CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Cobalt-Based Hardfacing on Hot Heading Machine Dies
Literature Overview
This study note examines the application of cobalt-based hardfacing on hot heading machine dies, reported by Xiong Xuehui and Fu Haijun from Harbin Bearing Factory in 1994. Hot heading machines are used to form bearing rings and other precision components by plastic deformation of hot billets. The dies are subjected to extreme conditions including high temperatures, high contact stresses, and cyclic loading. Cobalt-based hardfacing alloys, particularly Stellite-type alloys, offer excellent wear resistance, thermal stability, and hot hardness, making them ideal for die surface protection.
Core Technical Content
Hot Heading Process Conditions
Hot heading involves the following conditions:
- Billet temperature: 1000–1200 °C for bearing steel, requiring dies to withstand high temperatures.
- Contact pressure: 500–2000 MPa depending on the die design and forming force.
- Cyclic loading: Each heading cycle imposes a new load on the die surface.
- Thermal cycling: Temperature variations from ambient to several hundred degrees Celsius.
- Friction and wear: Severe abrasive and adhesive wear from contact with hot steel.
The die surface must resist all these degradation mechanisms simultaneously.
Cobalt-Based Alloy Selection
| Alloy Type | Key Elements | Hardness (as-cast) | Hot Hardness (1000 °C) | Application |
|---|---|---|---|---|
| Stellite 6 | Co, Cr, W, Mo | 40–45 HRC | High | General wear resistance |
| Stellite 21 | Co, Cr, W, Mo, C | 45–50 HRC | Very high | High temperature wear |
| Stellite 25 | Co, Cr, W, Mo, C | 45–50 HRC | Very high | Severe wear conditions |
| Stellite 31 | Co, Cr, W, Mo, C | 40–45 HRC | High | Moderate temperature wear |
Stellite 21 and Stellite 25 are commonly selected for hot heading dies due to their excellent hot hardness and wear resistance. The hardfacing layer thickness is typically 2–5 mm, sufficient to provide protection while maintaining die dimensional accuracy.
Overlay Process Selection
For hot heading dies, the preferred overlay methods include:
- Flame Hardfacing (Oxy-Fuel): Traditional method, suitable for thick layers, but with high dilution and residual stress.
- Submerged Arc Welding (SAW): Good for build-up, moderate dilution, suitable for thick layers.
- Plasma Transferred Arc (PTA) Surfacing: Precise composition control, low dilution, suitable for thin layers.
- Gas Metal Arc Welding (GMAW): Good control, moderate dilution, suitable for medium layers.
PTA surfacing is increasingly favored for hot heading dies due to its ability to produce high-quality overlay layers with minimal dilution and distortion.
Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Preheat temperature | 200–300 °C | Reduces cracking tendency |
| Heat input | 2.0–4.0 kJ/mm | Controlled to minimize dilution |
| Travel speed | 50–150 mm/min | Adjusted for layer thickness |
| Powder feed rate | 100–300 g/min | Controlled to maintain composition |
| Shielding gas | Argon or Argon-Helium | Provides inert atmosphere |
| Interpass temperature | ≤ 300 °C | Prevents overheating |
| Post-weld heat treatment | 800–900 °C for 1–2 hours | Stress relief, microstructure stabilization |
Defect Analysis and Quality Control
Common Defects in Cobalt-Based Hardfacing
| Defect | Cause | Impact | Countermeasure |
|---|---|---|---|
| Cracking | High residual stress, low toughness | Die failure, production stoppage | Preheat, stress relief, use compatible filler |
| Spalling | Poor bond strength, thermal fatigue | Surface damage, product defects | Optimize microstructure, control cooling rate |
| Excessive dilution | High heat input, thin layer | Reduced hardness, poor wear resistance | Control heat input, use PTA or low-heat processes |
| Hardness variation | Uneven cooling, composition variation | Inconsistent performance | Optimize process parameters |
| Incomplete fusion | Poor joint preparation, low heat input | Weak bond, delamination | Ensure good joint fit, increase heat input |
Quality Control Procedures
The quality of cobalt-based hardfaced dies is ensured through:
- Pre-weld inspection: Verification of base material, surface preparation, and dimensional accuracy.
- In-process monitoring: Control of process parameters, visual inspection of weld appearance.
- Post-weld testing:
- Hardness mapping: Ensuring uniform hardness across the hardfaced surface.
- Penetrant testing (PT): Detecting surface cracks and defects.
- Ultrasonic testing (UT): Evaluating bond strength and internal defects.
- Metallographic examination: Assessing microstructure and bond quality.
- Performance testing: Wear testing under simulated heading conditions.
Engineering Practice Insights
Application Examples
Cobalt-based hardfacing has been successfully applied to various hot heading die components:
- Impression dies: Providing wear resistance in the forming cavity.
- Punches and punches: Enhancing wear resistance in the punching area.
- Guide surfaces: Improving wear resistance in the guide area.
- Die shoulders: Protecting against wear in the shoulder area.
Economic Considerations
The economic viability of cobalt-based hardfacing compared to solid cobalt alloy dies depends on:
- Cost of base material: Carbon steel or low-alloy steel bases are significantly cheaper than solid cobalt alloys.
- Overlay cost: Powder cost, processing cost, and labor cost.
- Service life extension: Hardfacing can extend die life by 3–10 times compared to unhardfaced dies.
- Maintenance cost: Reduced replacement frequency and downtime.
Key Reflections
The application of cobalt-based hardfacing to hot heading dies represents a significant advancement in die manufacturing technology. The excellent hot hardness and wear resistance of cobalt-based alloys make them ideal for the severe conditions of hot heading. However, the success of the process depends critically on careful process parameter optimization, thorough quality control, and appropriate material selection. Engineers must understand the specific service conditions and select overlay materials accordingly. Furthermore, the integration of hardfacing into die manufacturing requires investment in equipment, training, and process development. The evolution from manual to automated hardfacing systems has further improved consistency and productivity, making hardfacing an increasingly attractive option for high-performance die applications. The future of die hardfacing lies in the development of advanced cobalt-based alloys with improved hot hardness and wear resistance, combined with advanced hardfacing processes offering even greater control and precision.