An investigation of accelerated graphitisation kinetics to improve the machinability of an experimental medium-carbon free-machining steel

Abstract

Recent studies have shown that the graphitisation process in a medium‑carbon steel during high temperature annealing can be accelerated by alloying; the key alloying additions are Si and Al. In addition, the nucleation, morphology and dispersion of graphite particles or nodules are observed to be affected by the pre-anneal microstructure, for example, either ferrite-pearlite, bainite or martensite, as well as the annealing time. The accelerated graphitisation kinetics and microstructural effects are revealed by light optical and electron microscopy, and in parallel with this, are also reflected by the hardness values as dictated by the normal tempering reactions and balance between the graphite and remaining carbides. Thus the extent of graphitisation is shown to influence the machinability of the experimental carbon steel. Indicative machinability was measured by tool wear during drill testing. The drill tests also indicated that optimisation of steel composition and heat treatment could potentially develop promising machinability comparable to existing commercial free-machining steel SAE 1117. Under the conditions of the drill tests conducted, graphitising from the starting bainitic microstructure also resulted in measured tool wear not too dissimilar to that of leaded free-machining steel SAE 12L14.