Raman spectroscopy study of the crystallinity of graphite formed in an experimental free-machining steel

Appropriate graphitisation of a carbon steel may provide an alternative route to developing more simply and inexpensively alloyed machining steels which are more recyclable than those which are currently available. The extent of crystallinity of graphite particles formed during a graphitisation anneal of an experimental carbon steel was studied by Raman spectroscopy. It is demonstrated that it was possible to record the progress of graphitisation with annealing time as well as to determine which starting microstructural conditions, either ferrite-pearlite, bainite or martensite, formed the most highly crystalline graphite particles during annealing. During a machining operation the graphite should act as an internal lubricant at the chip/tool interface and thus the extent of crystallinity may influence the final machinability of the steel, or effectively, optimisation of the heat treatment. It was revealed by the Raman spectra that the crystallinity of the graphite particles formed was strongly affected by the starting microstructures as well as the graphitising anneal periods. For a similar annealing time, a better degree of crystallinity was observed in graphite particles formed from a ferrite-pearlite starting microstructure rather than bainite and martensite starting microstructures. However, crystallinity could be gradually improved with increasing annealing time from all three of the starting microstructures examined.