Microstructure and Properties of Laser-CMT Hybrid and Submerged Arc Welded Joints in SUS304-Q235B Bimetallic Composite Spiral Pipes

Overview of the Study

This research addresses a critical manufacturing challenge in the production of bimetallic composite spiral pipes used in aggressive chemical processing environments, where the combination of corrosion-resistant stainless steel (SUS304) and structural carbon steel (Q235B) is required. The study examines the microstructure, mechanical properties, and bonding quality of joints produced using a hybrid laser-CMT (cold metal transfer) welding process for the first layer, followed by submerged arc welding (SAW) for subsequent fill layers. This multi-process approach is designed to combine the deep penetration and high precision of laser welding with the high deposition rate and low dilution characteristics of CMT and SAW processes.

Process Architecture and Technical Rationale

The hybrid laser-CMT process for the first layer serves a dual purpose: it establishes a metallurgical bond between the SUS304 cladding layer and the Q235B base pipe, and it creates a favorable composition profile for the subsequent SAW fill layers. The laser provides concentrated energy density (typically 10^5 to 10^6 W/cm^2) that enables deep, narrow penetration with minimal heat-affected zone, while the CMT arc contributes additional heat and filler metal deposition with controlled dilution rates.

The use of SUS308L or ER308L filler metal for the laser-CMT first layer is critical, as it provides sufficient chromium and nickel content to maintain the austenitic structure while accommodating the dilution from the Q235B base. The subsequent SAW layers use the same filler metal with appropriate flux coverage to ensure consistent composition and minimize carbon pickup from the flux.

Microstructural Analysis

The weld joint microstructure reveals several distinct zones, each with unique metallurgical characteristics. The fusion zone on the SUS304 side shows a fine austenite grain structure with occasional ferrite inclusions, resulting from the dilution by carbon steel elements. The weld metal itself, particularly in the first laser-CMT layer, exhibits a mixed austenite-ferrite microstructure with ferrite content typically in the range of 5 to 15 percent, which is acceptable for preventing hot cracking while maintaining adequate corrosion resistance.

The heat-affected zone (HAZ) on the Q235B side is the most critical region from a mechanical standpoint. The carbon steel experiences significant grain growth at the fusion boundary, with austenite grains growing from the original 20 to 50 micrometers to 100 to 200 micrometers in the coarse grain HAZ (CGHAZ). Upon cooling, these coarse austenite grains transform into a mixture of ferrite and pearlite, with the possibility of martensite formation in regions of rapid cooling near the fusion boundary. The presence of hard martensitic phases in the CGHAZ can reduce the local toughness and increase susceptibility to cracking during service.

The transition zone between the weld metal and the SUS304 base shows a gradient in composition, with chromium and nickel concentrations gradually increasing from the weld centerline toward the stainless steel side. This compositional gradient is essential for maintaining the passive film integrity and corrosion resistance of the cladding layer, particularly at the critical fusion boundary where intermetallic phases could form if the dilution ratio is too high.

Mechanical Performance and Bond Quality

Tensile testing of the composite weld joints reveals that the fracture typically occurs in the Q235B base material rather than in the weld or HAZ, which is the desired outcome for a bimetallic joint. The weld metal tensile strength is generally in the range of 520 to 580 MPa, with elongation of 25 to 35 percent, indicating adequate ductility despite the mixed microstructure. The hardness distribution shows a gradual transition from approximately 180 HV in the SUS304 base to 120 to 150 HV in the weld metal, with the lowest hardness occurring in the Q235B HAZ.

Bond strength testing, conducted according to ASTM E23 or equivalent standards, confirms full metallurgical bonding across the interface. No interfacial cracking or delamination was observed in properly executed welds, indicating that the hybrid laser-CMT process effectively establishes a robust metallurgical bond between the dissimilar materials.

Defect Analysis and Process Optimization

The following table summarizes the primary defects encountered and their root causes:

A particularly important finding is the relationship between the laser-to-arc power ratio and the dilution rate of the first layer. When the laser power exceeds 60 percent of the total input power, the dilution from the Q235B base increases significantly, potentially compromising the corrosion resistance of the weld metal. Conversely, when the CMT arc power dominates, the penetration depth decreases, risking incomplete bonding at the interface. The optimal power ratio was found to be approximately 50:50 to 60:40 (laser:CMT), which provides adequate penetration while maintaining acceptable dilution levels.

Engineering Practice and Standards Compliance

For pressure vessel and piping applications governed by ASME BPV Code Section VIII or NB/T 47002, the following quality assurance measures are essential:

1. The welding procedure qualification (WPQ) must include the hybrid laser-CMT first layer and SAW fill/cap layers as a single procedure, with adequate qualification coupons to demonstrate joint performance.
2. Non-destructive testing must include radiographic testing (RT) or phased array ultrasonic testing (PAUT) for the first layer to detect lack of fusion and porosity at the critical interface, supplemented by magnetic particle testing (MT) for surface and near-surface defects.
3. Hardness surveys must be conducted across the full width of the joint, with particular attention to the CGHAZ on the carbon steel side, to ensure no excessive hardening occurs.
4. Intergranular corrosion testing of the weld metal and HAZ on the SUS304 side is recommended, particularly for applications involving chloride-containing environments.

Study Insights and Reflections

This research demonstrates that the hybrid laser-CMT plus SAW approach is a technically viable and economically attractive solution for manufacturing bimetallic composite spiral pipes. The laser-CMT first layer provides a significant advantage over conventional SAW-only approaches by creating a narrower, more precise fusion zone with better control over dilution and composition. This is particularly important for maintaining the corrosion resistance of the SUS304 cladding layer, which is the primary reason for using a bimetallic construction in the first place.

The integration of two fundamentally different welding processes (laser-CMT and SAW) in a single weld joint introduces additional complexity in terms of process control and quality assurance. However, the benefits in terms of productivity and joint quality justify the additional complexity, provided that the process parameters are carefully optimized and the quality assurance protocols are appropriately rigorous.

From a broader perspective, this work represents a practical example of process hybridization in advanced manufacturing. The combination of high-energy-density laser processing with arc welding techniques is becoming increasingly common in industrial applications, and the lessons learned from this study can be applied to other bimetallic joining scenarios, including clad plate pressure vessels and heat exchanger tubes.