STUDY ON TIMKEN 16-25-6 TYPE HEAT-RESISTING ALLOY (III)

Change of the Properties during Tempering and Precipitates

Abstract

Change of the properties during tempering (0°-1000°C) of the solution-treated (1250°C×1h. W.C.) Timken 16-25-6 alloy were studied by measuremnt of hardness, differential dilatation, specific heat, electric resistance and microstructure.
By electron microstructural studies, X ray diffraction analysis and chemical analysis of electrolytically separated residues, the microconstituents of the aged (550°-900°C) Timken 16-25-6 alloy were studied.
The results were summerized as follows:
(1) Two Stages of contraction, heat evolution and decrease of electric resistance at 500°-550°C and 750°C-1000°C by the precipitation were observed during tempering of the solution-treated Timken alloy. The tbmperature range and the amount of changes of the latter was greater than that of the former. It seemed that at least there were two kinds of precipitates at 700°-1000°C and the transition temperature of these was at about 825°C.
(2) From the dilatation change, it seemed that the precipitate below 600°C dissolved once, and at above 700°C another precipitation occured, but the precipitate above 800°C existed with the precipitate below 800°C.
(3) The microconstituents of Timken 16-25-6 alloy were composed of double carbide M4C; {(Cr, Fe, Ni)₃Mo₁}C below 800°C and double nitride CrMoNx; {(Cr, Fe, Ni)₁Mo₁}N₁ as well as double carbide M₆C; {Cr, Fe, Ni)₅Mo₁} C above 800°C.
No evidence was found of the formation of sigma phase in this alloy.