Modern Steel Construction • June 2021 • STEEL INTERCHANGE

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High-Temperature Applications

I am currently evaluating steel structures that will be subjected to high temperatures. What guidance and information would you recommend?

Designing steel for elevated service temperatures, especially greater than 700 °F, is highly specialized. Material properties change at elevated temperatures, including the coefficient of expansion, modulus of elasticity, yield strength, and tensile strength. Additionally, the stress-strain curve becomes more rounded, causing a further strength reduction for members controlled by stability limit states.

Other potential effects that are generally considerations only for service temperatures exceeding 750 °F are creep (including creep rupture and creep buckling), temper embrittlement, graphitization, and oxidation/corrosion/scaling. For designing at temperatures greater than 700 °F, selecting proper materials to resist these effects is an essential first step. Additives such as molybdenum and chromium have shown highly improved resistance to graphitization, temper embrittlement, and creep rupture (Meier, 2014).

Material Properties

For fire conditions, Appendix 4 of the 2016 AISC Specification for Structural Steel Buildings (ANSI/AISC 360, aisc.org/specifications) contains information on material properties at elevated temperatures. However, these values should only be used when designing for fire conditions and other situations where large inelastic deformations are acceptable because the yield strength values are defined at 2% strain. This makes a significant difference because, at elevated temperatures, the stress-strain curve loses its well-defined yield point, and the curve becomes nonlinear at earlier stages of loading. At high temperatures, the curve for mild steel is shaped more like an aluminum stressstrain curve. Generally, 2% strain is not acceptable for design at elevated temperature service under otherwise normal conditions.

Brockenbrough and Merritt (1994) published accurate, designer-friendly equations to predict the elevated-temperature yield strength and modulus of elasticity for normal (0.2% strain) design conditions. In the newer editions of this handbook, these equations were removed, so try to locate the 2nd Edition. The equations were originally published in a journal paper by Brockenbrough (1970). Slightly more conservative values for yield strength, ultimate strength, and modulus of elasticity are tabulated in the 2010 ASME Boiler and Pressure Vessel Code (ASME, 2011).

Welds

The weld metal should be selected to match the base material. If the steel selected is not listed in AISC Specification, Section A3, you may need to design the welds according to the ASME Boiler and Pressure Vessel Code or another method.

Based on tensile tests of the deposited weld metal by Heuschkel (1954) for base metals listed in AISC Specification, Section A3, I have used the following design guidelines on past projects:

For temperatures up to 600 °F, no strength reduction is required. For temperatures above 600 °F, the reduction factor can be calculated using linear interpolation between points (1.0, 600 °F) and (0.0, 1,300 °F). This results in a reduction factor of 0.42 at 1,000 °F. The actual curves are nonlinear, so this approach may be too conservative at temperatures above 1,000 °F.

Recent tests on transverse welded joints (Conlon, 2009) showed a reduction factor of 0.60 at 1,000 °F. Table 5.6 in Conlon’s Thesis lists the reduction ratios at each temperature.

Stability

Another problem with elevated temperature design is this: The yield point on the stress-strain curve is not as well defined for elevated temperature design as it is for room-temperature design. This has a negative effect on member stability beyond the reduction due to the degraded yield strength and modulus of elasticity. Takagi and Deierlein (2007) developed design equations for the flexural buckling of columns and lateral-torsional buckling of beams at elevated temperatures, based on the AISC Specification, and their equations have been adopted into the provisions of Specification, Appendix 4.

Creep

Creep is the time-dependent permanent deformation that occurs when a material is subjected to sustained loading at temperatures in the creep range. Creep failure occurs due to excessive deformation, creep buckling in compression members, and creep rupture in tension members. Performance is dependent on stress, temperature, time, and the chemical composition of the steel. For carbon steels such as A36, A992, and A572 Grade 50, creep should be considered for temperatures exceeding 750 °F. At temperatures equal to or less than 700 °F, commonly available structural steel shapes and plates are usually the most economical materials. At temperatures greater than 700 °F, material selection is a compromise between cost and creep performance.

Bo Dowswell, principal with ARC International, LLC, is a consultant to AISC.

At sustained service temperatures greater than 700 °F, the effects of creep must be considered for most common structural steel shapes and plates. In design, this is typically done by reducing the allowable stresses. The allowable stress should be based on the time of sustained load at the sustained temperature during the useful life of the structure. In this case, creep deformation occurs, but failure is avoided until after the useful life period.

Another approach is to select materials that are resistant to creep at the service temperature. These alloy materials are usually costly compared to mild steels. These materials are typically available only for plates. Therefore, any shapes must be fabricated, built-up members, which further increases the cost.

Graphitization

When steels are subjected to elevated temperatures for prolonged periods, carbon migrates to the grain boundaries, forming graphite nodules that have an embrittling effect, known as graphitization: the breakdown of the chemical microstructure into its base elements of ferrite (iron) and graphite (carbon). This breakdown creates a localized weakened failure plane in the material, which leads to a higher potential for brittle fracture. Detrimental effects of graphitization include a considerable reduction in mechanical properties such as tensile strength, ductility, and creep resistance (Meier, 2014). The rate of graphite formation is affected by temperature, time, and chemical composition of the steel. According to Meier, “Temperatures below 800 °F may experience graphitization, but at such a marginal rate that it can be neglected over the design service life.”

Meier lists some typical materials (ASTM designations) used in flue-gas ducts and their suggested maximum temperatures to reduce the risk of graphitization:

Judgment will be needed to determine an appropriate material and design method for the project conditions. If the steel members will not resist sustained loads, creep is not a concern. However, the other effects should be considered. Researching the effects of temper embrittlement and oxidation/corrosion/scaling on the selected material is recommended.

Brockenbrough, R.L. (1970), “Theoretical Stresses and Strains from Heat Curving,” Journal of the Structural Division, American Society of Civil Engineers, Vol. 96, No. ST7.

Brockenbrough, R.L., and Merritt, F.S. (1994), Structural Steel Designers Handbook. Second Edition, McGraw-Hill, New York.

Conlon, K.A. (2009), Strength of Transverse Fillet Welds at Elevated and Post-Elevated Temperatures, Master’s Thesis, Lehigh University, May. (This report can be downloaded from the Lehigh website at https://tinyurl.com/conlonlehigh.)

Heuschkel, J. (1954), “Effects of Temperature on Weld Metal Properties,” Welding Research Supplement, August, pp. 388-s through 397-s.

Meier, A.A., Hammerschmidt, D.M. and Skibbe, E.R. (2014), “Graphitization Effects on High Temperature Ductwork,” Proceedings of the Structures Congress, American Society of Civil Engineers.

Takagi, J. and Deierlein, G.G. (2007), “Strength Design Criteria for Steel Members at Elevated Temperatures,” Journal of Constructional Steel Research, Vol. 63, pp. 1036–1050.

STEEL SOLUTIONS CENTER

Steel Interchange is a forum to exchange useful and practical professional ideas and information on all phases of steel building and bridge construction. Contact Steel Interchange with questions or responses via AISC’s Steel Solutions Center: 866.ASK.AISC | solutions@aisc.org

The opinions expressed in Steel Interchange do not necessarily represent an official position of the American Institute of Steel Construction and have not been reviewed. It is recognized that the design of structures is within the scope and expertise of a competent licensed structural engineer, architect or other licensed professional for the application of principles to a particular structure.

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