Welding Process for DNV Grade AH36 Shipbuilding Steel Plates

I. Common Welding Methods and Matching of Welding Consumables

All welding consumables must be certified by DNV and comply with the principle of equivalent strength matching to ensure that the mechanical properties of the weld match those of the base metal.

1. Shielded Metal Arc Welding (SMAW)

· Applications: Tacking, small components, field repairs, and welds in confined spaces.

· Welding Material Selection: Low-hydrogen sodium-type electrode E5015 (GB standard grade J507 marine grade), DNV-approved grade 3YH10, with excellent crack resistance and low-temperature toughness.

· Operating Requirements: DC reverse polarity; dry the electrodes at 350°C for 1 hour prior to welding; maintain at 100–150°C and use as needed.

2. Submerged Arc Welding (SAW)

· Applications: Flat plate joining, straight long welds, and flat welding positions on medium-to-thick plates; offers high deposition efficiency and good bead formation quality.

· Welding consumables: H10Mn2A welding wire (φ4.0/φ5.0 mm) paired with SJ101 sintered flux.

· Process Features: Enables double-sided welding without root penetration, significantly improving welding efficiency.

3. Gas Metal Arc Welding (GMAW/FCAW)

· Applications: Ship hull section welding and all-position automated welding; a mainstream, highly efficient process in shipyards.

· Solid Wire Solution: ER50-6 welding wire with a shielding gas mixture of 80% Ar + 20% CO₂.

· Flux-cored wire solution: E501T-1 flux-cored wire, with pure CO₂ gas shielding, offering excellent all-position weldability and high deposition efficiency.

4. Electric Gas Welding (EGW)

· Applications: Vertical butt welds on thick ship sections, automated continuous welding, with efficiency 5–10 times that of conventional vertical welding.

II. Pre-welding Preparation and Preheating Control

1. Groove Preparation and Cleaning

· Groove Types: Use I-grooves for thin plates (≤6 mm); prioritize X-grooves for medium- and thick plates to reduce filler metal volume and welding distortion; thick plates may use U-grooves or narrow-gap grooves.

· Preparation Methods: Flame cutting or machining; cut surfaces must be free of scale, slag, and burrs.

· Pre-weld cleaning: Thoroughly remove rust, oil, moisture, and scale from the groove and a 20 mm area on either side to prevent porosity and cold cracking defects.

2. Preheating Temperature Specifications

The primary purpose of preheating is to control the risk of cold cracking and reduce welding residual stresses. Adjustments must be made based on plate thickness, ambient temperature, and structural restraint:

· Plate thickness ≤ 30 mm: No preheating is required at ambient temperatures ≥ 5°C; preheat to 75°C when ambient temperature is 0–5°C; preheat to 75–100°C when ambient temperature is < 0°C.

· Plate thickness > 30 mm: Both butt and fillet joints must be preheated to 120–150°C; for highly constrained thick plate structures, the temperature may be appropriately increased to 150°C.

· Interpass temperature: For multi-pass welding, the interpass temperature should match the preheating temperature and not exceed 250°C; for TMCP-treated steel plates, it is recommended to maintain the temperature at ≤200°C to prevent grain coarsening in the heat-affected zone, which can lead to a decrease in toughness.

· Preheating zone: The preheating zone on each side of the weld should be no less than 3 times the plate thickness and no less than 100 mm; temperature measurements should preferably be taken on the back side of the weld.

III. Typical Welding Process Parameters

1. Manual Arc Welding (E5015 Electrode)

2. Submerged Arc Welding (H10Mn2A + SJ101, Flat Position)

Note: For DNV Grade AH36 steel plates produced using the TMCP process, the heat input per pass typically does not exceed 50 kJ/cm to prevent a decrease in toughness in the heat-affected zone. The specific upper limit is subject to the steel mill’s material certificate.

3. CO₂ Flux-Cored Wire Gas Shielded Arc Welding (E501T-1, φ1.2 mm)

IV. Key Points for Welding Process Control

· Tacking requirements: For high-strength steel, the length of tack welds must be ≥50 mm, with spacing of 300–500 mm; the welding consumables must be the same as those used for the main weld. Any cracks found in the tack welds must be thoroughly removed before re-welding.

· Arc-starting and extinguishing plates: Install arc-starting and extinguishing plates made of the same material and with the same groove configuration at both ends of the butt weld. The plates must be ≥80 mm in length; after welding, they must be cut off and ground smooth. It is strictly prohibited to strike the arc on non-welded areas of the base metal.

· Multi-pass Welding: For thick plates, use a multi-pass welding process. Thoroughly remove slag and spatter after each pass, and stagger interpass joints by ≥30 mm. Do not allow a single weld pass to be too thick, resulting in excessive heat input.

· Welding Sequence: Follow the principle of “symmetrical welding and stress relief.” For ship hull sections, adopt a welding sequence proceeding from the center outward, combined with reverse warpage and rigid clamping to control welding distortion.

· Environmental Control: When wind speed is ≥2 m/s, a windbreak must be erected for gas shielded welding; outdoor welding is prohibited during rain or when relative humidity exceeds 90%. If necessary, preheating and dehumidification measures must be taken.

V. Post-Weld Heat Treatment (PWHT)

· Standard Conditions: Post-weld heat treatment is generally not required for DNV Grade AH36 steel welds. Its low-carbon microalloyed design and TMCP process ensure that joint strength and toughness meet DNV code requirements.

· Special Conditions: Stress-relief annealing shall be performed only for highly constrained structures with plate thickness ≥50 mm, low-temperature service scenarios, or when explicitly required by design drawings or DNV surveyors. Process parameters are as follows: hold at 580–620°C; hold time is calculated as 1 hour per 25 mm of plate thickness; furnace cooling to below 300°C, followed by air cooling after removal from the furnace.

VI. DNV Compliance and Quality Inspection

· Process Qualification: All welding processes must undergo welding procedure qualification (WPQR) in accordance with DNV regulations. Corresponding welding procedure specifications (WPS) must be prepared and approved by DNV before construction may proceed.

· Personnel Qualifications: Welders must hold qualification certificates recognized by DNV for the corresponding welding methods and positions.

· Visual Inspection: Welds shall be uniformly formed, free of defects such as cracks, porosity, slag inclusions, undercut, or weld spatter. Excess height and misalignment shall comply with hull construction tolerance standards.

· Non-Destructive Testing: 100% ultrasonic testing (UT) shall be performed on critical butt welds, with additional magnetic particle testing (MT) on key areas. Acceptance criteria shall comply with DNV code requirements.