Zhuojin Pipe Fitting Co., LtdDissimilar Joining of AISI 304L and St37 Steels by TIG Welding Process
Literature Overview
The research by A. R. Khalifeh, A. Dehghan, and E. Hajjari, published in Acta Metallurgica Sinica (English Letters) (2013, Vol. 26, No. 6, pp. 721-727), investigates the TIG welding of dissimilar steel joints between austenitic stainless steel (AISI 304L) and ferritic carbon steel (St37). The study was conducted at Shiraz University and Shahid Chamran University in Iran. Four different austenitic filler metals were evaluated: ER308L, ER309L, ER316L, and ER310. The work addresses a common industrial challenge in piping fabrication where stainless steel and carbon steel components must be joined, such as in heat exchangers, chemical processing equipment, and transition joints in refinery piping.
Experimental Approach and Material Selection
The selection of four different filler metals reflects the practical reality that no single filler metal is universally optimal for all dissimilar steel weldings. Each filler metal offers a different balance of corrosion resistance, mechanical properties, and metallurgical compatibility.
| Filler Metal | Composition Type | Key Alloying Feature | Primary Application |
|---|---|---|---|
| ER308L | Austenitic, low carbon | Low C, Cr-Ni balanced | General 304L welding |
| ER309L | Austenitic, high Cr-Ni | Elevated Cr and Ni | Dissimilar steel welding |
| ER316L | Austenitic, Mo-bearing | Mo addition for pitting resistance | Corrosive environments |
| ER310 | Austenitic, very high Cr-Ni | Very high Cr and Ni | High temperature service |
The microstructural characterization employed optical microscopy, scanning electron microscopy, and ferritometry for delta ferrite quantification. Mechanical properties were evaluated through hardness testing, tensile testing, and impact testing.
Microstructural Analysis and Ferrite Number
The ferrite number (FN) of the weld metals varied between 0.5 and 9.5 depending on the filler metal used. This variation is significant because delta ferrite in austenitic stainless steel welds serves a dual role:
- Beneficial: Delta ferrite prevents hot cracking by absorbing sulfur and phosphorus segregants and providing a solidification path that accommodates thermal strains
- Detrimental: Excessive delta ferrite reduces impact toughness and may promote intergranular corrosion in certain environments
The study found a clear inverse relationship between delta ferrite content and impact toughness. As the amount of delta ferrite increases, the impact toughness of the weldments decreases. This is attributed to the fact that delta ferrite, being a body-centered cubic phase, is inherently less ductile than the face-centered cubic austenite matrix. At elevated delta ferrite levels, the ferrite-austenite interface becomes a preferential path for crack initiation and propagation.
Mechanical Property Comparison
The tensile and impact test results reveal important differences between the filler metal options:
- ER310 produces welds with the highest strength but the lowest impact toughness due to its very high delta ferrite content
- ER308L produces welds with moderate strength and moderate toughness but may have limited corrosion resistance at the weld
- ER309L and ER316L provide the best balance between mechanical and metallurgical properties for this specific dissimilar joint
The authors conclude that ER309L and ER316L are the preferred filler metals for AISI 304L/St37 dissimilar welding, providing a good combination of mechanical and metallurgical properties. This conclusion is consistent with industry practice and AWS D1.6 recommendations for dissimilar steel welding.
Welding Metallurgy Considerations
The dissimilar welding of austenitic stainless steel and ferritic carbon steel presents several metallurgical challenges:
- Dilution effects: The carbon steel side dilutes the weld metal, reducing the Cr and Ni content and potentially shifting the weld composition toward a duplex or even martensitic structure
- Carbon migration: Carbon from the carbon steel side can migrate into the stainless steel weld metal, causing sensitization and reducing corrosion resistance
- Thermal expansion mismatch: The different thermal expansion coefficients of austenitic stainless steel and ferritic carbon steel create residual stresses at the weld interface
- Heat affected zone softening: The carbon steel HAZ may experience softening due to grain growth and tempering, reducing the local strength
- Phase transformation: The carbon steel HAZ may undergo phase transformations during the welding thermal cycle, creating a complex microstructure with varying hardness
Engineering Practice and Standards Compliance
In piping fabrication, dissimilar steel joints are governed by several standards and codes:
| Standard | Relevant Requirement | Application |
|---|---|---|
| ASME B31.3 | Dissimilar metal weld requirements | Process piping |
| ASME B31.1 | Power piping dissimilar joints | Power piping |
| AWS D1.6 | Filler metal selection for stainless steel | Welding procedure qualification |
| ASME Section IX | Welding procedure qualification | Code welding |
| API 570 | Inspection of dissimilar joints | In-service inspection |
The selection of ER309L or ER316L filler metals for this dissimilar joint is consistent with AWS D1.6 recommendations. ER309L is specifically designed for welding dissimilar steels and provides adequate corrosion resistance while accommodating the dilution effects from the carbon steel side. ER316L offers additional pitting and crevice corrosion resistance due to its molybdenum content, making it suitable for more aggressive environments.
Practical Considerations for Pipe Fabrication
In practical pipe fabrication scenarios, the following considerations apply:
- Preheating may be required for the carbon steel side to reduce residual stresses and prevent cracking in the HAZ
- Post-weld heat treatment may be necessary to relieve residual stresses, particularly for thick-walled components
- The weld geometry should be designed to minimize dilution from the carbon steel side, such as by using a larger root opening on the stainless steel side
- Non-destructive examination methods should be selected to be compatible with both material types; ultrasonic testing may require separate calibration for each side
- Corrosion protection of the carbon steel HAZ is essential, as the HAZ may have reduced corrosion resistance due to microstructural changes
Study Insights and Conclusions
This research provides valuable quantitative data on the microstructural and mechanical behavior of AISI 304L/St37 dissimilar welds. The clear correlation between delta ferrite content and impact toughness is a critical finding that should guide filler metal selection in practical applications. The recommendation of ER309L and ER316L as optimal filler metals is well supported by the experimental data and is consistent with established industry practice.
The study underscores the importance of comprehensive characterization in dissimilar welding, including microstructural analysis, ferrite number measurement, and mechanical testing. In engineering practice, this level of characterization should be performed during welding procedure qualification to ensure that the selected filler metal and welding parameters produce acceptable results. The findings also highlight the need for careful consideration of the weld metal composition when dealing with dilution effects, as even small changes in composition can significantly affect the microstructure and properties of the weld.
Overall, this work contributes to the growing body of knowledge on dissimilar steel welding and provides practical guidance for engineers designing and fabricating joints between austenitic stainless steel and ferritic carbon steel components in piping systems and other pressure-containing equipment.