What are Fe-Cr Alloys - Definition

Fe-Cr alloys have chromium as the main alloying element. Chromium increases hardness, strength, and corrosion resistance. Fe-Cr Alloys

In metallurgy, stainless steel is a steel alloy with at least 10.5% chromium with or without other alloying elements and a maximum of 1.2% carbon by mass. Stainless steels, also known as inox steels or inox from French inoxydable (inoxidizable), are steel alloys, which are very well known for their corrosion resistance, which increases with increasing chromium content. Corrosion resistance may also be enhanced by nickel and molybdenum additions. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 10.5% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties.

Alloying Agents in Fe-Cr Alloys

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or silicon for instance) greatly increases its mechanical strength. Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties.

Carbon. Carbon is a non-metallic element, which is an important alloying element in all ferrous metal based materials. Carbon is always present in metallic alloys, i.e. in all grades of stainless steel and heat resistant alloys. Carbon is a very strong austenitizer and increases the strength of steel. In fact, it is the principal hardening element and is essential to the formation of cementite, Fe₃C, pearlite, spheroidite, and iron-carbon martensite. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. If it is combined with chromium as a separate constituent (chromium carbide), it may have a detrimental effect on corrosion resistance by removing some of the chromium from solid solution in the alloy and, as a consequence, reducing the amount of chromium available to ensure corrosion resistance.
Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. The high-speed tool steels contain between 3 and 5% chromium. It is ordinarily used for applications of this nature in conjunction with molybdenum.
Nickel. Nickel is one of most common alloying elements. About 65% of nickel production is used in stainless steels. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate. Nickel based alloys (e.g. Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base elements for superalloys, which are are a group of nickel, iron–nickel and cobalt alloys used in jet engines. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.

FeCrAl Alloy

FeCrAl alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %). These alloys are known under the trademark Kanthal, which is a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications.

References:

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See above:
Stainless Steel

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