1. Key Performance Characteristics
Q235NH is a , meaning its alloy composition (adding elements like P, Cr, Cu, Ni) is optimized for forming a protective rust layer in atmospheric conditions, not for retaining strength at high temperatures.
Like most carbon and low-alloy steels, Q235NH loses yield strength and tensile strength as temperature increases.
Around 400℃ (752℃F): This is a critical threshold. The steel's yield strength typically drops to about 50% of its room-temperature strength.
At 600℃ (1112℃F):10-20% of its original strength, making it incapable of supporting significant loads.
2. Comparison with Other Steels
| Q235NH (Weathering Steel) | 304 Stainless Steel | A242 / A588 (Similar Weathering Steels) | Fire-Resistant Steel (e.g., SFRM) | |
|---|---|---|---|---|
| Alloyed to retain ~60% YS at 600℃ |
Q235NH performs similarly to other structural carbon steels (like Q235B) at high temperatures. Its weathering elements do not confer any significant improvement in high-temperature strength retention.
3. The "Oxide Layer" Distinction
that forms at ambient temperatures in wet/dry cycles. This is what Q235NH is designed for.
High-Temperature Scale (Bad):570℃ (1058℃F), a different, non-protective iron oxide scale (primarily ) forms rapidly. This scale is brittle, can spall off, and exposes fresh metal to further oxidation, leading to rapid section loss.
4. Practical Implications and Design Considerations
For buildings, if Q235NH is used in structural members (columns, beams), its performance in a fire must be accounted for. It will (e.g., intumescent paint, fireproofing boards, concrete encasement) to insulate it and prevent it from reaching critical temperatures during a standard fire event (e.g., 1-2 hours).
It is not suitable for applications like furnace parts, exhaust systems, or boilers where sustained high temperatures are expected.
Temporary vs. Sustained Heat: It can withstand short-term exposure to high heat (e.g., from welding) if properly preheated and allowed to cool slowly. However, sustained exposure will lead to significant weakening.
