Yield Strength Derating Calculator
At elevated temperatures, yield strength decreases due to increased atomic mobility and dislocation activity. The derated yield strength is the product of the room-temperature yield strength and a temperature derating factor (a value between 0 and 1 representing the retained strength fraction). Derating factors must be obtained from official materials code tables (ASME Section II Part D, AISC, or material-specific standards); this calculator applies a user-supplied factor and computes the derated strength and an allowable design stress using an optional safety factor. For carbon steel A36 with sigma_y = 250 MPa at 300 C (derating factor = 0.80): derated yield = 250 * 0.80 = 200 MPa.
Yield strength derating formula
sigma_y(T) = sigma_y(RT) * f(T)
Allowable stress = sigma_y(T) / safety factor
Where sigma_y(RT) is the room-temperature yield strength (MPa), f(T) is the temperature derating factor from an official code table (dimensionless, between 0 and 1), and safety factor is the code-required design margin. Derating factors must be sourced from ASME, AISC, or the applicable material standard.
Carbon steel A36 derating guidance
AISC and ASME provide derating data for common structural materials. For carbon steel A36 (sigma_y = 250 MPa room temperature, as an illustrative example): 100 C: factor approximately 0.97. 200 C: approximately 0.90. 300 C: approximately 0.82. 400 C: approximately 0.65. 500 C: approximately 0.40. 600 C: approximately 0.20. Always use the official table for the specific grade and code. These figures are indicative only and not a substitute for code-compliant design.
Yield strength derating: frequently asked questions
Why does yield strength decrease at elevated temperature?
At elevated temperatures, increased thermal energy lowers the resistance to dislocation motion, reducing yield strength. Above approximately 0.3 to 0.4 times the absolute melting temperature, time-dependent creep effects also contribute. This is why high-temperature alloys (superalloys, stainless steels, creep-resistant steels) must be used above the capability range of common structural steels.
What is a temperature derating factor?
The temperature derating factor is the ratio of yield strength at a given elevated temperature to the room-temperature yield strength: f = sigma_y(T) / sigma_y(20 C). It is less than 1.0 for most metals above room temperature. Derating factors are tabulated in design codes such as ASME BPVC (pressure vessels), AISC (structural steel), and ASTM material specifications.
How much does steel yield strength decrease with temperature?
Carbon steel (A36): At 200 C, approximately 90% of room-temperature yield strength; at 400 C, approximately 65%; at 600 C, approximately 30%; at 700 C, approximately 15%. The rate of decrease depends on steel grade and alloying. HSLA and stainless grades retain strength better at intermediate temperatures.
Where can I find official derating factors?
ASME Section II Part D contains allowable stress and yield strength tables at elevated temperatures for pressure vessel materials. AISC Design Guide 19 covers fire-resistant structural steel design. ASTM material standards (A36, A572, A992) include elevated temperature properties or reference ASTM A370 testing requirements.
What is the difference between yield strength and allowable stress at temperature?
Yield strength at temperature is the material property (measured by test). Allowable stress is the design value after applying safety factors (typically 2/3 of yield or 1/3 of UTS in ASME, or design rules from AISC). Both decrease with temperature. The derating factor applies to the material property; the allowable design stress applies additional safety factors on top.
Official sources
- ASME Boiler and Pressure Vessel Code, Section II Part D, "Properties (Metric)": asme.org.
- AISC Design Guide 19, "Fire Resistance of Structural Steel Framing": aisc.org.
- ASTM A370, "Standard Test Methods and Definitions for Mechanical Testing of Steel Products": astm.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.