Cementite often is said to be metastable with respect to graphite but this represents the case where it is in contact with ferrite; when isolated, it is more stable than graphite. There are the grain boundary junctions and the boundaries themselves are not strongly facetted even though the crystal structure of cementite is anisotropic. Substitutional solutes affect the magnetic properties of cementite. The morphology of cementite when in steel is illustrated in the context of the other transformations, that there is nothing particularly unusual about the microstructure of polycrystalline cementite as a single phase. Other precipitates possible include copper or copper-rich particles that form within cementite during tempering. There are estimates using a combination of density functional theory and quantum Monte Carlo methods of the heat capacity of cementite. There has been discussion about whether it is the iron particles or the cementite particles that form subsequently, catalyse the carbon nanotubes.