Zhuojin Pipe Fitting Co., LtdEffect of Annealing Temperature on Ni60/WC Composite Coating Microstructure and Hardness on H13 Steel by Plasma Surfacing A Technical Study Note
Literature Overview
The paper by Chen Wen, Wang Huajun, Xie Bing, and Zhou Chunyang from Wuhan University of Technology's School of Materials Science and Engineering, published in Metal Heat Treatment (Vol. 44, No. 2, 2019, pp. 131-136), investigates the influence of annealing temperature on the microstructure and hardness of Ni60/WC composite coatings produced by powder plasma surfacing on H13 hot work steel substrates. Supported by the National Natural Science Foundation of China (Grant No. 51475346), this work provides detailed characterization of the coating, heat-affected zone (HAZ), and substrate microstructures at multiple annealing temperatures, along with comprehensive hardness profiling.
Technical Context and Coating System Design
The Ni60/WC composite coating system is designed for applications requiring simultaneous resistance to high-temperature wear, thermal shock, and corrosive environments. The coating composition combines:
- Ni60 matrix: A nickel-based superalloy with high-temperature strength, oxidation resistance, and good thermal fatigue performance.
- WC hard particles: Tungsten carbide particles providing extreme hardness (2300–2600 HV) through dispersion strengthening and composite toughening mechanisms.
The H13 substrate (equivalent to AISI H13, 4Cr5MoSiV1) is a hot work tool steel used for hot forging dies, extrusion dies, and casting molds. The combination addresses the common failure mode of hot work dies—surface degradation due to combined thermal fatigue, abrasion, and adhesive wear.
Microstructural Evolution with Annealing Temperature
As-Welded (As-Surfaced) Condition
The as-surfaced coating exhibits:
- Coating microstructure: Coarse dendritic structure with petal-shaped (rosette) dendrites characteristic of rapid solidification in Ni-based alloys. WC particles are unevenly distributed, with clustering at dendrite tips and coarser particles (20–50 μm) near the surface. Microcracks are present along dendrite boundaries, indicating high residual stress and cracking sensitivity.
- HAZ microstructure: Predominantly coarse plate martensite and lath martensite with retained austenite. The hard martensitic structure (600–700 HV) creates a brittle zone susceptible to cracking under thermal cycling.
- Substrate: Minimal microstructural change beyond the immediate HAZ boundary, retaining the original tempered martensite structure.
600 °C Annealing
At 600 °C, the following transformations occur:
| Region | Microstructural Change | Hardness Change |
|---|---|---|
| Coating | Slight WC redistribution; minor crack healing | Slight decrease (10–15 HV) |
| HAZ | Transformation to lath ferrite + acicular ferrite + granular cementite | Significant decrease (200–250 HV reduction) |
| Interface | Quenched hard zone softened; transition becomes more gradual | Hardness gradient established |
The 600 °C annealing effectively eliminates the brittle martensitic structure in the HAZ through tempering and partial recrystallization, replacing it with a tougher ferrite-cementite microstructure. However, the coating microstructure remains largely unchanged, as this temperature is below the recrystallization temperature of the Ni-based matrix.
800 °C Annealing
At 800 °C, more profound microstructural changes occur:
- Coating: The petal-shaped dendrites are refined through partial recrystallization and grain boundary migration. WC particles redistribute more uniformly through diffusion and liquid-phase redistribution. The microcracks observed in the as-welded condition are eliminated through stress relief and microstructural healing. The overall microstructure becomes more homogeneous with finer dendrite arm spacing.
- HAZ: Complete transformation to equiaxed ferrite with granular cementite (spheroidized structure). This represents a fully tempered condition with excellent toughness and ductility. The hardness in this region drops to approximately 350–400 HV.
- Interface: The transition from substrate to coating exhibits a smooth hardness gradient rather than an abrupt change. The quench-hardened zone is completely eliminated, and the hardness rises gradually from approximately 380 HV in the substrate to 600–650 HV in the coating.
Hardness Profiling and Performance Assessment
| Position | As-Welded (HV) | 600 °C Annealed (HV) | 800 °C Annealed (HV) |
|---|---|---|---|
| Substrate | 420–440 | 420–440 | 420–440 |
| HAZ | 650–720 | 450–500 | 380–420 |
| Coating base | 580–620 | 560–600 | 580–620 |
| Coating middle | 600–650 | 580–620 | 600–650 |
| Coating surface | 650–720 | 620–680 | 650–700 |
| Maximum hardness | 720 (surface) | 680 (surface) | 700 (surface) |
The 800 °C annealing condition produces the most favorable hardness distribution: uniform coating hardness with a smooth gradient to the substrate, elimination of the brittle HAZ, and retention of adequate coating hardness for wear resistance.
Engineering Practice Recommendations
For engineers implementing Ni60/WC plasma surfacing on H13 hot work tools, the following guidelines are recommended:
- Mandatory post-weld heat treatment: The as-surfaced condition is unsuitable for production service due to high cracking sensitivity and brittle HAZ. Post-weld annealing is not optional but essential.
- Recommended annealing schedule: 800 °C for 2 hours in air or protective atmosphere, followed by furnace cooling to below 200 °C. This eliminates cracking, refines the microstructure, and establishes an optimal hardness gradient.
- Alternative schedule for dimensional stability: For precision components where distortion is critical, 600 °C for 3 hours provides adequate HAZ toughening with minimal dimensional change, though the coating microstructure improvement is less pronounced.
- Quality verification: After annealing, verify the following:
- Hardness profile across coating-thickness (minimum 580 HV in coating)
- Visual inspection for surface cracking (should be crack-free)
- Magnetic particle testing (MT) of coating surface for subsurface cracks
- Cross-section metallography to confirm uniform WC distribution
- Service life expectation: With proper 800 °C post-weld annealing, the coating system is expected to provide 3–5 times the service life of the uncoated H13 substrate in hot forging and extrusion applications, with failure mode transitioning from surface fatigue to bulk die failure.
Study Insights and Reflections
This research provides a comprehensive understanding of how post-weld heat treatment transforms the performance of Ni60/WC composite coatings from a potentially unreliable as-welded condition to a robust, crack-free system suitable for demanding industrial applications. The finding that 800 °C annealing simultaneously addresses three critical issues—crack elimination, WC redistribution, and HAZ toughening—demonstrates the power of properly designed heat treatment as a unifying process step.
The hardness gradient achieved at the substrate-coating interface after 800 °C annealing is particularly significant from a fracture mechanics perspective. An abrupt hardness change creates stress concentration sites that initiate delamination under cyclic loading. The smooth gradient established by the recommended annealing schedule effectively eliminates this failure initiation mechanism, which is likely the primary reason for the extended service life observed in practice.
For engineers developing surfacing specifications for hot work tools, this work establishes clear, evidence-based guidelines for post-weld heat treatment parameters. The research demonstrates that the quality of the final component is determined not only by the surfacing process itself but critically by the subsequent heat treatment, which should be specified with the same rigor as the welding parameters themselves.