CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Annealing Temperature on Microstructure and Hardness of Ni60-WC Overlay on H13 Steel by Plasma Arc Welding
Literature Overview
This study by Chen Wen and colleagues from the School of Materials Science and Engineering at Wuhan University of Technology investigates the influence of post-weld annealing temperature on the microstructure evolution and hardness distribution of Ni60/WC composite overlay layers deposited on H13 hot-work die steel substrates via plasma arc welding (PAW). The research was supported by the National Natural Science Foundation of China (Grant No. 51475346) and published in the journal "Metal Heat Treatment" in 2019. The work addresses a critical practical challenge in die and mold engineering, where H13 tool steels are frequently subjected to abrasive and adhesive wear, necessitating hard overlay protection, yet the brittle carbide-rich overlay layer often cracks under thermal cycling conditions unless properly tempered.
Core Technical Content
The Ni60 alloy, a nickel-based superalloy containing approximately 6% carbon, is renowned for its exceptional work-hardening capability and high-temperature wear resistance. When combined with tungsten carbide (WC) particles, the resulting composite overlay can achieve surface hardness values exceeding 1500 HV after cold work. However, the as-deposited overlay typically contains a complex matrix of austenitic nickel, martensitic Ni3(Fe,Ni), and a network of primary and secondary carbides including M6C (Ni3W, Ni3Mo), MC (WC, W2C), and M7C3 (Fe,W)2C. The H13 substrate, a 1.2343 hot-work steel with approximately 0.4% C, 5% Cr, 1% Si, and 0.3% Mo, possesses a tempered martensitic structure in its delivery condition.
The study systematically examines annealing temperatures ranging from approximately 600 °C to 900 °C with soaking times of 1 to 2 hours, followed by air cooling or furnace cooling. The key metallurgical events during annealing include:
- Carbide dissolution and coarsening: Primary carbides begin to dissolve above 700 °C, leading to matrix softening.
- Phase transformation in the Ni60 matrix: The metastable martensite (ε and η phases) progressively transforms to equilibrium austenite and carbide phases.
- Diffusion bonding at the overlay-substrate interface: Carbon and alloying elements diffuse across the interface, forming a diffusion zone (DZ) that can either strengthen or weaken the bond depending on temperature.
- Residual stress relief: Thermal expansion mismatch between the Ni60 overlay and H13 substrate generates significant tensile residual stresses in the overlay; annealing reduces these stresses substantially.
Key Technical Parameters and Findings
| Annealing Temperature | Overlay Hardness (HV) | Substrate Hardness (HV) | Microstructural Feature | Bond Quality |
|---|---|---|---|---|
| As-deposited (no annealing) | 1500–1700 | 380–420 | Martensite + primary WC + M6C | High residual tensile stress |
| 600 °C | 1350–1500 | 370–400 | Partial martensite decomposition, carbide coarsening begins | Improved, stress reduced ~30% |
| 700 °C | 1100–1300 | 360–390 | Austenite + M6C + MC, significant carbide coarsening | Good, stress reduced ~60% |
| 800 °C | 900–1100 | 350–380 | Coarse M6C and MC, extensive diffusion zone | Acceptable, stress reduced ~80% |
| 900 °C | 700–900 | 340–370 | Very coarse carbides, intergranular cracking risk | Risk of bond degradation |
The optimal annealing temperature was identified near 700 °C, where a favorable balance between hardness retention and stress relief is achieved. Below 600 °C, insufficient stress relief leaves the overlay susceptible to cracking during subsequent service. Above 800 °C, excessive carbide coarsening and matrix softening compromise the wear resistance advantage of the overlay.
Process Considerations and Engineering Implications
The plasma arc welding process parameters used in this study typically include an arc current of 150–200 A, arc voltage of 20–25 V, travel speed of 150–250 mm/min, and a tungsten electrode diameter of 2.0–3.2 mm. The Ni60/WC composite powder is fed as a mixture where WC particles (typically 15–45 μm) are blended with Ni60 powder at mass fractions of 20–40%. A critical observation is that WC dissolution during the welding thermal cycle is inevitable; approximately 50–70% of the original WC particles dissolve into the molten pool, forming new M6C and MC carbides upon solidification. This means the final overlay hardness depends not only on retained WC but also on the newly formed carbide network.
From a practical standpoint, this research provides a clear guideline for die repair shops and maintenance engineers. When overlaying H13 dies with Ni60/WC via plasma arc welding, a post-weld annealing treatment at 700 °C for 1.5 hours in a protective atmosphere (nitrogen or argon) should be standard practice. This treatment reduces residual tensile stresses by approximately 60%, eliminates the risk of stress-corrosion cracking in the overlay, and maintains a surface hardness above 1100 HV, which is more than sufficient for most abrasive wear applications.
Critical Reflections and Independent Insights
One aspect that warrants deeper consideration is the role of the diffusion zone at the overlay-substrate interface. The study indicates that at higher annealing temperatures (above 800 °C), carbon diffusion from the Ni60 overlay into the H13 substrate creates a locally hardened zone (up to 500 HV) adjacent to the interface, which can promote micro-cracking initiation during thermal cycling. This phenomenon is analogous to the sensitization and intergranular cracking observed in stainless steel cladding systems, and it underscores the importance of limiting annealing temperatures to below 800 °C for Ni-based overlays on high-carbon tool steels.
Furthermore, the study does not extensively address the effect of multiple heat passes on the final microstructure. In practical welding of thick overlay layers (3–5 mm), multi-pass welding creates a thermally affected zone within the overlay itself, where earlier-deposited passes undergo repeated heating and cooling. This internal heat-affected zone can develop coarse carbides and reduced hardness, particularly near the top surface of the overlay. Engineers should consider incorporating a grinding or machining step after annealing to remove the top 0.5–1.0 mm of the overlay, thereby exposing a more homogeneous and harder layer.
Summary
This research provides valuable metallurgical guidance for the repair and protection of H13 hot-work dies using plasma arc welding of Ni60/WC composite overlays. The optimal annealing temperature of 700 °C offers the best compromise between hardness retention and residual stress relief. The findings reinforce the principle that post-weld heat treatment is not optional but essential for ensuring the long-term integrity and wear performance of hard overlay deposits on tool steel substrates.