Study Note on Microstructure and Property Comparison Between FSW and TIG Welded 316L Austenitic Stainless Steel Joints

Literature Overview

This 2010 paper by Wang Kuaishe, Lu Bin, and Yu Haifeng from Xi'an University of Architecture and Technology and the Stainless Steel Research Institute of Baoshan Iron & Steel Co., Ltd., published in Baosteel Technical Research, provides a comprehensive comparison of friction stir welding (FSW) and tungsten inert gas (TIG) welding for 316L austenitic stainless steel joints. The research is highly relevant to bimetal pressure vessel fabrication where 316L is commonly used as a cladding material and where the choice of welding process directly impacts the corrosion resistance and mechanical integrity of the final product.

Core Technical Content

Process Comparison Framework

The study compares FSW and TIG welding across multiple dimensions including microstructure, mechanical properties, corrosion resistance, and residual stress. The comparison provides engineers with decision-making criteria for process selection in bimetal pressure vessel fabrication.

Welding Parameters

Microstructural Analysis

FSW Weld Zone:

• The stir zone exhibits severe plastic deformation with refined grain structure (5-15 micrometers).
• Dynamic recrystallization occurs in the stir zone, producing equiaxed grains.
• No melting occurs; the weld is a solid-state joint with no segregation or grain boundary precipitation.
• The thermomechanical affected zone (TMAZ) shows elongated grains with varying degrees of recrystallization.
• Retained austenite content remains high (85-92%) throughout the weld zone.

TIG Weld Zone:

• The weld metal shows columnar dendritic structure growing from the fusion boundary.
• Grain size in the weld metal is 50-150 micrometers.
• The HAZ exhibits a coarse grain zone (100-300 micrometers) adjacent to the fusion line.
• Sigma phase precipitation may occur in the HAZ if interpass temperature exceeds 250°C.
• Retained austenite content decreases to 70-80% in the HAZ due to partial transformation during thermal cycling.

Mechanical Property Comparison

Corrosion Resistance Comparison

Relevance to Cladding and Bimetal Pressure Vessel Applications

Process Selection for 316L Cladding

The research provides clear guidance for process selection when fabricating 316L-clad pressure vessels:

Key Technical Considerations

1. Sensitization risk: TIG welding of 316L creates a sensitized HAZ when the material passes through the 450-850°C range for extended periods. This is the temperature range where chromium carbide (Cr23C6) precipitation occurs at grain boundaries, depleting the adjacent regions of chromium and reducing corrosion resistance.
2. Sigma phase formation: In TIG welds with multiple passes, sigma phase (Cr-rich intermetallic) can form in the HAZ if interpass temperature exceeds 250°C. Sigma phase is extremely brittle and severely reduces ductility and corrosion resistance.
3. Retained austenite stability: The FSW process preserves the retained austenite content of 316L, which contributes to strain-induced transformation hardening and improved corrosion resistance. TIG welding partially destabilizes retained austenite through thermal cycling.

Defect Analysis and Process Optimization

Study Insights and Engineering Practice Implications

The most important conclusion from this research is that FSW produces superior microstructural and corrosion resistance characteristics in 316L stainless steel joints compared to TIG welding. However, the practical limitations of FSW equipment (maximum plate thickness, joint geometry restrictions, and capital equipment cost) mean that TIG welding remains the dominant process for most bimetal pressure vessel fabrication applications.

For engineers specifying welding procedures for 316L-clad pressure vessels, the following recommendations emerge from this study:

1. For thin cladding layers (≤3 mm): Consider FSW where equipment is available, as it eliminates sensitization concerns entirely. The solid-state nature of the process preserves the as-received microstructure and corrosion resistance of the 316L cladding material.
2. For conventional TIG overlay welding: Limit total passes to minimize cumulative heat input. Use a single-layer, multi-pass approach with interpass temperature controlled below 150°C. Apply a final grinding and passivation treatment to remove any sensitized surface layer.
3. Post-weld treatment: For TIG-welded 316L joints in aggressive environments, a solution treatment at 1050°C followed by rapid water quenching can dissolve any precipitated carbides and restore full corrosion resistance. However, this treatment may not be feasible for large pressure vessels already in service.
4. Inspection requirements: For TIG-welded 316L cladding in critical applications, intergranular corrosion testing (ASTM A262 Practice E or A) should be performed on the HAZ region as part of procedure qualification. The results directly inform the acceptance criteria for production welds.

The research also highlights an important economic consideration: while FSW produces superior joints, the capital investment in FSW equipment (typically 2-5 million RMB) must be justified by the value of the improved corrosion resistance in the specific application. For pressure vessels operating in mild environments with adequate cathodic protection, TIG welding with proper procedure control may provide adequate performance at significantly lower cost.

This comparative study serves as an essential reference for welding procedure specification in bimetal pressure vessel fabrication. The data presented enables engineers to make informed decisions about process selection, parameter optimization, and quality assurance requirements based on the specific service conditions and performance requirements of each application. The continued advancement of both FSW and TIG welding technologies ensures that engineers have access to increasingly capable processes for fabricating high-performance bimetallic pressure vessels that meet the demanding requirements of modern chemical, petrochemical, and nuclear industries.