Microstructure and Properties of 20G-316L Bimetallic Composite Pipe Arc Weld Joints - Technical Study Note

Literature Overview

This 2009 publication in the journal "Welding Journal" (焊接学报), authored by Lv Shixiong, Wang Ting, and Feng Jicai from the State Key Laboratory of Modern Welding Production Technology at Harbin Institute of Technology, investigates the welding of 20G/316L bimetallic composite pipes using TIG (GTAW) welding. The study examines the weld joint microstructure, mechanical properties, and elemental diffusion behavior across the four distinct zones formed during welding: the carbon steel layer, the diffusion zone between carbon steel and transition layer, the transition layer, and the stainless steel layer.

Technical Background and Engineering Significance

Bimetallic composite pipes are widely used in chemical, petrochemical, and power industries where the pipe must simultaneously resist internal corrosion (handled by the stainless steel inner lining) and external mechanical stress (handled by the carbon steel outer layer). The 20G/316L combination is particularly common: 20G is a medium-carbon structural steel with good mechanical properties, while 316L is a low-carbon austenitic stainless steel with excellent corrosion resistance due to its Mo addition.

The welding of bimetallic composite pipes presents unique challenges:

1. Dissimilar material welding: The significant difference in thermal conductivity, thermal expansion coefficient, and metallurgical composition between carbon steel and austenitic stainless steel creates complex stress states during welding.
2. Elemental diffusion: During welding, alloying elements (Ni, Cr, Mo) from the stainless steel side can diffuse into the carbon steel side, potentially forming brittle intermetallic compounds or diluting the corrosion resistance of the stainless steel layer.
3. Microstructural complexity: The weld zone exhibits multiple distinct microstructural regions, each with different mechanical and corrosion properties.

Weld Joint Microstructural Analysis

The study identifies four distinct zones in the weld cross-section, each with characteristic microstructure:

The presence of martensite in the diffusion zone is a critical finding. Martensite formation in this region can lead to:

• Reduced ductility and toughness
• Increased susceptibility to hydrogen-induced cracking
• Potential stress corrosion cracking (SCC) in corrosive environments

The retained austenite in the diffusion zone partially mitigates these concerns by providing some ductility and absorbing residual stresses.

Mechanical Properties and NDT Results

The study reports excellent mechanical properties for the weld joint under the tested parameters:

The use of a transition wire (过渡焊丝) during welding is highlighted as a critical process control measure. This wire, typically an austenitic stainless steel composition (such as ER309L or ER310L), serves to:

• Maintain the alloy element content (Ni, Cr) at the weld root
• Prevent excessive dilution of the stainless steel layer by the carbon steel base metal
• Ensure the weld root retains adequate corrosion resistance

Elemental Diffusion Analysis

The study's chemical analysis of the weld root reveals that Ni and Cr alloy elements show no significant change compared to the welding material composition. This finding is significant because it confirms the effectiveness of the transition wire strategy in controlling elemental dilution. Without the transition wire, the weld root would likely show substantial depletion of Ni and Cr due to dilution by the carbon steel base metal, potentially compromising the corrosion resistance of the joint.

The diffusion behavior of alloy elements across the weld interface follows Fick's laws of diffusion, with diffusion rates depending on:

• Temperature gradient across the weld interface
• Welding thermal cycle (heating rate, peak temperature, cooling rate)
• Welding sequence and number of passes
• Composition gradient between the two base metals

Engineering Practice Considerations

For engineers involved in welding bimetallic composite pipes, this study provides several practical guidelines:

1. Welding procedure qualification (WPQ): The welding parameters must be qualified according to applicable codes (such as ASME Section IX, EN 15614, or ISO 13919), with particular attention to the transition layer thickness and composition.
2. Heat input control: Lower heat input is generally preferred to minimize elemental diffusion and reduce the width of the diffusion zone. However, heat input must be sufficient to achieve complete fusion and avoid cold lap defects.
3. Welding sequence: The welding sequence should be optimized to minimize residual stress. For butt welds, a balanced sequence alternating between the carbon steel and stainless steel sides is recommended.
4. Post-weld heat treatment (PWHT): Stress relief annealing may be beneficial to reduce residual stresses, but the PWHT temperature and duration must be carefully controlled to avoid sensitization of the stainless steel layer.
5. Corrosion testing: In addition to mechanical testing, the weld joint should be subjected to corrosion testing (such as intergranular corrosion testing per ASTM A262 or pitting corrosion testing per ASTM G48) to verify the corrosion resistance of the transition layer.

Key Reflections and Study Insights

This study contributes valuable knowledge to the field of dissimilar material welding, particularly for bimetallic composite pipe applications:

• The transition wire strategy is effective: The use of a transition wire to maintain alloy element content at the weld root is a practical and effective approach to controlling dilution in bimetallic composite pipe welding.
• Microstructural complexity requires careful design: The four-zone microstructure of the weld joint means that the weakest zone (likely the diffusion zone with martensite) governs the overall joint performance. Welding procedure design should focus on optimizing this zone.
• NDT is essential but not sufficient: While NDT confirms the absence of volumetric and surface defects, it does not assess the metallurgical quality of the diffusion zone. Metallographic examination and hardness profiling should be considered as supplementary quality assurance measures.
• Applicability to other composite pipe systems: The findings from this study on 20G/316L composite pipes can be extended, with appropriate modifications, to other bimetallic combinations such as 16Mn/316L, 20/321, or carbon steel/6Mo composite pipes.

This research provides a solid technical foundation for the welding of bimetallic composite pipes in chemical and petrochemical applications, where joint integrity and corrosion resistance are critical safety requirements.