Microstructure Analysis of Carbon Steel-Stainless Steel Cladding Welds Using Single-Source Dual-Wire Bypass Coupling High-Efficiency GMAW

Literature Overview and Research Background

The single-source dual-wire bypass coupling arc welding method represents an innovative approach to high-efficiency cladding that combines the advantages of dual-wire GMAW (high deposition rate) with the metallurgical control of a bypass arc configuration. This literature investigates the microstructural characteristics and mechanical properties of carbon steel-stainless steel cladding joints fabricated using this novel process. The method employs two welding wires fed simultaneously from a single power source, with one wire connected through a bypass circuit that introduces an additional arc path, effectively increasing the energy input and deposition rate without requiring a multi-source power supply.

The process is particularly relevant for the fabrication of carbon steel-stainless steel clad plates and pressure vessel components where high productivity is essential. Conventional single-wire GMAW overlay typically achieves deposition rates of 3–5 kg/h, while dual-wire configurations can reach 8–12 kg/h. The bypass coupling method aims to achieve deposition rates of 10–15 kg/h while maintaining metallurgical compatibility at the carbon steel-stainless steel interface.

Process Description and Parameter Configuration

The single-source dual-wire bypass coupling arc process operates by connecting two welding wires to a single GMAW power source. Wire 1 (typically a transition wire such as ER309L) is connected directly to the power source's positive terminal, while Wire 2 (the final overlay wire, e.g., ER316L) is connected through a bypass circuit that includes an inductor and/or diode, creating a secondary arc path. The two wires are fed simultaneously into the molten pool, with the bypass arc providing additional energy to melt the second wire.

The bypass inductance is a critical parameter that controls the current division between the two wires. A lower inductance (0.5 mH) favors equal current sharing, while a higher inductance (2 mH) directs more current to Wire 1 and less to Wire 2. The optimal bypass inductance for carbon steel-stainless steel cladding is typically 1.0–1.5 mH, which provides sufficient melting of both wires while maintaining a stable arc.

Microstructural Characteristics

Interface Region

The carbon steel-stainless steel interface is the most critical region from a metallurgical standpoint. In the bypass coupling process, the first layer deposited on carbon steel substrate uses ER309L wire, which produces a fully austenitic weld metal even with 20–30% base metal dilution. Metallographic examination reveals a columnar austenitic structure at the interface with grain size of 80–120 μm. The dilution zone shows a gradient from fully austenitic (0–0.3 mm from interface) to austenite-ferrite duplex (0.3–0.8 mm) as the base metal dilution decreases.

Overlay Layer

The subsequent overlay layers deposited with ER316L wire exhibit a fine-grained austenitic structure with occasional ferrite stringers (0.5–2% volume fraction) at the grain boundaries. The ferrite content is controlled by the Cr/Ni equivalent ratio, which in ER316L is approximately 1.0 (Cr_eq ≈ Ni_eq), resulting in a balanced austenite-ferrite structure with excellent crack resistance. The grain size in the overlay layer is 40–70 μm, finer than the interface region due to the lower dilution and more controlled thermal cycle.

Dilution Zone Chemistry

The bypass coupling process produces a distinct dilution profile compared to conventional single-wire GMAW. The dual-wire configuration creates a wider molten pool, which increases the total dilution but distributes it more uniformly. EDS analysis shows that the dilution zone (0.3–0.8 mm from interface) contains 14–16% Cr and 8–9% Ni, corresponding to a carbon equivalent of 0.45–0.52%. This is below the critical threshold for martensite formation (C_eq > 0.55%), confirming that the bypass coupling process effectively prevents harmful martensite in the dilution zone.

Mechanical Property Evaluation

Tensile Properties

The overlay layer exhibits ultimate tensile strength (UTS) of 520–580 MPa and elongation of 35–42%, comparable to ER316L weld metal deposited by conventional GMAW. The interface region shows slightly lower UTS (480–520 MPa) due to the coarser grain structure and higher carbon content from base metal dilution. The carbon steel base metal has UTS of 450–490 MPa and elongation of 22–26%.

Bond Strength

The bond strength of the overlay layer to the carbon steel substrate is evaluated by the bend test method (GB/T 11170) and the shear test method (ASTM A263). The bypass coupling process achieves a bend angle of ≥ 180° without cracking or delamination, meeting the acceptance criteria for clad plate. Shear test results show a bond strength of 280–320 MPa, exceeding the minimum requirement of 200 MPa specified in NB/T 47014.

Hardness Distribution

The hardness profile across the cladding joint shows a smooth transition from the carbon steel base (HV 180–220) through the interface (HV 160–180) to the overlay layer (HV 150–180). The absence of a hard martensitic band at the interface confirms the effectiveness of the ER309L transition layer. The maximum hardness variation across the joint is approximately 60 HV, which is within acceptable limits for stress distribution.

Comparison with Conventional Processes

The bypass coupling process offers a favorable balance between deposition rate and metallurgical quality. While the heat input is higher than single-wire GMAW, the dual-wire configuration provides better arc stability and more uniform heat distribution, resulting in superior interface quality compared to SAW and ESW overlay.

Defect Analysis and Countermeasures

Engineering Practice Implications

The bypass coupling GMAW method offers significant advantages for the fabrication of carbon steel-stainless steel clad plates and pressure vessel components. The high deposition rate (10–14 kg/h) reduces fabrication time by 50–60% compared to conventional single-wire GMAW, while maintaining excellent metallurgical quality. The process is particularly suitable for large-diameter pressure vessels where overlay thickness requirements (5–10 mm) would be prohibitively time-consuming with single-wire methods.

However, several practical considerations must be addressed. First, the bypass circuit requires careful tuning of the inductance value to ensure stable current sharing between the two wires. Second, the higher heat input necessitates stricter preheating and interpass temperature control to prevent hydrogen cracking in the carbon steel base metal. Third, the dual-wire torch geometry requires specialized equipment and operator training. Fourth, the process is best suited for flat or slightly curved surfaces; highly curved geometries (e.g., small-diameter tubes) may require additional process development.

Key Questions and Reflections

A fundamental question is whether the bypass coupling method can be extended to other cladding combinations, such as carbon steel-nickel alloy, carbon steel-titanium, or low-alloy steel-copper. The metallurgical compatibility of the transition layer (ER309L) is critical, and different base metal-alloy combinations may require different transition consumables. For example, carbon steel-nickel alloy cladding may require an IN718 or Monel 400 transition layer, while carbon steel-copper cladding may require a Cu-Ni (Monel) or Ag-Cu transition layer.

Another reflection concerns the scalability of the bypass coupling method. While the process has been demonstrated on flat plates and large-diameter vessels, its application to complex geometries (e.g., heat exchanger tube sheets, pipe fittings, and small-diameter tubes) requires further investigation. The dual-wire torch geometry may limit access to confined spaces, and the higher heat input may cause excessive distortion in thin-walled components.

Study Insights and Outlook

This literature demonstrates that the single-source dual-wire bypass coupling GMAW method is a promising high-efficiency process for carbon steel-stainless steel cladding. The process achieves deposition rates of 10–14 kg/h while maintaining excellent metallurgical quality, including a fully austenitic interface, controlled dilution, and high bond strength. The process parameters are well-defined and reproducible, and the defect prevention strategies are straightforward. For future development, the integration of real-time process monitoring (arc voltage/current sensing, thermal imaging) with automated bypass inductance control could further improve process stability and quality consistency. The extension of this method to other cladding combinations and complex geometries represents a significant opportunity for advancing high-efficiency cladding technology in the pressure vessel and heat exchanger industries.