Hardness changes after a 1100°C annealing treatment applied to creep-resistant HfC-containing Co-based alloys for improving their machinability

Author(s): Patrice Berthod, Elodie Conrath

Mechanical strengthening by hafnium carbides is an innovative way for obtaining refractory alloys with superior creep resistance. The level of mechanical properties achieved at elevated temperatures is particularly high when the alloys are based on cobalt. Unfortunately the presence of HfC, notably in cobalt alloys which are rather hard compared with ironbased ferritic alloys and nickel-based austenitic ones, is prejudicial for the room temperature machinability of these HfC-strengthened Co-based superalloys. In this work, three cobalt-based superalloys containing hafnium carbides in several volume fractions, known for their exceptional resistance against high temperature creep, were elaborated and tested by Vickers indentation in their as-cast condition. In parallel two similar ternary cobalt-based alloys with the same contents in chromium and carbon were also elaborated and tested in hardness for specifying the effect of the presence of HfC carbides on the room temperature hardness. The alloys were also heated to 1100°C during 50 hours for modifying a little their carbides’ volume fractions and morphologies in order to see whether such modifications may decrease a little their hardness. The obtained results show that this heat-treatment induced only little microstructure evolution but which was sufficient to lose up to 30 Vickers points, which may improve moderately the machinability of the alloys.

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