Effect of Annealing Temperature on Microstructure and Hardness of Ni60/WC Plasma Cladding on H13 Steel

Literature Overview and Research Background

This study examines the influence of post-weld annealing temperature on the microstructure, hardness distribution, and mechanical properties of a Ni60/WC composite plasma cladding layer deposited on H13 hot work die steel. H13 steel is widely used in hot extrusion dies, forging dies, and hot work tools where high-temperature strength and thermal fatigue resistance are critical. The Ni60/WC composite overlay provides excellent wear and corrosion resistance but typically exhibits high residual hardness and brittleness due to the rapid solidification inherent to plasma transferred arc welding. Post-weld annealing is therefore essential to relieve residual stresses, transform brittle phases, and achieve an optimal balance between hardness and toughness.

Microstructure Evolution with Annealing Temperature

As-Welded Condition

In the as-welded condition, the Ni60/WC plasma cladding exhibits a complex microstructure consisting of a dendritic nickel-based solid solution matrix, hard carbide phases (Ni3WC and Co3WC), and a eutectic region containing Ni and WC particles. The as-welded hardness typically ranges from 580 to 680 HV due to the rapid solidification and the presence of fine carbide precipitates. The microstructure also contains significant residual tensile stresses that can reach 300-400 MPa, which poses a risk of cracking during subsequent machining or service.

Effect of Annealing Temperature on Carbide Morphology

Annealing at 600°C for 2 hours produces moderate carbide coarsening with the primary carbide particle size increasing from approximately 1.8 μm in the as-welded state to 2.8 μm. The matrix retains a substantial fraction of fine dendritic features, and the overall microstructure shows partial stress relief without significant loss of hardness. This condition is suitable for applications where high wear resistance is required and moderate toughness is acceptable.

At 700°C, the carbide coarsening becomes more pronounced, with particle sizes reaching 3.5-5.0 μm. The dendritic structure begins to break down as the nickel matrix undergoes recrystallization, resulting in a more equiaxed grain structure. The hardness decreases to approximately 450 HV, representing a 27% reduction from the as-welded condition. The residual stress is significantly reduced to approximately 50 MPa, providing good dimensional stability.

At 800°C, extensive carbide coarsening occurs with particle sizes exceeding 7 μm, and the nickel matrix undergoes full recrystallization. The hardness drops to approximately 400 HV, which may be insufficient for severe wear applications. However, the toughness and fatigue resistance are substantially improved, making this condition suitable for applications where thermal cycling and impact loading are dominant failure modes.

Hardness Distribution and Mechanical Property Analysis

Through-Thickness Hardness Profile

The study reveals that the hardness distribution across the cladding thickness is non-uniform, with the surface region typically exhibiting higher hardness than the region adjacent to the fusion line. This gradient is attributed to the cooling rate variation: the surface cools more rapidly due to air exposure, while the fusion line region experiences slower cooling due to the thermal mass of the H13 substrate. After annealing at 700°C, the hardness gradient becomes less pronounced as the carbide coarsening occurs more uniformly throughout the thickness.

The dilution zone at the fusion interface between the Ni60/WC overlay and the H13 substrate is a critical region for mechanical integrity. The dilution typically ranges from 5% to 15% depending on the plasma arc parameters. Higher dilution results in a lower hardness in the transition region, which may create a soft band susceptible to wear. The study recommends controlling the dilution below 10% for applications requiring uniform wear resistance across the entire cladding thickness.

Wear Resistance Evaluation

Dry sliding wear tests against a Si3N4 ball at room temperature demonstrated that the as-welded condition offers the highest wear resistance, while annealing at 800°C results in a 45% increase in wear volume loss. The wear mechanism transitions from abrasive wear in the as-welded condition (dominated by hard carbide ploughing) to a mixed abrasive-adhesive wear mechanism in the annealed conditions (dominated by matrix deformation between coarsened carbides). This finding highlights the fundamental trade-off between hardness and toughness that must be managed in overlay design.

Engineering Practice Implications

For hot work die applications, the selection of annealing temperature must be guided by the specific failure mode expected in service. For dies subjected primarily to abrasive wear at elevated temperatures (such as extrusion dies), an annealing temperature of 600-650°C is recommended to maintain high hardness while partially relieving residual stresses. For dies subjected to thermal cycling and impact loading (such as forging dies), a higher annealing temperature of 700-750°C provides a better balance between wear resistance and thermal fatigue resistance.

A practical concern in production is the distortion risk associated with post-weld annealing of large H13 die components. The differential thermal expansion between the Ni60/WC overlay and the H13 substrate can induce warping during heating and cooling. The study recommends a slow heating rate of no more than 50°C/hour and controlled cooling in a furnace to minimize distortion. For critical dimensions, pre-machining of the die blank before cladding and final machining after annealing is the preferred approach.

Key Questions and Reflections

An important question is whether alternative post-weld heat treatment strategies, such as tempering or stress-relief annealing at lower temperatures, could achieve comparable results with less hardness loss. The literature suggests that a two-step heat treatment—first a low-temperature stress relief at 400°C, followed by a higher-temperature temper at 650°C—may provide a more controlled microstructure evolution than a single-step anneal. This approach warrants further investigation for industrial implementation.

Another reflection concerns the long-term stability of the annealed microstructure under service conditions. H13 dies often operate at temperatures approaching 600°C, which could trigger further carbide coarsening and hardness degradation over extended service life. Understanding the time-temperature relationship for carbide stability in the annealed Ni60/WC overlay is essential for predicting service life and planning maintenance intervals.

Summary

The annealing temperature exerts a decisive influence on the microstructure, hardness, and mechanical properties of Ni60/WC plasma cladding on H13 steel, with an optimal range of 600-700°C providing the best compromise between wear resistance, residual stress relief, and dimensional stability. Engineers must carefully match the annealing condition to the specific service environment of the die component, and the trade-off between hardness retention and toughness improvement must be explicitly considered in the design and qualification process.