ALTERNATIVE METHODS FOR DETERMINING MICROCLEANLINESS: A STUDY BASED ON HOT WORK TOOL STEELS

This study focuses on the development and comparison of standard and alternative methods to evaluate the cleanliness of hot work tool steels used for dies on a microscopic level. The steels subjected to this study are hot working tool steels produced by an electric arc furnace (EAF) and ladle furnace (LF). After re-melting by an electric slag (ESR) or a vacuum arc (VAR) furnace the forged or rolled specimen were taken as subject for the investigation with different methods. The standard investigation method for non-metallic inclusions (NMI) is light optical microscopy (LOM) according to different standards such as ASTM E45, ISO 4967, DIN 50602, EN 10247, JIS G0555, … . Following alternative methods were investigated: Giga cycle fatigue (GCF) tests including SEM-EDS-investigations, automated scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), electrolytic extraction method and X-ray computed tomography (XCT) with a maximum resolution of 1 µm voxel size. For comparative results the specimens used for XCT experiments (diameter of approx. 2 mm and length of approx. 20 mm) were destructively investigated with automated LOM. Additionally samples from the same lots were investigates acc. to LOM-standards to give a comparison of ASTM E45, ISO 4967, DIN 50602, EN 10247, JIS G0555 to the above mentioned alternative Methods. The correlations of different methods are illustrated in diagrams. Comparative results for the different production routes show a similar size of NMI within each method. Each method has its advantages and disadvantages concerning additional information, e.g. time for preparation and measurement and costs.