Effects of B and Ti addition and heat treatment temperature on graphitization behavior of Fe-0.55C-2.3Si steel

Abstract

We determined the effects of alloying elements and heat treatment temperature on the graphitization behavior of a medium-carbon high-silicon steel (Fe-0.55C-2.3Si, wt%). Trace amounts (<150 ppm) of B and Ti were individually added to base steel and graphitization heat treatment was applied to the three compositions (base steel, B-bearing steel, and Ti-bearing steel) at 700 and 750 °C. The grain size of the B-bearing steel was similar to that of the base steel, whereas that of the Ti-bearing steel was ∼37% lesser, which can be attributed to grain boundary pinning effects induced by TiN particles. For all three steels, cementite in pearlite decomposed during heat treatment and graphite formed mostly along the grain boundaries where carbon diffusion readily occurred. The graphitization rate follows the order of Ti-bearing steel > B-bearing steel > base steel, which is attributed to the numerous grain boundaries in the Ti-bearing steel and presence of BN particles in the B-bearing steel because the grain boundaries and BN particles act as major graphite nucleation sites. A higher abundance of nucleation sites for graphitization in the initial microstructure leads to higher graphite density but reduced average graphite size. For all three steels, higher heat treatment temperature promoted both cementite decomposition and graphite formation by accelerated carbon diffusion, which reduced the graphitization completion time, decreased the graphite number density, and increased the graphite size. Fine and uniformly distributed graphite forms within a short time (1 h) in the Ti-bearing steel treated at 750 °C, which significantly decreased its strength and improved its ductility.