Arc Surface Modification of TIG Weld Joints in 1Cr18Ni9Ti Stainless Steel

Literature Overview

This study by Wang Qiming from Hangzhou Oxygen Plant Group and Luo Wei from Zhejiang University, published in Welding in 1999, investigates the application of micro-beam plasma arc surface remelting to TIG weld joints of 1Cr18Ni9Ti stainless steel. The research focuses on improving the corrosion resistance of the weld joint through surface modification, addressing a well-known problem in stainless steel welding where chromium carbide precipitation at grain boundaries leads to intergranular corrosion susceptibility.

Core Technical Approach

The study employs a 4A micro-beam plasma arc to perform surface scanning remelting of TIG-welded joints. This technique involves passing a focused plasma beam over the weld surface at a controlled speed, causing the surface layer to melt and rapidly resolidify. The rapid solidification rate associated with this process produces a fine-grained microstructure with reduced microsegregation and suppressed chromium carbide precipitation at grain boundaries.

Process Parameters and Mechanism

The mechanism of corrosion resistance improvement is rooted in the metallurgy of stainless steel welding. During the TIG welding process, the high temperatures cause chromium to combine with carbon to form chromium carbides (primarily Cr₂₃C₆ and Cr₇C₃) at grain boundaries. This chromium depletion in the vicinity of grain boundaries reduces the local chromium content below the critical threshold (typically 12%) required for passive film formation, making the material susceptible to intergranular corrosion.

The micro-beam plasma arc remelting process addresses this problem by:

1. Rapid solidification: The high cooling rate of the remelted surface layer prevents the diffusion of carbon to grain boundaries, thereby suppressing chromium carbide precipitation.
2. Microstructure refinement: The rapid solidification produces a fine-grained microstructure with reduced microsegregation, which improves the uniformity of the passive film.
3. Surface homogenization: The remelting process redistributes alloying elements, reducing the compositional variations that can lead to localized corrosion.

Corrosion Performance Evaluation

The anodic polarization curves in 1M H₂SO₄ solution provide quantitative evidence of the improved corrosion resistance. The key indicators from these curves include:

• Corrosion potential (Ecorr): Shift to more noble (positive) values indicates improved thermodynamic stability of the passive film.
• Passivation current density (ipass): Lower values indicate a more stable and protective passive film.
• Pitting potential (Epp): Higher values indicate improved resistance to localized corrosion initiation.

The authors report a significant improvement in corrosion resistance after surface remelting, which is consistent with the metallurgical changes described above. This improvement is particularly important for applications in aggressive chemical environments, such as those encountered in oxygen plants, where 1Cr18Ni9Ti stainless steel is commonly used.

Comparison of Surface Conditions

Engineering Practice Integration

The application of this surface modification technique to stainless steel weld joints has practical significance in several industrial sectors:

1. Chemical processing: Equipment and piping in chemical plants often require improved corrosion resistance in weld joints to prevent premature failure.
2. Food and pharmaceutical processing: Stainless steel welds must meet stringent hygiene and corrosion resistance requirements.
3. Nuclear industry: Weld joints in nuclear applications must resist stress corrosion cracking and intergranular corrosion under demanding conditions.

From a quality assurance perspective, the surface modification process introduces an additional step that must be controlled and documented. The following quality considerations are relevant:

• Remelting depth control: The remelting must be limited to the surface layer to avoid affecting the bulk mechanical properties of the weld.
• Surface quality: The remelted surface must be smooth and free of defects to ensure effective passive film formation.
• Process consistency: The scanning speed, arc current, and gas flow must be maintained within tight tolerances to ensure uniform treatment.

Study Insights and Reflections

This work represents an early example of surface modification technology applied to improve the performance of welded joints. The concept of using rapid solidification to suppress deleterious microstructural features in weld metals is now well-established in welding metallurgy, but this 1999 study demonstrates a practical and effective implementation of the concept.

The choice of 1Cr18Ni9Ti (equivalent to AISI 321) is particularly significant because this grade is specifically designed to resist sensitization through the addition of titanium, which preferentially combines with carbon to form TiC, leaving chromium available for passive film formation. However, even in this stabilized grade, chromium carbide precipitation can still occur at grain boundaries under certain welding conditions, and the surface modification process provides an additional layer of protection.

The use of micro-beam plasma arc for surface remelting is an elegant solution because it provides the necessary energy input for melting while maintaining the rapid cooling rate required for microstructure refinement. The low current (4A) ensures that the remelting is limited to the surface layer, avoiding the thermal distortion and residual stress that would be associated with higher energy input. This approach demonstrates the principle of localized process application, where energy is concentrated precisely where it is needed, minimizing collateral effects on the surrounding material.