CLADDING TECHNOLOGY SHANXI CO., LTDBimetal Stamping Die Edge Overlay: New Materials and Process Research
Literature Overview
This 1996 paper by Jiang Lexin, Wang Yuncheng, and Du Yining from Hebei Institute of Mechanical and Electrical Engineering investigates the development of new overlay materials and processes for the working edges of bimetal stamping dies. The work addresses a significant industrial challenge in the sheet metal forming industry, where the cutting edges of stamping dies are subjected to extreme wear, impact, and thermal cycling during high-speed stamping operations. The research was motivated by the need to extend die life and reduce maintenance costs in automotive and rail vehicle manufacturing, where die replacement represents a major production cost.
Core Technical Content and Process Parameters
The bimetal stamping die edge overlay concept involves creating a hard, wear-resistant layer on the cutting edge of the die while maintaining a tougher base material for structural support. The paper explores several overlay material systems, including:
| Overlay Material System | Hardness (HV) | Wear Resistance | Impact Resistance | Application |
|---|---|---|---|---|
| Co-Cr-W (Stellite 6) | 450–500 HV | Excellent | Good | High-temperature forming |
| Cr-based (Cr₇C₃) | 1200–1500 HV | Very High | Moderate | Room-temperature stamping |
| High-speed steel (H13 equivalent) | 500–600 HV | Good | Excellent | General purpose |
| Ni-based (Inconel 625) | 300–400 HV | Moderate | Excellent | High-temperature forming |
| Ceramic composite (WC-Co) | 1500–1800 HV | Very High | Low | Low-impact applications |
The authors found that the optimal overlay material depends on the specific forming conditions, including the workpiece material, forming temperature, stamping speed, and lubrication conditions. For room-temperature stamping of carbon steel sheets, Cr-based overlay materials provided the best combination of wear resistance and cost-effectiveness. For hot stamping of high-strength steel, Co-Cr-W overlay materials were preferred due to their superior high-temperature wear resistance.
The welding process parameters for the overlay were optimized as follows:
| Parameter | Specification | Rationale |
|---|---|---|
| Welding process | Plasma Transferred Arc (PTA) or Laser Cladding | Low dilution, high quality |
| Shielding gas | Argon, 99.99% purity | Prevents oxidation |
| Heat input | Low to moderate | Minimizes dilution and distortion |
| Preheat temperature | 100–200°C | Prevents cracking in base metal |
| Interpass temperature | ≤150°C | Controls thermal stress |
| Overlay thickness | 1.0–2.5 mm | Sufficient for wear resistance |
| Post-weld treatment | Stress relief at 500–600°C | Reduces residual stress |
Process Analysis and Microstructural Considerations
The microstructure of the overlay deposit is critical to its performance. For Co-Cr-W overlays, the microstructure consists of a Co-rich matrix with dispersed Cr₇C₃ and WC carbides. The hardness and wear resistance are directly related to the size, distribution, and volume fraction of these carbide phases. Fine, uniformly distributed carbides provide the best combination of hardness and toughness.
For Cr-based overlays, the microstructure is dominated by Cr₇C₃ carbides in an austenitic or ferritic matrix. The hardness is extremely high (1200–1500 HV), but the impact resistance is limited due to the brittleness of the carbide phase. The overlay thickness must be carefully controlled to avoid cracking during service.
The authors emphasized the importance of the transition zone between the overlay and the base metal. A gradual transition in composition and microstructure is essential to prevent cracking at the interface. This can be achieved by using a multi-layer overlay approach, where the first layer is a transition alloy (e.g., a Ni-based alloy with intermediate composition), followed by the final hardfacing layer.
Engineering Practice and Application Results
The practical application of the bimetal stamping die edge overlay was demonstrated on automotive stamping dies for body panels and rail vehicle stamping dies for structural components. The overlay extended the die life by a factor of 3–5 compared to conventional single-material dies, resulting in significant cost savings and reduced production downtime.
A key practical consideration is the grinding and finishing of the overlay surface after welding. The overlay surface must be ground to a precise profile and surface finish to ensure proper die function. The grinding process can be challenging due to the hardness of the overlay material, and specialized grinding wheels (e.g., CBN or diamond) may be required. The authors recommended that the overlay thickness be designed with an additional 0.5–1.0 mm allowance for grinding.
The paper also discusses the importance of die maintenance and re-overlay capability. When the overlay is worn, the die can be re-overlay welded without requiring complete die replacement. This capability significantly reduces maintenance costs and production downtime. The re-overlay process requires careful preparation of the worn surface to ensure proper bonding with the new overlay deposit.
Study Insights and Reflections
This paper represents an important contribution to the field of bimetal die technology. The authors' systematic approach to material selection and process optimization provides a valuable framework for engineers designing overlay applications for stamping dies. The work demonstrates that the selection of the overlay material must be based on a thorough understanding of the specific service conditions, rather than a one-size-fits-all approach.
One area that the paper does not fully address is the residual stress state of the overlay weldment and its effect on die fatigue life. The welding process introduces significant residual stresses that can reduce the fatigue life of the die. While the paper mentions stress relief heat treatment, the effectiveness of this treatment in reducing residual stresses to acceptable levels is not thoroughly evaluated. Modern engineering practice would incorporate residual stress measurement (e.g., using X-ray diffraction or neutron diffraction) into the overlay quality assurance process.
Another important consideration is the effect of the overlay on the die's formability. The overlay creates a bimetallic structure with different mechanical properties, which can affect the die's behavior during stamping operations. The authors should have discussed the interaction between the overlay and the base metal during the forming process, particularly in terms of stress distribution and potential for cracking at the overlay-base interface.
The paper also implicitly raises the question of alternative die manufacturing technologies. While the paper focuses on welding overlay, modern alternatives such as surface hardening (e.g., carburizing, nitriding), thermal spray, and even additive manufacturing (3D printing) can offer competitive performance for stamping die applications. The selection of the appropriate technology should be based on a comprehensive comparison of performance, cost, and manufacturability.