The measles defect in CA-15 alloy steel castings

Abstract

An investigation was made to determine the effects of various mold variables on the surface condition of CA-15 alloy steel castings. This alloy often suffers from a peculiar defect known as measles, and an attempt was made to determine what reactions at the old-metal interface cause this defect. To this end, small cylindrical pins were rammed in sand molds and heated or melted in a small induction furnace. The type of sand and clay used were held constant while various atmospheres were provided in the mold flask. It was found that samples heated to 50°C below the melting point in an oxidizing atmosphere showed the characteristic measles defect encountered in casting CA-15 alloy in green sand molds. Samples melted, cooled to 50°C below the melting point, and held at that temperature in an oxidizing atmosphere showed a combination of the two defects. It was found that the measles defect is due to a liquid state reaction in which the chromium in the melt is oxidized to Cr₂O₃·Fe₂O₃ and Cr₂O₃·FeO. The mechanism was found to be akin to a pitting form of corrosion. The oxidizing atmosphere is provided by moisture in the molding material and carbon dioxide formed by the combustion of organic binders.