High-Efficiency Welding Process For 34–80mm Extra-Thick Steel Plate (FCAW+EGW Combined Technology)

1. Background: Bottlenecks of Traditional Thick Plate Welding In Shipbuilding

Large container ships adopt high-strength extra-thick steel for mid-hull structural parts due to high stress load characteristics. Traditional single-wire electro-gas welding (EGW) only supports a maximum plate thickness of 32–33mm, which cannot meet the processing demand of super-thick hull steel plates. Although double-wire EGW can reach 70mm thickness range, it brings extremely high heat input, and only dedicated high heat input resistant steel can match this process, which greatly increases material procurement cost.

If conventional flux-cored arc welding (FCAW) multi-layer multi-pass welding is adopted for thick vertical butt joints without special steel plates, the production cycle will be extremely long and seriously delay construction progress. To solve the conflict between thickness limit, material adaptability and welding efficiency, a novel FCAW+EGW combined welding technology is developed for 34–80mm heavy steel plates.

The process flow: Complete single-side forming by FCAW on the structural surface first, then implement EGW welding on the non-structural surface, combining the back forming advantage of FCAW and ultra-high deposition efficiency of EGW.

2 Core Technical Specifications of FCAW+EGW Combined Welding

2.1 Applicable Plate Thickness Range

This hybrid process targets 34–80mm thick steel plates. The lower limit is the maximum thickness of standard single-wire EGW, and the upper limit matches the thickest hull side shell plates used in mainstream container vessels, covering most heavy structural welding scenarios in shipbuilding.

2.2 Thickness Matching Principle

When designing the layered welding layout, two core factors must be balanced: maximize EGW high-efficiency advantages, and control the difference of total deposited metal volume on both sides within a reasonable range to avoid severe asymmetric welding deformation.

2.3 Optimized Joint & Groove Design

1. Groove Angle: The FCAW side adopts a moderately narrowed groove (X±5°) to reduce filling volume; EGW side sets variable angles based on plate thickness: Y±5° for 30–50mm plates, Z±5° for 51–80mm plates to cut down welding filler consumption.

2. Root Gap: Unified gap tolerance G±2mm to satisfy the operation requirements of both welding processes simultaneously.

3. Backing Strip: Conventional triangular backing strips are incompatible with this groove structure; round bar backing strips are required, with diameter selected according to actual assembly clearance.

2.4 On-Site Construction Operation Rules

1. Special Operator Training: Even technicians with mature EGW welding experience for medium plates need targeted training. The molten pool control and wire swing operation logic differ greatly when welding ultra-thick plates.

2. End Nondestructive Inspection: All weld start/stop segments must be inspected by RT or UT. Defects found shall be removed by air gouging and repaired with FCAW or SMAW.

3. Run-on & Run-off Tab: Tabs shall be at least 50mm long, with identical thickness and groove form matching base metal to avoid end defects.

4. Windproof Protection: Airflow disturbs shielding gas and introduces nitrogen into molten pool, causing pores and degraded joint mechanical properties. Wind barriers are mandatory during construction.

3 Process Qualification Test Standard

3.1 Test Conditions

All test pieces and matched welding consumables comply with shipbuilding standards; welding position is vertical 3G, and full parameter records are retained. Tests are supervised by surveyors from LR and CCS classification societies.

3.2 Nondestructive Testing Results

• PT: Weld surface edges are smooth, zero surface cracks, pores or undercut defects.

• UT: All test welds fully reach ISO 5817 Grade B acceptance standard.

• MT: No subsurface or surface linear defects detected on front and back weld surfaces.

3.3 Mechanical Property Test Data

Tensile, side bend, low-temperature impact and hardness tests are carried out in full accordance with marine specification requirements:

1. Tensile test: Four test specimens reach 580–595MPa tensile strength, all fracture on base metal instead of weld joint.

2. Side bend test (D=40mm, 180° bending): All four samples pass without cracks.

3. -20℃ impact toughness: Weld center, fusion line and 2/5mm outside fusion line all far exceed the minimum required value of 34J, showing excellent low-temperature crack resistance.

3.4 Formal Qualification Conclusion

All nondestructive and mechanical test indicators meet LR and CCS marine standards, and the FCAW+EGW combined welding process passes official procedure qualification.

4 Welding Efficiency Comparison

Take a 1-meter vertical butt weld as the comparison object:

• Traditional double-sided FCAW multi-pass welding: Total working time 250 minutes

• FCAW+EGW combined process: FCAW section 125 minutes + EGW section 18 minutes = total 143 minutes

  The innovative hybrid technology shortens construction time by nearly 43%, significantly lifting the throughput of thick plate hull workshops.

5 Conclusion

The FCAW+EGW combined welding process perfectly solves the three major pain points of thick plate vertical welding: single EGW thickness limitation, high heat input material restriction, and low efficiency of full FCAW multi-layer welding. It retains the stable back forming capacity of flux-cored wire welding while leveraging the ultra-high deposition rate of electro-gas welding, achieving dual gains of production efficiency and joint performance.

In actual ship production, strict control of groove processing accuracy, assembly clearance, matching welding consumables and standardized operation parameters is the prerequisite for stable mass application of this process. If you need customized thick plate hybrid welding process schemes for shipbuilding or heavy steel structure projects, contact the technical team at heavth.com for full parameter matching and process guidance.