Alonizing
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Alonizing is a diffusion metallizing process in that it is a thermochemical treatment that involves enriching the surface layer of an object with one or more metallic elements.[1] Specifically, alonizing is the diffusion of aluminum into the surface of a base metal through high temperature vapors.[2] The types of metals that can be alonized include all types of wrought and cast steels (i.e. plain carbon and low alloy grades, ferritic and austenitic steels, highly alloyed nickel-chromium steels, etc.).[3] This process results in an alloy with the surface properties of aluminum while retaining the base metal's inherent strength and rigidity.[4] Therefore, alonizing does not change the high-temperature mechanical properties of the base metal, which is the advantage of alonizing over simply creating an aluminum alloy (aluminizing the surface versus aluminum throughout the alloy).[5]
The process consists of first placing the metal in an airtight vessel, called a retort. The metal is then encircled by a blend of aluminum powders.[6] The retort is then closed and placed in a pressure-controlled kiln. At the elevated temperatures (typically greater than 300 °C, usually between 700 and 1100 °C) of the kiln, the aluminum is in its liquid phase and diffuses into the surface of the metal and forms an alloy with the substrate.[3] This alloy usually contains a minimum of 20 percent aluminum. Conventional case depth for carbon and alloy steel is 0.0127-0.0508 centimeters, and for stainless and nickel base alloys is 0.00508-0.0254 centimeters.[7]
After cooling, the metal is removed from the retort and from the excess powder. At this point, additional secondary operations are performed as needed. This process ensures uniform alloy protection over the entire surface of the base metal.[5]