Comparison of methods for characterising the steel cleanness in powder metallurgical high-speed steels

Powder metallurgical (PM) steels are characterised by a very high cleanness level. Single mesoscopic inclusions can nevertheless induce material failure under the extreme exposed stresses in the final product. Extensive knowledge about the cleanness of these steels is therefore essential. Various methods for inclusion analyses are available, providing different information about the non-metallic inclusion population present in the steel matrix. Several state-of-the-art methods of inclusion analysis are compared, considering morphological parameters and chemical composition of the detected inclusions as well as the required time effort and statistics behind the specific method. Tests were carried out with a standard PM steel HS6-5-3C. Combined with chemical extraction, automated SEM/EDS measurements enable a clear description of the microscopic cleanness level. Statistical analyses using the extreme value theory allowed the prediction of the maximum inclusion size in the investigated samples.