Zhuojin Pipe Fitting Co., LtdOverlay Welding Process and Economic Analysis for Large Cover Insert Molds
Literature Overview
This paper by He Bolin et al. from East China Jiaotong University and Luoyang First Tractor and Construction Machinery Co. (Mechanical Design and Manufacturing, 2009, No. 11, pp. 238-240) presents a comprehensive study of overlay welding technology applied to large cover insert molds for automotive body panel forming. The work combines technical evaluation with economic analysis, demonstrating the practical value of overlay welding in mold manufacturing and repair. The research was supported by the former Ministry of Machinery Industry Education Department Science and Technology Fund (95251214) and the Key Laboratory of Transport Equipment and Equipment.
Technical Background and Application Context
Large cover molds for automotive body panels are subjected to severe forming conditions involving high contact stresses, repeated impact loading, and abrasive wear from the sheet metal being formed. The critical wear surfaces are the cutting edges and forming surfaces of the insert blocks (镶块), which are traditionally manufactured from tool steel and subjected to overall heat treatment. This conventional approach is expensive and time-consuming, particularly for large mold assemblies where the insert blocks are numerous and complex in shape.
The alternative approach proposed in this study is to use ordinary cast iron as the base material for the insert blocks and apply a wear-resistant overlay layer to the critical surfaces. This approach leverages the excellent machinability and low cost of cast iron while providing the necessary wear resistance through the overlay layer.
Overlay Welding Process Design
The overlay welding process was designed to deposit a wear-resistant layer on the cutting edges of the cast iron insert blocks. The process parameters and design considerations are summarized below:
| Process Parameter | Specification | Rationale |
|---|---|---|
| Base material | Ordinary cast iron (HT200-HT300) | Low cost, good machinability |
| Electrode type | Cast iron repair electrode (e.g., E-ZrC-Fe) | High hardness, good bonding to cast iron |
| Welding process | Shielded metal arc welding (SMAW) | Flexibility, portability, low equipment cost |
| Preheat temperature | 250-400°C | Reduce cracking risk, improve ductility |
| Interpass temperature | 200-350°C | Control thermal stress |
| Post-weld treatment | Stress relief annealing | Reduce residual stress |
| Layer thickness | 3-5 mm | Adequate wear protection |
| Hardness requirement | 45-55 HRC | Sufficient for forming operations |
Microstructural Analysis of the Overlay
The overlay layer deposited on the cast iron base exhibits a microstructure that is critical for its wear resistance performance. The typical microstructure includes:
- Martensite matrix: Provides the primary source of hardness. The carbon content of the cast iron base and the alloying elements in the electrode flux contribute to the formation of martensite during cooling.
- Carbide particles: Hard carbide phases such as Fe3C, Cr7C3, or Cr23C6 are formed depending on the electrode composition. These particles provide additional wear resistance through dispersion strengthening and act as hard points that resist abrasive wear.
- Bonding zone: The interface between the overlay and the cast iron base is critical for the structural integrity of the repair. The bonding zone must be free of cracks, lack of fusion, and excessive dilution. The preheat temperature is adjusted to ensure good melting and bonding at the interface while minimizing the formation of brittle phases.
Economic Analysis and Comparative Evaluation
The economic analysis is a distinctive feature of this paper, providing a quantitative comparison between the overlay welding approach and the conventional tool steel insert approach.
Performance Comparison
| Metric | Overlay Welded Cast Iron Insert | Conventional Tool Steel Insert | Improvement |
|---|---|---|---|
| Service life | 25,000 pieces | 25,000 pieces | Equivalent |
| Comparison with D322 repair | 25,000 pieces | 19,000-19,200 pieces | 30-50% improvement |
| Hardness | 45-55 HRC | 50-55 HRC | Slightly lower but adequate |
| Design workload | Reduced by >80% | Baseline | Significant reduction |
| Machining equipment | Reduced by >70% | Baseline | Major reduction |
| Precious metal material | Reduced by >90% | Baseline | Major savings |
The achievement of equivalent service life (25,000 pieces) between the overlay welded cast iron insert and the conventionally heat-treated tool steel insert is the most significant technical result. This demonstrates that the overlay welding approach can fully replace the conventional method without sacrificing performance. The 30-50% improvement over D322 electrode repair further validates the superiority of the optimized overlay welding process.
Cost Structure Analysis
The economic advantages of the overlay welding approach are substantial:
| Cost Component | Conventional Approach | Overlay Welding Approach | Savings |
|---|---|---|---|
| Material cost | High (tool steel) | Low (cast iron + electrode) | >90% |
| Heat treatment cost | High (overall HT) | Low (stress relief only) | ~70% |
| Machining cost | High (tool steel) | Low (cast iron) | ~70% |
| Design cost | High (complex inserts) | Low (simplified design) | >80% |
| Total cost | Baseline | 30-40% of baseline | 60-70% |
The reduction in design workload by more than 80% is particularly significant from a project management perspective. Simplified mold designs with cast iron inserts and overlay welded surfaces reduce the complexity of the mold design process, shorten the design cycle, and reduce the risk of design errors. This translates into faster time-to-market for new vehicle models and lower engineering costs.
Engineering Practice Implications
The successful application of overlay welding to large cover insert molds has several important implications for engineering practice:
- Material substitution: The use of cast iron as a base material with overlay welded surfaces demonstrates a viable alternative to expensive tool steels for mold applications where the wear is localized to specific surfaces.
- Process flexibility: The SMAW process used for the overlay provides excellent flexibility for repairing worn surfaces in the field, without the need for expensive equipment or specialized facilities.
- Quality control: The overlay welding process requires careful attention to preheat temperature, interpass temperature, and post-weld stress relief to ensure the quality and durability of the overlay layer. Non-destructive testing (visual inspection, dye penetrant testing) should be performed to verify the absence of surface cracks and defects.
- Scalability: The approach is readily scalable to larger mold assemblies and can be applied to a wide range of mold types beyond automotive body panels, including stamping dies, forging dies, and forming tools.
Key Technical Insights and Reflections
This paper is notable for its integration of technical evaluation with economic analysis, providing a holistic view of the value proposition of overlay welding technology. The quantitative economic comparison, showing savings of 60-70% in total cost while maintaining equivalent performance, makes a compelling case for the adoption of overlay welding in mold manufacturing.
The achievement of 25,000 pieces of service life with the overlay welded cast iron insert is particularly impressive, as it matches the performance of the conventionally manufactured tool steel insert. This result validates the metallurgical design of the overlay process and demonstrates that the combination of cast iron base material and hardfacing overlay can produce a composite structure with the mechanical properties required for severe forming operations.
The reduction in design workload by more than 80% is a frequently overlooked but important benefit. Simplified mold designs not only reduce engineering costs but also reduce the risk of errors and improve the overall quality of the mold design. The ability to use standard cast iron inserts with overlay welded surfaces, rather than custom tool steel inserts, streamlines the mold manufacturing process and reduces the supply chain complexity.
Summary and Conclusions
This study demonstrates that overlay welding technology can be effectively applied to large cover insert molds for automotive body panel forming, achieving a service life of 25,000 pieces that is equivalent to conventionally manufactured tool steel inserts. The overlay welded cast iron inserts provide a 30-50% improvement in life over D322 electrode repairs, while offering dramatic economic advantages including 80% reduction in design workload, 70% reduction in machining equipment requirements, and 90% reduction in precious metal material costs. The total cost reduction of 60-70% makes this approach highly attractive for industrial applications. The key technical factors for success include proper preheat temperature control, appropriate electrode selection, and post-weld stress relief treatment. This work represents a practical and economically compelling application of overlay welding technology in the mold manufacturing industry, and its methodology can be extended to other mold and tool applications where localized wear is the primary failure mechanism.