EVALUATION OF STEEL RING STANDARDS FOR MAGNETIC PARTICLE INSPECTION

The Ketos tool steel ring has become part of most magnetic particle standards in the United States. The rings are used to verify system performance and evaluate the sensitivity of magnetic particle materials. In 1985, two supposedly identical rings yielded a difference in the number of holes detected under identical test conditions. An industry survey of over 100 rings indicated that there were basically two kinds of rings: those with five-hole or nine-hole detectability. The discrepancy in results was determined to be caused by a difference in permeability as a result of variations in annealing. It was found that when a five-hole ring was annealed, it became a nine-hole ring. In an attempt to standardize the rings, an SAE Aeronautical Material Specification (AMS) was drafted in 1986. Subsequent revisions have been made to the document and its release was delayed while test rings were made from 52100 steel which was considered a better choice of material. Comparative tests between the Ketos and 52100 steel rings was conducted resulting in the author's decision to write the AMS document around normalized and annealed tool steel (Ketos). This paper describes the history of the Ketos ring and reports on previous test results from other investigators. It also reports on the comparative test results from an improperly annealed and properly annealed Ketos ring and a 52100 steel ring.