Part I. The Metallurgy of: “Alloy Selection for Elevated Temperature Application”

Introduction

This is a complex subject in part due to the time dependant mechanical behaviour of metals under load at elevated temperature.

At room temperature, we understand that a part is failing once it is past it’s yield point – plastic deformation is occurring. The part is completely destroyed once Ultimate Tensile Strength (UTS) is reached – fracture occurs.

At elevated temperatures, persistent slow plastic deformation (creep) means that the part is “starting to fail” from the onset. A part undergoing creep has already past it’s ‘yield point’.

For High Temperature applications Creep Strength is roughly equivalent to Yield Strength and Rupture Strength is roughly equivalent to Ultimate Tensile Strength, with the added complexity of these material properties also being time dependant.

Elevated temperatures will also accelerate chemical reactions and physical chemistry changes. Usually design for high temperature applications requires determining the earliest mode of failure and designing the campaign life to be safely within this product life expectancy.

Scope

There are many considerations in selecting an alloy for high temperate application and the complexity increases with temperature cycles and temperature gradients. Factors to be considered in selecting an alloy for high temperature application include;

• Normal Operating Temperature
• Maximum operating temperature
• Type and magnitude of applied stresses
• Range of temperature cycles
• Frequency of Temperature Cycles
• Rate of temperature change
• Atmosphere
• Abrasive and wear conditions
• Manufacturing methodology for component
• Ease of inspection in situ
• Ease of replacement
• Cost

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.