Effect of TIG Remelting on Crane Beam Welded Joint Properties

Literature Overview

This paper published in the Welding journal (2005, Issue 2) by Qu Jinguang, Ma Chunxia, and Lu Jing from Shanghai Baoye Engineering Technology Company investigates the application of TIG remelting as a reinforcement technique for crane beam welded joints. Through mechanical property testing, residual stress measurement, hardness testing, metallographic examination, and fatigue testing, the authors demonstrate that TIG remelting significantly reduces residual stress and substantially improves fatigue performance.

Core Technical Points

TIG Remelting Principle

TIG remelting involves re-melting an existing weld bead using a non-consumable tungsten electrode without adding filler metal. The process relies on the following metallurgical mechanisms:

The existing weld metal is completely melted and re-solidified under controlled thermal conditions.
The re-solidification occurs with a different cooling rate and thermal gradient compared to the original welding, resulting in a refined microstructure.
The residual stress field is partially relieved through plastic deformation of the surrounding material during the remelting thermal cycle.

Performance Improvements Documented

Property	Before TIG Remelting	After TIG Remelting	Improvement
Residual stress (longitudinal)	High tensile stress	Significantly reduced	Substantial reduction
Hardness distribution	Non-uniform, high HAZ hardness	More uniform distribution	Reduced hardness gradient
Fatigue life	Baseline	Significantly improved	Substantial increase
Microstructure	Coarse grains, possible segregation	Refined equiaxed grains	Improved homogeneity

Residual Stress Reduction Mechanism

The residual stress reduction achieved through TIG remelting occurs through two primary mechanisms:

Thermal stress relief: The localized heating during remelting creates a thermal expansion of the remelted zone, which induces compressive stresses in the surrounding material. Upon cooling, the elastic recovery partially relieves the original tensile residual stress.
Plastic deformation: The thermal cycling causes the material near the remelted zone to undergo cyclic plastic deformation, which redistributes and partially relieves the residual stress field through strain hardening and recovery mechanisms.

Fatigue Performance Enhancement

The improvement in fatigue performance is attributed to the combined effects of reduced residual tensile stress and refined microstructure. In crane beams subjected to cyclic loading, the weld toe is typically the critical location for fatigue crack initiation. The TIG remelting process smooths the weld toe geometry, reduces stress concentration, and eliminates microstructural defects such as micro-porosity and segregation that serve as fatigue crack initiation sites.

Engineering Practice Implications

Crane beams in industrial facilities are subjected to severe cyclic loading conditions that frequently lead to fatigue failure at welded joints. The TIG remelting technique offers a non-destructive reinforcement method that can be applied to existing structures without disassembly, making it particularly valuable for maintenance and repair operations.

For engineering implementation, the following considerations are essential:

The remelting parameters (current, travel speed, torch angle) must be carefully calibrated to ensure complete remelting of the original weld bead without excessive penetration into the base metal.
Pre-remelting inspection should identify the extent of the weld bead to be remelted, and post-remelting inspection should verify the quality of the remelted joint through MT or PT.
The technique is most effective for fillet welds and butt welds with moderate weld geometry; excessively large weld beads may require multiple remelting passes.
Residual stress measurement using strain gauge method or hole drilling method should be conducted before and after remelting to quantify the stress relief achieved.

Study Insights and Reflections

This work demonstrates a practical and cost-effective approach to improving the fatigue life of existing welded structures. The TIG remelting technique is particularly attractive for crane beams because it does not require the removal of the component from service for extended periods, and it can be performed in situ. The key insight is that the fatigue life improvement is not merely due to stress relief but also due to the microstructural refinement that occurs during controlled re-solidification.

The technique aligns with the philosophy of structural health management, where existing structures are assessed, reinforced, and extended in service life rather than replaced. For piping and structural applications governed by ASME B31 codes, the remelting technique should be qualified through coupon testing that replicates the specific material, joint geometry, and loading conditions of the application. The combination of residual stress reduction and microstructural refinement makes TIG remelting a powerful tool in the engineer's toolkit for fatigue-critical welded joints.