A high-resolution study of graphite nodule formation in experimental medium-carbon machining steel

Abstract

Alloying a medium-carbon steel has accelerated graphite formation during annealing, for example, during high-temperature tempering. Electron microscopy and related techniques have been used to characterise the graphite particle structure and dispersion from pre-anneal starting microstructures of ferrite-pearlite, bainite and martensite. Graphitisation in the solid state in steels (and also, for useful comparison, in cast irons) has previously received little attention. However, FIB/FEGSEM has permitted specimen preparation of the relatively coarse microstructures, thus enabling high-resolution observation previously limited for graphite formation in cast irons and steels. The study has importance to the potential development of more economic and user-friendly machining steels, which would rely upon internal lubrication by graphite nodules within the microstructure. Many recently developed advanced steels are reduced in carbon but expensively alloyed to produce the desired properties, whereas an aim in this project is simply to use carbon, which is a very cheap and abundant alloying element.