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

Literature Overview

The paper by A.R. Khalifeh, A. Dehghan, and E. Hajjari, published in Acta Metallurgica Sinica (English Letters) (Vol. 26, No. 6, 2013, pp. 721-727), investigates the TIG welding of austenitic stainless steel (AISI 304L) and ferritic carbon steel (St37) using four different austenitic filler metals: ER308L, ER309L, ER316L, and ER310. The study examines microstructure characteristics and mechanical properties using optical and scanning electron microscopy, ferritometry, hardness, tensile, and impact tests. Ferrite numbers of the weldments vary between 0.5 and 9.5, with increased delta ferrite content reducing impact toughness. ER309L and ER316L electrodes provide the best combination of mechanical and metallurgical properties for this dissimilar welding application.

Core Technical Points

Dissimilar welding between austenitic stainless steel and carbon steel presents unique metallurgical challenges due to the significant differences in composition, thermal properties, and metallurgical behavior between the two materials. The selection of filler metal is critical to achieving acceptable weld properties, as it must accommodate the dilution from both base metals while maintaining adequate mechanical properties and corrosion resistance.

Filler Metal Selection Criteria

The selection of filler metal for dissimilar steel welding involves balancing multiple requirements:

Delta Ferrite Control

Delta ferrite formation in austenitic weld metals is a critical factor affecting mechanical properties, particularly impact toughness. The ferrite number (FN) is a standardized measure of delta ferrite content, and different applications require different FN ranges:

• FN < 5: Risk of hot cracking due to insufficient ferrite
• FN 5-15: Optimal range for most applications, balancing toughness and crack resistance
• FN > 15: Risk of reduced toughness and increased susceptibility to intergranular corrosion

In dissimilar welding, the dilution from the carbon steel base metal introduces carbon and manganese into the weld metal, promoting ferrite formation. The filler metal must be selected to achieve an acceptable FN despite this dilution effect.

Dilution and Compositional Evolution

The dilution ratio in dissimilar welding is typically higher than in similar welding due to the composition mismatch between base metals. This results in significant compositional evolution in the weld metal, which affects both microstructure and properties. The carbon content from the St37 base metal is particularly problematic, as it promotes carbide formation and reduces austenite stability.

Process Analysis and Engineering Relevance

Weld Microstructure

The weld microstructure in dissimilar stainless/carbon steel joints exhibits several characteristic features:

1. Weld metal: Composition depends on filler metal and dilution ratio; typically austenitic with varying ferrite content
2. Heat-affected zone (HAZ) in stainless steel: May experience sensitization if carbon migrates from carbon steel side
3. HAZ in carbon steel: May experience grain coarsening and martensite formation depending on cooling rate
4. Dilution zone: Gradual transition in composition and microstructure between weld metal and base metals

Mechanical Properties

The mechanical properties of dissimilar weldments are governed by the weakest link principle, with the carbon steel side typically limiting strength and the weld metal limiting toughness. Key considerations include:

• Tensile strength: Typically lower than either base metal due to dilution effects
• Impact toughness: Strongly affected by ferrite content; higher FN reduces toughness
• Hardness: May show significant gradients across the joint due to compositional variations

Corrosion Considerations

Dissimilar stainless/carbon steel joints are susceptible to several corrosion mechanisms:

1. Galvanic corrosion: Due to potential difference between austenitic and ferritic materials
2. Intergranular corrosion: Risk in HAZ if sensitization occurs due to carbon migration
3. Pitting and crevice corrosion: Risk in weld metal if ferrite content is excessive
4. Stress corrosion cracking: Risk in HAZ if residual stresses are high

Study Insights and Reflections

This study provides valuable guidance for filler metal selection in dissimilar stainless/carbon steel welding, with ER309L and ER316L identified as the optimal choices for general applications. The key insight is that filler metal selection must balance multiple competing requirements: ferrite content control, mechanical properties, corrosion resistance, and cost.

One important finding is the strong correlation between ferrite number and impact toughness, with higher FN values consistently associated with reduced toughness. This relationship is critical for applications requiring high toughness, such as pressure vessels and cryogenic equipment, where low-temperature impact performance is essential.

For pipe welding applications, dissimilar joints between stainless steel and carbon steel are common in heat exchangers, process piping, and instrumentation connections. The guidance provided in this study is directly applicable to these applications, with ER309L being the most widely used filler metal for general dissimilar welding and ER316L preferred for applications requiring enhanced corrosion resistance.

Reference Value and Outlook

The study by Khalifeh et al. provides a systematic evaluation of filler metal options for dissimilar welding of austenitic stainless steel and carbon steel, offering practical guidance for engineering applications. The identification of ER309L and ER316L as optimal filler metals aligns with industry practice and provides scientific validation for their widespread use. For pipe and fitting fabrication, this study reinforces the importance of proper filler metal selection in achieving acceptable weld properties and corrosion resistance in dissimilar joints. The emphasis on ferrite number control highlights the need for careful monitoring of weld metal composition, particularly in high-dilution applications where carbon steel base metal significantly influences weld metal properties.