1. Common Welding Defects in Q355GNH Welds
(1) Hydrogen-Induced Cracking (HIC) – The Most Critical Defect
Manifestation: Fine cracks (transgranular or intergranular) that form in the weld metal or HAZ hours to days after welding, often triggered by residual stress and hydrogen absorption.
Causes:
High hydrogen content from "dirty" welding materials (e.g., damp electrodes, unpurified shielding gas like CO₂ with moisture).
Q355GNH's Cu/Cr-rich composition increases its sensitivity to hydrogen embrittlement.
Rapid cooling after welding (e.g., in cold environments) traps hydrogen in the microstructure, increasing crack driving force.
(2) Lack of Fusion (LOF) and Lack of Penetration (LOP)
Manifestation: Unfused gaps between the weld metal and base metal (LOF) or incomplete penetration through the joint thickness (LOP), weakening the joint's load-bearing capacity.
Causes:
Insufficient heat input (e.g., low welding current, high travel speed) – Q355GNH's weathering alloying elements slightly increase its melting point, requiring higher heat than plain carbon steel.
Poor joint fit-up (e.g., excessive root gap, misalignment) or improper electrode/ wire angle.
(3) Grain Coarsening in the HAZ – Degrades Toughness
Manifestation: The HAZ (adjacent to the weld) develops coarse ferrite or bainite grains, leading to a sharp drop in low-temperature impact toughness (e.g., impact energy at -40°C falls below 27J).
Causes:
Excessive heat input (e.g., high welding current, slow travel speed) – prolonged exposure to high temperatures (above 950°C) causes austenite grains to grow uncontrollably.
Q355GNH's normalized microstructure is sensitive to heat; overheating disrupts its fine ferrite-pearlite balance.
(4) Corrosion Resistance Degradation – A "Hidden" Defect
Manifestation: The weld zone (including weld metal and HAZ) corrodes faster than the base metal, forming uneven rust layers – a failure of Q355GNH's core weathering function.
Causes:
Mismatched filler metal (e.g., using plain carbon steel electrodes instead of weathering steel-specific ones) – the weld lacks Cu/Cr/Ni to form a protective rust layer.
Post-weld oxidation (e.g., no passivation treatment) – the HAZ's coarse grains form porous, non-adherent rust.
2. Prevention Measures for Each Defect
(1) Prevent Hydrogen-Induced Cracking (HIC)
Control hydrogen sources:
Dry welding materials: Bake low-hydrogen electrodes (e.g., E5015-G for weathering steel) at 350–400°C for 1–2 hours; store unused electrodes in a heated holding oven (80–120°C).
Purify shielding gas: Use dry CO₂ or Ar-CO₂ (moisture content ≤50ppm); install a gas dryer if ambient humidity >60%.
Reduce residual stress:
Preheat the base metal to 80–150°C (critical for plates >12mm thick or in temperatures <5°C) to slow cooling and allow hydrogen escape.
Perform post-weld stress relief (550–650°C for 1–2 hours) for thick-walled components or high-stress joints.
Optimize cooling rate: Use interpass temperature control (maintain 150–250°C between weld passes) to avoid rapid cooling.
(2) Prevent Lack of Fusion/Penetration
Adjust heat input:
For manual arc welding (MMA): Use electrodes of diameter 3.2–4.0mm, current 100–160A, and travel speed 8–12cm/min.
For gas metal arc welding (GMAW): Use wire diameter 1.2mm, current 180–220A, voltage 22–26V, and travel speed 10–15cm/min.
Improve joint preparation: Ensure root gap 2–4mm, bevel angle 60°±5°, and clean joint edges (remove rust, oil, or scale with a wire brush or grinder).
Optimize welding technique: Maintain a electrode/wire angle of 70–80° (for fillet welds) or 90° (for butt welds); use "weave" motions (width ≤3× electrode diameter) to ensure full fusion.
(3) Prevent HAZ Grain Coarsening
Limit heat input: Keep heat input ≤25kJ/cm (for plates ≤20mm thick) – avoid oversized electrodes/wires or slow travel speeds.
Use low-heat welding processes: Prioritize GMAW or pulsed GMAW over submerged arc welding (SAW) for thin plates; SAW (high heat input) is only suitable for plates >25mm with post-weld normalization.
Control interpass temperature: Do not exceed 300°C – excessive interpass heat prolongs HAZ exposure to high temperatures.
(4) Prevent Corrosion Resistance Degradation
Use weathering-grade filler metals:
MMA: E5015-G (GB/T 5117) or E5016-G (weathering steel-specific electrodes with Cu/Cr/Ni).
GMAW: ER50-G (GB/T 8110) with Cu content 0.20–0.50% and Cr content 0.30–0.80%.
Post-weld surface treatment:
Remove weld spatter and slag with a chisel or grinder; wire-brush the weld zone to promote uniform rust layer formation.
Apply a passivation coating (e.g., epoxy primer) if the component is exposed to harsh environments (e.g., coastal salt spray) within 1 month of welding.
