TIG Cold Welding Remelting and Wire Deposition on 1Cr17 Stainless Steel Surface

Literature Overview

This 2021 paper by Zheng Shaoxian and colleagues from Lanzhou Jiaotong University and Lanzhou Lanshi Testing Technology Co., Ltd., published in Mechanical Engineering Materials (Vol. 45, No. 12), investigates TIG cold welding techniques for surface modification of 1Cr17 ferritic stainless steel. The research addresses two distinct applications: surface remelting for defect repair or property enhancement, and wire deposition for surface alloying or cladding. The work is supported by the National Natural Science Foundation of China (Grant 51765030) and the Gansu Provincial Natural Science Foundation (Grant 20JR5RA416), indicating its significance within the Chinese materials research community.

Core Technical Content

The TIG cold welding technique involves applying a short-duration welding current pulse to a preheated or ambient-temperature base metal surface, creating a localized molten pool that rapidly solidifies. The two process variants investigated are:

1. Cold welding remelting: The base metal surface is melted and rapidly resolidified without filler metal addition, creating a modified surface layer.
2. Cold welding wire deposition: ER347L stainless steel wire is fed into the molten pool during the brief welding pulse, creating an alloyed surface layer.

The authors systematically investigate the relationship between cold welding time and the minimum welding current required to achieve a well-formed remelted or deposited layer.

Process Parameter Optimization

The experimental results establish clear relationships between cold welding duration and required current:

The inverse relationship between welding time and required current reflects the fundamental heat balance in the welding process. Shorter welding times require higher current densities to achieve the same degree of melting within the reduced time window. The significantly higher currents required for wire deposition (compared to remelting) account for the additional energy needed to melt the filler wire.

Microstructural Analysis

The metallurgical examination reveals several important features:

Heat-Affected Zone (HAZ):

• The HAZ width does not exceed 250 μm for both remelting and deposition processes
• No significant grain coarsening is observed in the HAZ compared to the base metal
• This indicates excellent thermal control during the brief welding pulse

Remelted Layer:

• Grain structure is finer than the base metal
• Higher hardness than the base metal due to grain refinement and possible solid solution strengthening
• Improved corrosion resistance in 3.5% NaCl solution

Deposited Layer (ER347L):

• Finer microstructure than both the base metal and the remelted layer
• Highest hardness among all examined regions
• Best corrosion resistance performance

The corrosion testing in 3.5% NaCl solution provides practical relevance for marine and chemical processing applications. The improved corrosion resistance of both the remelted and deposited layers can be attributed to:

• Grain refinement reducing grain boundary attack susceptibility
• Possible precipitation of fine carbides that block corrosion propagation
• Surface homogenization eliminating micro-segregation from the original casting or rolling process

Comparison with Conventional Surface Treatment Methods

The TIG cold welding technique offers a favorable balance between equipment complexity, process control, and result quality. Compared to laser remelting, it requires less expensive equipment while achieving comparable HAZ control. Compared to flame remelting, it provides far superior thermal control and microstructural refinement.

Engineering Applications

The TIG cold welding technique described in this paper has several practical applications in piping and pressure vessel industries:

1. Surface repair: Remelting of surface defects (scratches, decarburization, minor cracks) in ferritic stainless steel piping without the need for complete welding and post-weld heat treatment.
2. Corrosion resistance enhancement: Creating a refined surface layer on 1Cr17 components exposed to corrosive environments.
3. Wear resistance improvement: Depositing a harder alloy layer on critical surfaces subject to erosion or abrasion.
4. Pre-weld preparation: Remelting of base metal surfaces before welding to homogenize composition and improve weldability.
5. Component refurbishment: Restoring worn or damaged surfaces on expensive components without replacement.

Quality Control Considerations

For implementation in production environments, the following quality control measures are recommended:

1. Current monitoring: Ensure welding current is at or above the minimum value for the selected welding time to guarantee complete melting.
2. Visual inspection: Verify the formation of a smooth, continuous remelted or deposited layer without cracking or porosity.
3. Hardness testing: Confirm that the surface hardness meets specifications and that the hardness transition is gradual.
4. Corrosion testing: Perform potentiodynamic polarization or salt spray testing to verify improved corrosion resistance.
5. Microstructural examination: Periodically verify HAZ width and grain structure to ensure process consistency.

Study Insights and Reflections

The work by Zheng and colleagues demonstrates the versatility of the TIG cold welding technique for surface modification. The ability to achieve significant property improvements with minimal thermal input represents a significant advancement over conventional remelting methods. The narrow HAZ (<250 μm) is particularly advantageous for thin-walled components where excessive heat input could cause distortion or property degradation in the unaffected base metal.

The use of ER347L wire for deposition is a practical choice, as this filler metal is specifically designed for welding austenitic stainless steels and contains low carbon content to minimize sensitization. The combination of ferritic base metal with austenitic deposited material creates a metallurgical couple that can accommodate thermal stresses through the ductility of the austenitic phase.

One area requiring further investigation is the long-term performance of the modified surfaces under cyclic thermal loading. The thermal expansion mismatch between the ferritic base metal and the austenitic deposited layer could lead to fatigue cracking at the interface over extended service life. Additionally, the effect of cold welding parameters on hydrogen pickup and delayed cracking susceptibility warrants further study, particularly for applications in hydrogen-containing environments.

Conclusion

This paper presents a systematic investigation of TIG cold welding techniques for surface modification of 1Cr17 ferritic stainless steel. The establishment of clear relationships between welding time and minimum current provides practical guidance for process setup. The achievement of narrow HAZ, refined microstructure, and improved corrosion resistance demonstrates the effectiveness of the technique for surface enhancement applications. For engineers working with ferritic stainless steel piping and components, this research offers a viable alternative to more expensive surface treatment methods while maintaining excellent quality control potential.