CLADDING TECHNOLOGY SHANXI CO., LTDEffect of TIG Welding Arc Re-Melting on Weld Joint Properties
Literature Overview and Research Background
The paper by Cao Yongsheng and Feng Wei, published in Coal Technology in 2011, investigates the effects of TIG welding arc re-melting on the properties of weld joints. Arc re-melting, also known as arc grinding or arc reprocessing, is a post-weld treatment technique where the weld surface is re-melted using a TIG arc without filler metal to improve the surface quality, reduce porosity, and refine the microstructure of the weld cap. This technique is particularly relevant in overlay welding and cladding applications where the surface quality and integrity of the overlay layer are critical for corrosion resistance and service performance. The research provides valuable insights into the mechanism of arc re-melting and its effects on the mechanical properties, microstructure, and surface characteristics of weld joints.
Technical Approach and Process Parameters
Arc re-melting is performed after the completion of the weld deposition, using a TIG arc to re-melt the surface of the weld cap without adding filler metal. The process parameters for arc re-melting are typically different from those used for the primary welding, with lower current and slower travel speed to ensure complete re-melting without excessive burn-through.
| Parameter | Primary Welding | Arc Re-Melting |
|---|---|---|
| Current | 100–200 A | 60–120 A |
| Voltage | 10–15 V | 8–12 V |
| Travel speed | 200–400 mm/min | 100–250 mm/min |
| Wire feed | Yes | No |
| Shielding gas | Argon, 15–25 L/min | Argon, 15–25 L/min |
| Re-melting passes | N/A | 1–3 passes |
The key to successful arc re-melting is achieving complete re-melting of the weld cap surface to a depth of 1–3 mm, which allows the re-solidification of the weld metal with a more uniform microstructure and reduced porosity. However, excessive re-melting can lead to burn-through, excessive dilution with the base metal, or distortion of the weld bead.
Effects on Weld Joint Properties
The study identified several key effects of arc re-melting on the weld joint properties:
Microstructural Refinement
Arc re-melting promotes the formation of a finer and more uniform microstructure in the re-melted zone. The re-melting process introduces additional thermal cycling, which can refine the grain size and promote a more homogeneous distribution of alloying elements. For austenitic stainless steel welds (such as 304 or 316L), the re-melting can reduce the amount of delta ferrite in the weld metal, which is beneficial for improving ductility and reducing susceptibility to stress corrosion cracking.
Reduction of Surface Porosity
One of the primary benefits of arc re-melting is the reduction of surface porosity. The re-melting process allows entrapped gas bubbles to escape from the molten pool, resulting in a denser and more homogeneous weld surface. The study reported a significant reduction in porosity density, from 5–10% porosity in the as-welded condition to less than 1% after arc re-melting.
Improvement of Surface Quality
Arc re-melting produces a smoother and more uniform weld surface, which is particularly important for overlay welding applications where the surface quality directly affects the corrosion resistance and appearance of the overlay layer. The re-melted surface exhibits fewer surface defects, such as craters, undercuts, and spatter, resulting in a more aesthetically pleasing and functionally superior weld cap.
Mechanical Property Changes
The mechanical properties of the weld joint after arc re-melting show mixed results:
| Property | As-Welded | After Arc Re-Melting | Change |
|---|---|---|---|
| Tensile strength | 500–600 MPa | 480–580 MPa | Slight decrease or unchanged |
| Elongation | 25–35% | 28–38% | Slight increase |
| Hardness | 180–220 HV | 170–210 HV | Slight decrease |
| Impact energy | 80–120 J | 90–130 J | Slight increase |
The slight decrease in hardness and tensile strength after arc re-melting is attributed to the grain refinement and reduction of delta ferrite, which promotes a more ductile microstructure. The increase in elongation and impact energy indicates improved toughness, which is beneficial for applications subject to impact loading or cyclic stress.
Defect Analysis and Countermeasures
While arc re-melting offers several benefits, it can also introduce new defects if not properly controlled. The following table summarizes common defects and countermeasures:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Burn-through | Excessive current or slow travel speed | Reduce current and increase travel speed |
| Excessive dilution | Deep re-melting into base metal | Limit re-melting depth to 1–3 mm |
| Surface cracking | Rapid cooling or high residual stress | Apply heat input control and post-weld stress relief |
| Arc blow | Magnetic field distortion | Use AC or pulsed current to reduce arc blow |
| Surface oxidation | Inadequate shielding | Ensure proper shielding gas flow and coverage |
Engineering Practice Applications
Arc re-melting is particularly applicable in the following engineering contexts:
- Overlay welding: The surface quality of the overlay layer is critical for corrosion resistance, and arc re-melting can significantly improve the surface finish and reduce porosity.
- Aesthetic welding: In applications where the weld appearance is important (such as food processing equipment or decorative structures), arc re-melting provides a smooth and uniform weld surface.
- Repair welding: Arc re-melting can be used to repair surface defects in existing welds without the need for grinding and re-welding.
- Pressure vessel fabrication: For pressure vessels subject to strict surface quality requirements, arc re-melting can be used as a post-weld treatment to improve the surface integrity of the weld cap.
However, several considerations must be addressed for practical implementation:
- Procedure qualification: Arc re-melting must be included in the welding procedure specification (WPS) and qualified according to the relevant standards (such as NB/T 47014 or ASME IX).
- Inspection requirements: The re-melted zone must be inspected for defects, and the acceptance criteria may differ from those for the primary weld.
- Residual stress management: Arc re-melting introduces additional thermal cycling, which can affect the residual stress distribution and must be considered in the overall stress analysis.
Key Reflections and Study Insights
This research highlights the potential of arc re-melting as a versatile post-weld treatment technique for improving weld joint properties. The key insight is that arc re-melting can simultaneously address multiple quality concerns (surface quality, porosity, microstructure) in a single operation, making it an efficient and cost-effective solution for critical applications.
For engineering practice, the following recommendations emerge:
- Parameter optimization: The arc re-melting parameters must be carefully optimized for each application, taking into account the base metal, weld metal, and desired outcome.
- Process integration: Arc re-melting can be integrated into the welding sequence as a final pass, eliminating the need for separate surface treatment operations.
- Quality assurance: The re-melted zone must be included in the quality assurance program, with appropriate inspection and testing to ensure compliance with the applicable standards.
- Cost-benefit analysis: The benefits of arc re-melting (improved quality, reduced rework) must be weighed against the additional time and cost of the re-melting operation.
The research provides valuable technical data for the implementation of arc re-melting in engineering practice, but further work is needed to develop standardized procedures and acceptance criteria for this technique in pressure vessel and structural applications.