1. Core Trend of Yield Strength vs. Temperature
High-temperature range (≥ 200°C):
As temperature rises, the atomic thermal motion in the steel's microstructure (ferrite + pearlite) intensifies, reducing the resistance to dislocation movement. The yield strength decreases gradually-for every 100°C increase, the yield strength typically drops by 15–25%. At 600°C (near the austenitization temperature), the yield strength drops to ~30% of its room-temperature value, and the steel loses structural load-bearing capacity.Room-temperature to low-temperature range (20°C to -60°C):
As temperature decreases, atomic motion slows down, and dislocation movement is hindered. The yield strength increases steadily-for example, Q355NH has a room-temperature yield strength of ≥355 MPa; at -20°C, this value rises to ~380–400 MPa; at -40°C, it can reach ~420–450 MPa.Brittle transition temperature (BTT):
When temperature drops below the BTT (typically -40°C to -60°C for standard weathering steels), the yield strength continues to rise sharply, but toughness drops drastically, leading to brittle fracture risk even at low stress. This is why low-temperature impact tests are required for weathering steels used in cold regions.
2. Key Influencing Factors on the Temperature Sensitivity
Alloy composition: Weathering steels contain Cu, Cr, Ni-these elements refine the grain structure and slightly reduce the temperature sensitivity of yield strength. High-strength grades (e.g., Q550NH) have higher temperature sensitivity than low-strength grades (e.g., SPA-H) due to their denser microstructure.
Microstructure: Steels with finer ferrite grains have more stable yield strength at low temperatures, as grain boundaries block dislocation movement more effectively without causing brittle failure.
Processing history: Cold working (e.g., flattening, bending) increases residual stress, which makes the yield strength more sensitive to temperature changes, especially at low temperatures.
3. Engineering Implications
For outdoor structural applications in temperate regions (temperature range -20°C to 100°C), the yield strength change is moderate and can be ignored in routine design (use room-temperature yield strength as the design basis).
For cold-region applications (temperature ≤ -20°C), design calculations must consider the increased yield strength, but also verify the low-temperature impact toughness to avoid brittle fracture.
For high-temperature applications (e.g., near industrial furnaces), weathering steels are not recommended-their yield strength drops significantly above 300°C, and high temperatures accelerate patina peeling and corrosion.
