An analytical electron microscopy study of paraequilibrium cementite precipitation in ultra-high-strength steel

摘要

To support quantitative design of ultra-high-strength(UHS) secondary-hardeningsteels, the precipitation of cementite prior tothe precipitation of the M2C phase is investigatedusing a model alloy. The microstructure ofcementite is investigated by transmission electronmicroscopy (TEM) techniques. Consistent withearlier studies on tempering of Fe-C martensite,lattice imaging of cementite suggestsmicrosyntactic intergrowth of M5C2 (hagg carbide).The concentration of substitutional alloyingelements in cementite are quantified by high-resolution analytical electron microscopy (AEM)using extraction replica specimens. Quantificationof the substitution of the substitutional elementsin cementite confirms its paraequilibrium (PE)state with ferrite at the very early stage oftempering. The implications of these results arediscussed in terms of the thermodynamic drivingforce for nucleation of the primary-strengthening,coherent M2C carbide phase, the ferrite-cementitePE condition reduces the carbon concentration inthe ferrite matrix with a significant reduction ofM2C driving force. The kinetics of dissolution ofPE cementite and its transition to otherintermediate states will also influence thekinetics of secondary hardening behavior in UHS steels.