Solution Aging Effects on WCp-Reinforced 18Ni300 Martensitic Age-Hardening Steel Composite Coating by Plasma Surfacing

Literature Overview

This study, published in the Welding Journal (2020, Vol. 41, No. 11, pp. 62-68) by Hu Yongjun, Luo Junwei, Yi Jianglong, Yi Yaoyong, and Niu Ben from Guangdong University of Technology and the Guangdong Institute of Welding Technology, investigates the microstructural evolution and tribological performance of WC-reinforced 18Ni300 (MS300) martensitic age-hardening steel composite coatings produced via plasma arc surfacing on Cr5 steel substrates. The research examines two WC particle mass fractions (25 wt% and 35 wt%) and evaluates the effect of solution treatment (900 °C × 1 h) followed by aging (490 °C × 5 h) on the coating microstructure, phase transformation, microhardness, and wear resistance.

Core Technical Findings

The key finding of this work is that the addition of WC particles to the MS300 powder fundamentally alters the solidification and phase transformation behavior of the cladding layer. In the as-deposited state, the WC/MS300 composite cladding is predominantly austenitic rather than martensitic, which contrasts sharply with the unalloyed MS300 baseline. This observation is significant because it implies that the carbon and alloying elements released from the decomposition of WC at the melting pool temperature modify the austenite stability and suppress the martensitic transformation during rapid solidification.

After solution treatment and aging, the baseline MS300 specimen experiences a decrease in hardness and wear resistance, which is consistent with over-aging of the precipitation-hardened martensite. In contrast, the WC/MS300 specimens undergo a γ→α-Fe transformation, resulting in substantial improvements in hardness and wear resistance. The 35 wt% WC specimen demonstrates the best wear performance among all tested conditions.

A particularly important microstructural observation is the formation of a thick diffusion layer around WC particles after solution aging. This diffusion layer significantly improves the interface bonding between the hard WC reinforcement and the matrix, which is critical for preventing particle pull-out during wear.

Process and Metallurgical Analysis

The plasma surfacing process used in this study is a powder-fed variant where the MS300 powder blended with spherical WC particles is delivered into the plasma arc. The melting pool temperature must be high enough to partially dissolve or at least heat the WC particles to promote interfacial wetting, yet controlled to avoid complete WC decomposition. The melting point of WC is approximately 2,870 °C, far exceeding the plasma arc temperature achievable in industrial surfacing (typically 10,000-15,000 °C at the arc core but much lower at the powder interaction zone). Therefore, WC particles in the cladding layer typically survive in a semi-molten or solid state, with limited dissolution contributing carbon and tungsten to the melt.

The solution treatment at 900 °C serves to homogenize the microstructure and dissolve any fine precipitates formed during rapid solidification. The subsequent aging at 490 °C promotes precipitation of Ni₃Mo-type strengthening phases in the MS300 matrix. However, the presence of WC particles introduces a competing mechanism: carbon released from the WC diffusion layer stabilizes austenite locally, but during the high-temperature solution step, this austenite is eventually transformed to martensite upon cooling, explaining the γ→α-Fe transformation observed after heat treatment.

Engineering Practice Implications

For engineers designing wear-resistant cladding systems, this study provides several actionable insights. First, the blending of WC with martensitic age-hardening steel powders creates a synergistic effect that neither material can achieve alone. Second, post-weld heat treatment is not merely optional but essential for unlocking the full performance potential of the composite coating. Third, the 35 wt% WC addition represents an optimal balance between reinforcement density and matrix continuity; further increases may compromise ductility and adhesion.

From a quality control perspective, verifying the presence and thickness of the diffusion layer around WC particles via metallographic examination or SEM-EDS mapping should be incorporated into the acceptance criteria for such coatings. The diffusion layer thickness serves as a direct indicator of interface bonding quality and, by extension, the long-term durability of the cladding under abrasive or adhesive wear conditions.

Study Insights and Reflections

One of the most thought-provoking aspects of this research is the counterintuitive result that the baseline MS300 coating loses hardness after solution aging while the WC-reinforced variant gains hardness. This underscores the principle that the interaction between reinforcement particles and matrix alloy is not simply additive but can be fundamentally transformative. The diffusion layer mechanism provides a clear metallurgical explanation: carbon and tungsten atoms migrate from the WC particle into the surrounding austenite during the 900 °C solution step, enriching the local matrix composition and shifting the phase transformation behavior during subsequent cooling.

This work also highlights the importance of particle morphology. The use of spherical WC particles, as opposed to irregular or angular particles, likely promotes more uniform distribution and better wetting during powder feeding. In practical plasma surfacing operations, the feed rate, torch travel speed, and arc current must be carefully calibrated to ensure that the WC particles are fully melted or at least sufficiently heated to achieve metallurgical bonding with the matrix, while avoiding excessive dilution of the substrate.

In summary, this study demonstrates that WC-reinforced MS300 composite coatings produced by plasma surfacing, when properly heat treated, offer a compelling solution for applications requiring combined hardness, wear resistance, and toughness. The diffusion layer mechanism at the WC-matrix interface is the key to achieving superior performance, and the 35 wt% WC addition represents an engineering optimum for the tested conditions.