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Selecting an Appropriate Heat-Resistant Alloy

Temperature limit is the first factor when choosing a heat-resistant alloy for a certain application. However, there are many more factors to consider for your application to succeed and to keep employees safe. An article from Industrialheating.com by Marc Glasser will give some good insight on heat-resistant alloy.

Oxidation

The first and foremost variable to consider is the oxidation limit of a particular alloy. A continuous layer of chromium oxide on the surface of austenitic alloys is responsible for promoting oxidation resistance. Silicon and aluminum, at high enough levels in an alloy, will allow the formation of subscales of silica or alumina, which will further enhance oxidation resistance. Finally, the addition of rare-earth and other heavy metals will add another level of oxidation resistance by adding an oxide that will bond to the other oxides to create a tighter, thinner, more adherent oxide that is harder to break. A thinner oxide scale is less prone to crack and spall than a thicker oxide.

Exposure to Other Atmospheres

In the heat-treating world, materials of construction can be exposed to other atmospheres, including carburizing, nitriding (and combinations of these two), vacuum, hydrogen, inert gas and more. In vacuum, and to a large extent inert-gas atmosphere, oxidation resistance is less important because the purpose of these atmospheres is to create an oxygen-free atmosphere. It should also be understood that products of combustion contain both carbon and nitrogen at high temperatures, which can lead to nitriding and carburizing. In commercial heat treating, carburizing and carbonitriding are generally performed in the temperature range of 1600-1750°F (871-954°C), while nitriding and ferritic nitrocarburizing are generally performed at 985-1050°F (530-565°C).

Creep and Rupture Strength

Tensile strength can no longer be used as a design parameter above 1000°F (538°C). Instead, two very important factors in deciding on a heat-resistant alloy are the ability of the alloy to resist sagging and breakage with an applied load at temperature. These two parameters are known as high temperature creep and rupture resistance, respectively. Simply stated, creep is the phenomenon of metal stretching from its own weight or from an applied load at an elevated temperature.

Embrittlement

High-chromium, low-nickel materials (stainless steels) change from ductile to brittle after anywhere from a few hundred to several thousand hours of service in the 1100-1600°F (593-871°C) range. This is due to the precipitation of a hard, brittle inter-metallic phase known as sigma phase. While sigma phase may not be harmful when the material is at temperature, it can completely embrittle the alloy at room temperature.

Thermal Cycling/Expansion

Thermal fatigue as it relates to heat-resistant alloys is cracking that occurs after repeated heating and cooling (quenching) of an alloy. Heat-resistant alloys have high coefficients of thermal expansion and low thermal conductivity. Simply stated, the metal surface heats and cools before the center does. During heating, the surface is expanding quicker than the center, which places strain on the center. Then during quenching, the surface is contracting faster than the center, placing more strain on the surface.

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