How does the yield strength of Q355NH weathering steel change with temperature?

The yield strength of Q355NH weathering steel shows a clear, systematic trend with temperature: it increases at sub‑zero (low) temperatures, decreases gradually at elevated (high) temperatures, and remains relatively stable near room temperature. This behavior is typical for low‑alloy and weathering structural steels.

1. Yield strength at sub‑zero temperatures (below 20 °C, down to −40 °C or lower)

As temperature decreases below room temperature, the yield strength of Q355NH increases noticeably.

At around 0 °C, yield strength is typically 5–10% higher than at 20 °C.

At −20 °C, it can be 10–15% higher than at room temperature.

At −40 °C, yield strength may rise by 15–25% compared to 20 °C.

This strengthening at low temperatures is accompanied by reduced ductility and toughness (lower elongation and impact energy), which is why low‑temperature toughness (e.g., V‑notch impact test at −20 °C or −40 °C) is an important supplementary requirement for Q355NH in cold environments.

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2. Yield strength at room temperature (20 °C)

This is the reference condition defined in standards (GB/T 4171‑2008).

The specified minimum yield strength depends on thickness:

≥ 355 MPa for t ≤ 16 mm

≥ 335 MPa for 16 < t ≤ 40 mm

≥ 325 MPa for 40 < t ≤ 60 mm

≥ 315 MPa for 60 < t ≤ 100 mm

Actual yield strength from mills is usually higher than these minimum values (often by 50–100 MPa) to provide a safety margin.

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3. Yield strength at moderately elevated temperatures (100 °C to 300 °C)

As temperature rises above 100 °C, yield strength starts to decrease gradually.

At 100 °C, yield strength is roughly similar to or slightly lower than at 20 °C.

At 200 °C, it is typically 10–20% lower than room‑temperature values.

At 300 °C, it may drop by 20–35% compared to room temperature.

Although strength decreases, Q355NH still maintains useful mechanical properties in this range, making it suitable for many non‑extreme elevated‑temperature applications (e.g., some industrial structures, pipelines, or equipment exposed to moderate heat).

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4. Yield strength at higher temperatures (above 300 °C, especially > 400 °C)

Above 300 °C, yield strength drops more rapidly.

At 400 °C, yield strength may be only 50–65% of the room‑temperature value.

Above 450–500 °C, the material enters a region of significant softening; yield strength falls sharply, and creep behavior becomes important.

For this reason, Q355NH is not designed for long‑term service at temperatures above ~350–400 °C, where creep, oxidation, and structural softening would limit its load‑carrying capacity and service life.

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Summary of the trend

Low temp (↓ T): Yield strength ↑ (but toughness ↓)

Room temp: Stable, standard‑specified values

Elevated temp (↑ T): Yield strength ↓, especially above 300 °C

This temperature dependence must be considered in engineering design, particularly for structures operating in cold climates, outdoor seasonal environments, or near heat sources.

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