Interphase precipitation of nanometer-sized carbides in a titanium-molybdenum-bearing low-carbon steel

The interphase-precipitated nanometer-sized carbides formed in a Ti-Mo-bearing steel after isothermal transformation in a temperature range of 630-720 °C for 30 min have been investigated by field-emission-gun transmission electron microscopy. The characteristic sheets of interphase precipitation were identified as planar and curved. The planar sheets of carbides have been analyzed and found to be oriented close to ferrite planes {2 1 1}, {2 1 0} and {1 1 1}; transmission electron microscopy results provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. Carbide size, sheet spacing and intercarbide spacing have been measured or estimated. The lower transformation temperature produces finer and denser carbides, and results in a higher hardness of ferrite. The contribution of the interphase precipitation hardening to the yield strength of the steel studied has been estimated, based on the Orowan mechanism.