Effect of Nanometer-Sized Carbides and Grain Boundary Density on Performance of Fe-C-Mo-M(M=Nb, V or Ti)Fire Resistant Steels

Fe- C- Mo- M steels(where M is Nb, V or Ti, ~0.1%, and Mo ≤0.2%) were produced by thermal mechanical control processing(TMCP), and then their performance was characterized in terms of failure temperature by means of constant load tensile test while heating from ambient temperature up to 800 oC with a heating rate 28oC/min. The boundary misorientation of the steels after TMCP was examined by electron back scattered diffraction(EBSD), and the precipitates of MC type carbides were characterized by transmission electron microscopy(TEM). The results show that the addition of 0.2% Mo in FeC-Nab/V steels increases the failure temperature of steels by 40℃. It is believed that the low-angle grain boundary provided the favorable nucleation site for MC type carbides, which in turn will accelerate the kinetics of precipitation process. The fine and dispersed precipitates of MC type carbides induce significant precipitation strengthening for the steels during the constant load tensile process, thus resulting in higherfailure temperature. Among the tested steels, the failure temperature of Ti-Mo steel is the highest due to its highest low- angle grain boundary density which results in the fast precipitation of MC type carbides.The failure temperature of Nb-Mo steel comes the second and that of the V-Mo steels is the lowest because of its lowest low angle grain boundary density leading to the lowest density of precipitated MC type carbides.