Zhuojin Pipe Fitting Co., LtdInfluence of Plasma Arc Overlay Parameters and Tungsten Carbide Content on Nickel-Based WC Composite Coating Performance
Literature Overview
Published in Mining and Metallurgical Engineering in 2020, this study by Wang Junjie and colleagues from Xi'an University of Architecture and Technology examines the effect of welding current and tungsten carbide (WC) content on the microstructure and tribological performance of Ni35-based WC composite coatings produced by plasma arc overlay welding on Q235A low-carbon steel. Funded by the National Natural Science Foundation of China, this work addresses a fundamental question in hardfacing engineering: how to balance coating hardness, wear resistance, and microstructural integrity by manipulating process parameters and filler composition.
Core Technical Findings
The study systematically varied two key factors: plasma arc welding current (195 A and 210 A) and WC content in the Ni35 composite powder. The results reveal a nuanced interaction between these parameters that defies simple linear expectations.
| Parameter Combination | Coating Hardness | Wear Volume | Key Microstructural Feature |
|---|---|---|---|
| 195 A, Low WC | Moderate | Moderate | Significant WC precipitation |
| 195 A, High WC | Highest | Lowest | Uniform WC distribution |
| 210 A, Low WC | Lower | Higher | Severe WC melting and precipitation |
| 210 A, High WC | High but lower than 195 A | Higher than 195 A | Partial WC dissolution |
The most critical finding is the non-monotonic relationship between WC content and wear performance, which depends on the welding current level. At the lower current (195 A), increasing WC content improves wear resistance because more intact WC particles are retained in the coating. At the higher current (210 A), increasing WC content paradoxically increases wear volume, likely because the excess heat causes partial melting and redistribution of WC particles, leading to a coarser and more brittle second-phase distribution.
Phase Composition and Strengthening Mechanisms
The coating microstructure consists of a γ-Ni(Cr,Fe) solid solution matrix with dispersed second phases including WC, Cr3C2, Cr7C3, FeNi3, and W2C. The strengthening mechanisms operative in these coatings include:
- Solid solution strengthening: Cr and Fe atoms dissolved in the Ni matrix increase the lattice strain and dislocation mobility resistance.
- Dispersion strengthening: Fine WC and carbide particles impede dislocation motion through the Orowan mechanism.
- Precipitation hardening: Cr-rich carbides (Cr3C2, Cr7C3) form during cooling and contribute additional strengthening.
The welding current directly controls the thermal input and, consequently, the extent of WC particle melting. At 210 A, the higher heat input causes significant WC dissolution, leading to precipitation of decomposition products (such as W2C and FeNi3) upon cooling. This redistribution reduces the number density of intact, fine WC particles that are most effective for wear resistance. At 195 A, the reduced heat input preserves more WC particles in their original morphology and size, maintaining a finer and more uniform dispersion.
Process Optimization Guidelines
Based on the study's findings, the following process guidelines can be extracted for practical application:
- Minimize heat input: Use the lowest practical welding current to preserve WC particle integrity. For Ni35-WC coatings on Q235A substrates, currents below 200 A appear to be optimal.
- Match WC content to heat input: At low currents, higher WC content is beneficial; at high currents, the benefit of additional WC is negated by particle degradation.
- Monitor dilution: The Q235A substrate contributes Fe to the dilution zone, which can form FeNi3 intermetallics. Excessive dilution should be controlled through proper preheating, travel speed, and filler wire geometry.
- Consider multi-pass strategies: Thin, multiple passes with reduced heat input per pass can achieve better WC retention than a single thick pass.
Engineering Practice Insights
In mining and material handling applications—where Ni35-WC coatings are commonly used for wear protection on shovel buckets, crusher components, and conveyor parts—the findings of this study have direct economic implications. A coating that is 30-50% harder but more brittle due to excessive WC content and high heat input may fail prematurely through spalling or delamination, negating the hardness advantage. The study's emphasis on the interaction between process parameters and composition is a reminder that hardfacing is not a simple "more WC is better" proposition. Engineers must balance hardness, toughness, and adhesion to achieve the desired service life.
The practical takeaway is that process parameter optimization should always be performed in conjunction with composition selection. A coating designed for maximum hardness at the expense of wear resistance is not a sound engineering solution. The optimal window lies where the WC particles remain intact and uniformly distributed, which requires careful control of thermal input.