Characterization of precipitates in a 2.25Cr-1Mo-0.25V steel for large-scale cast-forged products

In this paper, the precipitates formed during the heat-treatment processes for 2.25Cr-1Mo-0.25V steels were investigated by using an analytical transmission electron microscope (A-TEM). The results show that the complex precipitates containing several microalloyed elements (Ti, Nb, V) are dominant when the specimens are re-austenitized at 980 deg. C and 1200 deg. C. When the austenitization temperature is increased, the size and the quantity of the precipitates decrease. It is worth noting that Nb and V still exist in the precipitates even when the austenitizing temperature is as high as 1200 deg. C. It indicates that the composition of the complex precipitates has become homogeneous during prior thermal processing. Some vanadium is also preserved in the core of the complex precipitates. For the specimen quenched from 980 deg. C and tempered at 650 deg. C for 30 h, numerous carbides (e.g.; M23C6, M2C and M7C3) are formed along grain boundary or in the matrix, in which elements such as Fe, Mn, Cr, Mo and V are found. Moreover, the prior precipitates have become coarse due to the extended tempering time