1. Tempering After Quenching: The Main Scenario for Property Adjustment
Residual Stress Relief: Tempering (typically 200–600°C) softens the microstructure slightly, allowing internal stresses from quenching to dissipate. This reduces the risk of post-processing cracking (e.g., during welding or machining) and improves dimensional stability.
Toughness Improvement: By relieving stress concentrations and refining pearlite lamellae, tempering increases Charpy impact energy by 5–15% (e.g., from 30 J to 34–35 J at -40°C). The material becomes less prone to brittle fracture under dynamic loads.
Strength and Hardness Reduction (Mild): Quenching only marginally increases Q355GNH's strength (tensile strength +≤5%) and hardness (HB +5–10). Tempering reverses this slightly: tensile strength drops by 2–4%, and hardness decreases by 3–8 HB-a small trade-off for better toughness and stress relief.
Corrosion Resistance Preservation: Proper tempering (avoiding overheating >650°C) maintains the uniform distribution of Cu, Cr, and Ni. This ensures the steel still forms a dense protective patina, so weathering performance remains largely unchanged.
2. Tempering Without Prior Quenching: Minimal Impact
No Significant Strength Change: As-rolled/normalized Q355GNH already has a stable ferrite-pearlite microstructure. Tempering (even at 500–600°C) does not alter this structure enough to change tensile/yield strength (variations <2%).
Toughness and Hardness Unchanged: Without residual stresses from quenching, tempering cannot meaningfully improve toughness or reduce hardness. This process is unnecessary for as-rolled/normalized Q355GNH, as normalization already optimizes its balance of strength and toughness.
3. Key Factor: Tempering Temperature
Low Temperature (200–350°C): Relieves only partial stress; toughness improves slightly, but strength/hardness remain close to post-quenching levels.
Optimal Temperature (400–550°C): Achieves the best balance: maximum stress relief, noticeable toughness improvement, and only mild strength/hardness loss.
High Temperature (>600°C): Causes excessive softening (tensile strength drops by >5%, hardness by >10 HB) and may coarsen grains. This weakens the steel and risks disrupting alloy element distribution, harming corrosion resistance.
