M3C, M2X, MX, M7C3 and M23C6 Carbides in Alloy Steel

In the following, M stands for metallic elements and X represents the interstitial elements C or N.

M3C: Carbon is an interstitial solute in iron and hence has a much higher mobility than substitutional solutes or iron. It is natural, therefore, that iron carbides are the first to form when virgin martensite is tempered. In low and medium carbon steels containing dislocated martensite, cementite precipitates first in the tempering process. This cementite grows by a paraequilibrium mechanism; paraequilibrium is a state in which carbon achieves a uniform chemical potential across the interface, subject to the constraint that the substitutional-solute to iron atom ratio is maintained constant everywhere. The formation of paraequilibrium cementite during the tempering of martensite is considered to occur by a displacive mechanism.
The steps following the rapid precipitation of paraequilibrium cementite are complicated because the chemical composition of the cementite changes as it absorbs or rejects solutes in order to achieve its equilibrium composition. Mn and Cr can dissolve into cementite in large quantities. W, V, and Mo have limited solubility in cementite. The rate of enrichment will be fastest when the cementite particles are small and the ferrite is highly supersaturated in carbide-forming solute atoms. Cementite, although kinetically favoured, is less stable than many alloy carbides; consequently, while the cementite composition changes, alloy carbide precipitation commences and eventually leads to the dissolution of the cementite. These processes must all be considered to occur simultaneously in any model.

M2X: In many cases, M2X is the next phase to precipitate after cementite. It has a hexagonal structure and commonly precipitates as fine needles parallel to <100>alpha. The orientation relationship is that of Pitsch and Schrader:

{0001}M2X//{011}alpha and <11Z20<M2X//<100>alpha

M2X is generally considered to nucleate on matrix dislocations and martensite lath boundaries. Studies of an Fe-C-Mo alloy have shown that Mo2C can also nucleate on ferrite/cementite boundaries. The composition can vary widely with Mo, Cr and V soluble in significant quantities. In steels containing Mo, with no nitrogen and a low Cr content, M2X is often close to the ideal Mo2C composition.

MX: MX is a V- or W-rich carbide. It has an fcc (face-centred cubic) structure and commonly precipitates as fine disks on (100)alpha. The orientation relationship is that of Baker and Nutting:

{100}MX//{100}alpha and <001>MX//<011>alpha

MX particles form in a fine dispersion within the martensite laths, and it is therefore believed that they contribute significantly to hardening.

M7C3: M7C3 is a Cr-rich carbide with a trigonal crystal structure. Typical lattice parameters are a = 14.0 A and c = 4.5 A. Fe and Mn are also soluble in this phase. It usually occurs after M2C formation, or after cementite formation if there has not been any intermediate M2X precipitation. It is considered that M7C3 will only precipitate if the Cr content is sufficiently high compared to other alloying elements. If Mo is present, it is possible that M23C6, rather than M7C3, will form after M2C. Nucleation can occur either on fresh sites or in-situ at the ferrite/cementite interface.

M23C6M: M23C6 is also a Cr-rich carbide which may, in addition, contain W, Mo, V and Ni. It has a fcc crystal structure of which the typical lattice parameter is a = 10.7 A. It forms after either M7C3 or M2C and is often the equilibrium carbide in high-Cr (912 wt.%) steel. Nucleation occurs predominantly on prior austenite grain boundaries and lath boundaries.

M6C: In steels containing Mo and relatively low levels of Cr, M6C is the equilibrium carbide. It has an fcc crystal structure with a lattice parameter a = 11.0 A and is Mo-rich but may also contain Cr and V. M6C precipitates on the grain boundaries; therefore, it is thought that it nucleates separately from M2X, which precipitates in the laths. However, it is also reported that M6C nucleation can occur on M2X or M23C6 interphase boundaries. In addition, nucleation is possible on prior austenite grain and lath boundaries.

Shingo Yamasaki, Modelling Precipitation of Carbides in Martensitic Steels, University of Cambridge, Darwin College, PhD Thesis, 2004, pp. 5-6.

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