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

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.