Welding is the core process in ship construction. The total length of welds on an 85,000 DWT bulk carrier exceeds 100,000 meters. Weld quality directly determines the vessel's structural safety, watertight integrity, and service life. A single defective weld could suddenly fail decades after the vessel enters service, leading to incalculable consequences.
Jiangsu Haizhongzhou Shipping Industry Co., Ltd. consumes over 500 tons of welding consumables annually, with welding man-hours accounting for more than 30% of total production man-hours. Faced with such a substantial welding workload, the company has established a tiered, high-efficiency welding process system tailored to different weld types, plate thicknesses, and working conditions, achieving simultaneous improvements in efficiency and quality.
I. Differentiated Welding Process System
Flat and Horizontal Sections – Submerged Arc Automatic Welding (SAW)
For large flat and horizontal sections such as decks, bottom plates, and longitudinal bulkheads, the company employs gantry-type submerged arc automatic welding machines. This process buries the welding wire beneath a granular flux layer for welding. The arc is invisible, but thermal efficiency is high and penetration depth is significant. Welding current ranges from 600 to 1,200A, voltage from 30 to 38V, and welding speed can reach 30 to 60 meters per hour, which is 5 to 8 times faster than manual shielded metal arc welding.
The advantage of SAW extends beyond speed. Since welding parameters are automatically controlled by equipment, human interference is minimal, resulting in aesthetically pleasing weld appearance and stable mechanical properties. In the 85,000 DWT bulk carrier project, the first-pass acceptance rate for bottom plate butt welds using SAW reached 98.7%. A single pass can accommodate plate thicknesses ranging from 6 to 25mm. For thicker plates, double-sided welding is employed without the need for back gouging.
Vertical Butt Joints – Electrogas Welding (EGW)
Vertical butt joints present a challenge in hull construction. Traditional manual welding requires multiple layers and passes – an 8-meter-long vertical weld takes over three hours and demands exceptional skill from the welder. The company has introduced Electrogas Welding (EGW) technology, fundamentally transforming this challenging operation.
EGW uses water-cooled copper shoes as forming backing. The welding wire feeds vertically upward, and the arc melts the base metal and wire within the groove to form the weld. This process enables single-pass, double-sided formation for plate thicknesses of 8 to 32mm, achieving welding speeds 3 to 5 times faster than manual welding. An 8-meter-long vertical weld can now be completed in just 40 minutes, with the weld undergoing ultrasonic testing to ensure reliable and stable quality.
In the vertical butt joints of longitudinal bulkheads, the application of EGW has reduced man-hours for this process by over 60%.
Fillet Welds – CO₂ Gas Shielded Arc Welding
Fillet welds are the most numerous type of weld in hull structures, accounting for more than 50% of the total weld volume. The company has fully adopted CO₂ gas shielded arc welding to replace traditional manual shielded metal arc welding.
The advantages of CO₂ welding include: high welding speed, high deposition efficiency, controllable heat input, and minimal welding distortion. The use of flux-cored wire further improves deposition efficiency and process adaptability, maintaining good shielding effectiveness even in outdoor, windy conditions. For critical fillet welds, the company employs a twin-wire CO₂ welding process, where two wires are arranged in tandem – the leading wire fuses the root, and the trailing wire fills and caps – increasing welding efficiency by an additional 40%.
Critical Nodes – Manual Welding
Although automated welding covers the majority of welds, manual welding remains irreplaceable in the following scenarios: confined spaces and complex nodes; repair welding and rework; critical nodes of important structures (such as hatch coaming corners and connections between bulkheads and bottom plates); and dissimilar material welding between cast steel parts and steel plates.
For manual welding operations, the company enforces a certification and licensing system. Only welders who have passed the relevant qualification tests are permitted to perform welding on corresponding projects. Currently, the company employs 32 CCS-certified senior welders, capable of covering critical node welding for all vessel types and structures.
II. Welding Quality Control System
High efficiency must be built upon a foundation of high quality. The company has established a comprehensive quality management system covering the entire chain from consumable management, procedure qualification, and process control to non-destructive testing.
Welding Procedure Specification (WPS)
Every welding process must undergo rigorous procedure qualification before being put into service. In accordance with the requirements of CCS's "Rules for Materials and Welding," the company fabricates qualification test plates and conducts visual inspection, non-destructive testing, mechanical property tests (tensile, bend, impact, hardness), and macroscopic metallurgical examination. Currently, the company has established a process database containing over 80 WPSs, covering all combinations of base metal grades, plate thickness ranges, welding positions, and joint types.
Welder Qualification Management
The company maintains an electronic welder database, recording each welder's certified processes, welding performance history, quality metrics, and other information. Welder qualifications are renewed every two years through theoretical examinations and practical assessments. During the 85,000 DWT bulk carrier project, a peak workforce of 120 welders worked simultaneously, including 32 senior welders. All welders performing critical structural welds must pass project-specific training and assessment.
Non-Destructive Testing (NDT)
NDT of welds is implemented hierarchically according to importance levels: critical structures (such as deck butt joints, hatch coamings, and keels) undergo 100% ultrasonic testing; general structures have a sampling rate of no less than 20%; and watertight welds undergo 100% kerosene testing or vacuum testing. For any defects discovered, strict repair procedures are followed for removal and re-welding, with retesting until acceptance criteria are met.
The 85,000 DWT bulk carrier project accumulated over 5,000 meters of ultrasonic testing of welds, achieving a first-pass acceptance rate of 96.5%, exceeding the industry average. For the small number of welds requiring repair, the company has established a defect analysis and prevention mechanism to reduce the recurrence of similar issues at their source.
III. Technical Benefits and Future Outlook
The systematic application of high-efficiency welding processes has yielded significant comprehensive benefits:
Efficiency Improvement: Section fabrication cycle shortened by approximately 30%
Cost Reduction: Welding consumables reduced by 15%, rework man-hours decreased by 50%
Quality Improvement: First-pass NDT acceptance rate increased from 89% to 96.5%
Environmental Improvement: Fume emissions from automated welding significantly lower than manual welding
The company is actively advancing the application of welding robots. A pilot welding robot has been deployed on the sub-assembly production line for fillet welding of floor ribs and stiffeners. The company plans to achieve comprehensive robotic welding coverage for repetitive processes such as flat section fabrication and rib welding by 2025, targeting an automation rate of over 70% for major section welds.
The continuous upgrading of welding technology serves as a critical pillar supporting Jiangsu Haizhongzhou Shipping Industry Co., Ltd.'s progression toward intelligent manufacturing. The company will continue to increase investment in welding technology research and development, cultivate high-caliber welding talent, and fortify the quality defense line of every single weld – because every weld carries a commitment to safety.