CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Process for Cold Forging Dies
Literature Overview
This paper by Jiang Yucheng from Yantai Automobile Factory, published in 1989 in the journal "Welding," examines the application of overlay welding technology to cold forging dies. Cold forging dies are subjected to extreme conditions including high contact stresses, severe wear, and cyclic loading during the forming of metal components at room temperature. The authors investigated overlay welding as a surface engineering solution to extend die life and reduce manufacturing costs by enabling the use of economical base materials combined with wear-resistant overlay layers. This work represents an important early contribution to the understanding of overlay welding applications in tool and die engineering within the Chinese automotive industry.
Technical Challenges of Cold Forging Die Overlay Welding
Cold forging dies face a unique combination of service demands that make overlay welding both attractive and challenging. The overlay layer must simultaneously exhibit high hardness for wear resistance, adequate toughness to resist chipping under impact loading, and strong metallurgical bonding to the die substrate to prevent delamination during repeated forming cycles.
| Performance Requirement | Target Specification | Testing Method |
|---|---|---|
| Surface Hardness | 58–62 HRC | Rockwell C indentation |
| Core Toughness | > 10 J (Charpy V-notch) | Impact testing |
| Bond Strength | > 50 MPa (shear) | Pull-off or shear test |
| Wear Life | > 50,000 strokes (application dependent) | Field trial or laboratory simulation |
| Crack Resistance | No visible cracks after 10,000 cycles | Visual and MT inspection |
Substrate Preparation and Preheat Strategy
The base material for cold forging dies is typically a medium-carbon steel or low-alloy steel with adequate strength and machinability. Surface preparation is critical to ensuring metallurgical bonding; the authors emphasized thorough cleaning to remove scale, oil, and contaminants, followed by controlled preheating to reduce thermal gradients during welding. Preheat temperatures of 200–300°C were recommended to minimize the risk of cracking in the heat-affected zone, particularly for die geometries with high section thickness or complex shapes.
Overlay Layer Design and Welding Sequence
The overlay layer composition was selected to provide a combination of carbide-forming elements (Cr, Mo, V) for hardness and austenite-stabilizing elements (Ni, Mn) for toughness. The welding sequence was designed to minimize residual stresses and avoid cracking, with multiple passes applied in a controlled sequence. The authors recommended a multi-layer approach where the first layer (transition layer) provided a metallurgical bridge between the base material and the final overlay layer, reducing the risk of interfacial cracking.
Process Parameters and Quality Assessment
The overlay welding was performed using shielded metal arc welding (SMAW) with specialized overlay electrodes, which offered the flexibility and portability necessary for die repair and fabrication in a workshop environment. The authors documented the process parameters including electrode diameter, current, arc voltage, and travel speed, providing practical guidance for industrial implementation.
Defect Analysis and Countermeasures
Through metallographic examination and field performance evaluation, the authors identified several common defects and proposed countermeasures:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking at overlay-substrate interface | Excessive thermal gradient, hydrogen pickup | Increase preheat, reduce travel speed, post-weld bake |
| Surface cracks in overlay layer | High carbon equivalent, rapid cooling | Multi-pass welding with interpass temperature control |
| Poor bond strength | Incomplete cleaning, insufficient penetration | Thorough surface preparation, controlled first-pass parameters |
| Hardness non-uniformity | Inconsistent heat input, dilution variation | Standardized welding sequence, controlled electrode oscillation |
Engineering Application and Practical Outcomes
The overlay welding process was successfully applied to cold forging dies at Yantai Automobile Factory, with documented improvements in die life and reduction in maintenance costs. The authors reported that properly overlaid dies exhibited significantly extended service life compared to conventional die materials, with the overlay layer providing adequate wear resistance while maintaining sufficient toughness to withstand the cyclic loading of the forging process.
The economic analysis demonstrated that the overlay welding approach was cost-effective compared to replacing dies entirely with expensive tool steel materials. The process enabled the use of readily available and economical base materials while achieving the surface performance characteristics required for cold forging applications. This cost-benefit analysis remains relevant to modern manufacturing environments where die replacement costs and production downtime are significant economic concerns.
Key Technical Insights and Reflections
The work by Jiang Yucheng demonstrates the fundamental principle that overlay welding can effectively extend the service life of critical tooling components by combining the economic advantages of conventional base materials with the superior surface properties of specialized overlay alloys. The systematic approach to process development, from substrate preparation through quality assessment, provides a template that remains applicable to contemporary overlay welding applications in tool and die engineering.
A key insight from this work is the importance of balancing hardness and toughness in overlay layer design for cold forging applications. Excessive hardness without adequate toughness leads to brittle failure under impact loading, while insufficient hardness results in rapid wear. The multi-layer approach with a transition layer is particularly valuable for managing this balance and ensuring reliable metallurgical bonding.