1. Core Factor 1: Plate Thickness (the "Threshold" for Consideration)
Thickness <25mm: Welding heat input is relatively small, and residual stress is usually low. If the structure is simple (e.g., straight seams, no complex joints) and not under high load, stress relief treatment is often unnecessary.
Thickness ≥25mm: The welding heat-affected zone (HAZ) is wider, and the temperature gradient between the weld and the base metal is steeper. This leads to significant thermal expansion and contraction, resulting in high residual stress. Stress relief treatment is strongly recommended unless the structural design explicitly exempts it.
Thickness ≥50mm: Residual stress is highly concentrated, especially in multi-pass welding (common for thick plates). Stress relief treatment is almost mandatory to avoid post-welding deformation, cracking, or reduced service life.
2. Core Factor 2: Structural Design and Welding Conditions
Complex joints or high: If the Q355NH thick plate has closed welds, T-joints, or is constrained by other components (e.g., welded to a rigid frame), residual stress cannot be released through natural deformation. Stress relief treatment is required to prevent cracking.
High heat input welding: Processes like submerged arc welding (SAW) or multi-pass manual arc welding (SMAW) for thick plates generate more heat, increasing residual stress. Stress relief is needed to offset this effect.
Weld defects: If welding defects (e.g., incomplete fusion, porosity) exist, residual stress can concentrate around these defects, accelerating cracking. Stress relief treatment, combined with defect repair, is necessary.
3. Core Factor 3: Service Environment and Performance Requirements
High-stress service scenarios: If the Q355NH thick plate structure bears static/dynamic loads (e.g., bridge girders, pressure vessel shells, heavy machinery bases), residual welding stress can superimpose with working stress, leading to premature fatigue failure. Stress relief is mandatory.
Corrosive service environments: Q355NH relies on a protective rust layer (patina) for weather resistance. Residual stress can cause "stress corrosion cracking" (SCC) in humid, chloride-rich environments (e.g., coastal areas, industrial zones with pollutants). Stress relief treatment eliminates this risk and ensures the patina forms uniformly.
Precision or deformation-sensitive structures: If the structure requires strict dimensional tolerance (e.g., large-scale steel components for buildings), residual stress will gradually release over time, causing spontaneous deformation. Stress relief treatment prior to installation is essential.
