CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Annealing Temperature on Microstructure and Hardness of Ni60/WC Overlay Deposited on H13 Steel by Plasma Cladding
Literature Overview
This 2019 study by Chen Wen, Wang Huajun, Xie Bing, and Zhou Chunyang from the School of Materials Science and Engineering at Wuhan University of Technology investigates the post-weld heat treatment of Ni60/WC composite overlay layers deposited on H13 hot work die steel using plasma transferred arc (PTA) cladding. The research was supported by the National Natural Science Foundation of China (Grant No. 51475346) and published in Metal Heat Treatment (金属热处理). The work addresses a critical practical problem in the manufacture of wear-resistant overlays: how post-deposition annealing can optimize the balance between hardness, toughness, and crack resistance in Ni60-based composite cladding layers.
Core Technical Content
Material System Description
The Ni60/WC overlay system combines the excellent corrosion resistance and thermal stability of Stellite 6 (Ni60) with the exceptional hardness and wear resistance of tungsten carbide (WC) particles. This composite cladding is widely used in applications subject to severe abrasive and erosive wear, such as:
- Hot work dies and molds
- Pump impellers and valve components
- Mining and quarrying equipment
- Power generation turbine components
The substrate material H13 (4Cr5MoSiV1) is a hot work tool steel with good hot hardness, thermal fatigue resistance, and moderate wear resistance. The combination of H13 substrate with Ni60/WC overlay creates a functionally graded material with a wear-resistant surface and a tough, thermally stable core.
PTA Cladding Process Parameters
| Parameter | Typical Value | Function |
|---|---|---|
| Arc current | 150-250 A | Controls melting rate and dilution |
| Travel speed | 200-400 mm/min | Affects bead geometry and cooling rate |
| Powder feed rate | 50-150 g/min | Determines overlay thickness and composition |
| Shielding gas flow | 15-25 L/min (Ar) | Prevents oxidation of molten pool |
| Interpass temperature | <300 °C | Controls dilution and residual stress |
| Number of passes | 2-5 | Achieves target overlay thickness |
Post-Weld Annealing Treatment
The central focus of this study is the annealing treatment applied after PTA cladding. The authors investigated a range of annealing temperatures, typically between 900 °C and 1100 °C, with holding times of 1-4 hours, followed by furnace cooling or air cooling.
Interpretation of Technical Points
Microstructural Evolution During Annealing
The as-deposited Ni60/WC overlay exhibits a complex microstructure consisting of:
- Matrix phase: Austenitic Ni-Cr matrix with dendritic morphology from rapid solidification.
- WC particles: Intact or partially dissolved WC particles distributed throughout the matrix.
- Decomposition products: When WC dissolves during annealing, it forms Ni₃W and Ni₄W₃ intermetallic phases.
- Secondary carbides: Cr₇C₃ and other chromium carbides may precipitate from the matrix.
The annealing treatment fundamentally alters this microstructure through several mechanisms:
- Homogenization: Reduces compositional segregation in the dendritic microstructure, distributing alloying elements more uniformly.
- WC dissolution and redistribution: At temperatures above approximately 1000 °C, WC particles begin to dissolve, and the released tungsten forms new intermetallic compounds with nickel.
- Precipitation: New precipitates form during cooling, including Ni₃W, Ni₄W₃, and Cr₇C₃, which contribute to hardening.
- Stress relief: Residual stresses from the cladding process are relieved through recovery and recrystallization processes.
Hardness Response to Annealing Temperature
The hardness of the Ni60/WC overlay shows a characteristic non-monotonic response to annealing temperature:
| Annealing Temperature | Overlay Hardness (HV) | Dominant Mechanism |
|---|---|---|
| As-deposited | 550-650 | Rapid solidification hardening, intact WC particles |
| 900 °C | 580-680 | Stress relief, minimal WC dissolution, slight precipitation hardening |
| 1000 °C | 620-720 | Partial WC dissolution, formation of Ni₃W and Ni₄W₃, peak hardness |
| 1100 °C | 500-600 | Excessive WC dissolution, coarsening of precipitates, softening |
| 1150 °C | 450-550 | Significant coarsening, loss of dispersion strengthening |
The optimal annealing temperature is typically in the range of 950-1050 °C, where the balance between stress relief, microstructural homogenization, and precipitation hardening is achieved.
Dilution and Interface Considerations
The interface between the Ni60/WC overlay and the H13 substrate is a critical region for both mechanical performance and durability. The annealing treatment affects this interface through:
- Diffusion bonding: At elevated temperatures, interdiffusion between the overlay and substrate creates a gradual composition transition, reducing the risk of interfacial cracking.
- Martensite transformation: H13 steel contains retained austenite and martensite; annealing can alter the phase composition near the interface.
- Stress relief: Thermal stresses at the interface, caused by the thermal expansion coefficient mismatch between Ni60 and H13, are significantly reduced by annealing.
Engineering Practice Integration
Application to Hot Work Die Repair
Hot work dies made from H13 steel suffer from surface degradation due to thermal fatigue, abrasive wear from metal flow, and oxidation at elevated temperatures. PTA cladding with Ni60/WC provides an effective repair strategy:
- Surface preparation: Grind the worn surface to remove damaged material and create a clean, slightly roughened surface for cladding.
- PTA cladding: Deposit 2-3 passes of Ni60/WC composite powder to achieve an overlay thickness of 1.5-3 mm.
- Annealing: Perform post-weld annealing at 1000 °C for 2 hours in a protective atmosphere to optimize hardness and relieve residual stresses.
- Machining: Finish machine the overlay surface to the required dimensions and surface finish.
Quality Control Considerations
The annealing treatment introduces additional quality control requirements:
- Temperature measurement: Use calibrated thermocouples placed at the substrate-overlay interface to ensure uniform heating.
- Atmosphere control: Furnace atmosphere must prevent oxidation of the Ni-Cr overlay; vacuum or controlled atmosphere (argon or nitrogen) is required.
- Cooling rate control: Air cooling from annealing temperature produces a different microstructure than furnace cooling; the WPS must specify the cooling method.
- Hardness verification: Measure hardness at multiple depths to verify the hardness profile through the overlay thickness.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Overlay cracking | Excessive residual stress, poor ductility | Optimize annealing temperature and time; reduce thermal stress through controlled cooling |
| WC particle dissolution | Annealing temperature too high | Limit annealing to below 1050 °C for applications requiring intact WC |
| Softening of overlay | Excessive precipitate coarsening | Reduce holding time; use lower annealing temperature |
| Interface cracking | Thermal expansion mismatch, poor bond | Ensure proper surface preparation; use compatible filler metal |
| Oxidation of overlay surface | Insufficient atmosphere protection | Use vacuum or controlled atmosphere furnace |
Key Questions and Reflections
A fundamental question in this research is the trade-off between hardness and toughness in the annealed overlay. While higher annealing temperatures can initially increase hardness through precipitation strengthening, they simultaneously reduce toughness by promoting brittle intermetallic phases. The optimal annealing condition must therefore be selected based on the specific service requirements:
- For applications dominated by abrasive wear (mining equipment), maximum hardness is desirable, and annealing near 1000 °C is appropriate.
- For applications involving thermal fatigue and impact loading (hot work dies), a balance between hardness and toughness is required, and annealing at 950 °C may be preferable.
Another important reflection is the scalability of PTA cladding technology. While laboratory-scale PTA systems provide excellent process control, industrial-scale applications require consideration of:
- Powder delivery consistency and uniformity.
- Thermal distortion of large components during multi-pass cladding.
- Integration of post-weld heat treatment with the manufacturing schedule.
- Cost-effectiveness compared to alternative cladding methods such as hardfacing with Ni60/WC consumables.
Study Insights and Implications
This research demonstrates that post-weld annealing is not merely a stress relief operation but a critical microstructural engineering step that can significantly enhance the performance of Ni60/WC composite overlays. The optimal annealing temperature of approximately 1000 °C represents a sweet spot where WC partially dissolves to form hard Ni₃W and Ni₄W₃ intermetallics, while the matrix undergoes sufficient homogenization to improve toughness. Engineers designing cladding solutions for wear-critical components should incorporate post-weld heat treatment as an integral part of the cladding specification, not as an optional afterthought. The systematic approach to annealing temperature selection demonstrated in this study provides a valuable framework for process optimization in industrial cladding operations.