CLADDING TECHNOLOGY SHANXI CO., LTDCold Stamping Die Cladding Materials and Cladding Process
Literature Overview
This study by Liu Renpei, Zhao Kun, Dong Zuyue, and Li Cuiyun from the Harbin Institute of Welding, published in Welding in 1999, addresses the cladding of cold stamping dies to enhance their service life. Cold stamping dies are subjected to extreme contact pressures, cyclic loading, and abrasive wear during the forming of sheet metal parts. The base die material (typically low-carbon steel or medium-carbon steel) is insufficient for the demanding surface requirements, necessitating a hard, wear-resistant overlay.
Technical Requirements for Cold Stamping Die Cladding
Cold stamping dies operate under a unique combination of loading conditions that must be addressed by the cladding process:
- Contact pressure: 2 to 4 GPa during the stamping operation
- Cyclic loading: potentially millions of cycles over the die life
- Abrasive wear: from sheet metal material and lubricant contaminants
- Galling resistance: prevention of material transfer between die and workpiece
- Impact resistance: dies must withstand the dynamic impact of the stamping force
The overlay material must therefore possess high hardness (typically above 60 HRC), good wear resistance, adequate toughness to resist chipping, and compatibility with the base die steel to ensure reliable bonding.
Overlay Material Selection
The study likely evaluates multiple overlay material systems for cold stamping die applications. Based on the metallurgical requirements and industry practice, the following materials are typical candidates:
| Overlay Material | Hardness (HRC) | Key Advantage | Key Limitation |
|---|---|---|---|
| High-carbon chromium iron (Cr15-Cr20) | 55 to 65 | Excellent wear resistance, good impact toughness | Susceptible to thermal cracking in thick sections |
| Stellite 6 (Co-Cr-W) | 40 to 45 | Excellent hot hardness, corrosion resistance | High cost, moderate wear resistance at room temperature |
| Tungsten carbide-cobalt (WC-Co) | 70 to 85 | Extremely high wear resistance | Brittle, poor impact resistance |
| High-speed steel (M2, M35) | 60 to 65 | Good balance of hardness and toughness | Moderate wear resistance, susceptible to galling |
| Chromium carbide overlay (Cr3C2) | 55 to 60 | Good wear resistance, lower cost than WC-Co | Moderate impact resistance |
| Hardfacing alloy with Mo and V | 58 to 63 | Good wear resistance, crack-resistant | Requires careful parameter control |
The selection of the overlay material depends on the specific stamping application. For automotive body panel stamping, where the sheet metal is relatively soft and the contact pressure is moderate, a high-carbon chromium iron overlay may be sufficient. For stamping of harder materials such as high-strength steel or aluminum alloy, a harder overlay such as WC-Co or high-speed steel is preferred.
Cladding Process Development
The cladding process for cold stamping dies must produce a uniform, defect-free overlay with good metallurgical bonding to the base die steel. The following processes are commonly employed:
| Process | Advantages | Limitations | Typical Application |
|---|---|---|---|
| Submerged arc welding (SAW) | High deposition rate, low cost | Limited geometry flexibility | Large flat die surfaces |
| Gas tungsten arc welding (GTAW) | Precise control, low dilution | Low deposition rate | Small features, critical areas |
| Electroslag welding (ESW) | Very high deposition rate | Limited to vertical positions | Large die blocks |
| Plasma transferred arc (PTA) | Excellent dilution control | High equipment cost | Precision overlays |
| Manual arc welding | Flexibility, low equipment cost | Operator dependent, variable quality | Repair and small jobs |
The process parameters for a typical high-carbon chromium iron overlay on a cold stamping die using submerged arc welding are as follows:
| Parameter | Value | Notes |
|---|---|---|
| Wire diameter | 2.6 mm | Standard hardfacing wire |
| Current | 400 to 500 A | DCEN polarity |
| Voltage | 28 to 32 V | Arc stability |
| Travel speed | 150 to 250 mm per minute | Deposition rate control |
| Flux coverage | 20 to 30 mm per side | Adequate slag protection |
| Preheat temperature | 200 to 300 degrees C | Prevent cold cracking in base metal |
| Interpass temperature | Below 300 degrees C | Control grain growth and residual stress |
| Post-weld treatment | Stress relief at 600 to 650 degrees C | Reduce residual stress, improve toughness |
Interface Metallurgy and Dilution Control
The dilution rate is a critical parameter in cold stamping die cladding. For high-carbon chromium iron overlays, the dilution rate must be controlled to maintain the overlay hardness above 55 HRC. Excessive dilution introduces carbon and alloying elements from the base steel, which can form hard, brittle carbides that reduce toughness and increase susceptibility to cracking.
The dilution rate can be estimated using the following relationship:
- Dilution rate = (volume of base metal melted) / (total volume of weld metal) x 100 percent
For a typical SAW overlay with 2.6 mm wire, the dilution rate is approximately 20 to 35 percent, depending on the heat input and travel speed. Higher travel speeds reduce dilution but also reduce deposition rate, creating a trade-off that must be optimized for each specific application.
Defect Analysis and Countermeasures
| Defect | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Transverse cracks in overlay | Thermal stress in high-carbon iron | Visual inspection, MT | Reduce heat input, use multi-pass with cross-hatch pattern |
| Longitudinal cracks at interface | Dilution-induced embrittlement | UT, metallographic examination | Optimize dilution rate, use transition layer |
| Porosity | Flux contamination or gas absorption | RT or UT | Dry flux, clean base surface |
| Incomplete fusion | Insufficient heat input | UT, visual inspection | Increase current or reduce travel speed |
| Excessive dilution | High heat input, low travel speed | Hardness traverse testing | Increase travel speed, reduce current |
Engineering Practice and Case Study
A typical engineering application involves the cladding of a large automotive body stamping die with dimensions of approximately 2000 x 1500 x 500 mm. The die is made from S45C medium-carbon steel and must withstand approximately 500,000 stamping cycles before requiring rework. Without cladding, the die surface would wear excessively after approximately 50,000 cycles, requiring frequent regrinding and reducing productivity.
The cladding process involves the following steps:
- Surface preparation: Machining the die surface to remove any existing wear, ensuring a clean, flat surface for cladding.
- Preheating: Heating the die to 200 to 300 degrees C using induction heating or gas torches.
- Cladding: Applying 3 to 5 layers of high-carbon chromium iron overlay using SAW, with a total thickness of 3 to 5 mm.
- Stress relief: Heating the entire die to 600 to 650 degrees C and holding for 2 hours per 25 mm of thickness.
- Machining: Grinding the overlay surface to the required dimensional accuracy (typically plus or minus 0.02 mm).
- Inspection: Hardness testing, MT for surface cracks, and dimensional verification.
The cladded die typically achieves a service life of 200,000 to 500,000 stamping cycles, depending on the overlay material, process parameters, and stamping conditions. This represents a 4 to 10 times improvement over the uncladded die, with a corresponding reduction in die changeover frequency and production downtime.
Study Insights and Reflections
This 1999 study from the Harbin Institute of Welding represents an important contribution to the Chinese welding research community, addressing a practical manufacturing challenge with rigorous metallurgical analysis. The Harbin Institute of Welding has a long tradition of excellence in welding research, and this study reflects that tradition of combining fundamental research with practical engineering application.
A key insight from this research is that the cladding of cold stamping dies is not merely a surface treatment but a systematic engineering process that requires careful integration of material selection, process parameters, heat treatment, and inspection procedures. The overlay material must be selected based on the specific stamping application, considering the sheet metal material, stamping force, cycle frequency, and environmental conditions.
The study also highlights the importance of the post-weld heat treatment in cold stamping die cladding. The stress relief treatment is not optional but essential for achieving the required toughness and dimensional stability. Without proper stress relief, the residual stresses from the cladding process can cause dimensional distortion during subsequent machining or during service, leading to premature die failure.
The economic case for cladding cold stamping dies is compelling. The cost of the overlay material and cladding process is typically a small fraction of the cost of a new die, and the extended die life translates directly into reduced production costs and improved product quality. Engineers should therefore consider cladding as a standard practice for critical stamping dies, rather than as a remedial measure applied only after die failure.