Identification of Carbides in Sub-Zero Treated Vanadis 6 Ledeburitic Steel

Sub-zero processing is an add-on step of heat treatment that includes cooling down, holding at low temperature and re-heating up to room temperature. The aim of this process is to improve some mechanical properties of high-carbon steels. In this research, the Cr-V ledeburitic steel Vanadis 6 produced by powder metallurgy was used. As-received material as well as that after heat treatment contains various types of carbides, which are different in terms of both the nature and the origin. During the austenitizing, a part of the carbides (eutectoid, secondary) undergo dissolution in the austenite while other carbides (eutectic, secondary) remain in the material undissolved. The application of sub-zero treatment leads to increase in the volume fraction of small globular carbides, with a size below 0.3 μm. The volume fraction of secondary carbides was found to be constant, irrespectively to the use of sub-zero processing, but it decreases with increase of the austenitizing temperature. The nature of the carbides was found to be practically unaffected by the sub-zero treatment and it can be described as follows: The eutectic carbides are the vanadium rich MC-phase and the secondary ones are the M7C3. Regarding the nature of small globular particles, their nature is not doubtless because the electron diffraction fixed them as alloyed cementite but the EDX analysis revealed very high chromium content in the particles.