Zhuojin Pipe Fitting Co., LtdOverlay Welding Materials and Processes for Cold Stamping Dies
Literature Overview
The paper by Liu Renpei, Zhao Kun, Dong Zuyue, and Li Cuiyun from the Harbin Institute of Welding presents the development of a CrMoWVTi alloy-based overlay welding material for cold stamping dies. The authors formulated two consumable forms—TIG wire and covered electrode—and developed a complete overlay welding process for both repair and manufacturing of cold stamping dies. The material achieves a hardness of HRC ≥ 60, with wear resistance comparable to GCr15 bearing steel and crack resistance approaching that of hot working die materials.
Core Technical Findings
Material Design Philosophy
The CrMoWVTi alloy system was selected based on a systematic analysis of the working conditions and failure modes of cold stamping dies. Cold stamping dies operate under conditions of high contact pressure, abrasive wear from the workpiece material, and cyclic loading that can cause fatigue cracking. The alloy design addresses these requirements through:
- Cr (Chromium): Provides solid solution strengthening and forms hard carbides (Cr7C3, Cr23C6) that contribute to wear resistance.
- Mo (Molybdenum): Enhances secondary hardening and improves high-temperature strength, which is beneficial during the hot working phases of die manufacturing.
- W (Tungsten): Forms extremely hard WC carbides that provide superior wear resistance and contributes to the overall hardness of the overlay.
- V (Vanadium): Forms very hard VC and V2C carbides that refine the grain structure and improve toughness.
- Ti (Titanium): Forms TiC carbides that are among the hardest carbide phases and provide excellent resistance to abrasive wear.
Performance Characteristics
| Property | CrMoWVTi Overlay | GCr15 Bearing Steel | Hot Working Die Steel |
|---|---|---|---|
| Hardness | HRC ≥ 60 | HRC 60–65 | HRC 45–55 |
| Wear resistance | Comparable to GCr15 | Reference | Lower |
| Crack resistance | Approaches hot working die | Lower | Reference |
| Application | Cold stamping dies | Bearings | Hot forging dies |
The achievement of HRC ≥ 60 hardness with crack resistance approaching that of hot working die materials is a significant accomplishment. The high carbon equivalent of this alloy system typically leads to cold cracking susceptibility, and the inclusion of titanium and vanadium likely contributes to the improved crack resistance through carbide grain refinement and the formation of a more ductile matrix.
Process Development
The development of two consumable forms—TIG wire and covered electrode—provides process flexibility for different application scenarios:
- TIG wire: Suitable for precision overlay welding on complex die geometries, repair of small areas, and applications requiring low dilution and high metallurgical control. The TIG process provides excellent protection and produces a clean, smooth overlay surface.
- Covered electrode: Suitable for large-area overlay welding, repair of heavily worn dies, and field applications where portable equipment is required. The flux in the electrode provides additional alloying and deoxidation, but results in higher dilution and a rougher surface.
Engineering Practice Implications
Heat Treatment Considerations
The as-welded hardness of HRC ≥ 60 is achieved through the formation of a martensitic matrix with dispersed carbides. However, the residual stresses from welding must be relieved to prevent cracking during subsequent grinding and use. The following heat treatment sequence is recommended:
- Stress relief: 550–600 °C for 2 hours to relieve welding residual stresses without significantly reducing hardness.
- Tempering: If the as-welded hardness is excessive, tempering at 200–250 °C for 1 hour can reduce the hardness to HRC 55–58 while improving toughness.
- Aging: For critical applications, a multi-stage aging treatment at 400–500 °C can stabilize the carbide structure and reduce the risk of delayed cracking.
Process Window for Cold Stamping Die Overlay
| Parameter | TIG Overlay | Covered Electrode Overlay |
|---|---|---|
| Preheat temperature | 150–250 °C | 250–350 °C |
| Interpass temperature | ≤ 250 °C | ≤ 300 °C |
| Welding current (TIG) | 80–150 A | N/A |
| Welding current (SMAW) | N/A | 80–120 A |
| Travel speed | 30–60 mm/min | 40–80 mm/min |
| Bead width | 6–10 mm | 8–12 mm |
| Post-weld treatment | Stress relief at 550–600 °C | Stress relief at 550–600 °C |
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking at fusion zone | High cooling rate, hydrogen embrittlement | Increase preheat, use low-hydrogen consumable, post-weld bake |
| Surface porosity | Inadequate shielding, flux contamination | Improve gas flow, clean consumable, use vacuum arc welding |
| Excessive dilution | High travel speed, large bead size | Reduce current, increase travel speed, use smaller electrode |
| Soft spots | Incomplete fusion, low heat input | Increase current, reduce travel speed, ensure proper joint preparation |
Key Questions and Reflections
The paper does not address the effect of the overlay on the die's fatigue life under cyclic loading. Cold stamping dies are subjected to millions of loading cycles during production, and the overlay must maintain its integrity under these conditions. The interface between the overlay and the base die material is a critical location for crack initiation, and the residual stresses at this interface must be carefully managed.
Another important consideration is the effect of the overlay on the die's dimensional accuracy. Cold stamping dies require tight tolerances, typically ±0.01 mm or better, and the overlay must be ground to the required dimensions after welding. The grinding allowance must be sufficient to remove the weld bead contour while leaving adequate overlay thickness to provide wear protection.
Study Insights and Implications
The development of a specialized overlay welding material for cold stamping dies represents a significant contribution to the field of tool and die repair. The CrMoWVTi alloy system achieves an excellent balance of hardness, wear resistance, and crack resistance, which is the fundamental challenge in overlay welding for cold working applications. The availability of both TIG and SMAW consumable forms provides the process flexibility needed for different repair scenarios. The key lesson from this study is that the material and process must be developed together; a superior material cannot be utilized effectively without a matching process, and a well-designed process cannot compensate for an inadequate material. This integrated approach to material and process development should be adopted in all future overlay welding research.