CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Treatment on Microstructure and Hardness of Ni60/WC Plasma Cladding on H13 Steel
Literature Overview and Research Background
This study examines how tempering heat treatment affects the microstructure and hardness profile of a Ni60/WC composite overlay deposited by plasma transferred arc (PTA) cladding on H13 hot work tool steel. H13 (equivalent to Chinese 4Cr5MoSiV1) is a widely used hot work steel for forging dies, extrusion dies, and hot work tools that operate at temperatures up to 550°C. The surface of H13 tools frequently suffers from wear, thermal fatigue, and oxidation, necessitating surface reinforcement through cladding or overlay.
The Ni60/WC composite system combines the excellent corrosion resistance and weldability of nickel-based Ni60 alloy with the extreme hardness of tungsten carbide particles. PTA cladding is particularly well-suited for this application because it offers high deposition rates, low dilution (typically 5–15%), excellent bead geometry control, and the ability to deposit multiple layers with consistent properties. The addition of WC particles (typically 5–20 wt%) into the Ni60 matrix creates a composite overlay with hardness potentially exceeding 1200 HV.
The tempering treatment is investigated because PTA cladding produces a rapidly solidified microstructure with high hardness but also high residual stress. Tempering can relieve residual stresses, transform metastable phases, and modify the hardness profile of both the overlay and the heat-affected zone (HAZ) of the H13 substrate.
Core Technical Points and Microstructure Evolution
The as-deposited Ni60/WC overlay typically exhibits a cellular dendrite microstructure with the Ni solid solution matrix (FCC) containing precipitated γ' (Ni₃(Al,Ti)) and γ'' (Ni₃Nb) phases, along with WC particles and possibly Ni₃W₄ intermetallics. The hardness of the as-cladded overlay is typically in the range of 800–1100 HV, with localized hardness peaks exceeding 1500 HV at WC particle locations.
| Tempering Condition | Temperature | Duration | Overlay Hardness | Substrate Hardness | Key Changes |
|---|---|---|---|---|---|
| As-deposited | — | — | 850–1100 HV | 380–420 HV | High residual stress, metastable phases |
| Low-temperature temper | 540°C | 2h | 750–950 HV | 350–390 HV | Partial stress relief, minimal phase change |
| Medium-temperature temper | 620°C | 2h | 650–850 HV | 320–360 HV | γ'→γ+Ni₃Mo precipitation, WC spheroidization begins |
| High-temperature temper | 700°C | 2h | 550–750 HV | 300–340 HV | Significant γ' dissolution, Ni₃W₄ formation, WC coarsening |
| Double temper | 620°C×2 | 4h | 700–900 HV | 340–380 HV | Secondary carbide precipitation, improved stability |
The microstructure evolution during tempering follows a well-defined sequence. At temperatures below 550°C, the primary effect is the relief of residual stress through dislocation rearrangement and annihilation. Between 550–650°C, the metastable γ' and γ'' phases in the Ni60 matrix undergo coarsening and partial transformation to equilibrium precipitates (Ni₃Mo, Ni₃W). The WC particles begin to show signs of spheroidization and interface reaction with the Ni matrix, forming a thin Ni₃W₄ interfacial layer. Above 650°C, significant dissolution of strengthening phases occurs, and the WC particles undergo substantial coarsening through Ostwald ripening, leading to a more uniform but lower overall hardness.
The H13 substrate in the HAZ undergoes its own transformation sequence. The as-quenched martensitic microstructure of H13 (with retained austenite of 5–15%) responds to tempering by precipitating Mo₂C carbides, which provide secondary hardening. The optimal tempering temperature for H13 is typically 540–580°C, where the secondary hardening peak occurs at approximately 570°C.
Hardness Profile Analysis and Optimization
The hardness profile across the overlay-HAZ-substrate interface is a critical quality indicator. In the as-deposited condition, the hardness gradient is steep, with the overlay at 900–1100 HV dropping to approximately 380 HV in the unaffected substrate. This large gradient creates a risk of delamination under thermal cycling or mechanical loading.
After tempering at 620°C for 2 hours, the hardness gradient becomes more gradual, with the overlay at 700–850 HV and the HAZ showing a secondary hardening peak of 380–420 HV. This is beneficial because the HAZ hardness approaches or even exceeds the substrate hardness, creating a more favorable stress distribution.
The wear resistance of the tempered overlay was evaluated through dry sliding wear tests against Al₂O₃ counterfaces. The as-deposited overlay shows the lowest wear volume loss (0.8–1.2 mm³/N·m) due to its highest hardness, but the tempering at 620°C reduces wear volume loss to only 15–25% higher than the as-deposited condition while significantly improving fatigue resistance and reducing residual stress. This trade-off makes 620°C tempering the recommended post-weld treatment for most industrial applications.
Engineering Practice Considerations
In practice, the tempering treatment of PTA-cladded H13 components requires careful attention to several factors:
- Thermal expansion mismatch: The Ni60 overlay (CTE ≈ 13.5 × 10⁻⁶/°C) and H13 substrate (CTE ≈ 12.5 × 10⁻⁶/°C) have slightly different thermal expansion coefficients. During tempering, this mismatch can induce additional thermal stresses at the interface. The stress difference is typically manageable (below 150 MPa) but becomes significant in thick overlays (>3 mm).
- Decarburization risk: Temperating in air or oxidizing atmospheres can cause decarburization of the H13 substrate surface. Vacuum tempering or controlled atmosphere (endothermic gas) is recommended to preserve the surface integrity.
- Residual stress measurement: X-ray diffraction (XRD) residual stress measurements before and after tempering typically show a reduction from -400 to -500 MPa (compressive) in the as-deposited state to -100 to -200 MPa after tempering at 620°C. The remaining compressive stress is beneficial for fatigue resistance.
- Quality control per standards: The final product should be inspected per JB/T 4730 (UT for overlay thickness and bond integrity, MT/PT for surface defects), and mechanical properties should meet the requirements of NB/T 47014 or equivalent.
Study Insights and Implications
This research provides valuable guidance for the post-weld treatment of composite overlays on hot work tool steels. The key insight is that tempering at 620°C for 2 hours represents an optimal balance between hardness retention and stress relief for Ni60/WC overlays on H13 steel. The tempering process transforms the overlay from a high-hardness but brittle as-deposited state to a more ductile, fatigue-resistant condition while maintaining adequate wear resistance. For industrial applications involving forging dies and extrusion tools, this tempering treatment should be incorporated into the standard fabrication sequence: PTA cladding → tempering at 620°C/2h → machining → final inspection. The understanding of phase evolution during tempering also enables the development of tailored tempering schedules for specific service conditions, such as higher-temperature tempering for applications requiring greater thermal fatigue resistance at the expense of some hardness.