Effect of M_3C on the Precipitation Behavior of M_(23)C_6 Phase during Early Stage of Tempering in T91 Ferrltic Steel

摘要

Tempered martensitic structure is the service condition of T91 ferritic steel after adopting the austenitizing followed by tempering. Needle-like M_3C particles are precipitated during air cooling after austenization, while the precipitation of M_3C is suppressed during the water cooling. The effect of existence of M_3C on the precipitation behaviors of M_(23)C_6 during the early stage of tempering, as nucleation site, number density and size distribution, was investigated by means of TEM observation. The TEM results indicate that, upon the same tempering time, the size of M_(23)C_6 is smaller and its number density is higher in the sample pre-existing M_3C than in the sample without M_3C. This can be explained that existence of M_3C results in more M_(23)C_6 precipitates forming inside of grain, where a relatively low self-diffusion coefficient of alloy element leads to M_(23)C_6 hardly coarsening. However, with the prolongation of tempering time, this effect becomes weaken. Microhardness results indicate that the existence of M_3C phase results in the increase of hardness after tempering due to the precipitation of finer and denser M_(23)C_6 particles.