Due to the depth of this topic, this article can only serve as an introduction to the field of material selection for elevated temperature use. It will explore two broad criteria to consider when selecting metals for use at elevated temperature.

1.Mechanical Strength

2.Metallurgical Structural and Surface Film, Stability

Mechanical Strength

At room temperature, the yield strength of a material is the measure of stress required to start plastic deformation and generally limits the application of engineering alloys. In selecting a material, we consider “Stress’.

At high temperatures, the strength of metal decreases due to the increased mobility of atoms and vacancies. Plastic deformation (creep) will appear at stresses which would be considered inconsequential at room temperatures. Therefore, when considering high-temperature strength we must always consider ‘Time -Temperature-Stress’.

It is generally considered that creep becomes of engineering significance at a homologous temperature greater than 0.5. The homologous temperature expresses the temperature of a material as a fraction of its melting point using the absolute temperature scale.

The material property that is used in design is at high temperatures is Creep Strength. By consulting creep-strength curves (and Stress Rupture Curves) an alloy can be selected that allows for an amount of deformation which will not be objectionable during the design life of the component. When large deformations can be tolerated the rupture strength may be the limiting factor.

Surface Film Stability

Elevated Temperature makes available energy for reaction rates to be accelerated, therefore at elevated temperatures components are far more susceptible to the chemistry of their environment and reactions at the metal-environment interface and are therefore dependant upon the Surface Film Stability.

Oxidizing environments may resulting catastrophic oxidation, reducing atmospheres may break down passive oxide surface films. High temperatures will accelerate diffusion rates which may result in carburization or hydrogen embrittlement. The design engineer needs to ensure carful alloy selection to match the material to the environment.

High temperature environments which include an abrasive element, such as kiln gas flues, are very susceptible to an erosion-corrosion attack as the abrasive elements constantly wear away the reacted layer to expose unreacted material to the environment.

Metallurgical Structural Stability

Elevated Temperatures also make available energy for physical transformations to occur within the metal structure. Perhaps, most obviously, temperature can cause crystal structure phase changes which dramatically alter material properties and the phase diagram of the alloy systems under consideration must be consulted in material selection. Not so obvious is the recrystallization of cold worked materials and grain growth or the loss of strength in aged hardened materials due to the coarsening of the precipitated phase particles. Elevated temperature increase mobility of vacancies and increase diffusion rates resulting in increased homogenisation of the material.