DEFINITION:
Broadly speaking, high temperature alloys are metals designed to maintain strength above room temperature, and which generally operate between temperatures of 500°F and 2200°F.

HISTORY AND EVOLUTION:
In the early 1920’s, adding chrome to steel produced the first true high temperature materials, for resistance heating elements. Later, alloying with nickel was found to stabilize austenite, and promote protective chromia at lower chrome contents, giving the foundation for most of today’s high temperature alloys. Aerospace and gas turbine developments initially motivated the quest for improving the Fe-Cr-Ni alloys. This quest has since been accelerated by industry demands for continuous processing, reduced downtime, and the improved process efficiencies gained at increased temperatures. Today’s techniques used to improve high temp alloys include further alloying with molybdenum, cobalt, tungsten, aluminum, silicon, or rare earth elements (e.g. tantalum, cerium), and advanced melting techniques such as VIM-vacuum induced melting, VAR-vacuum arc remelting, and ESR-electroslag remelting.

• Thermal Expansion
• Thermal Conductivity
• Creep Strength
• Rupture Strength
• Thermal Fatigue Strength
• Thermal Shock Strength
• Erosion and Wear Resistance
• Corrosion Resistance

MATERIAL PROPERTIES AND HIGH TEMPERATURE ALLOYS
All metals expand when heated. Proper provision for expansion and contraction will reduce possibility of early mechanical failure. Thermal conductivity governs the rate at which localized heating is dissipated. Low thermal conductivity could produce distortion and burn-through in situations such as direct flame heating. Above 700°F, steel will flow continuously under applied load, rendering tensile and yield strengths inappropriate measures of material strength. Creep and stress-rupture are measures of elevated temperature strengths. Creep strength is commonly expressed as the stress to produce a 1% creep rate in 10,000 hours at a certain elevated temperature. The stress to produce rupture of a material over a certain amount of time, at a certain temperature, is called rupture strength. Thermal fatigue strength is the ability to survive cyclic temperature changes. Thermal shock resistance is the ability to survive rapid temperature change.
Erosion and wear resistance are important factors in abrasive or moisture-laden environments.
Corrosion and oxidation resistance are of primary importance, since high temperature materials must not deteriorate quickly at high temperatures.

TYPES OF CORROSION

The ability of stainless steel to resist corrosion rests primarily in the ability of chrome to combine with oxygen to form a passive, protective film over the material. At high temperatures, this passive film may fail to protect the material by the following types or modes of corrosion: