CLADDING TECHNOLOGY SHANXI CO., LTDMachining and Application of Surfaced Molds: A Technical Study Note
Literature Overview
This study note focuses on the machining processes and practical applications of hardfacing (surfacing) molds used in industrial manufacturing. Surfaced molds represent a critical intersection of welding technology and tool manufacturing, where weld overlay techniques are employed to deposit wear-resistant, corrosion-resistant, or impact-resistant alloy layers onto mold substrates to extend service life and improve surface quality.
Core Technical Principles
Hardfacing molds are fabricated by depositing specialized alloy overlays onto base mold materials (typically low-carbon steel or medium-carbon steel) using various welding processes. The overlay material provides enhanced surface properties while the base material retains structural integrity and machinability. The key principle is achieving a metallurgically sound bond between the overlay and substrate while maintaining the desired surface characteristics after subsequent machining operations.
Common Surfacing Processes for Mold Fabrication
| Process | Typical Overlay Material | Hardness (HV) | Machinability | Application |
|---|---|---|---|---|
| Submerged Arc Welding (SAW) | Stellite 6, Cr-Ni-C alloys | 400-500 | Moderate | Large flat molds |
| Gas Metal Arc Welding (GMAW) | Austenitic stainless, Ni-based | 300-450 | Good | Medium molds |
| Gas Tungsten Arc Welding (GTAW) | Tool steel, high-C alloys | 500-700 | Poor | Precision small molds |
| Plasma Arc (PTA) | Hardfacing powders | 600-800 | Poor | Wear-critical surfaces |
| Electroslag Welding (ESW) | Thick overlay builds | 350-450 | Moderate | Large block molds |
Machining Challenges After Surfacing
The machining of hardfaced mold surfaces presents unique challenges that distinguish this work from conventional mold making:
- Tool wear: Hardfacing alloys, particularly those containing carbides (WC, Cr7C3, Cr3C2), cause accelerated tool wear. Carbide-tipped inserts with cobalt binders or CBN (cubic boron nitride) tools are typically required.
- Heat generation: The high thermal conductivity of many overlay alloys combined with the difficulty of chip removal generates significant heat at the cutting interface, potentially causing thermal damage to the overlay surface.
- Residual stress effects: The welding process introduces residual stresses that may be partially relieved during machining, potentially causing dimensional instability if not properly managed.
- Microstructure sensitivity: Some overlay materials, particularly martensitic types, may undergo phase transformations during machining-induced heating, altering surface hardness and wear resistance.
Process Optimization Strategies
Based on practical experience and the principles outlined in the literature:
- Pre-machining heat treatment: Stress-relief annealing at 550-650°C for 2-4 hours before machining reduces residual stresses and improves dimensional stability.
- Progressive machining: Removing material in multiple passes with decreasing depth (e.g., 0.5 mm, 0.2 mm, 0.1 mm, 0.05 mm) minimizes work hardening and tool wear.
- Coolant selection: High-pressure mist coolant or soluble oil with extreme pressure additives is preferred over flood coolant to minimize thermal shock to the overlay surface.
- Cutting parameter selection: Lower cutting speeds (v = 30-60 m/min for carbide tools on Stellite overlays) with moderate feed rates balance productivity against tool life.
Application Cases
Case 1: Injection Molding Tools
Surfaced molds with Stellite 6 overlay (1-3 mm thick) followed by precision grinding achieve surface hardness of 45-50 HRC while maintaining the dimensional accuracy required for plastic injection molding. The overlay provides resistance to hot polymer erosion while the underlying steel ensures structural rigidity.
Case 2: Extrusion Dies
Aluminum extrusion dies with high-carbon martensitic overlay (e.g., D2-equivalent hardfacing) followed by EDM and grinding achieve surface hardness exceeding 60 HRC, extending die life by 3-5 times compared to conventional unhardened die steel.
Case 3: Casting Molds
Sand casting molds with Ni-Cr-C austenitic overlay provide excellent hot cracking resistance and thermal shock tolerance, with surface hardness maintained at 35-40 HRC after machining to ensure adequate machinability for pattern removal.
Quality Control Considerations
The quality of surfaced molds depends on multiple inspection stages:
| Inspection Stage | Method | Acceptance Criteria |
|---|---|---|
| Pre-machining | UT for overlay bonding | No lack of fusion, no cracks > 1 mm |
| Post-machining | Surface hardness measurement | Within 3 HRC of target |
| Post-machining | Surface roughness (Ra) | Ra ≤ 0.8 μm for precision molds |
| Final | Dimensional inspection | Within ±0.02 mm tolerance |
| Final | Surface integrity check | No grinding burns, no microcracks |
Study Insights
The machining of surfaced molds represents one of the most challenging applications of weld overlay technology because it requires the overlay to simultaneously provide functional surface properties and be amenable to subsequent precision machining. This dual requirement constrains the selection of overlay materials and processes considerably. The practical lesson is that the hardfacing specification must be developed in conjunction with the machining requirements, not independently. A hardfacing alloy that provides excellent wear resistance but cannot be machined to the required surface finish is ultimately useless for mold applications. This integrated approach to material selection and process planning is a recurring theme throughout hardfacing engineering and one that distinguishes successful industrial applications from laboratory demonstrations.