SURFACE BLOWHOLES AND SUBCUTANEOUS POROSITY IN STEEL CASTINGS.

A study was made of two types of defects caused by gases; rounded surface blowholes with dark oxidized surfaces and subcutaneous porosity of the type known as honeycombing, in which the individual defects are elongated and have a bright unoxidized surface. Hydrogen porosity can be prevented above all by controlling the water content of the molds and the hydrogen content of the steel, by drying the charge materials, ore and fluxes and the alloying and deoxidizing additives, removing rust from the scrap, reaching melt-down rapidly and promoting a vigorous boil, and killing the steel fully with sufficient aluminum to give a residual content of at least 0. 03 percent. The molding sand should contain no more than 5 percent water and its permeability should be at least 120; the molds and cores should be generously vented.