1. Main Causes of Quenching Cracks in SPA-H Steel
(1) High Carbon Equivalent (CE) & Hardenability
SPA-H has low carbon (≤0.12%) but contains Cu, Cr, and P, increasing hardenability.
Rapid cooling (water/oil quenching) creates martensite, leading to high internal stresses.
(2) Non-Uniform Cooling & Thermal Stress
Uneven cooling rates (e.g., thick vs. thin sections) cause thermal gradients, leading to cracking.
(3) Pre-Existing Defects
Inclusions, microcracks, or segregation (e.g., high P content) act as crack initiation points.
(4) Improper Quenching Temperature
Overheating (>950°C) leads to coarse grain structure, increasing brittleness.
(5) Lack of Tempering After Quenching
Untempered martensite is extremely brittle and prone to cracking under stress.
2. Preventive Measures to Avoid Quenching Cracks
(1) Avoid Unnecessary Quenching
SPA-H is not designed for quenching; use normalizing (if needed) instead (~900°C, air cooling).
(2) Controlled Cooling Rate
If quenching is required, use polymer or oil quenching instead of water to reduce thermal shock.
(3) Preheating Before Quenching (if applicable)
Heat to 400–600°C before final quenching to minimize thermal stress.
(4) Post-Quench Tempering (Critical!)
Immediately temper at 550–650°C to convert brittle martensite into tougher tempered martensite/ferrite-pearlite.
(5) Material Quality Control
Ensure low sulfur (S ≤ 0.030%) and phosphorus (P ≤ 0.15%) to reduce segregation risks.
(6) Stress-Relief Annealing (Alternative)
If hardening is not required, use 600–650°C stress relief instead of quenching.
3. Comparison with Other Weathering Steels
| Steel Grade | Typical Heat Treatment | Quenching Risk | Alternative Solution |
|---|---|---|---|
| SPA-H (JIS G3125) | As-rolled/Normalized | High (if quenched) | Use normalizing instead |
| Corten A (ASTM A588) | As-rolled | Moderate | Avoid quenching |
| S355J0WP (EN 10025-5) | Normalized | Low (if properly tempered) | Tempering recommended |
Conclusion
SPA-H should not be quenched in standard applications due to its weathering steel properties.
If quenching is unavoidable:
Use oil/polymer quenching (not water).
Always temper immediately (550–650°C).
Preheat & control cooling rates to minimize cracking.
For structural use, normalizing or as-rolled conditions are preferred.
