FeCrAl Resistance Heating Alloy-Scaling Resistance
FeCrAl alloy is used as catalytic substrate that is a metallic substrate offering various benefits as compare to ceramic substrates. A metallic substrate offers more resistance to shock and vibration, it offers higher conductance to heat rather than ceramic. Moreover,it can be manufactured easily in thin foil and offers honeycomb structure to render broad surface area and is light weighted.
Various FeCrAl resistance heating element alloys contain Yttrium that provides substantial oxidation resistance properties and retaining oxide layer, though Yttrium has some limitations. It is costly and loses weight while melting and pouring of iron alloys. Due to high reactivity of Yttrium, it reacts with other elements like oxygen and loses to slag and furnace refractory materials. Due to high reactivity of Yttrium, an expensive vacuum induction melting is utilized for fabrication of Iron-Chromium-Aluminum alloys that have Yttrium as a constituent. Moreover, while vacuum melting and casting, obtaining Yttrium in the alloy is generally less than 49.9% of that was included in the molten metal. Unfortunate delays, or issues that hinder the rapid pouring of melt, obtaining % may reduce significantly.
Furthermore, in fact vacuum induction heating method is not sufficient to receive significant content of Yttrium by remelting the scrap of FeCrAl alloys.
The catalytic system that consists of aluminum bearing ferrite steel acts as substrate, specifically a FeCrAl alloy that has Yttrium. FeCrAl resistance heating wire is exposed to possess the property of production of stable alumina film while the substrate surface heating that the alumina layer saves the steel from oxidation making it a resistance material.
To compensate the limitations of FeCrAl alloys containing Yttrium, other cheap alloying metals have replaced it. In U.S. Pat. No. 3,782,925, it has been stated that a FeCrAl alloy is a heat resistance material that contains silicon, titanium and other rare earth metals. FeCrAl alloys comprising of 10 to 15% chromium, 1 to 3.5% aluminum, 0.8 to 3% silicon, 0.01 to 0.5% calcium, cerium and other rare earth metals have scale adherence. The patent also needs a whole aluminum and silicon content from 2 to 5 %, titanium minimum 0.2% and combination of oxygen and nitrogen content minimum 0.05%.
An elevated temperature oxidation resistance FeCrAl alloys having small content of cerium offers long life of oxide coating. Such alloys contain cerium magnitudes from 0.01% to 0.37%. The FeCrAl alloy having lowest content of cerium of 0.01% gets spaling whereas FeCrAl alloy with cerium by .04% and 0.37% show no signs of spalling. Cerium is present in the latter alloys in the form of cerium-ferrous intermetallic compounds that is precipitated at the grain ends.
The FeCrAl resistance heating alloys that have cerium are called as electric resistance heating materials. There are about 5% inclusion of cerium and other materials and also 0.5% carbon and 0.05-0.5% nitrogen.
The major reason of manufacturing FeCrAl alloys is to enhance the oxidation resistance, scaling resistance and hardness at the high temperature more than 2102oF or 1150oC.