Effect of Quenching and Tempering on Ni60-WC Plasma Cladding Layer Microstructure and Hardness on H13 Steel

Literature Overview and Research Significance

The investigation of quenching and tempering effects on Ni60-WC plasma cladding layers deposited on H13 hot-work die steel addresses a critical challenge in surface engineering for high-temperature wear-resistant applications. H13 steel is widely used in hot forging dies, extrusion dies, and other hot-work tooling applications where the surface must resist wear, thermal fatigue, and oxidation at elevated temperatures. Plasma arc welding (PAW) cladding with Ni60-WC composite powders offers a promising approach to enhancing the surface properties of H13 steel, but the as-cladded microstructure often contains undesirable phases and may not achieve optimal hardness and wear resistance.

Post-weld heat treatment, specifically quenching and tempering, is a well-established method to refine the microstructure and enhance the mechanical properties of cladding layers. The literature under study provides systematic experimental data on the effects of different quenching and tempering parameters on the microstructure, phase composition, and hardness of Ni60-WC plasma cladding layers on H13 steel. This information is directly applicable to engineers designing and qualifying cladding processes for hot-work die repair and surface enhancement.

Baseline Microstructure of As-Cladded Ni60-WC Layer

The as-cladded Ni60-WC plasma cladding layer exhibits a complex microstructure that results from the rapid solidification conditions of plasma arc welding. The microstructure typically consists of a dendritic matrix of austenite and martensite, with carbide particles of Ni3C, Cr7C3, and WC dispersed throughout. The high cooling rates associated with plasma welding (typically 10 to 100 K/s) promote the formation of a fine dendritic structure with limited carbide coarsening.

However, the as-cladded microstructure has several limitations. The martensite in the Ni60 matrix is in a supersaturated state and is prone to cracking during subsequent mechanical loading or thermal cycling. The carbide distribution is often uneven, with some regions exhibiting excessive carbide agglomeration and others showing carbide-free zones. The dilution ratio between the Ni60-WC cladding material and the H13 substrate can range from 5 to 15 percent, depending on the welding parameters, which affects the final composition and phase balance of the cladding layer.

The as-cladded hardness of the Ni60-WC layer is typically in the range of 55 to 65 HRC, which is already significantly higher than the H13 substrate (approximately 45 to 50 HRC in the quenched and tempered condition). However, this hardness is not uniform across the layer thickness, and the overall wear resistance may not be optimized due to the presence of soft austenite phases and uneven carbide distribution.

Quenching and Tempering Parameters and Their Effects

The quenching and tempering process for Ni60-WC plasma cladding layers on H13 steel involves several critical parameters that must be carefully controlled. The quenching temperature, quenching medium, tempering temperature, and tempering time all influence the final microstructure and hardness of the cladding layer.

The quenching temperature is typically set in the range of 800 to 950 degrees Celsius, which corresponds to the austenitizing temperature for the Ni60 matrix. At this temperature, the martensite in the as-cladded layer dissolves and reforms, while the carbide particles undergo partial dissolution and reprecipitation. The quenching medium is typically oil or air, depending on the required cooling rate and the risk of cracking. Oil quenching provides a moderate cooling rate that minimizes cracking while still achieving full martensitic transformation.

The tempering temperature is the most critical parameter for controlling the final hardness and toughness of the cladding layer. Tempering temperatures in the range of 500 to 700 degrees Celsius are typically studied, with each temperature producing a distinct microstructure and hardness profile. At lower tempering temperatures (500 to 550 degrees Celsius), the martensite retains high hardness but with limited toughness. At higher tempering temperatures (650 to 700 degrees Celsius), the martensite transforms to tempered martensite with reduced hardness but improved toughness and thermal stability.

Microstructural Evolution During Heat Treatment

The microstructural evolution of the Ni60-WC cladding layer during quenching and tempering follows a well-defined sequence that is governed by the thermodynamics and kinetics of phase transformations. During quenching at 850 degrees Celsius, the as-cladded martensite dissolves and the carbide particles undergo partial dissolution. The dissolution of carbides releases carbon and alloying elements into the austenite matrix, which increases the hardenability and promotes the formation of a more uniform microstructure upon subsequent quenching.

Upon oil quenching, the austenite transforms to martensite, which is now more uniform in composition and distribution than the as-cladded martensite. The carbide particles that survived the austenitizing temperature remain in the matrix, providing hardening and wear resistance. The tempering process then refines this microstructure by promoting the precipitation of fine carbides from the supersaturated martensite.

At tempering temperatures of 500 to 550 degrees Celsius, the martensite retains its lath structure with fine carbide precipitates dispersed throughout. The hardness is maximized in this range, typically reaching 62 to 68 HRC, which represents an improvement of 5 to 10 HRC over the as-cladded condition. The fine carbide precipitates include Ni3C, Cr7C3, and un-dissolved WC particles, which provide excellent wear resistance.

At higher tempering temperatures of 600 to 650 degrees Celsius, the martensite begins to decompose, and the carbide particles coarsen through Ostwald ripening. The hardness decreases to 45 to 58 HRC, but the toughness and thermal stability improve significantly. This condition is suitable for applications where moderate wear resistance is required but thermal fatigue resistance is more important.

At the highest tempering temperature of 700 degrees Celsius, the microstructure transforms to a ferrite-cementite structure with spheroidized carbides. The hardness drops to 38 to 45 HRC, which is below the wear-resistant threshold for hot-work applications. This condition is generally not recommended for Ni60-WC cladding layers on H13 steel.

Hardness Profiling and Wear Resistance Assessment

The hardness profile of the Ni60-WC cladding layer after quenching and tempering provides critical information about the layer's performance characteristics. The hardness typically decreases with depth from the surface, reflecting the increasing dilution of Ni60-WC with the H13 substrate. The surface hardness is highest due to the lowest dilution ratio, while the hardness at the cladding-substrate interface is lower due to the higher dilution.

The optimal tempering temperature for maximum surface hardness is typically 500 to 550 degrees Celsius, where the surface hardness reaches 65 to 68 HRC. This represents a significant improvement over the H13 substrate hardness of 45 to 50 HRC, providing a wear-resistant surface layer that protects the underlying substrate from abrasive and adhesive wear. The hardness gradient from the surface to the interface is gradual and continuous, which helps to prevent delamination under mechanical loading.

Wear resistance testing, typically performed using pin-on-disk or block-on-ring wear testers, confirms the correlation between hardness and wear resistance. The Q + T1 condition (500 degrees Celsius tempering) exhibits the lowest wear rate, typically 0.5 to 1.0 times 10 to the minus 3 millimeters cubed per newton-meter, which is an improvement of 3 to 5 times over the as-cladded condition and 10 to 20 times over the untreated H13 substrate.

Engineering Practice and Process Optimization

From an engineering practice perspective, the quenching and tempering of Ni60-WC plasma cladding layers on H13 steel requires careful process planning and quality control. The following considerations are essential for successful implementation:

1. Pre-weld preparation: The H13 substrate should be preheated to 200 to 300 degrees Celsius to minimize thermal cracking during plasma welding. The surface should be cleaned and free of oxides, oils, and other contaminants to ensure good metallurgical bonding.
2. Plasma welding parameters: The welding current should be in the range of 150 to 250 amperes, with a travel speed of 200 to 400 millimeters per minute. The powder feed rate should be adjusted to achieve a dilution ratio of 5 to 10 percent, which provides a good balance between cladding composition and metallurgical bonding.
3. Quenching and tempering cycle: The component should be heated to 850 degrees Celsius at a controlled rate of 5 to 10 degrees Celsius per minute to avoid thermal shock. The holding time should be 1 to 2 hours to ensure uniform temperature distribution. Oil quenching should be performed immediately after the holding period, followed by tempering at the selected temperature for 2 hours.
4. Post-heat treatment inspection: Hardness testing should be performed at multiple depths to verify the hardness profile. Metallographic examination should confirm the microstructure and absence of cracks or porosity. If the component is subject to pressure or cyclic loading, non-destructive testing (MT or PT) should be performed to detect surface defects.

Key Questions and Reflections

The study raises several important questions for engineers working in the field of surface engineering and cladding. First, the optimal tempering temperature is application-specific and must be selected based on the relative importance of wear resistance versus thermal fatigue resistance. For hot forging dies that experience severe abrasive wear, the 500 degrees Celsius tempering condition is preferred. For extrusion dies that experience thermal cycling, the 550 to 600 degrees Celsius tempering condition may be more appropriate.

Second, the dilution ratio is a critical parameter that influences the final properties of the cladding layer. A lower dilution ratio (5 percent) results in a cladding layer that more closely matches the Ni60-WC composition, with higher hardness and better wear resistance. However, a higher dilution ratio (15 percent) may improve the metallurgical bond strength at the cladding-substrate interface, which is important for preventing delamination under high mechanical loads.

Third, the effect of multiple cladding passes on the microstructure and properties should be considered. Multi-pass cladding introduces inter-pass heating that can modify the microstructure of previously deposited layers. The thermal cycle from subsequent passes can partially temper the martensite in earlier layers, resulting in a gradient of hardness and microstructure through the layer thickness.

Study Insights and Implications for Practice

The study of quenching and tempering effects on Ni60-WC plasma cladding layers on H13 steel provides valuable insights for engineers designing surface engineering solutions for hot-work die applications. The key finding is that post-weld heat treatment can significantly enhance the hardness, wear resistance, and microstructural uniformity of the cladding layer, with the optimal tempering temperature being application-specific.

For engineers involved in bimetal product manufacturing and pressure vessel fabrication, the principles demonstrated in this study are directly applicable. The control of dilution ratio, the optimization of heat treatment parameters, and the verification of microstructure and mechanical properties are all essential aspects of quality assurance in cladding processes. The systematic approach to process development and qualification, as demonstrated in this literature, should be adopted as a best practice in all cladding and overlay applications.

In conclusion, the quenching and tempering of Ni60-WC plasma cladding layers on H13 steel is a powerful tool for optimizing the surface properties of hot-work dies. The selection of the optimal tempering temperature requires a careful balance between wear resistance and thermal fatigue resistance, and the process parameters must be qualified through systematic testing and inspection. Engineers who understand and apply these principles can significantly extend the service life of hot-work tooling and reduce maintenance costs in manufacturing operations.