Low-Temperature Welding Process for LR Grade D Marine Steel

1. Preparations for Low-Temperature Welding

1) Before welding LR Grade D steel in low-temperature environments, the surface preparation of the groove must be intensified. Rust, oil, scale, and moisture must be thoroughly removed within a 30 mm range on both sides of the groove to prevent cold weld defects caused by residual impurities.

2) Low-temperature marine welding consumables compatible with the base metal should be selected to ensure consistent low-temperature toughness of the joint.

3) When the ambient temperature is below 0°C and the plate thickness is ≥6 mm, preheating to 80–120°C must be performed prior to welding to reduce the risk of cold cracking.

2. Welding Processes and Parameter Control

LR Grade D steel is suitable for mainstream marine welding processes, using parameters optimized for low-temperature conditions:

Submerged Arc Welding (SAW): Suitable for large-area assembly of main hull structures in cold regions; it offers stable weld quality and high efficiency;

Gas Metal Arc Welding (GMAW): Suitable for low-temperature forming and welding of curved structural components;

Shielded Metal Arc Welding (SMAW): Suitable for confined spaces or localized repair welding.

During welding, the interpass temperature must be strictly controlled between 100–160°C to prevent excessive heat input, which can lead to grain coarsening in the joint and a decrease in low-temperature toughness.

3. Post-Weld Stress Relief Treatment

For critical load-bearing welded joints made of LR Grade D steel in cold regions, graded post-weld treatment should be implemented:

1) Conventional thin plates (≤20 mm): Allow natural slow cooling in a constant-temperature environment to avoid residual stresses caused by rapid cooling;

2) Thick plates (>20 mm) and long welds: Perform low-temperature annealing at 520–590°C to eliminate welding residual stresses, refine the microstructure of the heat-affected zone, stabilize low-temperature toughness, and prevent structural deformation and low-temperature fatigue failure.

4. LR Low-Temperature Welding Quality Inspection Standards

All LR Grade D steel welded joints must meet the requirements of the LR Low-Temperature Welding Inspection Specification:

1) Ultrasonic Testing (UT): Used for comprehensive inspection of internal defects (cracks, inclusions, etc.) in thick-plate welds;

2) Magnetic Particle Testing (MT): Used to detect microscopic surface cold cracks;

3) -20°C Low-Temperature Impact Test: A mandatory requirement used to verify whether the joint’s low-temperature toughness meets the base material standard; this is the core acceptance criterion for weld quality in cold regions.

5. Common Low-Temperature Welding Defects and Prevention Measures

Common defects in LR Grade D steel under low-temperature conditions primarily include cold cracks, insufficient joint toughness, and porosity. The main causes include insufficient preheating, moisture in welding consumables, and excessive welding speed.

Prevention and Control Measures:

1) Strictly follow the low-temperature preheating procedure;

2) Use thoroughly dried, specialized low-temperature welding consumables;

3) For thick plates, employ multi-pass welding in layers and strictly control heat input;

4) Ensure the performance of low-temperature welded joints is stable and reliable.