Weld Toe TIG Remelting Followed by Rapid Cooling Treatment Process

Literature Overview

This paper, published in the Journal of Dalian Jiaotong University in 2008 by Zhu Ping, Guo Changhong, Shi Chunyuan, Ding Chenggang, and Jiao Jianqiang from the School of Materials Science and Engineering, addresses a specific and practical challenge in the welding of clad and overlay joints — the control of weld toe microstructure and performance through post-weld local treatment. The authors propose a combined approach of TIG remelting of the weld toe region followed by rapid quenching, aiming to improve the mechanical integrity and corrosion resistance of the weld toe, which is widely recognized as the most vulnerable location for fatigue crack initiation in welded structures.

Core Technical Concepts

The weld toe region is inherently susceptible to stress concentration due to geometric discontinuities, residual stresses, and the presence of brittle phases formed during rapid solidification at the fusion boundary. In cladding and overlay applications, particularly where dissimilar metals are joined — such as stainless steel overlay on carbon steel pressure vessels — the weld toe becomes a critical zone where cracking and corrosion initiation are most likely to occur. The authors' approach involves two sequential operations: first, TIG remelting of the weld toe to modify the microstructure and eliminate surface defects; second, rapid cooling (quenching) to suppress the formation of coarse grain structures and brittle intermetallic phases.

TIG Remelting Parameters and Mechanism

The TIG remelting operation is performed with a non-consumable tungsten electrode, using a shielding gas (typically argon) to protect the molten zone from atmospheric contamination. The key parameters include welding current, travel speed, arc length, and heat input. The remelting is designed to penetrate only the outer portion of the weld toe — typically 1 to 2 mm deep — without disturbing the bulk weld metal or the base metal. This shallow remelting zone allows for the redistribution of alloying elements, the dissolution of coarse carbides or nitrides, and the refinement of the grain structure at the critical toe region.

Parameter	Typical Range	Purpose
Welding Current	100–200 A	Control remelting depth
Travel Speed	50–150 mm/min	Balance penetration and heat input
Arc Length	2–4 mm	Ensure stable arc and adequate shielding
Shielding Gas	99.99% Ar	Prevent oxidation and nitrogen pickup
Remelting Depth	1–2 mm	Avoid disturbing base metal
Preheat Temperature	150–250 °C	Reduce residual stress

Rapid Cooling Treatment

Following the TIG remelting, the weld toe is subjected to rapid cooling, typically achieved through water quenching or spray cooling applied immediately after the arc is extinguished. The cooling rate is critical — it must be high enough to suppress the formation of brittle phases but controlled to avoid excessive thermal stresses that could induce microcracking. The rapid cooling effectively transforms the microstructure toward a finer, more ductile configuration, improving fatigue resistance and reducing susceptibility to stress corrosion cracking.

Defect Analysis and Countermeasures

The following table summarizes common defects encountered in weld toe TIG remelting and rapid cooling processes, along with recommended countermeasures.

Defect Type	Cause	Countermeasure
Toe cracking	Excessive residual stress from rapid cooling	Reduce cooling rate; apply interpass preheat
Incomplete remelting	Insufficient heat input or excessive travel speed	Increase current or reduce travel speed
Overheating of base metal	Excessive heat input or prolonged dwell time	Use pulsed TIG; monitor temperature with pyrometer
Surface porosity	Inadequate shielding gas coverage	Improve gas flow rate; use trailing shield
Residual stress concentration	Uneven cooling rate across toe width	Apply uniform spray cooling pattern

Engineering Practice Integration

In the fabrication of bimetal pressure vessels — such as hydrogenation reactors clad with nickel-based alloys (Inconel 625) or stainless steel (316L) — the weld toe of the overlay weld is a critical inspection and quality control point. According to NB/T 47002 and ASME VIII Div.1, the weld toe must be free from cracks, undercut, and excessive roughness. The TIG remelting and rapid cooling technique provides an effective in-process improvement that can reduce the need for extensive post-weld grinding and polishing.

A practical application scenario involves the overlay of Inconel 625 on a low-alloy steel vessel for a hydrogen service environment. The overlay weld is deposited using submerged arc welding (SAW) or flux-cored arc welding (FCAW), followed by TIG remelting of the weld toe and rapid quenching. This sequence has been shown to improve the fatigue life of the overlay joint by 30% to 50% compared to untreated weld toes, as confirmed by fatigue testing under cyclic loading conditions.

Key Questions and Reflections

The authors' approach raises several important questions for engineering practice. First, how does the rapid cooling treatment affect the residual stress distribution in the overlay weld? While the cooling rate is increased, the thermal gradient may also increase, potentially inducing higher local residual stresses. Second, what is the optimal cooling rate for different overlay materials? For nickel-based alloys, excessive cooling rates may promote the formation of Laves phases or other brittle intermetallics, whereas for stainless steels, the risk is primarily related to sensitization and intergranular corrosion susceptibility. Third, how scalable is this technique for large-diameter pressure vessels where manual TIG remelting may be impractical?

The study contributes to the broader understanding of weld toe optimization in overlay applications. The combination of remelting and rapid cooling represents a practical, cost-effective approach that can be integrated into existing fabrication workflows without requiring specialized equipment beyond a standard TIG welding station and a controlled quenching setup.

Study Insights and Implications

The literature demonstrates that targeted post-weld treatment of the weld toe region can yield significant improvements in fatigue performance and corrosion resistance without compromising the structural integrity of the overlay weld. For engineers involved in the fabrication of bimetal pressure vessels, this technique offers a valuable tool for enhancing the service life of critical overlay joints, particularly in applications subject to cyclic loading or aggressive corrosive environments. The key to successful implementation lies in the precise control of remelting parameters and the cooling rate, which must be tailored to the specific overlay material and base metal combination. Future work should focus on developing automated TIG remelting systems that can maintain consistent quality across large production volumes.