CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of CMT and TIG Cladding at X80/2205 Bimetallic Composite Pipe Ends
Literature Overview
This study published in the Journal of China University of Petroleum (Science and Technology) (2022) by Li Liying, Sheng Xuezhen, Dai Guowen, Fu Xianqiao, Zhou Xin, and Han Bin investigates the microstructure and mechanical properties of transition welds deposited at the ends of X80/2205 duplex stainless steel bimetallic composite pipes using cold metal transfer (CMT) and gas tungsten arc welding (TIG) processes. The research was supported by multiple funding sources including the National Defense Science and Technology Innovation Special Zone Project, the National Natural Science Foundation of China (51771228), and the Shandong Provincial Natural Science Foundation (ZR2013EEQ027). This work addresses a critical engineering challenge in pipeline engineering: the reliable joining of dissimilar-material bimetallic pipes in high-pressure oil and gas transmission systems.
Core Technical Findings
Background: X80/2205 Bimetallic Composite Pipe
X80/2205 bimetallic composite pipes combine the high strength of X80 pipeline steel (minimum yield strength of 552 MPa) with the excellent corrosion resistance of 2205 duplex stainless steel (UNS S31803/S32205). These pipes are used in:
- Offshore oil and gas pipelines in chloride-containing environments
- High-pressure pipelines in sour service (H2S-containing)
- Deepwater production systems
- Subsea umbilicals and flowlines
The critical engineering challenge is creating a reliable transition weld at the pipe end where the bimetallic section connects to a single-material section (either X80 or 2205), requiring careful control of dilution, residual stress, and microstructural compatibility.
Microstructural Analysis of CMT Cladding
| Location | Phase Composition | Grain Structure | Hardness (HV) |
|---|---|---|---|
| X80 base metal | Ferrite + pearlite + bainite | Fine-grained | 200-250 |
| 2205 base metal | Ferrite + austenite (45:55) | Fine equiaxed | 250-320 |
| CMT weld centerline | Ferrite + austenite (40:60) | Columnar, elongated | 260-300 |
| CMT weld near X80 side | Ferrite + austenite + martensite traces | Mixed, fine | 280-340 |
| CMT weld near 2205 side | Ferrite + austenite | Columnar | 250-310 |
| HAZ (X80 side) | Bainite + martensite | Coarsened | 300-380 |
Microstructural Analysis of TIG Cladding
| Location | Phase Composition | Grain Structure | Hardness (HV) |
|---|---|---|---|
| TIG weld centerline | Ferrite + austenite (42:58) | Fine columnar | 255-295 |
| TIG weld near X80 side | Ferrite + austenite | Fine, slightly coarsened | 270-310 |
| TIG weld near 2205 side | Ferrite + austenite | Fine columnar | 250-300 |
| HAZ (X80 side) | Bainite + martensite | Moderately coarsened | 290-360 |
Comparison of CMT and TIG Processes
| Parameter | CMT Process | TIG Process |
|---|---|---|
| Welding current | 80-150 A (pulsed) | 100-200 A (DC) |
| Travel speed | 150-300 mm/min | 50-120 mm/min |
| Heat input | 0.8-1.5 kJ/mm | 1.2-2.5 kJ/mm |
| Dilution ratio | 25-35% | 30-45% |
| Deposition rate | High (3-5 kg/h) | Low (0.5-1.5 kg/h) |
| Microstructure uniformity | Good | Excellent |
| HAZ width | Moderate (1-2 mm) | Narrow (0.5-1.5 mm) |
| Cracking tendency | Low | Very low |
| Productivity | High | Low |
Process Analysis and Engineering Considerations
Filler Metal Selection
The selection of filler metal for the transition weld is critical for achieving the desired metallurgical compatibility:
| Filler Metal | Composition | Application |
|---|---|---|
| ER2209 | Duplex SS matching 2205 | Primary choice for transition welds |
| ER309L | 309L austenitic SS | When X80 side is thicker |
| ER316L | 316L austenitic SS | For low-stress applications |
| ER80S-D2 | Matching X80 | When connecting to X80 section |
Residual Stress Management
| Stress Source | Magnitude (MPa) | Management Strategy |
|---|---|---|
| Thermal contraction | 300-500 | Preheating, low heat input |
| Phase transformation (X80 HAZ) | 200-400 | Post-weld stress relief |
| Dilution-induced mismatch | 100-300 | Multi-pass strategy, controlled dilution |
| Restraint effects | Variable | Fixture design, sequence planning |
Defect Analysis and Prevention
| Defect Type | Cause | Detection Method | Prevention |
|---|---|---|---|
| Hot cracking | High sulfur, wide freezing range | PT, RT | Low-S filler, proper heat input |
| Cold cracking | High hardness in X80 HAZ | MT, UT | Preheat, PWHT |
| Lamellar tearing | Through-thickness restraint | UT (TOFD) | Through-thickness toughness verification |
| Porosity | Hydrogen from moisture | RT, UT | Dry electrodes, preheat |
| Lack of fusion | Low current, poor technique | RT, UT | Adequate current, proper fit-up |
Quality Assurance and Code Compliance
Applicable Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| ASME VIII Div.1 | Pressure vessels | WPS/PQR, NDE, material specs |
| ASME IX | Welding qualification | WPS qualification, welder qualification |
| NB/T 47014 | Chinese welding qualification | Procedure and welder qualification |
| API 934 | Piping welding | Procedure qualification for pipelines |
| GB/T 150 | Chinese pressure vessels | Design, fabrication, inspection |
| NACE MR0175 | Sour service materials | Materials and welding requirements |
Inspection Requirements for Bimetallic Pipe End Welds
- 100% visual inspection of all welds for surface quality
- 100% magnetic particle testing (MT) for surface and near-surface defects
- Radiographic testing (RT) or ultrasonic testing (UT) per code requirements (typically 10-100% depending on service criticality)
- Dye penetrant testing (PT) for the 2205 side where MT is not applicable
- Hardness survey across the weld and HAZ to detect excessive hardness (>350 HV typically triggers PWHT)
- Post-weld heat treatment (PWHT) when hardness exceeds limits or when specified by design
Study Insights and Reflections
This research addresses a highly relevant engineering problem in the oil and gas industry, where the reliable connection of bimetallic composite pipes is essential for the integrity of offshore and subsea systems. The comparison between CMT and TIG processes provides valuable practical guidance for fabrication engineers.
The key finding is that while TIG welding produces superior microstructural quality with finer grains and more uniform phase distribution, CMT offers significantly higher productivity with acceptable metallurgical quality. For large-scale pipeline projects where fabrication schedule is critical, CMT represents a viable alternative provided that the WPS is properly qualified and the dilution ratio is carefully monitored.
From a pressure vessel and pipeline engineering perspective, the transition weld between dissimilar materials represents one of the most critical welds in the system. The combination of thermal stress, corrosion stress, and mechanical stress in these regions demands rigorous qualification and inspection. The research confirms that both CMT and TIG can produce acceptable transition welds when properly controlled, but the selection should be based on the specific service conditions, applicable code requirements, and fabrication constraints.
The study also highlights the importance of post-weld heat treatment in managing residual stresses and controlling the microstructure of the X80 heat-affected zone, which is particularly susceptible to martensite formation and subsequent hydrogen-induced cracking in service.