Effect of Annealing Temperature on Ni60/WC Overlay Microstructure and Hardness on H13 Steel - A Study Note

Literature Overview

This paper by Chen Wen, Wang Huajun, Xie Bing, and Zhou Chunyang, published in Heat Treatment of Metals (Vol. 44, No. 2, 2019, pp. 131-136), investigates the effect of post-weld annealing temperature on the microstructure and hardness of Ni60/WC composite overlay coatings produced by plasma powder cladding on H13 hot-work die steel. The research was conducted at the School of Materials Science and Engineering, Wuhan University of Technology, and was funded by the National Natural Science Foundation of China (Grant No. 51475346).

Material System and Process Parameters

The Ni60/WC composite overlay system combines the excellent castability and corrosion resistance of the Ni-based alloy Ni60 with the exceptional hardness of tungsten carbide (WC) particles. This combination is widely used in applications requiring both wear resistance and corrosion resistance, such as pipeline pump impellers, valve seats, and hot-work die surfaces.

Ni60/WC Composite Overlay System

The plasma powder cladding process was used to deposit the overlay, which offers several advantages over arc overlay welding:

• Lower heat input, reducing dilution and HAZ damage.
• Better control over the powder feed rate and composition.
• More uniform coating thickness.
• Reduced residual stress.

Effect of Annealing Temperature on Microstructure

The study systematically investigates annealing temperatures of 600°C and 800°C, in addition to the as-welded condition. The microstructural evolution is examined in three distinct zones: the substrate, the heat-affected zone (HAZ), and the overlay coating.

As-Welded Condition

In the as-welded state, the overlay HAZ consists primarily of coarse plate-like martensite, lenticular martensite, and retained austenite. This hard, brittle microstructure is a consequence of the rapid cooling rate associated with the plasma cladding process. The retained austenite fraction is significant, which provides some toughness but also introduces dimensional instability during subsequent service.

The overlay coating itself in the as-welded condition contains WC particles distributed within a Ni-based dendritic matrix. However, the distribution of WC particles is often non-uniform, with agglomeration occurring in certain regions and depletion in others. This non-uniformity is a consequence of the rapid solidification and the density difference between WC and the Ni-based liquid.

600°C Annealing

After annealing at 600°C, the HAZ microstructure transforms from martensite to a mixture of lenticular ferrite, acicular ferrite, and granular carbides. This transformation is driven by the recovery and recrystallization of the martensitic structure, which relieves the residual stresses and reduces hardness. The overlay coating hardness decreases slightly, and the quenched-hardened zone is mitigated.

800°C Annealing

After annealing at 800°C, the HAZ microstructure transforms to equiaxed ferrite and granular carbides, which represents a fully recrystallized and spheroidized condition. This microstructure offers the best combination of toughness and hardness. In the overlay coating, the petal-shaped dendrites are refined, the microstructure becomes more uniform, and the WC distribution is improved. The hardness distribution becomes uniform, the quenched-hardened zone disappears, and the hardness at the substrate-overlay interface rises in a gradient manner.

Microstructural Comparison Across Zones

Hardness Distribution Analysis

The hardness profile across the substrate-HAZ-overlay interface is a critical quality indicator. In the as-welded condition, a sharp hardness transition exists at the interface, which creates a stress concentration zone that is susceptible to cracking during thermal cycling or mechanical loading.

The 800°C annealing produces a hardness profile that rises gradually from the substrate through the HAZ into the overlay, eliminating the sharp hardness discontinuity that exists in the as-welded condition. This gradient is highly beneficial for fatigue resistance and thermal fatigue resistance because it reduces stress concentration at the interface.

Engineering Practice Implications

For engineers involved in the overlay repair of hot-work die components, pump impellers, and pipeline valves, this research provides critical guidance on post-weld heat treatment:

Recommended Annealing Parameters

Quality Control Considerations

1. Crack elimination: The study confirms that annealing effectively eliminates the crack sensitivity present in the as-welded overlay. This is a critical quality requirement for components subjected to cyclic loading or thermal cycling.
2. WC distribution: The improvement in WC distribution after annealing is attributed to the dissolution and re-precipitation of WC particles during the high-temperature exposure. This process allows the particles to redistribute more uniformly throughout the matrix.
3. Interface quality: The gradient hardness transition achieved at 800°C is essential for preventing delamination during service. A sharp hardness discontinuity at the interface creates a zone of high residual stress that can initiate interfacial cracking.
4. Dimensional stability: The 800°C annealing reduces the retained austenite fraction, which improves dimensional stability during subsequent service. This is particularly important for precision components such as valve seats and pump impellers.

Study Insights and Independent Reflection

The most significant finding of this research is the demonstration that a single annealing step at 800°C can simultaneously address multiple quality concerns: crack elimination, microstructure refinement, WC redistribution, and hardness gradient optimization. This efficiency is valuable in industrial settings where processing time and cost are critical considerations.

The study also highlights an important concept in overlay welding quality: that the as-welded condition is often not the optimal condition for service, and that post-weld heat treatment is not merely a stress-relief step but a fundamental process that transforms the microstructure from a metastable, high-energy state to a stable, low-energy state that offers superior mechanical properties.

For pipeline applications, the findings have direct relevance to the overlay repair of pump impellers and valve components that are subjected to both wear and corrosion. The Ni60/WC composite overlay provides excellent wear resistance, while the 800°C annealing ensures that the overlay is crack-free, dimensionally stable, and well-bonded to the substrate. The elimination of the quenched-hardened zone is particularly important for components that are subjected to impact loading, such as pump impellers operating with slurry flows.

The research also underscores the importance of considering the entire cross-section (substrate, HAZ, and overlay) when evaluating overlay quality. The HAZ is often the weakest link in the system because it experiences the most severe microstructural changes without the benefit of the overlay's alloy additions. The 800°C annealing effectively addresses this weakness by transforming the HAZ into a tough, ductile microstructure that can accommodate the thermal and mechanical strains imposed during service.

In conclusion, this study provides a comprehensive and practical framework for optimizing the post-weld heat treatment of Ni60/WC composite overlays on H13 steel substrates. The recommended 800°C annealing treatment offers the best balance of hardness, toughness, crack resistance, and microstructural uniformity, and should be considered as the default post-weld treatment for high-performance Ni60/WC overlay applications in pipeline and pump repair.