Abstract: The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with 3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needleshaped carbides/ferrite matrix at secondary hardening peak tempered at 482℃ for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are fully coherent with ferrite and have individual crystal structure, and the coarsened carbides with moir fringe are partially coherent with matrix. The orientation relationship between M2C and ferrite matrix is directly observed and identified with the well established P-S relationship [001]α //[ ]c.

Key words: Ultrahigh strength steel, Carbide precipitation, HRTEM