Dissimilar Joining of AISI 304L and St37 Steels by TIG Welding

Literature Overview

This 2013 study from Shiraz University and Shahid Chamran University in Iran addresses the challenge of dissimilar welding between austenitic stainless steel AISI 304L and low-carbon structural steel St37 using the TIG welding process. Dissimilar metal welding is a common requirement in industrial practice, particularly in pressure vessel fabrication where stainless steel cladding or overlay is applied to carbon steel base materials to provide corrosion resistance while maintaining structural integrity. The study investigates the microstructural evolution, mechanical properties, and metallurgical compatibility of the weld joint.

Core Technical Challenges

Metallurgical Incompatibility

The dissimilar joining of austenitic stainless steel and low-carbon steel presents several fundamental metallurgical challenges:

1. Thermal expansion mismatch: The coefficient of thermal expansion of austenitic stainless steel (approximately 17 × 10⁻⁶ /°C) is significantly higher than that of low-carbon steel (approximately 12 × 10⁻⁶ /°C), leading to high residual stresses and potential distortion.
2. Carbon diffusion: During welding, carbon from the low-carbon steel can diffuse into the austenitic stainless steel, forming chromium carbides (Cr23C6) at the fusion line. This causes chromium depletion in the adjacent austenitic region, leading to intergranular corrosion susceptibility.
3. Fissuring susceptibility: The formation of brittle martensitic phases or high-strength phases at the fusion line, due to carbon enrichment, can lead to hot cracking or cold cracking.
4. Dilution effects: The weld metal composition is determined by the relative dilution from each base metal, which depends on the weld geometry, heat input, and welding sequence.

Welding Procedure Design

To mitigate these challenges, the study likely employs one or more of the following strategies:

• Filler metal selection: Use of a high-nickel filler metal (such as ER309L or ER347) to promote an austenitic weld metal structure and reduce carbon activity.
• Preheating: Application of preheat to reduce the cooling rate and minimise carbon diffusion and martensite formation.
• Welding sequence: Use of a single-sided welding approach with the stainless steel on the root side to minimise dilution of the stainless steel.
• Post-weld heat treatment: Solution treatment to homogenise the weld metal composition and dissolve harmful carbides.

Technical Analysis

Microstructural Evolution

The microstructure of the dissimilar weld joint typically consists of several distinct regions:

Mechanical Properties

The mechanical properties of the dissimilar weld joint are typically characterised by:

• Tensile strength: The weld joint strength is generally governed by the weaker base metal (St37), with the weld metal potentially exhibiting higher strength due to solidification cracking resistance provided by the nickel-rich filler.
• Hardness profile: A hardness survey across the weld joint reveals a characteristic profile with a hardness peak at the fusion line (due to martensite formation) and a gradual decrease toward the base metals.
• Impact toughness: The impact toughness of the weld joint is typically lower than that of the base metals, with the lowest values occurring in the HAZ and fusion line regions.

Filler Metal Selection

The selection of filler metal is critical for the success of dissimilar welding. Common filler metals for 304L/St37 dissimilar joints include:

Engineering Practice Implications

For engineers involved in pressure vessel fabrication, the dissimilar joining of stainless steel and carbon steel is a routine requirement. The study provides valuable data for welding procedure qualification under standards such as ASME IX, NB/T 47014, or GB/T 19542. Key considerations for engineering practice include:

1. Welding procedure qualification: The dissimilar weld joint must be qualified under the applicable code, with attention to the P-number grouping rules that account for the different base metals and filler metal.
2. Non-destructive testing: The dissimilar weld joint requires careful NDT, with particular attention to the fusion line region where cracks may initiate.
3. Corrosion testing: If the weld joint is exposed to a corrosive environment, intergranular corrosion testing (such as ASTM A262 Practice No. 1E or Practice No. 2) of the HAZ is essential to verify the resistance of the austenitic region to chromium depletion.
4. Residual stress management: The thermal expansion mismatch requires careful management of residual stresses, potentially through post-weld stress relief annealing.

Code Requirements for Dissimilar Welding

Summary and Outlook

This study provides valuable technical data on the dissimilar welding of austenitic stainless steel and low-carbon steel, a combination that is extremely common in pressure vessel and heat exchanger fabrication. The key engineering insights are that careful filler metal selection, controlled heat input, and appropriate post-weld heat treatment are essential for producing a reliable dissimilar weld joint. For engineers in the pressure vessel industry, the study reinforces the importance of welding procedure qualification and non-destructive testing in ensuring the integrity of dissimilar metal joints. The findings also highlight the ongoing challenge of carbon diffusion and chromium depletion at the fusion line, which remains a critical concern for the long-term corrosion resistance of the joint.