Tempering Treatment Effects on Microstructure and Hardness of Plasma-Deposited Ni60/WC Overlay on H13 Steel

Literature Overview

Chen Wen et al. (Wuhan University of Technology, 2020) investigated the effects of tempering treatment on the microstructure and hardness of a Ni60/WC composite overlay deposited onto H13 hot-work die steel via powder plasma arc welding (PPAW). Supported by the National Natural Science Foundation of China (Grant 51475346), this study was published in Hot Working Technology (Vol. 49, No. 6, pp. 109–112). The work addresses a common engineering challenge: the cracking susceptibility and hardness gradient issues inherent in Ni60-based overlay weldments, and proposes tempering as an effective post-weld remedy.

Core Technical Approach

Ni60 (Stellite 6) is a widely used cobalt-chromium alloy overlay for hot-section components due to its excellent wear, corrosion, and oxidation resistance at elevated temperatures. However, when deposited via PPAW, the rapid solidification rates produce a dendritic microstructure with high carbon supersaturation, leading to inherent cracking susceptibility and a steep hardness gradient between the overlay and substrate. The addition of WC particles to the Ni60 powder is intended to enhance wear resistance through carbide dispersion, but the resulting overlay often exhibits network-type carbide segregation along dendrite boundaries and microcracks.

The authors applied a tempering treatment (quenching followed by tempering) to the as-deposited Ni60/WC overlay on H13 steel and examined the resulting microstructural evolution and hardness profile. The tempering process involves reheating the overlay-substrate assembly to a temperature below the Ac1 point (typically 600–700 °C for this system), holding for a sufficient duration, and then air cooling.

Microstructural Evolution and Hardness Analysis

The tempering treatment produced several significant microstructural changes in the Ni60/WC overlay:

The elimination of cracking after tempering is attributed to the relief of residual stresses and the dissolution of brittle phases that formed during rapid solidification. The formation of Cr₅B₃ hard boride phases provides additional dispersion strengthening, contributing to the observed hardness increase. The dissolution of petal-shaped dendrites and their replacement by strip and block borides indicates that the tempering temperature was sufficient to partially dissolve the dendritic network while promoting the precipitation of thermodynamically stable boride phases.

The hardness profile analysis is particularly important from an engineering standpoint. In the as-deposited condition, the steep hardness gradient at the overlay-substrate interface creates stress concentrations that can initiate delamination under thermal or mechanical cycling. After tempering, the smooth hardness transition reduces these stress concentrations and improves the overall mechanical integrity of the overlay-substrate system.

Engineering Practice Implications

For engineers deploying Ni60/WC overlays on hot-work die components, this study provides several practical recommendations:

1. Mandatory post-weld heat treatment: Tempering should be specified as a standard part of the Ni60/WC overlay process specification. The cost of tempering is minimal compared to the cost of overlay failure due to cracking or delamination.
2. Tempering temperature optimization: The tempering temperature must be carefully selected to avoid exceeding the Ac1 point of the H13 substrate (approximately 820 °C), which would cause substrate softening. A typical tempering range of 600–700 °C for 1–2 hours is recommended, with the exact parameters validated through hardness and microstructure verification.
3. Multi-pass overlay considerations: For thick overlays, the tempering treatment may need to be performed after each pass or after the final pass, depending on the total overlay thickness and the cracking susceptibility of each individual pass.
4. Quality verification: Post-tempering inspection should include hardness profiling across the overlay-substrate interface, metallographic examination for residual cracks, and, if applicable, fatigue or thermal cycling tests to validate the overlay performance.

Study Insights and Reflections

This study reinforces a fundamental principle in overlay welding engineering: the as-deposited microstructure is rarely the optimal microstructure for service. Post-weld heat treatment is not an optional add-on but a critical process step that transforms a potentially brittle, cracked overlay into a durable, high-performance surface layer. The specific case of Ni60/WC on H13 steel illustrates how tempering can simultaneously address multiple performance issues—cracking, hardness gradient, and carbide distribution—through a single thermal cycle. From a process development perspective, this work suggests that future Ni60-based overlay systems should be designed with post-weld heat treatment in mind from the outset, rather than treating tempering as a remedial measure for problematic as-deposited conditions. The formation of Cr₅B₃ boride phases during tempering also highlights the importance of boron as an alloying element in Ni-based overlay alloys, as it contributes to both hardness and thermal stability.