M2C CARBIDE PRECIPITATION IN AF1410 STEEL

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Abstract
The multicomponent M2C carbides which produce secondary hardening in a high cobalt and nickel steel (AF1410 : Fe-14Co-10Ni-2Cr-1Mo- 0.16C - w%) have been studied using atom-probe/field-ion microscopy (AP/FIM). It is demonstrated that great care must be taken to establish the correct conditions for studying these carbides. The effect of temperature, pulse fraction and image gas pressure are presented. A pulse fraction of 0.20 is found to be necessary in order to obtain an accurate composition with a tip temperature of 80K. The presence of image gas lowers the apparent carbon composition of the carbides. The carbides in alloys which have been heat treated to produce good mechanical properties are coherent with the matrix. The measurement of the compositions of coherent carbides using the atom probe and of larger carbides by scanning transmission electron microscopy agrees well with thermochemical calculations of the expected composition trajectories as the carbide size increases

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