1. Cold Cracks (Hydrogen-Induced Cracks)
Causes:
High hydrogen content: Moisture in welding electrodes/flux, or oil/rust on the base metal surface, decomposes into atomic hydrogen during welding. This hydrogen diffuses into the weld zone and accumulates at grain boundaries.
High residual stress: S355J2W has a relatively high CEV (typically 0.35–0.45%), leading to poor weld ductility. Rapid cooling after welding creates large tensile stress in the joint.
Brittle microstructure: The weld heat-affected zone (HAZ) cools quickly, forming hard martensite or bainite-microstructures that are prone to cracking when combined with hydrogen and stress.
Impacts: Cold cracks are often invisible (e.g., internal cracks in the HAZ) and can expand under load, leading to sudden joint failure. They also destroy the continuity of the protective rust layer, accelerating local corrosion.
2. Weld Porosities
Causes:
Gas entrapment: Gases like CO₂ (from flux decomposition), H₂ (from moisture), or O₂ (from poor shielding gas coverage) are generated during welding but cannot escape in time as the weld metal solidifies.
Contaminated base metal/wire: Oil, paint, rust, or oxide layers on S355J2W's surface react with welding heat to produce gas. Low-quality welding wire with high impurity content can also release gas during melting.
Improper process parameters: Too high welding speed (leaving no time for gas to escape) or too low arc voltage (poor molten pool fluidity) increases porosity risk.
Impacts: Porosities reduce the effective load-bearing area of the weld, lowering joint strength (by 10–30% for severe cases). They also act as "corrosion channels," allowing moisture/salt to penetrate the weld, undermining the steel's weather resistance.
3. Lack of Fusion & Lack of Penetration
Causes:
Insufficient heat input: S355J2W has higher thermal conductivity than ordinary carbon steel. If welding current/voltage is too low, or travel speed is too fast, the base metal/weld metal does not reach the melting point for full fusion.
Poor joint design: Narrow groove angles, excessive root gaps, or incorrect electrode/wire angles prevent the arc from reaching the root of the joint, leading to incomplete penetration.
Contaminated fusion surfaces: Oxide layers (from S355J2W's pre-rusted surface) or slag inclusions block fusion between the weld and base metal.
Impacts: Both defects create weak "interfaces" in the joint, making it prone to cracking under stress. Lack of penetration also leaves gaps in the joint, which trap moisture and cause localized corrosion-critical for S355J2W used in outdoor/coastal environments.
4. Slag Inclusions (Secondary Common Defect)
Causes: Incomplete slag removal between multi-pass welds, too high welding speed (slag cannot float to the surface), or improper arc manipulation (slag is pushed into the molten pool).
Impacts: Slag weakens the weld's mechanical properties and disrupts the protective rust layer, causing pitting corrosion around the inclusion.
