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

Literature Overview

This 2020 study by Chen Wen et al., published in Hot Working Technology, examines the effects of tempering treatment on the microstructure and hardness profile of a plasma-arc transfer (PAT) Ni60/WC overlay coating deposited on H13 hot-work die steel. The research addresses a practical concern in die and mold manufacturing: the susceptibility of plasma-overlay coatings to cracking during and after deposition, and the potential of post-deposition heat treatment to mitigate this issue while improving mechanical performance. The findings are directly applicable to engineers involved in the repair and refurbishment of hot-work dies, extrusion tools, and forging equipment.

Core Technical Findings

As-Deposited Coating Characteristics

The as-deposited Ni60/WC plasma overlay coating on H13 steel exhibits the following characteristics:

Crack sensitivity: The as-deposited coating is susceptible to cracking, likely due to the high residual stresses introduced during the rapid solidification of the plasma-arc process and the thermal mismatch between the coating and the substrate.
WC distribution: Tungsten carbide particles are present but may exhibit non-uniform distribution, with some particles dissolved into the matrix during the high-temperature deposition process.
Microstructure: The coating microstructure consists of dendritic Ni solid solution with dissolved alloying elements (Cr, Mo, Fe) and partially dissolved or retained WC particles.
Hardness profile: The as-deposited coating shows a hardness gradient from the surface to the substrate, with potential discontinuities at the interface.

Effects of Tempering Treatment

Feature	As-Deposited	After Tempering
Crack sensitivity	High (cracks present)	Eliminated
WC particle distribution	Non-uniform	Improved uniformity
Dendritic morphology	Petal-shaped dendrites (center)	Eliminated (strips, blocks)
New phases	None	Cr5B3 hard phase precipitates
Hardness profile	Discontinuous transition	Smooth, gradual transition
Overall hardness	Lower	Higher

The tempering treatment achieves several beneficial metallurgical transformations:

Crack healing and stress relief: The tempering process relieves the residual stresses that drive cracking in the as-deposited coating. The elevated temperature allows for stress redistribution and crack closure, effectively eliminating the crack sensitivity.
Microstructural refinement: The petal-shaped dendritic morphology in the coating center is replaced by finer strip-like and block-like boride phases. This refinement results from the dissolution and re-precipitation of boron-containing phases during tempering.
Cr5B3 precipitation: A significant volume fraction of Cr5B3 hard boride phases precipitates during tempering. These phases are thermodynamically stable at the tempering temperature and provide additional hardening and wear resistance.
Hardness improvement: The tempering treatment increases the overall coating hardness and, critically, creates a smooth hardness gradient from the coating surface to the H13 substrate. This smooth transition is essential for preventing interfacial cracking under thermal and mechanical cycling.

Hardness Profile Analysis

The hardness profile is a key performance indicator for overlay coatings, as it determines the coating's ability to resist delamination and spalling under service loading. The study reports that:

As-deposited: The hardness profile shows abrupt changes, particularly at the coating-substrate interface, indicating poor metallurgical compatibility and high residual stress.
After tempering: The hardness profile exhibits a gradual, continuous transition from the high-hardness coating surface to the lower-hardness H13 substrate. This smooth gradient is a direct indicator of reduced residual stress and improved interfacial integrity.

Engineering Practice Implications

Application to Die and Mold Repair

H13 steel is the most widely used hot-work die steel in the world, employed in forging dies, extrusion dies, hot-work tooling, and die-casting molds. These components are subjected to extreme thermal cycling (repeated heating to 400-500 degrees Celsius and rapid cooling), mechanical loading, and abrasive contact with metal chips. Surface coatings are routinely applied to extend die life, and plasma-overlay Ni60/WC coatings are a common choice due to their excellent combination of hardness, toughness, and thermal fatigue resistance.

The findings of this study have direct implications for die repair and refurbishment practice:

Mandatory tempering after deposition: The study demonstrates that tempering is not optional but essential for producing a crack-free, high-performance Ni60/WC coating on H13 steel. The tempering parameters should be specified in the coating procedure specification.
Tempering temperature selection: The tempering temperature should be selected to be high enough to relieve residual stresses and promote Cr5B3 precipitation but low enough to avoid excessive softening of the coating. Typical tempering temperatures for Ni60-based coatings on H13 steel are in the range of 600-700 degrees Celsius.
Post-tempering hardness verification: Hardness profiling across the coating-substrate interface is a critical quality control step. A smooth hardness gradient (no abrupt jumps exceeding 100-150 HV between adjacent measurement points) indicates successful tempering.
Thermal cycling resistance: The smooth hardness gradient achieved through tempering is directly related to the coating's resistance to thermal fatigue cracking. In service, the coating will experience repeated thermal expansion and contraction cycles. A gradual hardness transition allows for differential strain accommodation without interfacial cracking.

Comparison with Other Post-Deposition Treatments

Treatment	Effect on Cracking	Effect on Hardness	Effect on Microstructure	Practicality
Tempering (this study)	Eliminates cracks	Increases hardness	Cr5B3 precipitation, dendrite refinement	High (standard furnace treatment)
Hot isostatic pressing (HIP)	Eliminates porosity	May reduce hardness	Densification, pore closure	Moderate (expensive equipment)
Shot peening	Induces compressive stress	May increase surface hardness	Surface work hardening	High (common post-treatment)
No post-treatment	Cracks remain	Lower hardness	As-deposited dendritic structure	Not recommended

Study Insights and Outlook

This study provides valuable evidence that tempering is an effective and practical method for improving the performance of plasma-overlay Ni60/WC coatings on H13 steel. The elimination of crack sensitivity, the formation of Cr5B3 hard phases, and the creation of a smooth hardness gradient are all significant improvements that translate directly to enhanced service life in hot-work die applications. The study underscores the importance of post-deposition heat treatment in coating engineering, which is often neglected in favor of simply optimizing the deposition parameters.

For engineers specifying plasma-overlay coatings for die and mold repair, the recommendation is to include a mandatory tempering step in the coating procedure, with hardness profiling as a post-treatment quality control requirement. The Cr5B3 precipitation phenomenon identified in this study opens up interesting possibilities for future research: by controlling the boron content in the Ni60/WC powder feedstock and the tempering parameters, it may be possible to tailor the volume fraction and morphology of Cr5B3 phases to optimize the hardness-toughness balance for specific service conditions. Additionally, the study should be extended to other base materials (such as D2, A2, and H11 die steels) and other coating compositions (such as Ni60/Al2O3, Ni60/SiC, and Co-based alloys) to establish a comprehensive database of tempering effects on plasma-overlay coatings. The principles demonstrated here are broadly applicable to the design and optimization of surface engineering solutions for demanding industrial applications.