Welding Process for DNV Grade A Ordinary-Strength Shipbuilding Steel Plates

DNV Grade A is a standard-strength marine structural steel certified by the Norwegian Ship Classification Society, with a yield strength of ≥235 MPa and a tensile strength of 400 - 520 MPa. It is primarily used for the construction of non-critical structural components on ships (such as superstructures, deck accessories, and compartment bulkheads) under ambient temperature conditions. This steel grade has a low carbon content (≤0.21%) and few alloying elements. Its carbon equivalent (CEV) is typically ≤0.40%, and its cold cracking sensitivity index (Pcm) is ≤0.20%. It offers excellent weldability and can be welded using all conventional fusion welding methods. Welding procedures require PQR qualification and witness by a DNV surveyor, and welding must be performed by certified welders.

I. Pre-welding Preparation

1. Base Material and Welding Consumables

The base material has a low carbon equivalent (CEV ≤ 0.40%) and good weldability; preheating is generally not required (except for thick plates or low-temperature environments). The steel surface must be free of defects such as cracks, delamination, and rust.

Storage of welding consumables: Low-hydrogen electrodes and flux should be stored in a dry, well-ventilated warehouse at a temperature of ≥15°C and relative humidity of ≤60%.

2. Groove Preparation and Cleaning

Groove Types

Selected based on plate thickness and welding method. Common types include V-groove (plate thickness 6 - 20 mm), X-groove (plate thickness > 20 mm), and single-sided V-groove (for fillet joints). The groove angle is generally 60° ± 5°, with a root face of 2 - 3 mm and a gap of 2 - 4 mm.

Cleaning Requirements

The area within 20 - 30 mm on both sides of the weld must be ground to reveal a metallic luster, thoroughly removing contaminants such as oil, rust, scale, and moisture.

Mechanical methods (such as edge milling or planing) are preferred for groove preparation; plasma cutting may also be used, but the hardened layer must be ground off after cutting.

3. Drying and Holding of Welding Consumables

Low-hydrogen electrodes (e.g., E7018): Drying temperature 350 - 400°C, holding time 1 - 2 hours; remove only as needed.

Submerged arc welding flux: Drying temperature 300 - 350°C, hold for 1 - 2 hours.

Dried welding consumables should be stored in an insulated container maintained at 100 - 150°C for no more than 4 hours; if stored longer, they must be re-dried (re-drying should not exceed two times).

4. Preheating Requirements

DNV Grade A shipbuilding steel plates have excellent weldability and generally do not require preheating; however, preheating is required under the following conditions:

Plate thickness > 50 mm: Preheating temperature 50 - 80°C.

Ambient temperature < 0°C: Preheating temperature 50 - 100°C.

High structural restraint (e.g., thick plate fillet joints, T-joints): Preheating temperature 50 - 80°C.

Preheating Zone: ≥1.5 times the plate thickness on each side of the weld, with a minimum of 100 mm.

Preheating Temperature Measurement: Measure 75 mm from the arc on the back of the weld; when using flame heating, measure after the flame has been removed.

II. Welding Methods and Welding Material Matching

DNV Grade A can be welded using all conventional fusion welding methods. Common methods and corresponding welding materials in the shipbuilding industry are as follows:

Note: The mechanical properties of the deposited metal for all welding consumables shall not be lower than those of the base metal, i.e., yield strength ≥235 MPa, tensile strength 400 - 520 MPa, and elongation ≥22%.

III. Welding Process Parameters

1. Shielded Metal Arc Welding (SMAW)

2. CO₂ Gas Metal Arc Welding (GMAW)

3. Submerged Arc Welding (SAW)

4. General Parameter Requirements

Interpass Temperature: ≤250°C (static load structures), ≤230°C (dynamic load structures), and not lower than the preheating temperature.

Heat Input: Generally controlled at ≤50 kJ/cm; for joints requiring toughness, heat input should not exceed 35 kJ/cm.

Welding Polarity: Direct Current Electrode Positive (DCEP), which provides a stable arc and good penetration.

IV. Key Welding Operation Points

Arc Striking and Extinguishing

Arc striking should be performed within the groove or on an arc-striking plate; striking the arc arbitrarily on the base metal surface is prohibited. When extinguishing the arc, the crater must be filled to prevent crater cracks.

Multi-pass Welding

For plate thicknesses > 12 mm, multi-pass welding should be used, with each pass thickness ≤ 4 mm and each weld bead width ≤ 15 mm; Slag and spatter must be thoroughly removed between passes.

Welding Sequence

Employ methods such as symmetrical welding and segmented back-stepping to minimize welding distortion and residual stress; for long welds, the segmented skip-welding method may be used.

Root Pass

Ensure full penetration to avoid defects such as lack of penetration and slag inclusions; root cleaning on the reverse side should be performed using carbon arc gouging or grinding with a grinding wheel, with a root cleaning depth of ≥2 mm.

Environmental Control

For gas shielded welding, a windbreak must be installed when wind speeds exceed 2 m/s.

Welding should be suspended when relative humidity exceeds 90%.

Outdoor welding is prohibited during rain or snow.

V. Post-Welding Treatment

DNV Grade A welded joints generally do not require post-weld heat treatment; however, stress-relief heat treatment may be considered in the following special cases:

Critical structures with plate thickness > 100 mm.

Joints with extreme structural restraint prone to delayed cracking.

Components with special dimensional stability requirements.

Stress-relief heat treatment parameters: Heating temperature 580 - 620°C; holding time calculated based on plate thickness (2 - 2.5 min/mm), followed by slow cooling to room temperature.

VI. Quality Control and Inspection

1. Welding Procedure Qualification (WPS)

All welding procedures must be qualified in accordance with ISO 15614-1 or DNV specifications to obtain valid PQRs (Procedure Qualification Records) and WPSs (Welding Procedure Specifications).

The WPS shall include all critical information, such as base metal grade, welding method, welding consumable type, preheating temperature, interpass temperature, welding parameters, and post-weld treatment.

2. Welder Qualification

Welders must hold a qualification certificate recognized by DNV, with examination standards based on ISO 9606-1 or DNV standards.

The qualification certificate is valid for 2 years, during which the welder must maintain continuous welding work and have no major quality incidents.

3. Non-Destructive Testing (NDT)

Visual Inspection

All welds must undergo 100% visual inspection. No defects such as cracks, lack of penetration, lack of fusion, porosity, or inclusions are permitted. Undercut depth must be ≤0.5 mm, with a continuous length of ≤100 mm.

Internal Defect Inspection:

Critical welds: 100% ultrasonic testing (UT) or radiographic testing (RT).

General Welds: Proportional sampling inspection (typically 10% - 20%).

Inspection Standard: DNV-RU-SHIP-Pt2Ch4:2021; defect acceptance criteria shall comply with ISO 5817 Class B or DNV AL2.

VII. Preventive Measures for Common Defects

VIII. Special Requirements of DNV Regulations

All welding work must be performed under the supervision of a DNV surveyor. The DNV surveyor must be notified in advance to witness the welding and inspection of critical welds.

Any change in welding materials requires a new welding procedure qualification.

Repair welding shall not exceed two times. If more than two repairs are required, approval from the DNV surveyor must be obtained, and a specific repair procedure must be established.