Welding Process for LR AH32 Shipbuilding Steel Plates

1. Standard Pre-Welding Preparation and Selection of Welding Materials

The welding area must be thoroughly prepared prior to welding:

a) Remove rust, oil, scale, and moisture within a 40 mm radius on both sides of the groove to eliminate potential causes of defects;

b) Select LR-certified high-strength marine welding materials compatible with the AH32 base metal to ensure that the joint matches the base metal in strength and toughness;

c) For plates with a thickness of ≥ 8 mm, preheating to 80–120°C is required when the ambient temperature is below 5°C to prevent cold cracks; preheating is not required under normal room temperature conditions to improve construction efficiency.

2. Applicable Welding Processes and Parameter Control

LR AH32 shipbuilding steel plates are suitable for mainstream, high-efficiency marine welding processes:

a) Submerged Arc Welding (SAW): Preferred for large-area assembly of the ship’s main structure, offering stable weld quality and high efficiency;

b) Gas Metal Arc Welding (GMAW): Suitable for the forming and welding of curved structural members and complex components;

c) Stick Welding (SMAW): Used for localized repair welding in confined spaces.

d) During welding, the interpass temperature must be strictly controlled between 100 and 160°C, and heat input must be appropriately adjusted to avoid overheating, which can lead to grain coarsening and reduced joint toughness.

3. Post-Weld Residual Stress Relief Treatment

a) Conventional thin plates (≤ 20 mm) and non-load-bearing joints: Use natural slow cooling combined with heat retention measures to eliminate surface cooling stresses;

b) Thick plates (> 20 mm) and welds in primary load-bearing structures: Perform low-temperature annealing at 500–560 °C to thoroughly eliminate welding residual stresses, refine the microstructure of the weld, stabilize the mechanical properties of the joint, and prevent long-term fatigue failure under marine alternating loads.

4. LR Standards for Welding Quality Inspection and Acceptance Requirements

All LR AH32 welded joints must comply with the LR Code for Welding of High-Strength Marine Steels:

a) Ultrasonic Testing (UT): Perform full-coverage testing on primary welds in thick plates to detect internal microcracks, slag inclusions, and delamination defects;

b) Magnetic Particle Testing (MT): Used to screen for minute surface welding defects;

c) Core Acceptance Criteria: The joint’s tensile strength, ductility, and low-temperature impact toughness at 0°C must all meet the LR AH32 base material performance standards, which serve as the basis for the final assessment of weld quality.

5. Common Welding Defects and Targeted Prevention and Control Measures

Common defects in the welding of LR AH32 shipbuilding steel plates include minor cold cracks, insufficient joint toughness, and porosity. The primary causes include insufficient preheating, moisture in the groove, and excessive welding speed.

Targeted Measures:

a) Strictly enforce the graded preheating specifications for low-temperature construction;

b) Use thoroughly dried, qualified welding consumables;

c) For thick plates, employ a multi-pass, low-heat-input welding process;

d) Standardize post-weld holding and slow-cooling procedures to systematically eliminate common defects.