Welding Procedure Specifications for BV Grade D Shipbuilding Steel Plates

I. Technical Specifications for Base Material

BV Grade D is a general-strength shipbuilding structural steel that must pass the Charpy V-notch impact test at -20°C, with a yield strength of ≥235 MPa and a tensile strength of 400 - 520 MPa.

Delivery conditions: hot-rolled (AR), controlled-rolled (CR), TMCP, or normalized (N). For plate thicknesses >40 mm, normalizing is recommended to refine the grain structure and ensure low-temperature toughness.

Chemical Composition (Upper Limits): C ≤ 0.21%, Mn 0.60%, Si 0.10% - 0.35%, S ≤ 0.035%, P ≤ 0.035%. Strictly control sulfur and phosphorus content to reduce cold brittleness and susceptibility to weld cracking.

Applicable plate thickness range: 4 - 260 mm. For critical joints with plate thicknesses greater than 40 mm, it is recommended to use Z15, Z25, or Z35 grade laminar tear-resistant steel plates.

Process Qualification Standards: To be carried out in accordance with EN ISO 15614-1; the Welding Procedure Qualification Record (WPQR) must be submitted to a BV surveyor for approval and validation.

II. Welding Methods and Selection of Welding Consumables

Note:

1) Low-hydrogen welding consumables must be used for thick plates and highly constrained joints;

2) Low-hydrogen electrodes must be baked at 350°C for 1 - 2 hours before use, then stored in a heat-retaining bucket and used as needed at 110°C.

III. Pre-welding Preparation

1. Groove Types

Flat Butt Joints:

t ≤ 6 mm: Type I groove, root gap 2 - 3 mm; single-sided welding with root cleaning on the reverse side, or use ceramic backing to achieve single-sided welding with double-sided build-up;

6 < t ≤ 20 mm: Type V groove, groove angle 60°, blunted edge 2 mm;

t > 20 mm: X-type double-sided groove; weld alternately on the front and back sides to control distortion.

T-joint and rib joints: Single-sided groove; root cleaning is required for critical full-penetration joints.

Butt joints of plates with unequal thickness: A gentle transition bevel with a slope of ≤ 1:3 must be machined on the thicker plate side.

2. Base Metal Preparation

The groove and the areas on both sides of the weld, each extending ≥ 30 mm, must be thoroughly ground to remove scale, rust, oil, cutting spatter, and paint. Damp steel plates must be dried in advance.

3. Assembly and Tacking

Misalignment Control: Misalignment of the main hull butt welds must be ≤ 10% of the plate thickness and ≤ 3 mm.

Tacking: Approved welding consumables of the same grade must be used; tack welds must be ≥ 30 mm in length, spaced 80 - 120 mm apart. If cracks or porosity occur, the affected area must be ground clean and rewelded.

IV. Preheating Before Welding

When the ambient temperature is ≥ 5°C:

t ≤ 30 mm: No preheating required;

30 < t ≤ 50 mm: Preheat to ≥ 60°C;

t > 50 mm: Preheat to 80 - 120°C.

When the ambient temperature is < 5°C or when welding outdoors in humid conditions: Preheat all thicknesses to ≥ 80°C.

V. Interpass Temperature Control

The interpass temperature should be controlled between 100 - 250°C. For multi-pass welding, wait until the weld has cooled to within this range before continuing; the interpass temperature must not exceed 250°C to prevent grain coarsening in the heat-affected zone, which would reduce impact toughness at  - 20°C.

VI. Post-Weld Heat Treatment

General structures with t ≤ 40 mm: No stress-relief heat treatment is required.

Highly constrained structures with t > 50 mm (main hull, hatch corners, keel joints, etc.): Stress-relief annealing must be performed at a temperature of 550 - 600°C, with the holding time calculated at 25 mm/h, followed by slow cooling.

VII. Welding Process Parameters

1. Manual Metal Arc Welding (SMAW)

2. Submerged Arc Welding (SAW)

Welding wire φ4.0 mm: Current 550 - 680 A, Voltage 28 - 32 V, Welding Speed 35 - 50 cm/min.

Heat Input Control: Strictly controlled between 12 and 35 kJ/cm; high heat input can cause grain coarsening in the heat-affected zone and reduce impact toughness at -20°C.

3. CO₂ Gas Metal Arc Welding (GMAW)

Welding wire φ1.2 mm: Current 260 - 320 A, voltage 28 - 31 V, CO₂ flow rate 18 - 25 L/min.

4. Welding Sequence Requirements

Weld butt joints first, then fillet welds; weld the free-end welds first, then the restrained-end welds;

For long welds exceeding 2 m in length, the segmented back-welding or skip-welding method should be used to minimize residual welding distortion;

Cross welds on the hull should be staggered by ≥ 200 mm to avoid triaxial stress concentration;

For double-sided welding, after one side is completed, the root must be cleaned using carbon arc gouging and ground until sound metal is exposed before welding the opposite side.

VIII. Welding Environment Control

Wind Speed Limits: For gas shielded welding, a windbreak must be installed when the ambient wind speed exceeds 2 m/s; for manual welding and submerged arc welding, wind protection must be provided when the wind speed exceeds 10 m/s.

Open-air welding is prohibited during rainy weather, high humidity, or when condensation forms on the steel plate surface.

Minimum welding ambient temperature: -5°C.

IX. Post-Weld Inspection and Non-Destructive Testing (NDT) Standards

Visual Inspection: Weld bead height shall be 0 - 3 mm, with no undercut, surface porosity, cracks, or spatter. The quality grade for main structure welds shall be Class B per ISO 5817, and Class C for secondary structures.

NDT Coverage:

High-stress areas such as the main deck, longitudinal main butt joints of the bottom plates, and hatch corners: 100% UT or RT;

Transverse welds on the bottom plates: ≥ 25% random inspection;

Secondary framework structures: ≥ 10% random inspection;

Non-load-bearing attachments on the superstructure: Visual inspection only.

X. Key Points for Preventing Common Defects