On carbide dissolution in an as-cast ASTM F-75 alloy

The solution treatment of an as-cast ASTM F-75 alloy was investigated. Micr ostructural evolution was followed during thermal processing, in particular with regard to the content and type of carbides formed. To evidence any pr obable carbide transformations occurring during the heating stage, as well as to clarify their effect on the carbide dissolution kinetics, three heati ng rates were studied. Image analysis and scanning electron microscopy tech niques were used for. microstructural characterization. For the identificat ion of precipitates, these were electrolytically extracted from the matrix and then analyzed by X-ray diffraction. It was found that the precipitates in the as-cast alloy were constituted by both a M23C6 carbide and a g inter metallic phase. The M23C6 carbide was the only phase identified in solution -treated specimens, regardless of the heating rate employed, which indicate d that this carbide dissolved directly into the matrix without being transf ormed first into an M6C carbide, as reported in the literature. It was foun d that the kinetics of dissolution for the M23C6 carbide decreased progress ively during the solution treatment, and that it was sensitive to the heati ng rate, decreasing whenever the latter was decreased. Because the M23C6 ca rbide was not observed to suffer a phase trans form ati on prior to its dis solution into the matrix, the effect of the heating rate was associated to the morphological change occurred as the specimens were heated. The occurre nce of the observed phases was analyzed with the aid of phase diagrams comp uted for the system Co-Cr-Mo-C. (C) 2001 John Wiley & Sons, Inc.