Microstructure and Properties of CMT and TIG End Cladding on X80/2205 Bimetallic Composite Pipe

Literature Overview

This 2022 study published in the Journal of China University of Petroleum (Natural Science Edition) by researchers from China University of Petroleum (East China), National Pipeline Network Group Southwest Pipeline Co., Ltd., and China Petroleum Engineering Corporation addresses a critical challenge in oil and gas pipeline engineering: the end cladding of bimetallic composite pipes using Cold Metal Transfer (CMT) and Gas Tungsten Arc Welding (TIG) processes. The X80 carbon steel/2205 duplex stainless steel composite pipe is designed for sour service environments where the carbon steel provides structural strength while the duplex stainless steel offers superior resistance to chloride stress corrosion cracking and sulfide stress corrosion cracking.

Technical Background and Significance

Bimetallic composite pipes are increasingly used in oil and gas production systems where the combination of high-strength carbon steel for pressure containment and corrosion-resistant alloy linings for fluid contact provides an optimal balance of mechanical performance and corrosion resistance. The X80/2205 combination represents a modern approach to sour service, with X80 providing minimum yield strength of 552 MPa and 2205 duplex stainless steel offering excellent resistance to chloride stress corrosion cracking at temperatures up to approximately 60 degrees Celsius.

The end cladding of these composite pipes is a critical manufacturing step that ensures the integrity of the corrosion-resistant lining at pipe joints. Inconveniently, the end cladding must bridge the metallurgical dissimilarity between the carbon steel outer layer and the duplex stainless steel inner lining, creating unique challenges in terms of dilution control, cracking susceptibility, and mechanical property matching.

CMT Process Characteristics for End Cladding

Cold Metal Transfer (CMT) welding represents an advanced form of Gas Metal Arc Welding (GMAW) that employs a pulsed current with controlled wire feed velocity to achieve low heat input and excellent process stability. The key advantage of CMT for end cladding of bimetallic pipes is the ability to precisely control the heat input, thereby minimizing dilution of the corrosion-resistant lining and reducing the risk of cracking in the weld overlay.

The CMT process parameters optimized in this study included:

The low heat input characteristic of CMT welding results in a heat affected zone (HAZ) that is significantly narrower than that produced by conventional GMAW or TIG processes. This narrow HAZ minimizes the thermal distortion of the pipe and reduces the volume of base metal affected by the welding thermal cycle, which is particularly important for maintaining the mechanical properties of the X80 carbon steel.

TIG Process Characteristics for End Cladding

Gas Tungsten Arc Welding (TIG) was also evaluated as an alternative end cladding process, offering precise heat control and the ability to deposit layers with minimal spatter. The TIG process provides superior control over the weld pool geometry, which is advantageous for the complex geometry of pipe end cladding where the overlay must transition smoothly from the inner duplex lining to the outer carbon steel surface.

TIG process parameters investigated included:

Microstructural Analysis

The microstructure of the end cladding deposits was examined using optical microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD) to characterize the phase distribution, grain morphology, and crystallographic orientation.

For the CMT deposits, the microstructure consisted of:

• A fine-grained ferrite-austenite duplex structure in the weld metal with approximately 50 percent ferrite and 50 percent austenite.
• Columnar grains growing perpendicular to the fusion boundary, transitioning to equiaxed grains in the center of the deposit.
• Mottled ferrite morphology within the austenite matrix, providing good resistance to hot cracking.
• A narrow HAZ with minimal grain growth, preserving the mechanical properties of the base materials.

For the TIG deposits, the microstructure showed:

• Similar duplex structure but with slightly coarser grain size due to lower cooling rates.
• More pronounced columnar grain growth near the fusion boundary.
• Potential for sigma phase formation in regions of excessive heat input, which would compromise corrosion resistance.
• Better control over the dilution profile, resulting in a more gradual composition gradient between the overlay and the base materials.

The key microstructural difference between CMT and TIG deposits lies in the grain size and phase distribution. CMT produces finer grains due to its higher cooling rate, while TIG offers better composition control but at the expense of slightly coarser microstructure.

Mechanical Properties and Performance

The mechanical properties of the end cladding deposits were evaluated through tensile testing, hardness profiling, and impact testing to ensure adequate performance for pipeline service conditions.

The CMT deposits exhibited slightly higher strength but lower ductility compared to TIG deposits, which is consistent with the finer grain structure and higher cooling rate. Both processes produced deposits with mechanical properties that meet or exceed the requirements for pipeline application, with adequate toughness at low temperatures relevant to subsea or arctic service conditions.

Defect Analysis and Quality Control

Several defect types were identified during the end cladding process, each requiring specific countermeasures:

• Cracking at the fusion boundary: Caused by thermal expansion mismatch between the carbon steel and duplex stainless steel. Countermeasure: Use of a transition layer with intermediate thermal expansion coefficient, or application of proper preheat and post-weld heat treatment.
• Porosity: Resulting from gas entrapment during solidification. Countermeasure: Ensure proper shielding gas coverage, clean filler wire, and avoid excessive travel speed.
• Incomplete fusion: Occurring at the interface between the overlay and the base metal. Countermeasure: Adequate heat input and proper joint preparation.
• Excessive dilution: Leading to degradation of the corrosion-resistant properties of the duplex lining. Countermeasure: Optimize process parameters to minimize dilution, particularly important for CMT where lower heat input is achievable.
• Sigma phase formation: Observed in TIG deposits with excessive heat input. Countermeasure: Control heat input and avoid excessive dwell time at the critical temperature range of 600-800 degrees C.

Engineering Practice and Standards Compliance

The end cladding of bimetallic composite pipes must comply with applicable standards including API 934 for the manufacturing of bimetallic composite pipe and ASME B31.4 for pipeline construction. The welding procedure specification (WPS) must be qualified according to ASME IX or equivalent national standards, with specific attention to the following requirements:

1. Qualification of the welding procedure for dissimilar metal welding with appropriate dilution limits.
2. Performance qualification through mechanical testing of the weld overlay, including tensile, hardness, and impact testing.
3. Non-destructive examination (NDE) of the end cladding through ultrasonic testing (UT) or radiographic testing (RT) to detect volumetric defects.
4. Verification of the corrosion-resistant lining integrity through dye penetrant testing (PT) or magnetic particle testing (MT) of the surface.
5. Hydrostatic testing of the completed pipe assembly to verify the integrity of the end cladding under pressure.

The study demonstrates that both CMT and TIG processes can produce end cladding deposits that meet the stringent requirements for sour service pipeline application. The selection between CMT and TIG should be based on the specific requirements of the application, with CMT preferred for high-volume production where speed and consistency are paramount, and TIG preferred for complex geometries or applications requiring precise composition control.

Study Insights and Reflections

This research addresses a practical and important challenge in the oil and gas industry, providing valuable data on the microstructure and properties of end cladding deposits for bimetallic composite pipes. The comparison between CMT and TIG processes reveals that both techniques can produce acceptable results, but with different trade-offs in terms of production efficiency, cost, and microstructural characteristics.

One particularly significant finding is the demonstration that CMT welding, with its inherent low heat input, can effectively minimize dilution while maintaining adequate deposition rates. This is crucial for preserving the corrosion-resistant properties of the 2205 duplex lining, which is the primary reason for using bimetallic composite pipes in sour service environments.

The study also highlights the importance of understanding the interaction between the welding process, the metallurgical behavior of the dissimilar materials, and the resulting service performance. The narrow HAZ achieved with CMT welding not only preserves the mechanical properties of the base materials but also reduces the risk of stress corrosion cracking in the heat affected zone, which is a significant concern for carbon steel in sour environments.

Future work should focus on long-term service evaluation of the end cladding deposits under simulated sour service conditions, including exposure to hydrogen sulfide and carbon dioxide at elevated temperatures and pressures. Additionally, the development of automated CMT welding systems for end cladding could significantly improve production efficiency and quality consistency for large-scale pipeline manufacturing operations.

In summary, this literature provides a comprehensive technical basis for the selection and optimization of end cladding processes for X80/2205 bimetallic composite pipes, with the CMT process emerging as a particularly promising technology for high-volume production with excellent control over dilution and microstructure. Engineers working on bimetallic pipe fabrication should carefully evaluate both CMT and TIG options based on their specific production requirements and quality standards.