LR Grade E Cryogenic Welding Process and Quality Control

1. Enhanced Pre-Welding Preparation (Arctic Low-Temperature Environments)

Before welding in polar ultra-low-temperature environments (below -20°C), enhanced pre-treatment must be performed:

a) Thoroughly remove rust, oil, scale, frost, and residual moisture within a 50 mm range on both sides of the groove to eliminate potential causes of defects;

b) Select specialized -40°C polar-resistant welding consumables compatible with LR Grade E base metal to ensure the joint’s low-temperature toughness matches that of the base metal;

c) For plates with a thickness of ≥6 mm, when the ambient temperature is below 0°C, perform staged preheating at 100–140°C; when the ambient temperature is below -20°C, increase the preheating temperature to 120–160°C to completely eliminate the risk of cold cracking.

2. Applicable Welding Processes

LR Grade E marine steel is suitable for mainstream marine welding processes, and parameters must be optimized for ultra-low-temperature conditions:

a) Submerged Arc Welding (SAW): Preferred for large-area jointing of primary load-bearing structures on polar vessels, offering stable weld quality and high efficiency;

b) Gas Metal Arc Welding (GMAW): Suitable for forming welds on curved structural members and complex components;

c) Shielded Metal Arc Welding (SMAW): Used for localized repair welding in confined spaces.

During welding, the interpass temperature must be strictly controlled between 120–180°C to avoid excessive or insufficient heat input, which could lead to grain coarsening and deterioration of low-temperature toughness in the joint.

3. Post-Weld Ultra-Low-Temperature Stress Relief Treatment

All LR Grade E load-bearing welded joints must undergo graded post-weld stress relief treatment:

a) Conventional thin plates (≤20 mm): Use isothermal slow cooling and holding to prevent stress concentration caused by rapid cooling in ultra-low-temperature environments;

b) Thick plates (>20 mm) and long continuous welds: A high-precision low-temperature annealing treatment at 530–600°C must be performed to thoroughly eliminate residual stresses, refine the microstructure of the weld, stabilize the joint’s low-temperature toughness at –40°C, and prevent long-term fatigue failure.

4. Polar Cryogenic Welding Quality Inspection Standards

All LR Grade E welded joints must comply with the LR Polar Marine Cryogenic Welding Inspection Code:

a) Ultrasonic Testing (UT): Perform full-coverage inspection of thick plate welds to detect internal microcracks, slag inclusions, and delamination defects;

b) Magnetic Particle Testing (MT): Used to screen for microscopic cryogenic cold cracks on the surface;

Core Acceptance Criteria: Weld joints must pass the -40°C cryogenic impact test, and their impact toughness must meet the requirements for LR Grade E base metal; this serves as the final basis for determining weld quality.

5. Common Cryogenic Welding Defects and Prevention Measures

The most common defects in LR Grade E marine steel under polar cryogenic conditions include:

cryogenic cold cracks, insufficient joint toughness, and internal porosity. The primary causes include insufficient preheating, moisture and frost on the groove, moisture in the welding consumables, and excessive welding speed.

Targeted Prevention and Control Measures:

a) Strictly implement a graded cryogenic preheating system;

b) Use thoroughly dried, specialized polar welding consumables;

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

d) Strengthen post-weld heat retention and controlled cooling measures to eliminate defects at the source.