抄録
The induction heating inspection is a rather simple and robust system to detect automatically surface flaws of square billets in steels including non-magnetic ones. In this system the induced current goes around crevice cracks in the surface on the rapid RF induction heating and the immediate thermographical mapping reveals flaws as the lower-temperature streaks around the cracks.
The metallic bright scratches marked on conveyers are so low in emmisivity (ε) that they resemble the flaws as the lower-temperature streaks on the map. In order to make the emmisivity uniform all over the surface, authors had applied the spray coating with the resin powder on the billets prior to the induction heating and therewith succeeded in reducing such fake images.
In this paper authors optimized the spray coating with the acrylic resin powder on the square billets in an austenitic stainless steel. The coverage (the coated area/the bare area) increases linearly the emmisivity of the metallic bright surface and 90% is the minimum to equalize it with ε≅1. On the other hand, the coating retards the thermal diffusion through the layer proportionally to its thickness and 20μm is nearly the maximum to avoid the substantial delay in measuring the metal surface temperatures by a radiation thermometer. The highest S/N ratio on the flaw detection is obtained through the coating about 90% in coverage and about 20μm in thickness.
