Zhuojin Pipe Fitting Co., LtdExperimental Study on Overlapping Welding Process of Tubular Cast Tungsten Carbide
Literature Overview
The paper published in the journal Heat Treatment (2025, Vol. 40, No. 5, pp. 23-28) by He Kailin, Wang Chen, and Shi Xiaohua from Jiangsu Fengshang Intelligent Technology Co., Ltd. addresses a highly practical engineering challenge: the overlapping welding (surfacing) of tubular cast tungsten carbide components. The authors investigated how surfacing process parameters and post-weld heat treatment influence the microstructure, hardness, and wear resistance of the overlay layer. The study is particularly relevant to engineers working on surface engineering for wear-resistant components in mining, petroleum, and heavy machinery sectors, where tubular cast tungsten carbide parts are widely used for their exceptional abrasion resistance but suffer from limited service life due to edge wear and surface degradation.
Core Technical Findings
The researchers tested two different base materials for surfacing, followed by various heat treatment cycles including medium-frequency quenching and carbonitriding. The key findings can be summarized as follows:
Effect of Overlay Layer Thickness
The study demonstrates that both excessively thin and excessively thick overlay layers are detrimental to the uniform distribution of cast tungsten carbide particles. When the overlay layer is too thin, the molten pool fails to fully engulf and redistribute the carbide particles, resulting in localized clustering and insufficient metallurgical bonding. Conversely, when the molten pool is too thick, the high dilution rate dissolves the tungsten carbide particles into the matrix, transforming them into complex carbides and reducing the volume fraction of the primary hard phase. This finding directly informs process parameter selection in production settings, where maintaining an optimal dilution ratio is critical.
Dilution and Carbide Transformation
After surfacing, the original cast tungsten carbide (WC) particles undergo significant dilution. The degree of dilution varies depending on the welding heat input and the base material's thermal properties. The dissolved tungsten reacts with chromium and iron in the matrix to form complex carbides such as M₆C and M₂₃C₆. The authors note that varying levels of dilution correspond to different matrix hardness values, with higher dilution generally producing a softer matrix due to the replacement of discrete WC particles with finer, more dispersed carbide phases.
Metallurgical Bonding and Heat Treatability
A critical conclusion from the study is that the overlay layer forms true metallurgical bonding with the base material, which means the composite part can undergo post-weld heat treatment without delamination risk. This is an important distinction from thermal spray coatings, which typically rely on mechanical interlocking and cannot be heat treated.
Carbonitriding Enhancement
Among the heat treatments tested, carbonitriding (渗碳淬火) was found to be the most effective for improving overlay performance. The carbonitriding process enriches the near-surface region with carbon and nitrogen, forming additional fine carbides within the matrix and increasing the matrix hardness. This synergistic effect between the residual WC particles and the newly formed carbides results in a significant enhancement of wear resistance.
Process Parameter Analysis
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Overlay thickness | Moderate (neither too thin nor too thick) | Ensures adequate carbide distribution and minimizes excessive dilution |
| Heat input | Controlled to limit dilution | Prevents complete dissolution of WC particles |
| Post-weld treatment | Medium-frequency carbonitriding | Maximizes matrix hardness and carbide dispersion |
| Base material selection | Two materials tested | Different thermal properties affect dilution rate |
Engineering Practice Integration
From a practical standpoint, this study provides actionable guidance for manufacturers producing tubular cast tungsten carbide components. The following engineering considerations should be noted:
- Process window control: The acceptable overlay thickness window is narrow, requiring precise control of welding speed, wire feed rate, and torch travel speed. In automated surfacing setups, closed-loop thickness monitoring via laser displacement sensors is recommended.
- Heat treatment scheduling: Since the overlay can withstand heat treatment, a carbonitriding cycle should be incorporated into the production workflow. Typical carbonitriding parameters for this application would be 820-880°C for 2-4 hours in a carbon-rich atmosphere, followed by oil quenching and tempering at 200-250°C.
- Quality inspection: Vickers hardness testing should be performed at multiple depths across the overlay to verify the hardness profile. Metallographic examination should confirm the presence of undissolved WC particles and the absence of excessive complex carbide formation.
- Defect prevention: Cracking at the fusion line is a potential concern, particularly if the base material has high carbon content or if the cooling rate is too rapid. Preheating the base material to 150-250°C can mitigate this risk.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the optimal dilution ratio for maximizing wear resistance is not precisely quantified. Future work should systematically vary the dilution rate and correlate it with tribological performance metrics such as specific wear rate and friction coefficient. Second, the study does not address the long-term stability of the overlay under cyclic loading conditions, which is critical for components subjected to repeated impact and abrasion. Third, the effect of overlay layer count (single pass vs. multi-pass) on carbide distribution and bonding quality remains to be explored.
Another reflection worth noting is that the use of medium-frequency induction heating for the quenching step represents a cost-effective approach compared to conventional furnace-based carbonitriding, particularly for batch production of tubular components where inductive heating can be applied selectively to the overlay region without affecting the base material.
Study Insights and Implications
This research contributes valuable process knowledge to the field of tungsten carbide surface engineering. The confirmation that metallurgical bonding is achievable and that subsequent heat treatment is feasible opens new pathways for enhancing the service life of cast tungsten carbide tubular components. For production engineers, the most actionable takeaway is the clear demonstration that carbonitriding after surfacing provides the most significant improvement in overlay hardness and wear resistance. This finding should be incorporated into standard operating procedures for any facility manufacturing tungsten carbide overlaid tubular parts, as it offers a straightforward and cost-effective method to extend component life without requiring changes to the base casting process.