Title

GRAPHITIZATION AS A POTENTIAL ROUTE TO IMPROVED MACHINABILITY OF CARBON STEELS

GRAPHITIZATION AS A POTENTIAL ROUTE TO IMPROVED MACHINABILITY OF CARBON STEELS

Authorship

EDMONDS, D.V.; HE, K.

EDMONDS, D.V.

I am this author

HE, K.

I am this author

DOI

10.5151/2594-5327-2005-14483-0286

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Abstract

Exchanging cementite for graphite in carbon steel could be an alternative route to improved machinability, and also result in better cold workability, than the customary practice of adding elements such as Pb, S, Te, Se and Bi, some of which impair cold forgeability or make recycling more difficult. However, the annealing times required for graphitization are generally too long for industrial processing of high volume products. In the present work, the graphitization process has been accelerated by alloying with Si and Al, and the microstructural evolution during graphitization treatment at 680°C monitored by microanalytical transmission electron microscopy (TEM). Electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM) have been used to reveal information on the development of graphite nodules and on the accompanying dissolution of the cementite phase. The overall graphitization process in the experimental steel was virtually complete within a time scale ~2-4 hours, indicating a promising acceleration of graphitization kinetics. A circumstantial link between dissolution of cementite and nucleation of graphite in the experimental steel was identified. Different graphitization kinetics, and hence different graphite dispersions, has also been detected between different starting microstructures, for example, between bainite and martensite, which it is proposed, provides additional, indirect evidence for the importance of the dissolving cementite phase in the transformation process.

Exchanging cementite for graphite in carbon steel could be an alternative route to improved machinability, and also result in better cold workability, than the customary practice of adding elements such as Pb, S, Te, Se and Bi, some of which impair cold forgeability or make recycling more difficult. However, the annealing times required for graphitization are generally too long for industrial processing of high volume products. In the present work, the graphitization process has been accelerated by alloying with Si and Al, and the microstructural evolution during graphitization treatment at 680°C monitored by microanalytical transmission electron microscopy (TEM). Electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM) have been used to reveal information on the development of graphite nodules and on the accompanying dissolution of the cementite phase. The overall graphitization process in the experimental steel was virtually complete within a time scale ~2-4 hours, indicating a promising acceleration of graphitization kinetics. A circumstantial link between dissolution of cementite and nucleation of graphite in the experimental steel was identified. Different graphitization kinetics, and hence different graphite dispersions, has also been detected between different starting microstructures, for example, between bainite and martensite, which it is proposed, provides additional, indirect evidence for the importance of the dissolving cementite phase in the transformation process.

Keywords

Carbon steel; Graphitization; EELS/EFTEM.

Carbon steel; Graphitization; EELS/EFTEM.