CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Quenching and Tempering on Microstructure and Hardness of Ni60/WC Cladding on H13 Steel via Plasma Transferred Arc
Literature Overview
This study investigates the microstructural evolution and hardness distribution of a Ni60/WC composite cladding deposited on H13 hot work die steel using Plasma Transferred Arc (PTA) welding, with particular focus on the effect of post-deposition quenching and tempering (QT) heat treatment. The relevance of this work is significant for engineers working on die and mold repair, where H13 steel components are frequently used and subjected to abrasive and erosive wear. The Ni60 (Stellite 6) matrix reinforced with tungsten carbide (WC) particles is a well-established system for hardfacing applications, yet the interaction between the base metal's heat treatment state and the cladding microstructure remains an area requiring deeper understanding.
Core Technical Content
The study examines how the QT treatment of the base H13 steel influences the dilution, microstructure, and hardness of the PTA-deposited Ni60/WC overlay. H13 steel, a medium-carbon hot work tool steel containing approximately 1.05–1.40% C, 4.75–5.50% Cr, and 1.40–1.65% Mo, is typically supplied in a quenched and tempered condition with a hardness of 30–40 HRC. The PTA process parameters typically include an arc current of 80–120 A, arc voltage of 22–28 V, travel speed of 100–200 mm/min, and powder feed rate of 0.5–1.2 kg/min, with an argon shielding gas flow of 15–20 L/min.
The Ni60/WC composite powder consists of a Ni-Cr-Mo-Co alloy matrix (approximately 60% Ni balance Cr, Mo, Co, and Fe) with WC particles typically sized 15–45 μm added at 20–40 wt%. During PTA deposition, the high thermal gradient and rapid solidification rates lead to columnar dendritic growth in the dilution zone and equiaxed dendritic growth in the upper layers. The WC particles may partially dissolve, reacting with the molten pool to form hard carbides such as W₂C, WC, and M₇C₃ (where M = Cr, Mo, W).
Microstructural Analysis
| Condition | Dilution Zone Microstructure | Overlay Microstructure | Surface Hardness (HV) |
|---|---|---|---|
| As-deposited (no QT) | Columnar dendrites + M₇C₃ network | Equiaxed dendrites + WC/W₂C + M₇C₃ | 1100–1400 |
| QT at 850°C / 600°C | Tempered martensite + retained austenite | Coarsened carbides, reduced retained austenite | 950–1200 |
| QT at 850°C / 500°C | Fine tempered martensite | Moderate carbide coarsening | 1050–1350 |
The key finding is that the QT treatment of the base metal does not directly alter the overlay microstructure but significantly affects the dilution zone. When the base H13 is in a tempered martensitic state, the dilution zone exhibits a mixture of martensite and retained austenite with dispersed carbides. The tempering temperature of the base metal influences the amount of retained austenite in the dilution zone, which in turn affects the overall hardness gradient across the cladding interface.
Key Observations
- The hardness of the overlay layer is primarily governed by the Ni60/WC composition and PTA process parameters rather than the base metal heat treatment state.
- QT treatment at 600°C leads to moderate carbide coarsening in the dilution zone, resulting in a slight decrease in hardness at the interface region (approximately 80–120 HV reduction compared to the as-deposited condition).
- Lower tempering temperatures (500°C) preserve finer carbide distributions and higher hardness in the dilution zone, maintaining better bonding integrity.
- The hardness profile across the cladding shows a typical gradient: highest at the surface (1200–1400 HV), gradually decreasing through the dilution zone (600–800 HV), and reaching the base metal hardness (300–350 HV at 30–40 HRC).
Engineering Practice Implications
From a practical standpoint, this study highlights several important considerations for engineers involved in die and mold repair:
- Base metal pre-treatment matters: The heat treatment state of the H13 base steel should be carefully considered before applying PTA cladding. A tempering temperature of 500–550°C is generally preferred to maintain a fine microstructure in the dilution zone without introducing excessive retained austenite.
- Dilution control: The PTA process inherently produces a dilution zone of 1–3 mm depth. Engineers should account for this when planning multi-layer cladding, as the first layer will have the highest dilution and the most pronounced microstructural transition.
- Hardness uniformity: For applications requiring uniform hardness across the entire cladding thickness (such as in extrusion die repair), multiple passes with controlled overlap (30–50%) are recommended to minimize hardness variation.
- Residual stress management: The QT treatment of the base metal can partially relieve residual stresses introduced during PTA deposition, reducing the risk of cracking in thick cladding builds.
Key Questions and Reflections
One critical question that arises from this study is whether the QT treatment should be applied to the base metal before cladding or whether a post-cladding heat treatment of the entire component would yield superior results. The answer likely depends on the specific application requirements, but it is clear that the interaction between the base metal microstructure and the dilution zone microstructure is a factor that should not be overlooked in process development.
Another important consideration is the effect of the WC particle size distribution on the cladding performance. The study suggests that finer WC particles (15–25 μm) may provide more uniform dispersion and less likelihood of particle agglomeration, but this needs to be balanced against the melting behavior of WC during PTA deposition, as larger particles may survive the thermal cycle with less dissolution.
Study Insights and Outlook
The study reinforces the principle that cladding performance is not solely determined by the overlay composition and process parameters but is also significantly influenced by the base metal condition. For engineers working on H13 steel die repair with Ni60/WC PTA cladding, the recommendation is to maintain the base metal in a tempered condition at 500–550°C before deposition, use a multi-pass approach with controlled dilution, and verify the hardness profile across the full cladding thickness after deposition. Future research should explore the combined effects of base metal pre-treatment, PTA process parameters, and post-cladding heat treatment on the long-term wear resistance and fatigue life of the cladding system, particularly under thermal cycling conditions typical of hot work die applications. The integration of computational modeling with experimental validation would further enhance our ability to predict and optimize cladding performance for specific service environments.