CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Arc Welding Joints of 20G-316L Bimetallic Composite Pipe
Literature Overview
This study focuses on the microstructural characteristics and mechanical properties of arc welding joints in 20G-316L bimetallic composite pipe, a critical component in hydrogenation reactors, heat exchangers, and pressure vessels operating under corrosive conditions. 20G is a carbon steel with good mechanical properties and weldability, while 316L is an austenitic stainless steel offering excellent resistance to pitting and intergranular corrosion. The bimetallic composite pipe combines the structural integrity of the carbon steel with the corrosion resistance of the stainless steel, making it ideal for high-pressure, high-temperature service environments.
The welding of such composite pipes is challenging due to the significant metallurgical differences between the two metals. The weld metal composition, dilution rate, and microstructural evolution at the interface are critical factors determining the long-term reliability of the joint. The study employs both gas tungsten arc welding (GTAW/TIG) and submerged arc welding (SAW) for the composite pipe joints, examining how the welding process parameters influence the joint performance.
Core Technical Points
The key technical challenge in welding 20G-316L composite pipe is controlling the dilution of carbon into the stainless steel weld metal. Excessive carbon dilution from the 20G base metal can lead to the formation of chromium carbides (Cr₂₃C₆ and Cr₇C₃), which cause intergranular corrosion susceptibility in the weld metal and the adjacent heat-affected zone. The study demonstrates that the dilution rate can be controlled by adjusting the welding current, travel speed, and the sequence of welding layers.
The recommended welding strategy involves using a high-carbon-free filler metal (such as ER316L) for the first pass on the stainless steel side to minimize carbon pickup, followed by subsequent passes with a balanced filler composition. The study also evaluates the effect of welding sequence—starting from the stainless steel side versus the carbon steel side—on the dilution rate and microstructural evolution.
| Welding Parameter | GTAW (TIG) | SAW | Effect on Joint |
|---|---|---|---|
| Current | 120–180 A | 350–500 A | Higher current increases dilution |
| Travel Speed | 50–80 mm/min | 200–350 mm/min | Higher speed reduces dilution |
| Shielding Gas | 99.99% Ar | Flux (rutile type) | Ar provides better protection |
| Filler Metal | ER316L (1.6 mm) | ER316L (2.4 mm) | Low-C filler minimizes carbide formation |
| Preheat Temperature | 50–100°C | 100–150°C | Reduces cracking tendency |
| Interpass Temperature | <150°C | <200°C | Controls cooling rate |
Microstructural Analysis
The weld microstructure of the 20G-316L composite pipe joint exhibits a complex gradient of phases. In the fusion zone, the microstructure is predominantly austenitic with some ferrite (approximately 8–15% δ-ferrite), which is beneficial for preventing hot cracking. The ferrite content is determined by the dilution rate and the composition of the filler metal. The study shows that when the dilution rate exceeds 30%, the weld metal composition shifts toward a higher carbon equivalent, promoting the formation of more δ-ferrite and increasing the risk of intergranular corrosion.
The heat-affected zone (HAZ) on the 20G side exhibits a coarse-grained martensitic structure near the fusion boundary, transitioning to a fine-grained ferrite-pearlite structure further away. The HAZ on the 316L side shows a slight grain growth and the formation of chromium carbides along the grain boundaries, particularly near the fusion boundary. The carbide formation is directly related to the carbon dilution from the 20G base metal and is the primary concern for long-term corrosion resistance.
The interface between the 20G and 316L layers in the composite pipe, if present, shows a clean, well-bonded interface with no significant intermetallic formation, provided that the welding process does not exceed the recommended temperature limits. The bonding strength of the composite interface is maintained above 95% of the base metal strength.
Mechanical Properties and Corrosion Resistance
The tensile properties of the welded joint demonstrate good matching with the base metals. The weld metal UTS is approximately 550–620 MPa, which is slightly lower than the 316L base metal (620–700 MPa) but acceptable per ASTM A263 requirements. The elongation is approximately 35–42%, indicating good ductility. The hardness of the weld metal is approximately 200–220 HV, while the HAZ on the 20G side exhibits a peak hardness of approximately 300–350 HV due to the martensitic transformation.
The intergranular corrosion resistance of the weld metal and HAZ was evaluated using the ASTM A263 Method 5 (65% ethanolic oxalic acid test) and the ASTM G28 test. The study reveals that when the dilution rate is controlled below 25%, the weld metal passes the intergranular corrosion test, indicating acceptable corrosion resistance. However, when the dilution rate exceeds 35%, the weld metal and adjacent HAZ fail the test due to excessive chromium carbide precipitation.
| Test Method | Dilution Rate <25% | Dilution Rate >35% | Acceptance Criteria |
|---|---|---|---|
| ASTM A263 Method 5 | Pass | Fail | No intergranular attack |
| ASTM G28 | Pass | Fail | <20 μm attack depth |
| Salt Spray (ASTM B117) | >500 h | <200 h | No pitting corrosion |
Defect Analysis and Countermeasures
The primary defects in 20G-316L composite pipe welding include intergranular corrosion susceptibility, hot cracking, cold cracking, and lack of fusion. Intergranular corrosion is the most critical defect, caused by chromium carbide precipitation due to carbon dilution. The countermeasure involves using low-carbon filler metals (ER316L, E316L) and controlling the dilution rate below 25% through parameter optimization.
Hot cracking is associated with excessive δ-ferrite formation and the presence of low-melting-point inclusions. The countermeasure involves controlling the dilution rate to maintain the δ-ferrite content within the 8–15% range, which provides optimal crack resistance. Cold cracking in the 20G HAZ is related to the formation of hard martensitic structures; preheating to 100–150°C and maintaining interpass temperatures below 200°C effectively mitigates this risk.
Study Insights and Reflections
The study provides valuable guidance for the welding of bimetallic composite pipes in pressure vessel applications. The key insight is that the dilution rate is the single most important parameter governing the joint's long-term corrosion resistance, and it must be carefully controlled through a combination of filler metal selection, welding sequence, and parameter optimization. From an engineering practice perspective, the recommended approach is to start welding from the stainless steel side with a low-dilution first pass, followed by subsequent passes that gradually build up the weld metal. This strategy minimizes the carbon pickup and ensures that the weld metal maintains its austenitic structure with acceptable δ-ferrite content. The study also emphasizes the importance of post-weld stress relief annealing at 1050°C for 30 minutes to dissolve any chromium carbides that may have formed during welding, thereby restoring the full corrosion resistance of the joint.