Welding of CCS Grade A Shipbuilding Steel Plates

1. Basic Properties of the Base Material

1.1 Applicable Standards

CCS ‘Classification Rules for Steel Seagoing Ships’, GB/T 712-2022.

1.2 Mechanical Properties

Yield strength ≥ 235 MPa, tensile strength 400 - 520 MPa, elongation ≥ 22%; no low-temperature impact requirements (applicable ambient temperature ≥ 0 °C).

1.3 Chemical Composition (Typical, wt%)

C ≤ 0.21, Mn 0.80 - 1.20, Si ≤ 0.5, S ≤ 0.035, P ≤ 0.035; low-carbon, low-manganese, with excellent weldability.

2. Selection of Welding Consumables

2.1 Shielded Metal Arc Welding (SMAW)

General structures: J422 (E4303, acidic), J427 (E4315, low-hydrogen).

Critical joints / thick plates: Low-hydrogen type J427/E4315 must be used (diffusion hydrogen ≤ 5 mL/100 g).

Drying specifications: Acidic: 100 - 150°C × 1 h; Low-hydrogen: 300 - 350°C × 1 h; storage cylinder temperature ≥ 100°C; take as required.

2.2 Submerged Arc Welding (SAW)

Welding wire: H08A/H08MnA (φ4.0 - 5.0 mm).

Flux: HJ431 (fused)/CHF43 (sintered), dried at 250°C for 1 hour.

2.3 Gas Shielded Arc Welding (GMAW/FCAW)

Solid wire: ER50-6 (φ1.0 - 1.2 mm), CO₂ purity ≥99.5%.

Flux-cored wire: E501T-1.

3. Pre-welding Preparation

3.1 Groove Design (Grooving is mandatory for plate thickness ≥ 12 mm)

3.2 Surface Preparation

Remove rust, oil, paint and scale to bare metal within a 20 mm radius on both sides of the groove.

No oil or moisture residue is permitted to prevent porosity and cracking.

3.3 Preheating and Interpass Temperature

Plate thickness < 20 mm: Weld at ambient temperature; no preheating required (ambient temperature ≥ -5 °C).

Plate thickness 20 - 50 mm: Preheat to 50 - 80 °C when ambient temperature is < 5 °C.

Plate thickness > 50 mm: Preheat to 80 - 100 °C; interpass temperature ≤ 200 °C.

3.4 Tacking

Weld length: 30 - 50 mm, spacing 200 - 300 mm.

Tack welds: ≤ 2/3 of the design value, free from cracks, porosity and slag inclusions.

Prior to final welding: Grind both ends of the tack welds to a gentle slope.

4. Welding Process Parameters

4.1 Shielded Metal Arc Welding (SMAW, J427/E4315)

4.2 Submerged Arc Welding (SAW, H08MnA+HJ431)

4.3 Gas Metal Arc Welding (GMAW, ER50-6, CO₂)

5. Welding Process Control

5.1 Welders

Must hold a CCS-approved qualification certificate, appropriate for the project

5.2 Welding Sequence

Weld short seams first, then long seams; vertical welding first, then flat welding; weld symmetrically to minimise distortion.

5.3 Passes

Multi-pass welding, with each pass thickness ≤4 mm; remove slag and grind between passes; no slag inclusion.

5.4 Heat Input

Control within 10 - 25 kJ/cm to avoid grain coarsening or embrittlement.

5.5 Process Qualification

Complete CCS welding procedure qualification (WPS/PQR) tests prior to welding.

5.6 Environment:

Temperature: ≥0°C; preheat to 50 - 80°C when below 5°C.

Humidity: ≤85%, no rain or snow.

Wind protection: Wind speed ≤2 m/s for gas-shielded welding; erect a windbreak.

6. Post-welding Treatment and Inspection

6.1 Post-welding Cleaning

Remove slag, spatter and undercut; grind the weld to ensure a smooth transition and reduce stress concentration.

6.2 Post-welding Heat Treatment

For plate thicknesses > 50 mm or high-stiffness joints: post-weld heat treatment at 150 - 200 °C for 1 - 2 hours, followed by slow cooling to relieve hydrogen and prevent cracking.

Critical structures (e.g. outer side plates): Full stress-relief annealing at 600 - 650°C.

6.3 Non-Destructive Testing

Visual inspection (VT): 100%, free from cracks, porosity, inclusions and undercut (≤0.5 mm).

Ultrasonic testing (UT): 100% of critical butt joints, tested 24 hours after welding (to prevent delayed cracking).

X-ray (RT): Random inspection of critical joints (5% - 10%), defect rating ≤ Grade II.

6.4 Repair:

Defect removal: Grind/air-chamfer until defect-free, with gentle bevels at both ends.

Repair welding: Same as original process, including preheating and interpass temperature control.

Re-inspection: 100% NDT after repair.

7. Common Issues and Prevention

Defect  Cause  Preventive Measures

Porosity  Moisture in the groove/welding consumables, impure shielding gas  Thorough drying, cleaning, and compliance with shielding gas specifications.

Inclusion  Inadequate slag removal, improper electrode movement  Interpass slag removal, correct electrode movement.

Lack of Penetration  Small groove angle, low current, insufficient gap  Adjust the groove angle, current and gap.

Cracks  Insufficient preheating, high hydrogen diffusion, high restraint  Preheat thick plates, use low-hydrogen welding consumables, follow a rational sequence.

Distortion  Improper welding sequence, high heat input  Symmetrical welding, low heat input, rigid clamping.

8. Key Points of CCS Specifications

All welding must be carried out in accordance with a WPS (Welding Procedure Specification) and be approved by CCS (WPQR testing).

Welding consumables, welders and equipment must all be CCS-certified or approved.

Weld quality shall be in accordance with the CCS ‘Specifications for Materials and Welding’.

Summary

The core principles of CCS Grade A shipbuilding steel plate welding are: matching welding consumables, adequate preheating, compliant grooving, controlled processes and rigorous post-weld inspection. Strict adherence to CCS specifications and approved WPSs can effectively prevent defects and ensure weld quality and ship safety.