CMT and TIG Overlay Welding of X80/2205 Bimetallic Composite Pipe Ends Microstructure and Performance

Literature Overview

This paper by Li Liying and colleagues from China University of Petroleum (East China) and Southwest Pipeline Co., Ltd., published in the Journal of China University of Petroleum (2022), investigates the application of Cold Metal Transfer (CMT) and Tungsten Inert Gas (TIG) overlay welding to the pipe ends of X80/2205 bimetallic composite pipes. Bimetallic composite pipes combine a high-strength carbon steel outer layer (X80) with a corrosion-resistant stainless steel inner lining (2205 duplex stainless steel), providing both structural integrity and corrosion resistance for harsh pipeline environments such as sour gas (H2S-containing) and high-chloride offshore conditions. The pipe end preparation for welding is a critical step in pipeline construction, and the overlay welding process must ensure metallurgical compatibility, mechanical integrity, and corrosion resistance at the transition zone. This study compares two welding processes—CMT and TIG—and evaluates their effects on microstructure, hardness, and corrosion resistance.

Core Technical Findings

CMT Process Parameters and Weld Geometry

The study systematically varied the welding speed to determine its effect on weld bead geometry:

The CMT process is characterized by a pulsed current waveform that allows the molten wire to transfer in small droplets at the moment of pinch-off, resulting in low spatter, low heat input, and excellent bead profile control. These characteristics make CMT particularly suitable for overlay welding where precise control of the deposition geometry is required.

Microstructure Analysis

A key finding of this study is the comparison of grain boundary character distribution (GBCD) between CMT and TIG overlay layers:

The higher fraction of Σ3 coincidence site lattice (CSL) boundaries in the CMT overlay is significant because Σ3 boundaries (twin boundaries) are known to resist grain boundary sliding, intergranular corrosion, and crack propagation. In the context of 2205 duplex stainless steel, the ferrite:austenite ratio is a critical microstructural parameter because it directly affects the pitting and stress corrosion cracking (SCC) resistance. The CMT overlay's ferrite:austenite ratio of 54:46 is closer to the nominal 2205 composition (typically 40-60% ferrite) than the TIG overlay's 37:63 ratio, which is austenite-rich and may be more susceptible to pitting corrosion.

Interface Characterization

The study identified a critical microstructural feature at the interface between the overlay layer and the X80 carbon steel substrate: a raised, brittle, hard phase with a body-centered cubic (BCC) crystal structure identified as martensite. This martensitic phase has fine grains and predominantly high-angle grain boundaries, resulting in a sharp hardness gradient at the interface. This hardness transition is a potential crack initiation site under cyclic loading or impact conditions.

The formation of martensite at the interface is attributed to the high cooling rate and the dilution of carbon from the X80 substrate into the weld pool. X80 steel typically contains 0.05-0.10% carbon, and even small amounts of carbon can promote martensite formation in iron-based alloys when the cooling rate is sufficiently high. The CMT process, despite its lower heat input, may still produce adequate cooling rates at the interface to form martensite.

Corrosion Resistance

The pitting corrosion performance was evaluated through electrochemical testing:

The superior pitting resistance of the CMT overlay is attributed to the combined effect of a more balanced ferrite:austenite ratio and a higher fraction of corrosion-resistant Σ3 grain boundaries. This finding has direct implications for pipeline integrity in aggressive environments such as sour gas service (API 5L NACE MR0175) and offshore platforms exposed to chloride-containing seawater.

Engineering Practice Implications

Process Selection for Pipeline Applications

The comparison between CMT and TIG provides clear guidance for process selection in bimetallic composite pipe welding:

For large-scale pipeline construction, CMT is the preferred process due to its superior corrosion resistance, higher efficiency, and better microstructural control. However, for remote or field locations where CMT equipment is not available, TIG remains a viable alternative provided that the austenite-rich microstructure is accepted and appropriate corrosion monitoring is implemented.

Interface Management

The formation of martensite at the X80/overlay interface is a critical concern that requires active management:

1. Preheating: Increasing the substrate preheat temperature to 200-300 °C can reduce the cooling rate at the interface and suppress martensite formation. However, excessive preheat may affect the mechanical properties of the X80 substrate.
2. Interpass temperature: Maintaining a controlled interpass temperature of 150-250 °C can moderate the thermal cycle and reduce the driving force for martensite transformation.
3. Post-weld heat treatment: A low-temperature anneal (600-700 °C) can transform martensite to bainite or ferrite-pearlite, reducing hardness and improving toughness. However, this may affect the corrosion resistance of the overlay layer.
4. Filler metal selection: Using a low-carbon filler metal (e.g., ER309L or ER319L) can reduce carbon dilution from the substrate and minimize martensite formation.

Pipeline Integrity Considerations

For bimetallic composite pipes in service, the overlay weld at the pipe end is a potential location for failure initiation. The hardness gradient at the interface creates a stress concentration that can initiate cracking under:

• Thermal cycling: Differential thermal expansion between the carbon steel and stainless steel layers generates cyclic stresses at the interface.
• Internal pressure: The hoop stress in the pipe wall creates a tensile stress component at the interface.
• Corrosion fatigue: In sour gas environments, hydrogen ingress can promote cracking at the hard, brittle martensitic interface.

A comprehensive integrity assessment program should include:

• Ultrasonic testing (UT) of the overlay weld for lack of fusion and cracking.
• Hardness mapping across the interface to identify the extent of the hard zone.
• Corrosion testing (e.g., ASTM G150 for sulfide stress cracking) of the overlay weld in representative service environments.
• Finite element analysis (FEA) of the interface region under combined mechanical and thermal loading.

Key Questions and Reflections

1. Long-term corrosion performance: The electrochemical testing provides short-term corrosion data, but long-term immersion testing (e.g., 1000+ hours in simulated sour gas environments) is essential to validate the corrosion resistance predictions.
2. Mechanical property gradient: The study reports hardness data but does not provide a complete mechanical property profile (tensile strength, yield strength, elongation) across the interface. The ductility of the martensitic zone is likely very low, which could be a crack initiation site.
3. Effect of welding sequence: The study does not address the welding sequence for multi-pass overlay deposition. The thermal history of subsequent passes affects the microstructure of previously deposited passes, and the final microstructure depends on the welding sequence.
4. Comparison with other composite pipe end treatments: Alternative approaches to pipe end preparation include machining the composite pipe to expose the full thickness of both layers and then welding with a suitable filler metal, or using a transition fitting. The relative merits of overlay welding versus these alternatives should be evaluated for specific applications.

Study Insights and Implications

This study provides valuable comparative data on CMT and TIG overlay welding for bimetallic composite pipe ends, with clear conclusions favoring CMT for its superior corrosion resistance, higher efficiency, and more favorable microstructural characteristics. The identification of martensite at the X80/overlay interface is a critical finding that highlights the need for active thermal management during welding to prevent the formation of brittle, hard phases at the interface. For pipeline engineers, the key takeaway is that the selection of overlay welding process must be driven by a holistic assessment of corrosion resistance, mechanical integrity, and process efficiency, rather than by any single criterion. The CMT process, while requiring more expensive equipment, offers a compelling combination of performance attributes that justify its deployment for critical pipeline applications involving aggressive environments. Future work should focus on long-term corrosion testing, mechanical property profiling across the interface, and validation through full-scale pipeline component testing.