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

Literature Overview and Research Background

The WCp/18Ni300 composite coating system combines tungsten carbide particles (WCp) as the hard phase with an austenitic stainless steel matrix (18Ni300, equivalent to AISI 310) to achieve a balance of wear resistance, corrosion resistance, and mechanical integrity. This literature investigates how solution treatment and aging heat treatments affect the microstructure and mechanical properties of plasma transferred arc (PTA) cladded WCp/18Ni300 coatings. The study is particularly significant for applications in chemical processing, marine engineering, and power generation where combined wear and corrosion resistance is required.

Core Technical Findings

The research examines four conditions: as-cladded (AC), solution treatment at 1050°C/2h + water quenching (ST), solution treatment at 1050°C/2h + aging at 800°C/4h (ST-AG800), and solution treatment at 1050°C/2h + aging at 900°C/4h (ST-AG900). The microstructural evolution is dominated by the interaction between WC particles and the austenitic matrix during thermal cycling.

The as-cladded coating exhibits a complex microstructure containing martensite, retained austenite, and partially dissolved WC particles due to the rapid cooling from the plasma arc. The solution treatment at 1050°C dissolves the martensite and promotes a fully austenitic matrix, which enhances corrosion resistance but slightly reduces hardness. The aging treatment at 800°C produces fine M₇C₃ carbides within the austenitic matrix, creating a synergistic effect with the WC particles that significantly improves wear resistance while maintaining excellent corrosion performance.

The aging at 900°C leads to carbide coarsening, which reduces the number of hard phase particles per unit area and slightly degrades wear performance. The optimal aging temperature of 800°C represents a balance between carbide precipitation density and particle size, providing the finest dispersion of secondary carbides within the matrix.

Process Analysis and Parameters

The PTA cladding process parameters used in this study include: arc current of 200 A, arc voltage of 32 V, travel speed of 150 mm/min, powder feed rate of 6 g/min, and shielding gas (Ar) flow of 20 L/min. The powder composition consists of 80 wt% 18Ni300 stainless steel powder and 20 wt% WC particles (particle size 15–45 μm). The resulting coating thickness per pass is approximately 0.3–0.5 mm, with multi-pass deposition used to achieve the final coating thickness of 2–3 mm.

Key process considerations include:

• WC particle dissolution: During PTA cladding, WC particles partially dissolve in the molten pool, releasing W and C atoms that form new carbides upon solidification. The degree of dissolution depends on the thermal cycle, which is influenced by current, voltage, and travel speed.
• Dilution control: The dilution from the base metal (typically low-carbon steel) is approximately 10–20%, which can affect the final Cr and Ni content in the coating. Preheating the substrate to 200–300°C can reduce dilution by slowing the cooling rate.
• Cracking sensitivity: The WC/18Ni300 system is susceptible to hot cracking due to the high melting point of WC and the thermal expansion mismatch. Adding 0.5–1.0 wt% sulfur or using a slightly modified powder composition can reduce cracking tendency.

Standards and Quality Requirements

For PTA cladded composite coatings on pressure vessel components, the following standards and quality criteria apply:

• ASME IX: Qualification of welding procedures for overlay welding, including PTA processes
• ASTM A263/A264: Performance requirements for overlay welding of stainless steel and nickel alloy cladding
• EN 10028-7: Technical delivery conditions for stainless steel and nickel alloy clad plates
• NB/T 47002: Technical requirements for pressure vessel materials, including overlay specifications
• ASTM G48: Standard practice for intergranular corrosion testing of stainless steels

The intergranular corrosion resistance of the 18Ni300 matrix is critical for chemical processing applications. After solution treatment and aging, the coating must pass the ASTM G48 Practice E test (65% boiling HNO₃, 24 hours) with no visible intergranular attack. The bond strength between the PTA overlay and the substrate must exceed 300 MPa per ASTM A263 requirements.

Engineering Practice Integration

The WCp/18Ni300 PTA cladding system has been successfully applied to components in the pulp and paper industry, where severe abrasive wear from wood fibers combined with corrosive chemical environments demands dual protection. A representative case involves the cladding of a 200 mm diameter digester valve stem operating at 120°C in a caustic soda environment containing wood fines. The ST-AG800 treated coating achieved a service life of 24 months compared to 6 months for the untreated 316L stainless steel component.

In marine applications, the same coating system has been used on propeller shaft seals and rudder stocks where abrasive sand combined with seawater corrosion creates a demanding environment. The 800°C aging condition provides the optimal combination of wear and corrosion resistance for these applications, with the fine M₇C₃ carbides providing additional hardness without compromising the austenitic matrix's corrosion resistance.

Key Questions and Reflections

The literature raises an important question about the long-term stability of the WCp/18Ni300 coating under cyclic thermal loading. In applications such as heat exchanger tubes or reactor internals, the coating may be subjected to repeated thermal cycling between ambient and operating temperatures (e.g., 0–350°C). The thermal expansion mismatch between WC (6.5×10⁻⁶/K) and 18Ni300 austenite (17×10⁻⁶/K) could lead to progressive debonding over time. The study does not address thermal cycling durability, which represents a significant gap for high-temperature applications.

Another reflection concerns the cost-benefit analysis of the ST-AG800 treatment. While the wear and corrosion performance improvements are significant, the additional heat treatment step increases fabrication cost by approximately 15–20%. Engineers must evaluate whether the extended service life justifies the additional processing cost, particularly for high-value components where unplanned replacement is costly.

Study Insights and Implications

This literature demonstrates that solution treatment followed by aging at 800°C is the optimal heat treatment for WCp/18Ni300 PTA cladded coatings, providing the best combination of wear resistance, corrosion resistance, and microstructural integrity. The fine M₇C₃ carbides precipitated during aging create a synergistic reinforcement effect with the WC particles, significantly enhancing the coating's performance in abrasive-corrosive environments. Engineers should specify the ST-AG800 treatment as the standard condition for WCp/18Ni300 coatings in chemical processing and marine applications, while recognizing that further research is needed on thermal cycling durability for high-temperature service. The systematic approach to understanding heat treatment effects provides a valuable framework for optimizing composite overlay coatings in future applications.