CLADDING TECHNOLOGY SHANXI CO., LTDDissimilar Joining of AISI 304L and St37 Steels by TIG Welding
Literature Overview
This paper, published in Acta Metallurgica Sinica (English Letters) in 2013, addresses the challenging problem of joining dissimilar steels—specifically AISI 304L stainless steel and St37 carbon steel—using the TIG welding process. The research was conducted by scholars at Shiraz University and Shahid Chamran University in Iran. Dissimilar metal welding is a common requirement in industrial applications involving repair welding, bimetallic component fabrication, and the joining of legacy carbon steel structures with new stainless steel replacements.
Core Technical Content
The fundamental challenge in dissimilar joining of austenitic stainless steel (304L) and carbon steel (St37) lies in the significant differences in thermal expansion coefficients, thermal conductivity, and solidification behavior. The thermal expansion coefficient of austenitic stainless steel is approximately 17×10⁻⁶/°C, compared to 12×10⁻⁶/°C for carbon steel. This mismatch leads to high residual stresses and potential cracking in the weld zone.
Weld Metal Composition and Solidification Behavior
The weld metal composition is determined by the dilution ratio between the two base metals. For a symmetric joint with equal plate thickness, the dilution ratio is approximately 50:50. The resulting weld metal composition falls in the austenite-ferrite two-phase region of the Schaeffler diagram, producing a dual-phase microstructure of austenite and ferrite.
| Weld Metal Composition Factor | Value | Implication |
|---|---|---|
| Cr equivalent | ~14–16% | Sufficient for corrosion resistance |
| Ni equivalent | ~5–7% | May be insufficient for full austenite stability |
| Ferrite number (FN) | 15–30 | Moderate ferrite content acceptable for crack resistance |
| Dilution ratio (304L:St37) | ~50:50 | Results in mixed composition |
| Ferrite content | 15–25% | Provides resistance to hot cracking |
The presence of 15–30% delta ferrite in the weld metal is beneficial for resisting solidification cracking (hot cracking), which is a major concern when welding high-carbon steel with stainless steel. However, excessive ferrite (above 40%) can lead to reduced corrosion resistance and potential 400°C embrittlement.
Microstructural Analysis
The weld zone can be divided into three regions:
- Weld metal zone: A mixed austenite-ferrite microstructure with varying ferrite content depending on the local dilution ratio. Near the stainless steel side, the microstructure is predominantly austenitic; near the carbon steel side, the ferrite fraction increases.
- Heat-affected zone (HAZ) – stainless steel side: The HAZ in 304L may experience sensitization if the thermal cycle passes through the critical temperature range of 450–850 °C. Carbide precipitation at grain boundaries depletes chromium, reducing local corrosion resistance.
- Heat-affected zone (HAZ) – carbon steel side: The St37 HAZ may experience grain coarsening, martensite formation in high-carbon regions, and reduced toughness. The thermal cycle from TIG welding, with its moderate heat input, generally produces a narrower HAZ compared to higher-energy processes.
Welding Procedure Considerations
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Welding current | 100–150 A (DCEN) | Adequate penetration without excessive heat input |
| Travel speed | 250–350 mm/min | Balances penetration and HAZ width |
| Tungsten electrode | WC-20La or WC-10Ce, 3.2 mm | Stable arc, good arc force |
| Shielding gas | 100% Ar or Ar-5% O₂ | 5% O₂ improves arc stability and wetting |
| Filler metal | ER309L (32% Cr, 26% Ni) | High Ni content ensures ferrite control in dilution |
| Preheat temperature | 50–100 °C | Reduces residual stress without sensitizing 304L |
| Interpass temperature | <150 °C | Prevents sensitization in 304L HAZ |
The selection of ER309L filler metal is critical. The high nickel content (26%) compensates for the dilution from the carbon steel side, ensuring that the weld metal maintains sufficient austenite stability. The low carbon content (0.03% max) minimizes sensitization risk in the weld metal.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking (solidification) | High carbon dilution from St37 side | Use ER309L filler; maintain 15–25% ferrite |
| Cold cracking (hydrogen-induced) | Carbon steel HAZ martensite + hydrogen | Preheat 50–100 °C; use low-hydrogen consumables |
| Sensitization (304L HAZ) | Thermal cycle through 450–850 °C | Limit interpass temperature; consider PWHT |
| Poor fusion | Thermal conductivity mismatch | Increase current on stainless steel side |
| Excessive weld spatter | Arc instability | Use pure argon shielding; maintain proper gas flow |
Engineering Practice Implications
In pressure vessel fabrication, dissimilar metal welds are encountered in:
- Thermal boundary zones where austenitic stainless steel cladding meets carbon steel base material.
- Repair welding of carbon steel components with stainless steel patches.
- Transition sections between carbon steel and stainless steel piping systems.
For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, the following requirements must be met:
- The weld procedure must be qualified per ASME IX Section IX or equivalent, with impact testing of the weld metal and HAZ when required by the design temperature.
- The weld metal composition must be verified by spectroscopic analysis to ensure adequate ferrite content and corrosion resistance.
- Non-destructive examination (NDE) must include RT or UT for volumetric defects and MT or PT for surface defects, with acceptance criteria per ASME Section V or JB/T 4730.
Key Questions and Reflections
A critical issue not fully addressed in this research is the long-term creep behavior of dissimilar metal welds at elevated temperatures. In pressure vessel applications operating at temperatures above 300 °C, the differential thermal expansion between the two metals leads to progressive creep deformation at the weld interface. The resulting creep cavitation and intergranular failure can lead to premature rupture. Engineers should consider the applicability of creep-resistant filler metals (such as ER347 or specialized low-alloy fills) for high-temperature dissimilar joints.
Additionally, the effect of weld geometry on residual stress distribution deserves attention. The asymmetric thermal expansion between 304L and St37 creates complex residual stress patterns that are difficult to predict analytically. Finite element analysis (FEA) of the welding process can provide valuable insights into the residual stress state and guide post-weld stress relief procedures.
Summary
This research provides valuable insights into the metallurgical challenges and solutions for TIG welding of dissimilar AISI 304L/St37 steel joints. The selection of ER309L filler metal, combined with careful control of thermal input and interpass temperature, offers a practical pathway to achieving sound dissimilar metal welds with acceptable mechanical and corrosion properties. For engineers involved in pressure vessel fabrication and repair, this work underscores the importance of understanding dilution effects and microstructural evolution in dissimilar joints. The findings should be integrated into welding procedure development, with particular attention to post-weld heat treatment and non-destructive examination protocols to ensure long-term service reliability.