Effect of Solution Aging on Microstructure and Properties of Plasma-Surfaced WCp/18Ni300 Steel Composite Coating

Literature Overview

This paper by Hu Yongjun and colleagues, published in the Transactions of the China Welding Institute in 2020 (Vol. 41, No. 11, pp. 62–68), investigates the influence of solution aging heat treatment on the microstructure, phase transformation, microhardness, and tribological properties of plasma-surfaced WC-reinforced 18Ni300 (MS300) martensitic aging steel composite coatings deposited on Cr5 steel substrates. The study compares two WC particle additions at mass fractions of 25% and 35% and examines the effect of a solution treatment at 900 °C for 1 hour followed by aging at 490 °C for 5 hours. The work originates from the Guangdong University of Technology and the Guangdong Provincial Institute of Welding Technology, supported by provincial innovation-driven development funds.

Core Technical Findings

The plasma surfacing process was employed to deposit the composite coating, which is notable for its high deposition efficiency and low dilution ratio compared to conventional arc surfacing methods. The base MS300 martensitic aging steel is a high-strength material whose strength derives primarily from age-hardening precipitates formed during the solution-aging cycle. The introduction of spherical WC particles into the MS300 powder fundamentally alters the solidification and phase transformation behavior of the surfacing layer.

The key finding is that the WC/MS300 composite surfacing layer exhibits a predominantly austenitic microstructure immediately after deposition, which is markedly different from the martensitic structure expected in a pure MS300 deposit. This austenitic retention is attributed to the carbon activity modification and solidification rate effects introduced by the WC particles. Upon solution aging treatment, the base MS300 specimen (without WC) experiences a decrease in both hardness and wear resistance, which is counterintuitive and suggests that the solution treatment partially dissolves the age-hardening precipitates without sufficient re-precipitation during aging.

However, in the WC/MS300 specimens, the solution aging treatment triggers a γ-to-α-Fe transformation in the matrix, leading to a significant improvement in hardness and wear resistance. The specimen with 35% WC addition achieves the best wear performance. The microstructural evolution of WC particles is particularly instructive: after solution aging, a thick diffusion layer forms around the WC particles, which substantially improves the interface bonding between the hard particles and the matrix. This diffusion layer formation is a critical mechanism for enhancing coating integrity and wear resistance.

Phase Transformation and Interface Chemistry Analysis

The phase transformation behavior in these composite coatings is governed by a complex interplay between the carbon content in the matrix, the presence of WC particles, and the thermal history. In the as-deposited condition, the rapid solidification rate inherent to plasma surfacing promotes austenite retention, particularly in the presence of WC particles which act as carbon reservoirs and modify the local cooling rate. The solution treatment at 900 °C dissolves carbides and homogenizes the composition, while the subsequent aging at 490 °C for 5 hours promotes precipitation of fine carbides and intermetallic compounds.

The formation of the diffusion layer around WC particles is a metallurgically significant phenomenon. During the solution treatment, chromium and iron atoms diffuse from the matrix into the WC particles, while carbon may migrate from the WC toward the matrix. This creates a gradient zone at the particle-matrix interface that acts as a mechanical key, reducing the tendency for particle pull-out during wear. The thickness of this diffusion layer increases with WC content, which explains why the 35% WC specimen exhibits superior wear resistance.

From an engineering perspective, this study demonstrates that the combination of plasma surfacing with post-deposition solution aging can produce coatings with properties that are not achievable through either process alone. The solution aging step is not merely a standard post-weld treatment but is integral to the design of the composite coating system. Engineers selecting WC-reinforced coatings for high-wear applications should consider the synergistic effect of particle addition and heat treatment rather than treating them as independent variables.

Engineering Practice Implications and Reflections

For practitioners in the surface engineering field, this study offers several actionable insights. First, the dilution ratio in plasma surfacing must be carefully controlled to ensure sufficient WC particle incorporation; excessive dilution would reduce the effective WC content below the threshold needed for the austenite-retention effect. Second, the solution aging parameters (900 °C × 1 h + 490 °C × 5 h) represent a specific process window that may need adjustment for different substrate geometries and thicknesses to avoid distortion or cracking. Third, the preference for spherical WC particles over irregular ones should be considered, as the morphology affects both the solidification behavior and the diffusion kinetics during aging.

A critical question that arises from this study is whether the thick diffusion layer around WC particles, while improving interface bonding, might also reduce the effective volume fraction of intact WC particles over time, particularly under cyclic loading or at elevated temperatures. Long-term durability testing under thermal cycling conditions would be valuable to confirm the stability of the diffusion layer. Additionally, the study focuses on laboratory-scale specimens; scale-up to industrial components such as pump impellers, valve seats, or mining equipment would require addressing issues of thermal distortion, residual stress management, and coating thickness uniformity.

This literature represents a well-conducted investigation into the microstructure-property relationships of plasma-surfaced composite coatings. The finding that solution aging can reverse the trend of property degradation in WC-reinforced coatings is particularly noteworthy and opens a pathway for designing coatings with tailored thermal-mechanical properties through controlled heat treatment.