High-Resolution Metallography of a Coarse Microstructure: Graphite Formation in the Solid-State in Steel

Abstract

ABSTRACT:The equilibrium form of carbon in iron and steel is graphite. In higher carbon cast irons graphite is normally formed from the liquid state. In lower carbon steels carbon invariably exists in the form of metastable cementite, due to slow graphitisation kinetics in the solid, but in fact, graphite formation is also more likely to be prevented by alloying to stabilise the microstructure for service. Relatively few high-resolution studies of graphite formation have thus been made, in the past also because of probable difficulties associated with specimen preparation of a relatively coarse aggregate microstructure. More recently, new investigative techniques and methods have allowed closer examination of graphite formation in steels. Examples are given of HRTEM, EELS, ELNES, EFTEM and FIB/FEGSEM recently applied to observations of graphite nucleation and growth in the solid state in steel. This has importance, for example, in the potential development of more economic and user-friendly machining steels which would rely upon internal lubrication by graphite particles in the microstructure.

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

“…The present work complements this earlier work by focusing solely upon the behaviour of the graphite particles themselves during annealing. The approximate size of graphite particles formed after a graphitising anneal in each of the three starting microstructures was ≤ 20, ≤ 5 and ≤ 2 μm from ferrite-pearlite, bainite and martensite, respectively, as reported by Inam et al [8][9][10][11][12].…”

“…Figure 2 shows SEM micrographs typical of the starting microstructures, ferrite-pearlite, bainite, and martensite, respectively. These background microstructures in the same experimental carbon steel and their detailed transformation during the graphitisation annealing process, including the identification and formation of the graphite particles, have previously been studied in detail by light optical, and scanning and high resolution electron microscopy [5][6][7][8][9][10][11][12][13]. The present work complements this earlier work by focusing solely upon the behaviour of the graphite particles themselves during annealing.…”

“…[1][2][3][4]. A potential alternative route to improve carbon steel for machining that is being explored [1,[5][6][7][8][9][10][11][12][13] is to avoid extensive alloying by using carbon itself, already in the steel, and which is also a comparatively cheap element. This may be achievable by inducing carbon to form in its free state in the steel, as graphite, rather than in its more usual form which is combined with iron as a carbide (θ-Fe3C, cementite).…”

“…Currently we can consider two acknowledged methods of graphitisation: (1) Destabilisation of cementite by enhancing Si and Al alloying [5][6][7][8][9][10][11][12][13] and avoiding carbide stabilizing elements such as Mn and Cr [e.g. [15][16][17]; (2) Providing heterogeneous nucleation sites for graphite using carbides, nitrides and oxides [15], for example, Al2O3, AlN, BN, SiO2,TiC, ZrN, Nb(C,N), V(C,N), ZrC [18] and Mo2C [19].…”

Graphite Formation and Dissolution in Ductile Irons and Steels Having High Silicon Contents: Solid-State Transformations

“…Subcritical annealing of hypoeutectoid steel has gained renewed interest. The most active European group is led by Prof. Edmonds at Leeds University; see chosen examples in [21][22][23][24][25].…”