1. Core Mechanism of Stress Relief Annealing for SPA-H Steel
Atomic diffusion is enhanced slightly, allowing dislocations (the source of residual stress) to rearrange and eliminate internal stress.
The ferrite and pearlite phases remain stable-no grain coarsening, phase transformation, or carbide precipitation occurs.
The process only relieves stress, not recrystallizes the steel (recrystallization requires temperatures above 700°C for SPA-H).
2. Specific Effects on Key Mechanical Properties
| Mechanical Property | Change After Stress Relief Annealing | Mechanism |
|---|---|---|
| Residual Stress | Reduced by 60–80% | Dislocation rearrangement and stress relaxation at 550–620°C eliminate thermal/mechanical residual stress from hot working (e.g., welding, bending, stamping). |
| Yield Strength & Tensile Strength | Slight decrease (≤ 3–5%) | Minor dislocation annihilation reduces the steel's work-hardening effect, but the change is negligible for engineering applications. Room-temperature yield strength remains ≥345 MPa (compliant with JIS G 3115). |
| Ductility (Elongation) | Slight increase (1–2%) | The removal of residual stress reduces crack initiation risks, allowing the steel to deform more uniformly before fracture. Elongation typically rises from ~22% to 23–24%. |
| Low-Temperature Impact Toughness | Significant improvement (10–15%) | Residual stress is a key trigger for brittle fracture at low temperatures (-20°C to -40°C). Annealing eliminates this stress, increasing Charpy V-notch absorbed energy from ≥27 J to ≥30–31 J. |
| Hardness | Slight decrease (≤ 2–3 HBS) | Correlated with the minor drop in strength-hardness remains in the range of 180–220 HBS, which is acceptable for SPA-H's application scenarios. |
| Corrosion Resistance (Patina Formation) | No negative impact; slight improvement in uniformity | The elimination of residual stress avoids localized stress corrosion cracking, ensuring the patina layer forms evenly across the steel surface (no preferential corrosion at stress concentration points). |
3. Key Limitations: What Annealing Does NOT Do
Does not increase strength: Unlike quenching or tempering, stress relief annealing does not enhance yield/tensile strength-if higher strength is required, other processes (e.g., controlled rolling) are needed.
Does not coarsen grains: Low annealing temperature prevents grain growth, so the steel's toughness remains stable.
Does not reduce weather resistance: The alloy elements (Cu, Cr, Ni) that drive patina formation remain uniformly distributed, so the steel's core corrosion resistance is preserved.
4. Engineering Implications
For critical structural components (e.g., load-bearing welded joints, deep-stamped 3D signs, cold-region applications), the improvement in toughness and stress reduction far outweigh the minor strength loss. Annealing is strongly recommended to prevent long-term deformation or brittle fracture.
For non-critical decorative components (e.g., flat panels, shallow-bent signs), the mechanical property changes are minimal, so annealing can be skipped to save production costs.
