1. Low-temperature stress relief annealing: The most common heat treatment for Q295GNH
Process parameters: Heat Q295GNH to 250–350°C, hold for 1–2 hours (depending on the thickness of the workpiece, thicker parts need longer holding time), then cool slowly to room temperature (cooling rate ≤50°C/h).
Performance improvement effects:
Eliminates 60%–80% of internal stress generated during cold working (e.g., cold bending of Q295GNH for guardrails) or welding (e.g., welding joints of Q295GNH building brackets), avoiding "stress corrosion cracking" in humid/acidic environments.
Reduces "cold work hardening" (surface hardness increase, plasticity decrease) after cold forming, making subsequent secondary processing (e.g., drilling, tapping) easier without cracking.
Does not damage the weather resistance of Q295GNH: The low temperature avoids changing the distribution of weather-resistant alloy elements (Cu, P, Cr) in the steel, so the ability to form a protective rust layer remains unchanged.
2. Normalization: Improves uniformity of structure and plasticity (for specific scenarios)
Process parameters: Heat to 880–920°C (above the Ac3 phase transition temperature of Q295GNH), hold for 0.5–1 hour to ensure complete austenitization, then cool in air to room temperature.
Performance improvement effects:
Refines the grain structure of Q295GNH (converts coarse ferrite-pearlite to fine-grained structure), increasing elongation by 5%–10% (from the original ≥23% to ≥28%), making it less likely to crack during large-angle cold bending (e.g., 180° bending for Q295GNH decorative panels).
Reduces "banded segregation" (local uneven distribution of elements) in hot-rolled steel, avoiding local brittle fracture during use.
Note: Normalization has little effect on the strength of Q295GNH (yield strength remains around 295MPa, in line with standard requirements) and will not reduce its weather resistance.
3. What heat treatment is not suitable for Q295GNH?
Q295GNH has low carbon content (≤0.16%) and few alloy strengthening elements. Quenching (rapid cooling after heating) cannot form martensite (a high-strength structure), and tempering will not significantly improve strength, but may instead increase production costs and cause deformation of the workpiece.
High-temperature quenching may destroy the distribution of weather-resistant elements (e.g., Cu precipitates dissolve into the matrix), reducing the ability of Q295GNH to form a stable protective rust layer and weakening its weather resistance.
