1. Core Principles: Why Hot Working of SPA-H Steel May Require Stress Relief Annealing
Uneven heating/cooling: Rapid cooling after hot forging, welding, or flame cutting causes thermal gradients between the surface and core of the steel, leading to thermal residual stress.
Severe plastic deformation: Heavy hot bending, deep stamping, or thick plate welding creates mechanical residual stress in localized areas.
Dimensional distortion during subsequent processing or long-term outdoor service.
Stress corrosion cracking (SCC) when the steel is exposed to humid, salt-laden environments (e.g., coastal areas).
Reduced low-temperature toughness, increasing the risk of brittle fracture in cold regions.
2. Scenarios Requiring Stress Relief Annealing for SPA-H Steel Plates
Heavy structural welding: Welded joints of load-bearing components (e.g., bridge guardrails, port equipment supports) generate high residual stress in the heat-affected zone (HAZ). Stress relief annealing eliminates this stress to prevent weld cracking.
Deep hot bending/stamping: Components with large bending radii (≤5× plate thickness) or deep embossing (e.g., 3D sign characters with depth >5 mm) accumulate localized residual stress that can cause shape distortion during patina formation.
Precision components with tight dimensional tolerance: Parts requiring stable flatness (e.g., decorative panels for high-end buildings, precision instrument brackets) need annealing to avoid post-processing warpage.
Service in harsh environments: SPA-H steel used in coastal, industrial, or cold regions (-20°C and below) should be annealed to reduce the risk of stress corrosion and brittle fracture.
3. Scenarios Where Stress Relief Annealing Is Unnecessary
Light hot working: Simple hot cutting (plasma/laser cutting) of thin plates (≤6 mm), shallow bending (radius >10× plate thickness), or surface grinding-residual stress is minimal and will dissipate naturally during outdoor weathering.
Non-load-bearing decorative components: Ornamental panels, garden signs, or art installations that do not bear structural loads-minor dimensional changes do not affect their functionality or aesthetics.
Post-working artificial patina acceleration: The controlled oxidation process during chemical patina acceleration can also relieve minor residual stress, making additional annealing redundant.
4. Standard Stress Relief Annealing Parameters for SPA-H Steel Plates
Heating temperature: 550–620°C (below the pearlite transformation temperature to preserve the ferrite-pearlite microstructure).
Holding time: 1–2 hours per 25 mm of plate thickness (ensure uniform temperature penetration).
Cooling rate: Slow furnace cooling to ≤200°C, then air cooling to room temperature (rapid cooling will reintroduce residual stress).
Key note: Avoid annealing above 650°C-this will coarsen the grain structure, reducing the steel's strength and toughness.
