Zhuojin Pipe Fitting Co., LtdPrecision Forging Machine Hammer Head Machining and Overlay Welding Process Research
Literature Overview
Published in Forging and Stamping Technology (2013, Vol. 38, No. 6), this study from North University of China and the Shanxi Provincial Deep Hole Machining Engineering Technology Research Center investigates the machining and overlay welding processes for precision forging machine hammer heads. The research was supported by a 2013 Shanxi Provincial International Science and Technology Cooperation Project on "Key Technologies for Laser Cladding Remanufacturing of Aero-Engine Components." The study addresses two critical engineering challenges: the high cost of imported hammer heads and the need for economical repair and remanufacturing solutions when the hammer head surface becomes worn during forging operations.
Core Technical Points
Forging Machine Hammer Head Design and Requirements
Precision forging machine hammer heads are subjected to extreme cyclic loading during the forging process. Each forging stroke subjects the hammer head to impact loads of several hundred kilonewtons, resulting in surface fatigue, plastic deformation, and abrasive wear. The hammer head must therefore possess:
- High impact toughness to resist fracture under cyclic loading
- High surface hardness to resist abrasive wear from the workpiece
- Sufficient core toughness to prevent crack propagation
- Dimensional accuracy to ensure proper die filling and forging quality
The study developed a forging and machining process that optimizes the microstructure and mechanical properties of the hammer head to meet these requirements while reducing the cost compared to imported components.
Forging and Machining Process
| Process Step | Key Parameters | Purpose |
|---|---|---|
| Material selection | 42CrMo or 38CrMoAlA | High strength and toughness |
| Billet heating | 1150–1200 °C | Uniform temperature for forging |
| Pre-forging | Multi-hit, progressive reduction | Break down grain structure |
| Final forging | Controlled reduction ratio | Achieve desired shape and grain flow |
| Normalizing | 860–880 °C, air cooling | Refine grain structure |
| Quenching and tempering | 860 °C oil quench, 580–620 °C temper | Achieve target hardness (28–32 HRC) |
| Machining | CNC milling and turning | Achieve dimensional accuracy (±0.05 mm) |
| Surface treatment | Shot peening or overlay | Improve surface integrity |
Overlay Welding for Repair and Remanufacturing
When the hammer head surface becomes worn or damaged during service, the overlay welding process can restore the original dimensions and improve surface properties. The study evaluated several overlay approaches:
| Overlay Method | Material | Hardness (HRC) | Application |
|---|---|---|---|
| SMAW | Hardfacing electrode (e.g., D266) | 55–60 | Quick repair, moderate wear resistance |
| SAW | Flux-cored wire with alloy powder | 50–58 | Large area repair, good penetration |
| Plasma arc surfacing | Composite powder (Ni-Cr-C) | 45–55 | Precision repair, good surface finish |
| Laser cladding | Ni-based or Co-based alloy | 40–60 | High precision, low dilution |
The study found that a multi-pass overlay approach combining a transition layer (e.g., 309L stainless steel) with a hardfacing overlay (e.g., Ni-Cr-C alloy) provides the best combination of bonding strength and wear resistance. The transition layer accommodates the thermal expansion mismatch between the base steel and the hardfacing material, reducing the risk of cracking.
Performance Validation
The study validated the hammer head performance through actual forging trials. After 13 days of continuous forging, producing approximately 2000 axles, the hammer head surface exhibited only fine cracks with no significant cracking or wear. This performance meets the production requirements and demonstrates the effectiveness of the developed process.
Engineering Practice Integration
The findings of this study have direct implications for the remanufacturing and maintenance of forging equipment in heavy industry. Key practical considerations include:
- Preventive maintenance: Regular inspection of the hammer head surface for early signs of wear or cracking, using techniques such as magnetic particle testing (MT) or ultrasonic testing (UT).
- Overlay thickness management: The overlay layer should be thick enough to accommodate material loss over the intended service interval, typically 3–5 mm for forging hammer heads.
- Heat treatment after overlay: A post-weld heat treatment is essential to relieve residual stresses and prevent delayed cracking. The temperature should be carefully controlled to avoid softening the hardfacing layer.
- Surface finish requirements: The overlay surface should be machined or ground to achieve the required surface roughness (typically Ra 1.6–3.2 μm) to ensure proper contact with the die and workpiece.
FMEA Analysis for Hammer Head Failure Modes
| Failure Mode | Cause | Effect | Detection Method | Preventive Action |
|---|---|---|---|---|
| Surface cracking | High residual stress, thermal shock | Reduced service life, potential catastrophic failure | MT or PT inspection | Proper preheat, controlled cooling, post-weld stress relief |
| Delamination | Poor bonding, thermal expansion mismatch | Overlay spalling during service | UT or TOFD | Use transition layer, control dilution |
| Excessive wear | Insufficient hardness, wrong material selection | Reduced forging accuracy, increased maintenance cost | Visual inspection, dimensional measurement | Select appropriate hardfacing material, monitor wear rate |
| Core cracking | Excessive quenching severity, poor material quality | Catastrophic failure | UT inspection | Optimize heat treatment parameters, use appropriate material |
Key Questions and Reflections
The study's focus on the hammer head as a repairable component aligns with the broader industry trend toward remanufacturing and circular economy principles. However, several questions remain open:
- Fatigue life prediction: The study does not provide a detailed fatigue life analysis of the overlay layer under cyclic impact loading. Engineers should consider supplementing the reported data with fatigue testing to predict the service life of the overlay under specific operating conditions.
- Long-term performance: The 13-day forging trial is a short-term validation. Long-term performance data over multiple repair cycles would be valuable for establishing reliable maintenance schedules.
- Cost analysis: A detailed cost comparison between the developed process and imported hammer heads, including the cost of overlay repair materials and labor, would strengthen the economic case for the approach.
Study Insights and Implications
This study demonstrates a practical and cost-effective approach to the manufacturing and repair of precision forging machine hammer heads. The developed forging and machining process reduces reliance on expensive imported components, while the overlay welding approach provides a viable remanufacturing solution for worn or damaged hammer heads. For engineers involved in forging equipment maintenance, the study reinforces the importance of proper material selection, process optimization, and quality control in ensuring the reliability and service life of critical forging components. The findings also highlight the potential of advanced overlay technologies, such as laser cladding, for further improving the performance and extendability of forging equipment components.