A new method of inspection on non-metallic inclusions using hydrogen charge

  Giga-cycle fatigue damage, an important issue for martensitic high-strength materials such as a valve spring for reciprocating engines, is caused by large-sized nonmetallic inclusions inside the material (Al₂O₃ and SiO₂). Although materials with extremely high cleanliness have been developed in recent years, steel materials that are completely free of relatively large inclusions are not technically feasible and it is essential for both steel material suppliers and product designers to determine the size of large inclusions in steel materials.

  Conventional inclusion rating methods are the method of detecting inclusions in a sliced section of steel by using a microscope and the method of detecting inclusions by observing the fracture surface generated by high-cycle fatigue testing. The former has few chances to encounter large inclusions; the latter, although the largest inclusion in the test section of the material will be detected, requires a large amount of time and cost.

  “Rating Method for Nonmetallic Inclusions using Hydrogen Charge” proposed by Professor Yukitaka Murakami of Kyushu University, our technical advisor, gives us not only highly-reliable inclusion rating but also an effective inspection method for inclusions.

  This method uses technology patented by NSK Ltd. KMTL is the only laboratory in Japan which NSK has approved to use the method for nonmetallic inclusion rating.