ON FLUCTUATIONS IN THE CHEMICAL COMPOSITION OF THE COARSE PRIMARY M7C3 CARBIDES IN AISI D2 TOOL STEEL

Changes and fluctuations in the chemical composition of the coarse primary M7C3 carbides in tool steels have been one of the Authors’ main task of the studies. The purpose of this study was to supplement the revealed fluctuations in the composition of M7C3 carbides of D2 tool steel after a standard soft annealing and consecutive austenitizing this steel at 1150ºC in the air atmosphere. The study was performed on the decarburized layer, on the depth of about 0.4 mm from the surface. The hardened tool steel samples were soft annealed and then austenitized for the two periods of time: 30 minutes, and 90 minutes. The studies allowed to find the coarse primary M7C3 carbides differ significantly both after annealing and austenitizing. The difference in D2 tool steel relates to the contents of Cr, Mo, and V. The highest differences were found in the contents of chromium and molybdenum. The average concentration of chromium was found to rise successively with the growth of austenitizing time. After first 30 minutes of austenitizing the concentrations of molybdenum and vanadium are lower than these concentrations in the carbides of the annealed D2 tool steel. Prolongation of the austenitizing time to 90 minutes results in lowering the per cent fraction of Mo while that of V increased in the coarse carbides. That unusual behavior of these two elements was also observed in other Authors’ studies on other steels.