Investigative Methods for Determining Phases in High Speed Steels Both carbide studies and matrix studies were carried out for X-ray phase determination on the high speed steel S 6-5-2. The carbide phase was isolated chemically, the isolate yield being 10.5% by weight. The isolate was analysed and its phases investigated X-radiographically. The X-ray diagram showed that M6C carbide is to be considered the main constituent of the carbide. VC is present in small amounts. The two types of carbide in the cut surface could be distinguished metallographically by a potentiostatic etching procedure. Microprobe studies gave the composition of the two carbide types. The M6C carbide is essentially a Fe4(WMo)2C with slight V and Cr impurities in the iron group. The VC is a V-rich mixed carbide in which about 25% of Fe, Cr, Mo and W are dissolved. The proportionate amounts of M6C and VC in the isolate were determined by a balance calculation using the composition of the carbides and isolate analysis obtained with the microprobe. After corresponding conversions the results obtained were compared with the carbide-type constituents determined metallographically. The isolate was utilized as standard for X-ray determination of carbide. The calculation of the absorption coefficients necessary for X-ray deter- mination of carbides was carried out on the basis of the composition of carbide types for Cr and Mo radiation determined with the microprobe.