Zhuojin Pipe Fitting Co., LtdPlasma Arc Overlay Welding Technology for Mold Repair
Literature Overview
This paper, published in 1997 in the journal Forging and Stamping Technology (锻压技术), authored by Li Jian from Northwestern Polytechnical University, presents a new technology for mold repair using plasma arc overlay welding. The work was supported by the National Natural Science Foundation of China (Youth Science Fund) and addresses the challenge of short mold life in high-volume production of small forged parts.
The study proposes using H10Mn2SiMnMo steel as the overlay material for plasma arc welding repair of molds, and it investigates the relationship between microstructure, mechanical properties, and mold life extension. This work represents an early contribution to the field of plasma arc overlay welding for tool and die repair.
Core Technical Content
Mold Wear in Small Forged Part Production
Small forged parts are produced in large quantities using closed-die forging operations. The molds (die sets) are subjected to:
- High contact pressure: The forging force is concentrated on a small contact area, leading to high contact stresses (often exceeding 2000 MPa).
- High temperature: The workpiece temperature during forging can reach 1000–1200°C, and the mold surface temperature can reach 400–600°C.
- Cyclic loading: Each forging stroke applies a cyclic load, leading to fatigue damage accumulation.
- Thermal cycling: The temperature gradient between the hot workpiece and the relatively cooler mold creates thermal stresses.
- Chemical interaction: Diffusion of carbon and alloying elements between the workpiece and the mold can alter the mold surface composition.
The combination of these factors leads to several wear mechanisms:
| Wear Mechanism | Description | Typical Location |
|---|---|---|
| Adhesive wear | Material transfer between die and workpiece | Die cavity surface |
| Abrasive wear | Hard particles in workpiece abrade die surface | Die cavity surface |
| Fatigue wear | Surface cracking and spalling from cyclic loading | Die cavity surface, corners |
| Thermal fatigue | Cracking from repeated thermal expansion/contraction | Die surface, edges |
| Erosion | Material removal by impact of metal flow | Die cavity edges, corners |
Plasma Arc Overlay Welding (PAOW)
Plasma arc overlay welding is a specialized welding process that uses a constricted plasma arc to achieve high energy density and precise control of the weld pool. The key features of PAOW include:
| Feature | Description | Advantage for Mold Repair |
|---|---|---|
| High energy density | Focused plasma arc with high power density | Low dilution, minimal heat input to base |
| Narrow weld bead | Precise control of bead width and profile | Conformal coating on complex geometries |
| Low dilution | Limited melting of base metal | Better retention of overlay properties |
| High deposition rate | Multiple wire feeds possible | Efficient thick layer application |
| Good positional flexibility | Can be applied in various positions | Access to complex die geometries |
Overlay Material: H10Mn2SiMnMo
The selection of H10Mn2SiMnMo as the overlay material is based on the following considerations:
| Property | H10Mn2SiMnMo | Typical Mold Steel (e.g., H13) |
|---|---|---|
| Carbon content | 0.9–1.1% | 0.35–0.45% |
| Hardness (quenched) | 60–65 HRC | 48–54 HRC |
| Toughness | Moderate | Good |
| Wear resistance | Excellent | Good |
| Thermal fatigue resistance | Good | Moderate |
| Cost | Moderate | High |
The higher carbon content of H10Mn2SiMnMo provides superior wear resistance compared to conventional mold steels, while the alloying elements (Mn, Si, Mo) contribute to hardenability, toughness, and thermal stability.
Interpretation of Key Technical Points
Microstructure and Properties Relationship
The paper investigates the relationship between the microstructure of the overlay weld and the resulting mechanical properties. The key microstructural features include:
- Martensite: The primary phase in the overlay, providing high hardness and wear resistance. The morphology and size of martensite plates affect toughness.
- Retained austenite: A small amount of retained austenite can improve toughness by transforming to martensite under stress, absorbing energy.
- Carbides: M7C3 and MC-type carbides contribute to wear resistance. The size, shape, and distribution of carbides are critical.
- Dilution zone: The transition region between the overlay and the base metal, where dilution is highest. This zone may have different properties from both the overlay and the base.
Plasma Arc Welding Parameters
| Parameter | Typical Range | Effect on Overlay |
|---|---|---|
| Plasma arc current | 100–300 A | Higher current → deeper penetration, higher dilution |
| Arc voltage | 20–35 V | Higher voltage → wider bead |
| Travel speed | 100–500 mm/min | Higher speed → thinner deposit |
| Wire feed rate | 2–8 m/min | Higher rate → thicker deposit |
| Plasma gas flow | 5–15 L/min | Insufficient flow → arc instability |
| Shielding gas flow | 10–20 L/min | Insufficient flow → oxidation, porosity |
| Shielding gas type | Ar, Ar/He mix | He increases heat input |
| Torch standoff | 5–15 mm | Larger standoff → reduced penetration |
Multi-Pass Overlay Strategy
For thick overlay layers (typically 3–10 mm), a multi-pass strategy is employed:
- First pass: Lower heat input to minimize dilution and ensure good bonding to the base.
- Intermediate passes: Moderate heat input to build up thickness efficiently.
- Final pass: Controlled parameters to achieve the desired surface profile and hardness.
The interpass temperature should be maintained below 250°C to avoid excessive grain growth and property degradation.
Process and Standards Analysis
Welding Procedure Qualification
The welding procedure for mold repair must be qualified according to relevant standards:
| Standard | Scope |
|---|---|
| AWS D10.9 | Overlay welding procedure qualification |
| AWS D1.1 | Structural welding code (general welding principles) |
| ISO 15614 | Qualification testing of welding procedures |
| GB/T 19866 | Chinese standard for welding procedure qualification |
The qualification test must verify:
- Hardness: Surface hardness ≥ specified value (e.g., ≥60 HRC).
- Dilution: Dilution rate ≤ 20–30% (measured by metallography).
- Cracks: No cracks in the overlay or the heat-affected zone (measured by PT/MT).
- Porosity: No porosity exceeding specified size (measured by RT or UT).
- Impact toughness: Charpy V-notch impact energy ≥ specified value (if required).
Quality Control Plan
| Inspection Step | Method | Acceptance Criteria |
|---|---|---|
| Pre-weld surface preparation | Visual inspection | Clean, no rust, no oil |
| Welding parameters | Process monitoring | Within qualified range |
| Interpass temperature | Infrared thermometer | ≤ 250°C |
| Post-weld hardness | Rockwell C hardness | ≥ 60 HRC (surface) |
| Crack detection | Dye penetrant (PT) | No cracks |
| Porosity detection | Radiographic (RT) or UT | No porosity > 1 mm |
| Dimensional verification | CMM or micrometer | Within tolerance |
Integration with Engineering Practice
Case Study: Small Forged Part Production
In the production of small forged parts (e.g., fasteners, fittings, hardware components), the mold life is a critical economic factor. The following example illustrates the impact of plasma arc overlay welding on mold life:
| Parameter | Without Overlay | With PAOW Overlay |
|---|---|---|
| Mold material | H13 | H13 base + H10Mn2SiMnMo overlay |
| Overlay thickness | N/A | 3–5 mm |
| Surface hardness | 48–52 HRC | 60–65 HRC |
| Die life (strokes) | 5,000–10,000 | 20,000–50,000 |
| Repair frequency | Every 10,000 strokes | Every 40,000 strokes |
| Cost per stroke | Higher | Lower |
Practical Considerations
- Geometry complexity: Plasma arc welding can be applied to complex die geometries, but access to deep cavities and sharp corners may be limited.
- Thermal distortion: The heat input from plasma arc welding can cause distortion of the mold. Preheating and post-weld stress relief are recommended.
- Surface finish: The as-welded surface is rough and must be machined or polished to the required finish for forging.
- Multiple repairs: The mold may undergo multiple overlay repair cycles. Each cycle should be planned to ensure adequate base metal remains.
Economic Analysis
| Item | Cost |
|---|---|
| New mold cost | 100% (baseline) |
| PAOW repair cost | 10–25% of new mold |
| Life extension | 3–5× original life |
| Cost per stroke reduction | 60–80% |
The economic benefit of PAOW repair is substantial, particularly for high-volume production where mold life directly impacts productivity and unit cost.
Key Questions and Reflections
The study raises several questions for further investigation:
- How does the number of overlay repair cycles affect the long-term properties of the mold? Is there a limit to the number of repairs before the base metal is exhausted?
- Can the overlay material be optimized for specific wear conditions (e.g., higher carbon for abrasive wear, more alloying for thermal fatigue)?
- What is the effect of plasma arc parameters on the dilution rate and the resulting hardness profile?
- Can the overlay welding process be automated for consistent quality and reduced labor cost?
The paper demonstrates that plasma arc overlay welding is a promising technology for mold repair, but it also highlights the need for careful process control and quality assurance.
Study Insights and Implications
This study is a pioneering work in the application of plasma arc overlay welding for mold repair. It demonstrates that the technology can significantly extend mold life and reduce production costs in high-volume forging operations.
For engineers involved in tool and die maintenance, the key takeaways are:
- Plasma arc overlay welding offers superior dilution control compared to conventional arc welding processes, making it suitable for overlay applications where property retention is critical.
- The selection of overlay material should be based on the specific wear mechanism and operating conditions, not simply on hardness.
- Quality control is essential to ensure that the overlay meets the required hardness, dilution, and defect criteria.
- The economic benefit of overlay repair is substantial and should be considered in the life-cycle cost analysis of molds.
This paper serves as an early reference for the development of overlay welding technologies for mold repair, and its principles remain relevant in modern practice.