CLADDING TECHNOLOGY SHANXI CO., LTDExperimental Study on Tubular Casting Tungsten Carbide Cladding Process with Heat Treatment
Literature Overview
This research focuses on the development and optimization of a tubular casting tungsten carbide (WC) cladding process, with particular emphasis on the heat treatment parameters that govern the final microstructure and tribological performance of the overlay. The study is conducted by researchers from Jiangsu Fengshang Intelligent Technology Co., Ltd., addressing a practical industrial need for durable wear-resistant tubular components used in mining, cement, and aggregate processing industries. The cladding process involves depositing a WC-reinforced composite layer onto tubular substrates, followed by controlled heat treatment to optimize the carbide distribution and matrix hardness.
Process Description and Parameters
The tubular casting cladding process involves several key stages: substrate preparation, preheating, cladding material application, casting, and post-casting heat treatment. The substrate material is typically low-carbon steel (Q235 or 20# steel), and the cladding material is a WC-cermet composite containing 40 to 60 wt% WC particles embedded in a cobalt or nickel-based binder matrix.
Heat Treatment Parameter Matrix
| Parameter | Range Tested | Optimal Value | Effect |
|---|---|---|---|
| Heating Temperature | 800-1100 degrees C | 950 degrees C | Carbide dissolution and redistribution |
| Heating Rate | 5-20 degrees C/min | 10 degrees C/min | Thermal stress control |
| Dwell Time | 1-4 hours | 2 hours | Homogenization completion |
| Cooling Method | Air / Furnace / Oil quench | Controlled furnace cool | Residual stress management |
| Post-tempering | 200-600 degrees C | 400 degrees C | Binder matrix softening |
The heat treatment process is critical because the as-cast microstructure of the WC composite typically contains coarse, irregularly shaped carbides and may exhibit significant residual stresses from the casting solidification. The heat treatment serves to dissolve excess free carbides, redistribute the carbide phase, and relieve internal stresses without compromising the overall hardness of the overlay.
Microstructural Analysis
Metallographic examination reveals that the as-cast overlay contains a mixture of primary WC particles (5 to 50 micrometers), secondary carbides formed during solidification, and a cobalt-rich binder phase. After heat treatment at 950 degrees C for 2 hours followed by controlled furnace cooling, the microstructure shows significant improvement. The free carbides that were irregularly shaped and poorly distributed in the as-cast condition become more uniformly dispersed, with a reduced average size of approximately 8 micrometers. The binder phase undergoes solid solution strengthening, with carbon atoms dissolved in the cobalt matrix reducing the binder hardness from approximately 200 HV to 180 HV while increasing its ductility.
The interface between the cladding layer and the substrate is another critical area examined in this study. In the as-cast condition, the interface exhibits a diffusion zone of approximately 50 to 100 micrometers with a gradual transition in composition. After heat treatment, this diffusion zone expands to 150 to 200 micrometers, indicating enhanced metallurgical bonding between the overlay and substrate. The increased diffusion zone contributes to improved bond strength, which is essential for tubular components subjected to impact loading during operation.
Mechanical Property Comparison
| Condition | Overlay Hardness (HV30) | Wear Index (mm3/Nm) | Bond Strength (MPa) | Fracture Toughness (MPa·m^0.5) |
|---|---|---|---|---|
| As-cast | 1450 | 0.0008 | 180 | 8.5 |
| 800C/2h air cool | 1380 | 0.0010 | 200 | 9.2 |
| 950C/2h furnace cool | 1520 | 0.0005 | 245 | 11.0 |
| 1100C/2h oil quench | 1480 | 0.0007 | 210 | 9.8 |
| 950C/2h + 400C temper | 1500 | 0.0006 | 250 | 11.5 |
The data clearly demonstrates that the optimal heat treatment condition of 950 degrees C for 2 hours with controlled furnace cooling, followed by tempering at 400 degrees C, provides the best combination of hardness, wear resistance, bond strength, and fracture toughness. This condition represents the engineering optimum for tubular WC cladding applications.
Defect Analysis and Countermeasures
The study identifies several common defects that can occur during the tubular casting cladding process and their relationship to heat treatment parameters. Porosity is the most frequently observed defect, occurring primarily at the cladding-substrate interface and within the overlay layer. Porosity formation is attributed to gas evolution from moisture in the preheated substrate or from decomposition of organic binders in the cladding material. The recommended countermeasure is thorough substrate cleaning and preheating to 200 to 300 degrees C to remove surface moisture before cladding application.
Cracking at the overlay-substrate interface is another critical defect, particularly problematic for tubular geometries where the curvature creates additional thermal stresses during cooling. The study shows that a heating rate exceeding 15 degrees C/min during heat treatment significantly increases the risk of interface cracking due to differential thermal expansion between the overlay and substrate. Maintaining a heating rate of 10 degrees C/min or lower, combined with a gradual cooling rate, effectively mitigates this defect.
Hot tearing within the overlay layer is associated with excessive free carbide content and insufficient binder wetting of the WC particles. The heat treatment process partially addresses this by dissolving excess free carbides and improving binder distribution. However, the study recommends that the initial cladding composition be optimized to limit free carbide content below 15 vol% to minimize hot tearing susceptibility.
Engineering Practice Integration
For industrial implementation, the heat treatment process for tubular WC cladding requires careful thermal management. Tubular geometries present unique challenges compared to flat plates because of the variation in wall thickness around the circumference and along the length. The study recommends using induction heating for uniform temperature distribution around the tube circumference, with thermocouples placed at multiple axial positions to monitor temperature uniformity. The maximum allowable temperature gradient across the wall thickness should not exceed 50 degrees C to prevent differential expansion-induced cracking.
For large-diameter tubes (above 300 mm), the heat treatment process should include a stress relief step at 550 to 600 degrees C for 1 hour before the main heat treatment cycle. This pre-stress-relief step reduces the residual stresses from the casting process and minimizes the risk of distortion during subsequent heat treatment. For smaller diameter tubes (below 100 mm), the entire heat treatment cycle can be performed in a single step without pre-stress relief.
Study Insights and Implications
This study provides a comprehensive process window for the heat treatment of tubular WC cladding, establishing clear relationships between thermal parameters and final performance. The finding that 950 degrees C with controlled furnace cooling provides the optimal balance of hardness and toughness is particularly valuable for engineers selecting heat treatment schedules for production environments. The study also highlights the importance of the cladding-substrate interface as a critical region that requires careful thermal management to ensure long-term service reliability. For engineers involved in the design and manufacture of wear-resistant tubular components, the practical takeaway is that the heat treatment step is not merely a post-processing convenience but a critical process parameter that directly determines the service life of the cladded component. The recommended process parameters provide a reliable starting point for production implementation, with the understanding that specific applications may require further optimization based on service conditions and loading requirements.