CLADDING TECHNOLOGY SHANXI CO., LTDEffect of TIG Arc Re-melting on Weld Joint Properties
Literature Overview and Process Description
This 2011 study from Heilongjiang Forestry Vocational and Technical College and Gansu University of Technology investigates the effects of TIG arc re-melting on weld joint properties. Arc re-melting, also known as weld remelting or arc re-passing, is a post-weld heat treatment technique in which the solidified weld metal is re-melted by the TIG arc without additional filler metal. This technique is employed to refine the microstructure, eliminate porosity, reduce residual stresses, and improve the mechanical properties of welded joints. The study is particularly relevant to carbon and low-alloy steel welding, where microstructure refinement and stress relief are critical for achieving acceptable mechanical properties and fatigue resistance.
Core Technical Principles and Mechanisms
The TIG arc re-melting process involves passing the TIG arc over the previously solidified weld bead at controlled parameters—typically lower current and slower travel speed than the original welding pass. The arc re-melts the surface layer of the weld (typically 1–3 mm deep) and allows it to resolidify under different thermal conditions. Several mechanisms contribute to the property improvements observed after re-melting:
- Microstructure refinement: The re-melting and resolidification process creates a new thermal gradient and solidification rate, which can refine the grain structure and break up coarse columnar grains formed during the original welding.
- Porosity elimination: Gas pores and shrinkage cavities in the weld metal can be partially or fully eliminated as the molten metal resolidifies under the protective effect of the shielding gas, allowing dissolved gases to escape.
- Residual stress relief: The localized heating and cooling during re-melting introduces new thermal stresses that partially counteract the original welding residual stresses, resulting in a net reduction of peak stress values.
- Segregation mitigation: Microsegregation of alloying elements and impurities, which occurs during the original solidification, can be partially homogenized by the re-melting process.
| Re-melting Parameter | Typical Value | Effect on Properties |
|---|---|---|
| Re-melting Current | 100–180 A (60–80% of original) | Controls melt depth and thermal input |
| Travel Speed | 3–8 mm/s (slower than original) | Allows complete re-melting of target depth |
| Number of Passes | 1–3 | More passes provide greater refinement |
| Inter-pass Time | 5–15 min | Allows partial stress relief between passes |
| Shielding Gas | Argon or Argon-Helium | Prevents oxidation of re-melted surface |
| Melt Depth | 1–3 mm | Determines the volume of microstructure refined |
Microstructural Changes and Property Improvements
The microstructural changes induced by TIG arc re-melting are material-dependent but follow general trends. For low-carbon and low-alloy steels, the original weld metal typically exhibits a coarse, columnar ferrite-pearlite microstructure with possible martensite formation in regions of high cooling rate. After re-melting, the microstructure becomes finer and more equiaxed, with a more uniform distribution of ferrite and pearlite phases. In higher-alloy steels, re-melting can reduce the fraction of hard, brittle martensite by promoting a more tempered microstructure.
The following table summarizes the typical property improvements observed after TIG arc re-melting:
| Property | Before Re-melting | After Re-melting | Improvement |
|---|---|---|---|
| Hardness (HV) | 220–280 | 180–220 | 15–25% reduction |
| Tensile Strength (MPa) | 450–550 | 480–580 | 5–10% increase |
| Elongation (%) | 15–20 | 20–28 | 20–40% increase |
| Impact Energy (J, -20°C) | 20–40 | 40–80 | 50–100% increase |
| Residual Stress (MPa) | 200–350 | 100–200 | 30–50% reduction |
| Porosity (RT indication) | Moderate | Reduced | 30–60% reduction |
Defect Analysis and Limitations
While TIG arc re-melting offers significant benefits, it is not without limitations and potential drawbacks. The following defect mechanisms and limitations must be considered:
- Surface oxidation: If the shielding gas coverage is inadequate during re-melting, the re-melted surface can become oxidized, leading to a brittle oxide layer that may crack during subsequent service.
- Surface cracking: Rapid cooling of the re-melted surface, particularly in higher-carbon or higher-alloy steels, can lead to surface cracking due to thermal stresses and the formation of hard martensite.
- Incomplete re-melting: If the current or travel speed is not properly controlled, the re-melting may be incomplete, resulting in a partially melted surface with a discontinuous microstructure that can act as a crack initiation site.
- Distortion: The additional thermal input from re-melting can cause further distortion of the weldment, particularly in thin sections or heavily constrained joints.
- Coarse grain formation: Excessive re-melting (too many passes or too high current) can lead to grain coarsening, which negates the benefits of microstructure refinement and may reduce toughness.
Engineering Practice and Quality Assurance
The application of TIG arc re-melting in engineering practice requires careful process planning and quality control. The following considerations are important:
- Material selection: Arc re-melting is most effective for low-carbon and low-alloy steels (e.g., Q345, 16Mn, A516 Gr.70, P91). For high-carbon steels or highly alloyed materials, the risk of cracking may outweigh the benefits.
- Process parameter optimization: The re-melting parameters should be optimized through trial welds and verified through metallographic examination and mechanical testing before production application.
- Post-re-melting inspection: Visual inspection (VT), dye penetrant testing (PT), and magnetic particle testing (MT) should be performed after re-melting to detect surface cracks and other defects.
- Documentation: The re-melting process should be documented as part of the welding procedure specification (WPS) and welding procedure qualification record (WPQR), per standards such as GB/T 150, NB/T 47014, and ASME Section IX.
- Applicability: Arc re-melting is particularly useful for repair welding, where the original weld may have defects that need to be addressed without removing the entire weld. It is also beneficial for improving the fatigue performance of critical welds in pressure vessels and structural components.
Study Insights and Concluding Remarks
The TIG arc re-melting technique represents a practical and economical post-weld improvement method that can significantly enhance weld joint properties without the need for expensive equipment or complex procedures. The technique is particularly valuable in situations where post-weld heat treatment (PWHT) is impractical due to component size, geometry, or cost constraints. However, engineers must exercise caution in applying this technique, as improper parameters can introduce new defects and degrade the very properties the technique is intended to improve. A thorough understanding of the metallurgical mechanisms underlying arc re-melting, combined with rigorous process qualification and quality control, is essential for successful implementation in production welding environments. The continued development of arc re-melting as a standardized process, with well-defined parameter windows and acceptance criteria, would enhance its acceptance and application across the welding industry.