TEM of the tempering reactions in 2.25 Cr 1Mo steel

Abstract

The precipitation of carbides in the ferritic steels, has been the subject of several investigations. This paper discusses the precipitation of carbides in 2.25Cr-1Mo steels. The sequence of changes that occur in the morphology, structure and composition of carbides as a function of tempering temperatures and carbon content, 0.06%C(A) and 0.11%C(B) has been examined in detail.The steels were austenitized at 1323K for 1h and quenched in flowing argon gas(AQ), followed by tempering in the range 823-1023K for varying durations of time. Structural and chemical information of carbides were obtained using electron diffraction and analytical microscopy respectively. The Cliff-Lorimer ratio method, incorporating theoretically calculated correction factors was employed in the analysis of spectra.Following AQ, both steels exhibited a mixture of bainite and polygonal ferrite. Fig.1 shows the presence of coarse distribution of carbides in bainite and relatively carbide-free ferrite matrix in steel B. The repartitioning of carbon takes place during AQ and transforms carbon rich regions into bainite. The inset in Fig.1 is the SAD pattern obtained from the bainite, the analysis of which confirms that the precipitates are Fe3C.