Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Arc Welded Joints in 20G/316L Bimetallic Composite Pipe
Literature Overview
The 2009 paper by Lü Shixiong, Wang Ting, and Feng Jicai from Harbin Institute of Technology's State Key Laboratory of Advanced Welding Production Technology, published in the Transactions of the Welding Institute of China (Vol. 30, No. 4), investigates the TIG welding of 20G/316L bimetallic composite pipes. These pipes combine the economic strength of carbon steel (20G) with the corrosion resistance of austenitic stainless steel (316L), making them ideal for applications requiring both mechanical integrity and resistance to aggressive media. The study provides comprehensive characterization of the weld joint through mechanical testing, non-destructive examination, and microstructural analysis.
Bimetallic Composite Pipe Background
Bimetallic composite pipes are widely used in chemical processing, oil and gas production, and power generation where the process fluid is corrosive but the external environment requires high mechanical strength. The 20G/316L combination is particularly common in:
- Chemical reactor piping systems
- Acid service pipelines
- Heat exchanger tubes
- Pharmaceutical process lines
The welding of such composite pipes presents unique challenges because the weld must maintain both the structural integrity of the carbon steel layer and the corrosion resistance of the stainless steel layer, while managing the metallurgical incompatibility between the two materials.
Weld Joint Microstructure
The paper identifies four distinct zones in the weld cross-section:
| Zone | Location | Microstructure | Key Characteristics |
|---|---|---|---|
| Carbon steel zone | Outer layer | Ferrite + pearlite | Base metal structure, unaffected |
| Diffusion zone | Between carbon steel and transition layer | Martensite + retained austenite | High hardness, potential brittleness |
| Transition layer | Between diffusion zone and SS layer | Austenitic | Good toughness, corrosion resistant |
| Stainless steel zone | Inner (316L) layer | Cellular dendrites | Typical weld metal structure |
The formation of martensite in the diffusion zone is a critical finding. This occurs because carbon diffuses from the 20G steel into the weld pool during welding, increasing the carbon equivalent of the deposited metal and promoting martensitic transformation during cooling. The presence of retained austenite partially mitigates the brittleness associated with fully martensitic microstructures.
Transition Wire Strategy
A key process innovation highlighted in this study is the use of a transition filler wire. The authors found that at the weld root, the Ni and Cr alloy element concentrations remained essentially unchanged compared to the filler metal composition. This indicates that the transition wire effectively controlled dilution at the root, preventing excessive carbon steel dilution that would compromise the corrosion resistance of the root weld.
| Parameter | Root Weld (with transition wire) | Cap Weld |
|---|---|---|
| Ni content | Maintained at filler wire level | Standard SS filler |
| Cr content | Maintained at filler wire level | Standard SS filler |
| Carbon dilution | Controlled | Standard |
| Corrosion resistance | Adequate | Excellent |
Mechanical Performance and NDE Results
The weld joints demonstrated excellent mechanical properties across all tested parameters:
- Tensile strength: Adequate for pressure vessel service, meeting or exceeding the lower strength of the two parent materials.
- Bend test: No cracking or delamination observed, indicating good ductility and metallurgical bonding.
- Impact test: Satisfactory toughness, with the austenitic transition layer providing good energy absorption.
- Pressure test: No leakage, confirming dimensional integrity and full penetration.
- NDE (RT/UT): No defects detected, confirming sound weld quality.
Engineering Practice Considerations
For practitioners welding bimetallic composite pipes, the following points emerge from this study:
- Filler metal selection is critical: The transition wire composition must be carefully designed to resist dilution from the carbon steel side while maintaining adequate austenite stability. A typical transition wire might contain 20–25% Ni and 18–22% Cr to ensure full austenite in the dilution zone.
- Welding direction matters: Welding from the stainless steel side toward the carbon steel side generally provides better control of dilution and reduces the risk of carbon contamination of the stainless steel layer.
- Heat input control: Lower heat input (typical of TIG welding) is preferred to minimize the diffusion zone width and reduce the volume of martensitic microstructure.
- Post-weld inspection: Both RT and UT should be employed to detect lack of fusion at the bimetallic interface, which is a common defect in composite pipe welding.
- Corrosion testing: Even if mechanical properties are adequate, intergranular corrosion resistance and stress corrosion cracking susceptibility should be evaluated, particularly for the diffusion zone where sensitization may occur.
Key Reflections
This paper provides a systematic framework for understanding the metallurgical challenges of bimetallic pipe welding. The four-zone microstructural model is particularly useful for quality assessment, as it provides clear criteria for evaluating weld acceptability. The success of the transition wire approach validates the concept of tailored filler metal composition as a tool for managing dilution in dissimilar metal welding. For modern practice, the integration of PAW welding with higher deposition rates and better arc stability could further improve productivity while maintaining the metallurgical control demonstrated in this study.