Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 0 1 8 Fig. 5 — Schematic representation of the decomposition process; bainitic ferrite sheaves decompose into equiaxed ferrite grains, whereas prior austenite regions retain their initial shape and precipitate carbides internally. Fig. 4 — Microstructure after annealing for 6 h, the bainitic sheaves decomposed into new ferrite grains. Fig. 6 — (a) Coalescence of bainitic ferrite plates into thicker plates (6 h). (b) Fully annealed bainitic region (24 h). (c) Cemen- tite within prior γ region. Even though graphitized steels with improved machinability were pro- posed more than half a century ago, their industrial applications remained highly constrained due to their chemi- cal compositions with limited harden- ability. The graphite-forming tendency increases with the concentrations of Si and Al, but is counteracted by carbide forming elements such as Mn, Cr, and Mo, whereas the role of carbon con- centration is thought to be negligible. The majority of nanostructured bain- itic steels contain rather high concen- trations of Si and Al, often in excess of 1.5% [6] . This could make them suitable for graphitization heat treatments, were it not for the usually lengthy procedures of obtaining fully carbide-free lower bainitic microstructures. During annealing at the graphitiza- tion temperature, the morphology tran- sitions from lenticular bainitic sheaves toward a uniform distribution of very fine grains of ferrite with particles of cementite and graphite sit- uated at the prior sheave and grain boundaries, whereas the prior marten- site regions are marked by fine particles of cementite, as can be seen in Fig. 4. The annealed micro- structure after 6 h con- Fig. 3 — Hardness decreases during holding at the annealing temperature of 680°C.