition zone with mixed columnar and equiaxed grains, and a free-growth zone with fully columnar dendrites. The intermetallic phase M6C carbides (Cr,Fe)6C form preferentially along dendrite boundaries, providing the primary wear resistance mechanism through their high hardness (approximately 1400 HV) and thermal stability.
At ambient temperature, the dominant wear mechanism is abrasive wear where hard carbide particles resist material removal through ploughing resistance. As temperature increases above 400 degrees Celsius, adhesive wear begins to contribute significantly. The formation of a protective chromium oxide scale (Cr2O3) becomes the critical factor determining long-term performance. Stellite 6, with its higher chromium content, forms a more stable and continuous oxide layer at elevated temperatures, which explains its superior performance above 600 degrees Celsius despite having fewer carbides.
Engineering Practice Implications
For engineers selecting cladding alloys for high-temperature applications, the following decision framework emerges from this literature:
- For temperatures below 500 degrees Celsius, Stellite 21 offers better wear resistance with comparable cost efficiency.
- For temperatures above 600 degrees Celsius, Stellite 6 provides superior oxidation and wear resistance due to its protective oxide scale.
- PTA cladding should be preferred over conventional arc surfacing when dilution control is critical, particularly for thin overlay requirements (1-3 mm).
- The number of PTA passes significantly affects microstructure refinement; a minimum of 4-6 passes is recommended for optimal wear properties.
The study also highlights that preheating temperature during PTA cladding (recommended 150-250 degrees Celsius for carbon steel substrates) influences the HAZ microstructure and residual stress distribution, which directly impacts the long-term fatigue and wear performance of the cladded component.
Key Reflections and Study Insights
The most valuable takeaway from this research is the temperature-dependent reversal of wear performance between Stellite grades. Traditional alloy selection based solely on room-temperature hardness is inadequate for high-temperature applications. Engineers must consider the entire service temperature spectrum and the corresponding dominant wear mechanisms. Additionally, the process advantage of PTA in dilution control represents a significant quality improvement over conventional methods, justifying the higher equipment investment for critical applications. The interplay between microstructure, oxidation behavior, and wear mechanism at elevated temperatures remains an area requiring further investigation, particularly for multi-cycle thermal loading conditions common in industrial service.