Composition Optimization and Wear Resistance of Plasma Surfacing Coatings

Literature Overview

Published in Materials in Mechanical Engineering (2006, Vol. 30, No. 3, pp. 35–37), this study addresses the optimization of plasma surfacing coating composition for enhanced wear resistance. The research team from the Welding Engineering Research Institute at Shanghai Jiao Tong University and the School of Materials Science and Engineering at Tianjin University developed a new alloy powder formulation based on Ni60 substrate alloy with additions of chromium, manganese, and tungsten powders. The study employed a three-factor, three-level orthogonal experimental design (L9) with hardness as the primary response variable, followed by wear testing to validate the optimized composition. The plasma surfacing trials were conducted on Q235 steel plates.

Experimental Design Methodology

The orthogonal experimental design approach represents a systematic and statistically rigorous methodology for multi-parameter optimization with minimal experimental runs:

The L9 orthogonal array requires only 9 experimental runs to evaluate three factors at three levels, compared to 27 runs for a full factorial design. This efficiency is particularly valuable when each experimental run requires powder preparation, plasma surfacing, and comprehensive characterization.

The response variable (hardness) was selected as the primary optimization criterion because hardness correlates strongly with wear resistance in abrasive wear mechanisms. This assumption is validated by the subsequent wear testing, which confirms that the optimized composition also provides superior wear performance.

Optimized Composition and Results

The orthogonal analysis identified the optimal powder formulation as:

• ω(Cr) = 10 wt%
• ω(Mn) = 4 wt%
• ω(W) = 7 wt%
• Balance: Ni60 base alloy powder

The wear testing results confirmed that the wear resistance of the Q235 steel surface improved significantly, and that wear resistance increased with increasing surfacing layer hardness. This validates the hardness-as-proxy approach used in the orthogonal optimization.

Ni60 base alloy characteristics: The Ni60 (ASTM B337) base alloy is a well-established hardfacing alloy containing approximately 57–63% Ni, 56–65% Fe, 2.5–3.5% C, 1.5–2.5% Si, and 0.5–1.5% B. Its microstructure consists of austenite matrix with M7C3 carbides and a boride network. The austenitic matrix provides excellent toughness and thermal shock resistance, while the carbides provide wear resistance.

Effect of Cr addition: Chromium enhances wear resistance through multiple mechanisms:

• Formation of additional carbides (Cr7C3, Cr3C)
• Solid solution strengthening of the austenitic matrix
• Improved oxidation resistance at elevated temperatures
• Enhanced thermal hardness retention

Effect of Mn addition: Manganese serves as an austenite stabilizer, maintaining a higher fraction of retained austenite in the as-deposited microstructure. This austenite transforms to martensite during deformation (strain-induced transformation), providing work-hardening capacity during wear.

Effect of W addition: Tungsten contributes to wear resistance through:

• Formation of hard carbides (WC, W2C)
• Solid solution strengthening
• Refinement of the microstructure
• Enhancement of thermal stability

Microstructural Analysis

The plasma surfacing process produces a microstructure distinct from both the base metal and the powder composition due to the rapid solidification rates (typically 100–1000 K/s at the solidification front):

The plasma surfacing process, operating at arc temperatures of 10,000–30,000 K with deposition rates of 1–5 kg/h, produces deposits with fine grain structures and minimal dilution (typically 5–15% depending on process parameters). The low dilution preserves the intended alloy composition in the deposited layer, which is critical for achieving the designed microstructure and properties.

Engineering Practice Applications

For pipeline and piping applications, plasma surfacing with optimized Ni-Cr-Mn-W compositions offers specific advantages:

1. Valve seat hardfacing: The Ni60-based alloy with Cr and W additions provides excellent resistance to erosion-corrosion in valve seats exposed to flowing fluids containing solid particles.
2. Pump impeller repair: The combination of Ni-based toughness and carbide hardness makes this formulation suitable for pump impellers operating in abrasive slurries.
3. Flange face hardening: For flanges subject to galling and scoring during assembly, the optimized coating provides surface hardness with adequate toughness to resist cracking during bolt loading.
4. Pig launcher/receiver hardfacing: The wear-resistant coating extends service life of pig launcher and receiver components subject to repeated pig passage.

However, practical implementation requires attention to:

• Plasma torch calibration: Arc power, gas flow rates, and powder feed rate must be precisely controlled to maintain consistent deposit properties.
• Base metal preparation: Surface cleanliness and geometry directly affect powder adhesion and deposit quality.
• Interpass temperature control: Excessive interpass temperatures may cause carbide coarsening and reduced hardness in subsequent passes.
• Post-deposition treatment: Stress relief at 400–500°C may be required for thick deposits to prevent cracking during subsequent machining or service.

Key Questions and Methodological Reflections

The orthogonal experimental design approach, while efficient, has limitations that engineers should recognize:

1. Interaction effects: The L9 orthogonal array does not fully capture two-factor and three-factor interactions. In reality, the effects of Cr, Mn, and W are not independent—Cr and W compete for carbon to form different carbides, and Mn affects the austenite stability which in turn influences carbide precipitation behavior. A full factorial design or response surface methodology (RSM) would provide more complete optimization information.
2. Hardness as sole response: While hardness correlates with wear resistance, it does not capture all aspects of wear behavior. Abrasive wear, adhesive wear, erosive wear, and tribochemical wear each have different mechanisms and may respond differently to composition changes. The study's wear testing provides validation but does not explore wear mechanism transitions.
3. Deposition quality variability: Plasma surfacing deposits can exhibit porosity, lack of fusion, and composition segregation that affect final properties. The study likely assumes ideal deposition conditions, but practical deposits will have quality variations that reduce the achievable property envelope.
4. Thermal cycling behavior: The hardness and wear resistance measured at ambient temperature may not represent performance at elevated service temperatures. The thermal hardness retention of Ni-Cr-Mn-W alloys is generally good, but quantitative assessment at service temperatures would be valuable.
5. Long-term stability: The carbide phases in the deposit may coarsen during prolonged exposure to elevated temperatures, reducing hardness and wear resistance over time. This is particularly relevant for applications involving sustained thermal cycling.

Study Insights and Reference Value

This research demonstrates the effectiveness of orthogonal experimental design for rapid optimization of plasma surfacing alloy compositions. The key insight is that strategic addition of Cr (10%), Mn (4%), and W (7%) to a Ni60 base alloy powder produces a composition with superior hardness and wear resistance compared to the base Ni60 alloy alone. The approach is particularly valuable for engineers who need to develop custom hardfacing alloys for specific service conditions but lack the resources for extensive full-factorial experimentation.

For the pipeline industry, this work provides a practical framework for developing specialized hardfacing coatings tailored to specific wear environments. The Ni60-based formulation offers particular advantages in corrosive-abrasive environments where both chemical and mechanical degradation mechanisms are active. The austenitic matrix provides corrosion resistance, while the optimized carbide population provides mechanical wear resistance—a combination that is difficult to achieve with purely iron-based or cobalt-based hardfacing alloys. Engineers should note that the optimized composition represents a starting point for further refinement based on specific service requirements, and that practical implementation requires process qualification and performance validation under actual operating conditions.