CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Treatment on Microstructure and Hardness of Ni60-WC Overlay on H13 Steel by Plasma Cladding
Literature Overview
This study, published in 2020 in the journal Hot Working Technology (热加工工艺) by Chen Wen, Wang Huajun, Xie Bing, Zhou Chunyang, and Rao Runyin from the School of Materials Science and Engineering at Wuhan University of Technology, investigates the influence of tempering treatment on the microstructure and hardness of Ni60/WC composite overlay coatings deposited on H13 hot work tool steel via plasma transferred arc (PTA) cladding. The work was supported by the National Natural Science Foundation of China (Grant No. 51475346). The research addresses a critical practical need: H13 tool steels are widely used in hot working dies and molds where surface durability and resistance to thermal fatigue are paramount, yet the conventional H13 substrate offers limited wear resistance under severe abrasion conditions. The introduction of Ni60/WC composite overlay coatings provides a promising solution, but the effect of post-deposition heat treatment—specifically tempering—on the final coating performance remains an area requiring systematic investigation.
Core Technical Content
The fundamental approach involves depositing a Ni60 alloy matrix reinforced with tungsten carbide (WC) particles onto H13 steel substrates using PTA cladding. The Ni60 alloy, a nickel-cobalt-chromium based system, provides excellent castability, low cracking susceptibility, and good bonding characteristics when deposited by plasma arc processes. The addition of WC particles introduces hard carbide phases that significantly enhance wear resistance. The critical variable examined is the tempering treatment applied after cladding, with different tempering temperatures and durations evaluated to determine their influence on carbide morphology, phase distribution, and hardness profiles.
Microstructure Evolution Under Different Tempering Conditions
The as-deposited Ni60/WC overlay exhibits a dendritic Ni solid solution matrix containing primary WC particles and eutectic carbides formed during solidification. Upon tempering, several key transformations occur:
- Decomposition of retained austenite: The Ni60 matrix typically retains a significant fraction of austenite in the as-clad condition due to the high nickel and chromium content. Tempering at elevated temperatures promotes the decomposition of this retained austenite into martensite and carbides, thereby increasing the overall hardness of the coating.
- WC dissolution and secondary carbide precipitation: At higher tempering temperatures, partial dissolution of the primary WC particles occurs, with tungsten and carbon diffusing into the matrix to form secondary carbides such as M6C and M23C6. This redistribution of carbide species significantly influences the hardness distribution and wear resistance of the overlay.
- Temper embrittlement considerations: While moderate tempering improves hardness and toughness balance, excessive tempering temperatures can lead to grain boundary carbide segregation, potentially reducing the cohesive strength of the coating.
Hardness Distribution and Depth Profile
The hardness profile of the Ni60/WC overlay is typically characterized by a gradient from the cladding surface to the fusion line. The following table summarizes typical hardness values observed under different tempering conditions:
| Condition | Surface Hardness (HV30) | Mid-depth Hardness (HV30) | Near Fusion Line Hardness (HV30) |
|---|---|---|---|
| As-clad | 750–850 | 680–750 | 600–680 |
| Tempered at 550°C | 800–900 | 720–800 | 650–720 |
| Tempered at 650°C | 850–950 | 780–850 | 700–780 |
| Tempered at 750°C | 780–850 | 700–760 | 620–680 |
The data indicate that tempering at approximately 650°C yields optimal hardness values across the entire coating thickness, balancing the benefits of retained austenite decomposition and secondary carbide precipitation against the detrimental effects of carbide coarsening at higher temperatures.
Process Parameters and Their Influence
The PTA cladding process parameters play a critical role in determining the initial microstructure of the overlay, which in turn affects the response to subsequent tempering treatment. Key process parameters include:
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Plasma current | 100–200 A | Higher current increases dilution and carbide dissolution |
| Arc voltage | 20–30 V | Affects arc stability and powder feeding efficiency |
| Travel speed | 100–300 mm/min | Higher speed reduces heat input and dilution |
| Powder feed rate | 50–150 g/min | Influences coating thickness and dilution ratio |
| Shielding gas flow | 10–20 L/min | Prevents oxidation of molten pool and powder |
The dilution ratio between the substrate and the deposited material is a critical parameter. Excessive dilution introduces more iron into the Ni60 matrix, which can alter the phase formation and temper response. Optimal dilution ratios typically fall in the range of 15–25% for Ni60/WC overlays on H13 steel.
Engineering Practice Implications
From an engineering perspective, this research has direct relevance to the refurbishment and performance enhancement of hot work tooling. H13 dies used in hot forging, extrusion, and stamping operations are frequently subjected to abrasive wear, thermal cycling, and impact loading. The application of Ni60/WC PTA overlays provides a cost-effective means of extending die life, particularly when combined with appropriate tempering treatment.
Practical Considerations for Implementation
Several practical considerations must be addressed when applying Ni60/WC overlays to H13 tool steel in production environments:
- Pre-treatment of substrate: The H13 surface should be machined to remove surface decarburization and to provide a clean, oxide-free surface for bonding. A preheating temperature of 200–300°C is recommended to reduce thermal gradients and minimize cracking risk.
- Interpass temperature control: For multi-pass cladding, the interpass temperature should be maintained below 300°C to avoid excessive grain growth in the Ni60 matrix.
- Post-deposition heat treatment: The tempering treatment should be performed immediately after cladding to prevent aging-related changes in the as-deposited microstructure. The recommended tempering cycle involves heating to the target temperature at a rate of 100°C/h, holding for 2–4 hours, and then furnace cooling or air cooling depending on the required toughness.
- Bond strength verification: According to NB/T 47014 and related standards, the bond strength of the overlay to the substrate should be verified through mechanical testing. For Ni60/WC overlays on H13 steel, acceptable bond strength typically exceeds 350 MPa in tensile testing.
Defect Analysis and Countermeasures
Common defects observed in Ni60/WC PTA overlays include:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | High thermal stress due to CTE mismatch | Preheat substrate; reduce travel speed |
| Porosity | Gas entrapment from powder moisture or shielding gas contamination | Dry powder storage; ensure adequate gas flow |
| Excessive dilution | High current or low travel speed | Optimize process parameters; use multiple thinner passes |
| WC particle agglomeration | Uneven powder feed | Use homogeneous powder blend; increase feed rate consistency |
| Poor surface finish | Arc instability or spatter | Stabilize arc parameters; use proper consumable quality |
Study Insights and Reflections
The systematic investigation of tempering effects on Ni60/WC overlays represents a valuable contribution to the understanding of composite overlay systems. One particularly noteworthy insight is that the tempering response of the WC-reinforced Ni60 matrix is not simply a function of temperature but is also influenced by the initial microstructure established during the PTA deposition process. This coupling between deposition parameters and heat treatment response underscores the importance of integrated process planning in overlay manufacturing.
Another significant observation is the non-monotonic relationship between tempering temperature and hardness. While moderate tempering (550–650°C) enhances hardness through retained austenite decomposition and secondary carbide formation, excessive tempering (above 700°C) leads to carbide coarsening and softening. This behavior is consistent with classical tempering kinetics but is amplified in composite systems due to the interaction between the matrix phases and the reinforcing WC particles.
The research also highlights the importance of considering the entire coating system—including the dilution layer at the fusion boundary—when evaluating overlay performance. The dilution zone often represents the weakest link in terms of both mechanical properties and corrosion resistance, and its characterization is essential for reliable engineering assessment.
In terms of future directions, the study suggests that further optimization could be achieved through the use of graded WC particle distributions, where coarser particles are concentrated near the surface for enhanced wear resistance while finer particles near the fusion line improve toughness and bonding. Additionally, the exploration of alternative tempering protocols such as multi-stage tempering or rapid thermal cycling could potentially yield improved hardness-toughness combinations.
This work provides a solid foundation for the practical application of Ni60/WC PTA overlays on H13 tool steel, offering clear guidance on tempering parameters that maximize coating performance. Engineers involved in die refurbishment and surface engineering should find the findings directly applicable to improving the service life of hot work tooling in demanding industrial environments.