Effect of Composition of Substrates and Constituent of Chromizing Powder on Properties of Chromized Layer Formed on Heat-resistant Alloys

Abstract

Effects of chemical composition of substrates and constituents of chromizing powder on the thickness and chromium concentration of chromized layer have been investigated. The thickness of chromized layer on austenitic alloy is determined by contents of carbon and carbide forming elements in the substrate. The thickness of chromized layer of Cr-Ni austenitic steel which contains more than 0.2% C is limited to a small value because chromium carbides formed on boundaries of chromized layer and the substrate become a barrier against chromium diffusion. But chromized layer of more than 100 μ is obtained on high carbon super alloys such as S590 and S816, primarily because these alloys contain large amounts of carbide forming elements.
A clear relationship is found between the thickness of the chromized layer on ferritic stainless steel and the chromium equivalent of the substrate steel. A remarkable increase of thickness of the layer is detected when chromium equivalent of the substrate steel exceeds 7.5. It has been already reported that addition of iron powder to Cr-Al₂O₃ chromizing powder gives a pronounced effect on growth of chromized layer of austenitic alloys, but the use of ferro-chromium power is not so effective as the addition of iron powder.