Handbook-Heat & Heat Treating

8 Flame-Priming and Flame Descaling Steel which is hot-rolled or cast, and then allowed to cool in air, is usually coated with a layer of scale. This is actually the black oxide of iron and sometimes it helps protect the steel surface from further oxidation. However, on castings which must be machined, or subject to critical inspection for cracks and other defects, this scale is a drawback. It can cause excessive wear on sharp cutting tools; it can hide defects. To remove it, many different methods are used. Flame-descaling is one. Scale is a rather poor conductor of heat. If the outside surface of the scale is heated rapidly by a row of high-velocity oxy-acetylene flames, the rapid expansion of the surface layer will cause the scale to pop loose at the junction between the scale and the steel. A layer of scale which is tightly bonded to the surface of rolled steel need not be removed before the steel is painted. However, scale absorbs moisture readily. To remove scale which is not tightly bonded, to drive all moisture out of the tightly bonded scale, and to burn off any other contaminating materials on the surface, the flame-priming process was developed. As in flame-descaling, a head which produces a row of closely-spaced, high- velocity oxy- acetylene flames is used, although the size of each flame (that is, the gas consumption rate) is less in flame- priming than in descaling. If the steel is wire-brushed immediately, to remove loose particles, and then painted while still warm, before the surface has cooled enough to pick up moisture from the air, the life of the paint job can often be doubled or tripled. For both flame-descaling and flame-priming, heads are made in several widths, with skids at each end of the head which permit the head to be dragged over the surface of the steel.