Inspecting rolled prods. for surface defects - where prods. are coated with indicator soln. and subjected to high frequency induction heating which reveals the defects

摘要

The surface of the rolled prod. is coated with a substance (A) forming a background for a second substance (B) which changes colour when heated. High frequency heating is then used so that defects on or near the surface of the prod. cause a thermochemical change in substance (B) altering its colour so the defects are clearly visible. When substance (B) is used as a soln., drying is pref. used for coating (B) at a temp. below that causing its colour to change. Substances (A,B) may alternatively be combined in an aq. colloidal suspension, contg. 30-160 g/l KOH in soln. and 5-20 g/l soluble potato starch. Used for inspection of ingots and slabs which still have a scaled surface due to hot rolling, or for examining profiles, e.g. rails or girders. The invention can be used without prior surface treatment of the rolled prods.