Alloy Surfac ing of Molds: Process, Equipment, and Economic Analysis

Literature Overview

This paper by Wang Qingguo from Northeast Light Alloy Co., Ltd. (2003) provides a practical overview of alloy surfacing technology applied to mold manufacturing. The study focuses on the economic and technical advantages of depositing alloy steel or hard alloy layers onto ordinary carbon structural steel mold bases, as an alternative to manufacturing entire molds from expensive alloy materials. The work covers process parameters, welding equipment selection, and cost-benefit analysis for punch dies and forging dies.

Core Technical Concepts

The fundamental principle of mold surfacing is the combination of a low-cost, machinable, and weldable base material (typically Q235 or Q345 carbon steel) with a wear-resistant, hot-work resistant, or corrosion-resistant surfacing layer applied to the working surfaces. This approach leverages the complementary properties of both materials:

Common surfacing materials for mold applications include:

• Cold work die steels: D2, Cr12MoV, 65Nb for punch and die applications
• Hot work die steels: 5CrNiMo, 4Cr5MoSiV1 for forging and casting dies
• Hardfacing alloys: Stellite 6, cobalt-based alloys for extreme wear conditions
• Cemented carbides: WC-Co alloys for ultra-high wear resistance

Process Considerations

The surfacing process for molds requires careful attention to several critical factors:

1. Base material preparation: The base material should be preheated to 200-400°C depending on thickness to reduce thermal stresses and prevent cracking at the interface.
2. Surfacing method selection: Manual metal arc welding (SMAW), shielded metal arc welding with flux-cored wire (FCAW), or submerged arc welding (SAW) may be used depending on the geometry and production volume.
3. Layer thickness control: The surfacing layer thickness is typically 3-10 mm, sufficient to provide wear resistance while maintaining the structural integrity of the base material.
4. Post-weld machining: After surfacing, the layer is machined to final dimensions, which requires consideration of the machinability of the surfacing material and the hardness of the heat-affected zone.

Welding Equipment Selection

The choice of welding equipment is critical for mold surfacing applications:

• Transformer-type welding machines: Provide the high current required for thick surfacing layers and good arc stability. Suitable for large forging dies.
• Rectifier-type welding machines: Offer better arc characteristics and reduced spatter. Preferred for precision punch and die applications.
• Pulse welding machines: Allow precise heat input control, reducing distortion and improving the quality of thin surfacing layers.
• Submerged arc welding equipment: Enables high deposition rates and consistent quality for large-scale production.

Economic Analysis

The economic advantages of mold surfacing are substantial:

• Material cost reduction: Using carbon steel for the base material reduces raw material costs by 50-70% compared to solid alloy steel molds.
• Maintenance cost savings: Worn surfacing layers can be rebuilt by additional surfacing, extending mold life without replacement.
• Production flexibility: Different surfacing materials can be selected for different working surfaces of the same mold, optimizing performance.
• Manufacturing time: Carbon steel base materials are more readily machined than alloy steels, reducing initial manufacturing time.

Engineering Practice Insights

From my experience in mold manufacturing, several practical considerations emerge:

• Dilution control: The dilution of base material into the surfacing layer is typically 5-15% for the first layer and decreases for subsequent layers. This dilution must be accounted for when selecting surfacing materials, as it affects the final composition and properties.
• Crack prevention: The high carbon equivalent of the base material and the thermal stresses from surfacing can lead to cracking. Preheating, controlled cooling, and multiple thin layers are essential mitigation strategies.
• Dimensional accuracy: Thermal distortion during surfacing can be significant, especially for thin-walled mold components. Sequential surfacing of opposing surfaces and symmetrical layering help minimize distortion.
• Quality verification: Hardness testing, visual inspection for cracks, and dimensional measurement should be performed after each surfacing operation.

Key Reflections

The paper effectively communicates the practical value of mold surfacing technology, but it could benefit from more detailed discussion of failure modes and their prevention. In my experience, the most common failure modes in surfaced molds are:

1. Spalling: Caused by high residual tensile stresses at the interface, leading to delamination of the surfacing layer.
2. Cracking: Initiated at the fusion zone due to insufficient preheating or excessive cooling rate.
3. Wear: Occurs when the surfacing layer is too thin or when the material selection does not match the service conditions.

A systematic FMEA (Failure Mode and Effects Analysis) approach should be applied to mold surfacing operations, identifying potential failure modes, their causes, and implementing preventive measures at each stage of the manufacturing process.

The economic argument for mold surfacing is compelling, but engineers must ensure that the technical requirements are met. A poorly executed surfacing operation can result in premature mold failure, which is more costly than manufacturing a solid alloy mold. Quality assurance procedures, including welder qualification, process parameter control, and non-destructive testing, are essential for successful implementation.