Hot hardness of (Fe, Cr)3C and (Fe, Cr)7C3 carbides

Abstract

Hot hardness was measured on the primary carbides, (Fe, Cr)3C and (Fe, Cr)7C3, in unidirectionally solidified iron-carbon-chromium hypereutectic alloys with chromium more than 4.8 wt %. The hardness-temperature relation was represented by two Ito-Shishokin formulae,Hv =A(— BT), and thus was drawn by two lines on a semilogarithmic graph. The inflection temperature where the two lines intersected was found at 730 to 860 K for (Fe, Cr)3C carbide containing 0 to 14 wt % Cr, increasing with an increase in the chromium concentration in the carbide, and at about 910 K for (Fe, Cr)7C3 carbide containing 36 to 76 wt % Cr. With increasing chromium concentration in each carbide, the hardness of the carbide increased and the thermal softening coefficients decreased. The effect of chromium on the hardness, the inflection temperature and the thermal softening coefficients was more pronounced for (Fe, Cr)3C carbide than for (Fe, Cr)7C3 carbide. Each of the thermal softening coefficients,B1(T Tt), the inflection temperature,Tt, room-temperature hardness,Hv(TRT), and the hardness atTt,Hv(Tt), related linearly to the chromium concentration in the carbides, and hence the hot hardness of the carbides could be expressed as functions of temperature and chromium concentration in the carbides. The relationships betweenHv(TRT) andHv(Tt) and between the thermal softening coefficient,B2, and the activation energy for creep,Qc(kJ mol−1), were represented by the following equations:Hv(Tt)≃0.7Hv(TRT),B2=1.26/Qc.