Welding Process for CCS Grade AH32 Shipbuilding Steel Plates

I. Base Metal Properties

1. Chemical composition: C ≤ 0.18%, Mn 0.9% - 1.6%, microalloyed with Nb and V.

2. Carbon equivalent (CE) ≤ 0.40%; good weldability.

3. Mechanical Properties: Yield strength ≥315 MPa, tensile strength 440 - 590 MPa, impact energy at 0°C ≥31 J.

4. Major Welding Risks:

a) Cold cracks are prone to occur in thick plates (≥38 mm) or in low-temperature environments.

b) Excessive heat input leads to grain coarsening and reduced toughness.

II. Selection of Welding Consumables (CCS certification required)

1. Shielded Metal Arc Welding (SMAW)

Electrodes: E5015, E5015-G

Baking: 350°C × 1 h, then held at 150°C until use.

2. CO₂ Gas Shielded Arc Welding (GMAW / FCAW)

Solid wire: ER50-6 (φ1.0/1.2/1.6 mm)

Flux-cored wire: E71T-1

Shielding gas: Pure CO₂ (≥99.9%), flow rate 18 - 25 L/min

3. Submerged Arc Welding (SAW)

Welding wire: h20Mn2 (φ3.2/4.0/5.0 mm)

Flux: SH331, PFH-60A, CHF101 (sintered type)

Baking: Flux at 350°C for 1 hour; welding wire must be derusted and protected from moisture.

III. Pre-welding Preparation

1. Groove Design

2. Preheating and Interpass Temperature (Key Measures to Prevent Cracking)

Preheating Conditions

Interpass Temperature Control

Interpass temperature: ≤250°C; avoid overheating and embrittlement.

Temperature measurement location: 50 mm on either side of the groove, using surface contact temperature measurement.

3. Surface preparation

The groove and the area within 50 mm on either side must be free of oil, rust, water and scale, with a metallic sheen exposed.

Plasma/flame-cut surfaces must be ground to remove the hardened layer.

IV. Recommended Welding Procedure Specifications (WPS)

1. Shielded Metal Arc Welding (SMAW)

2. CO₂ Gas Metal Arc Welding (GMAW)

3. Submerged arc welding (SAW, φ4.0 mm)

V. Key Points for Welding Process Control

Multi-pass welding: Thick plates ≥3 layers, single-pass thickness ≤4 mm, thorough slag removal between passes.

Welding sequence: Symmetrical welding and segmented back-welding to control distortion and residual stress.

Crater filling: Fill the crater completely upon arc termination to prevent crater cracks.

Environmental control:

a) Wind speed ≤2 m/s (a windbreak must be provided for CO₂ welding).

b) Ambient temperature ≥0°C; if below 0°C, preheat to 150°C.

VI. Post-welding Treatment and Inspection

1. Post-welding Slow Cooling

Following welding of thick plates or in low-temperature environments, cover the weld with insulating wool and allow it to cool slowly for ≥4 hours to prevent cracking due to rapid cooling.

2. Visual Inspection

The weld bead shall be uniform in shape, free from cracks, porosity, slag inclusions, undercut (≤0.5 mm) and weld spatter.

3. Non-Destructive Testing (NDT)

Butt welds: 100% UT + MT/PT; RT to be added for critical structures.

Fillet welds: MT/PT sampling; 100% for critical locations.

4. Mechanical Property Testing

Tensile, bending and 0°C impact tests; results must comply with CCS specifications.

VII. Common Defects and Countermeasures

VIII. Key Requirements of CCS

1. Welders must hold CCS-recognised qualifications, and WPSs must pass CCS procedure qualification.

2. Welding consumables, welding wire and flux must be accompanied by CCS certificates, with traceable batch numbers.

3. Preheating to 120 - 150°C is mandatory in the following circumstances:

Low-temperature welding (≤0°C)

Thick plates (≥38 mm)

Joints with high restraint

IX. Typical WPS Example (20 mm thick AH32 butt joint, V-groove)

Welding method: CO₂ gas shielded arc welding (φ1.2 mm)

Preheating temperature: 120°C

Interpass temperature: ≤200°C

Shielding gas: Pure CO₂, 20 L/min

Post-weld treatment: Slow cooling with insulation blanket for 4 h

NDT: 100% UT + MT