Graphitising

Fig. 11. Schematic models for the structure of A, graphitising carbons, and B, non-graphitising carbons [104].

Fig. 13. Increase in stack width parameter, L with heat treatment temperature, HTT, for some graphitising cokes, [Adapted from 1 12],...

Fig. 16. The mechanism of graphitisation (Reprinted from [116] by courtesy of Marcel Dckker Inc.

The question might be addressed as to whether the hemispheroidal structures observed here may provide an indication that complete toroids may be formed from graphite. The structures appear to form as a result of optimal graphitisation of two adjacent concentric graphene tubes, when further extension growth is no longer feasible (i.e., as in the case shown in Fig. 2 at b-b where a four-walled tube section must reduce to a two-waUed one.) This suggests that perfect toroids... 

Fig. 1. Chemical structure of the starling materials for low-temperature graphitisation (1) is 2-methyl-1,2 -naphthylketone mainly considered in this chapter.

Pitting of passive metals such as the stainless steels, aluminium alloys, etc., in the presence of specific ions, e.g. Ci ions Dezincification deaiuminification graphitisation... 

In principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

Cast iron behaves in a manner similar to steel at alkaline pH values but at low pH values it is subject to graphitisation. 

One of the outstanding properties of the austenitic irons is their resistance to graphitic corrosion or graphitisation . In some environments ferritic cast irons corrode in such a manner that the surface becomes covered with a layer of graphite. This compact graphite layer, being more noble than the matrix, markedly increases the rate of attack. The austenitic irons rarely form this... 

Another cause of growth which is of equal importance with graphitisation is the penetration of oxides into the metal along the graphite flakes. This presumably takes place because oxidising gases can be adsorbed on to the... 

All the remarks so far made have been concerned with conditions of cyclic reheating. When an alloy is held at a steady temperature above the critical range, some growth will arise from graphitisation, partly offset by the contraction involved in the ferrite-austenite transformation, but most of the growth will be due to oxide penetration. 

Work carried out by Gilbert on irons maintained at 500°C for 64 weeks (Fig. 7.15) has shown that in ordinary unalloyed flake irons graphitisation and oxidation cause roughly equal amounts of growth, and that as the carbon content increases the effect of oxidation becomes more important and the overall rate of growth increases. Nodular graphite irons grow very slowly under these conditions. 

Comments Heat treatment g 1000 C Oraphmsed 2600-3000- C As deposited at -2000°C Graphitised 3000 C... 

The characteristic mode of corrosion of some alloys may be the formation as a corrosion product of a redeposited layer of one of the alloy constituents, as in the case of the brasses that dezincify, or of a residue of one of the components, as in the case of the graphitic corrosion of cast iron. Particularly in the case of the dezincified brass, the adherent copper is not likely to be removed with the other corrosion products, and therefore the mass-loss determination will not disclose the total amount of brass that has been corroded. This is especially important because the copper layer has very little strength and ductility and the extent of weakening of the alloy will not be indicated by the mass loss. In these cases, also, the mass-loss determinations must be supplemented by, or replaced by, mechanical tests or metallographic examination, or both, to reveal the true extent of damage by corrosion. Difficulties in obtaining accurate mass losses of heavily graphitised specimens have been reported... 

Detailed consideration of the structure of many of the advanced and complex alloys which are of considerable technological importance (high-strength titanium alloys, nickel-base superalloys, etc.) is beyond the scope of this section, other than to point out that no new principles are involved. Certain titanium alloys, for example, exhibit a martensitic transformation, while many nickel-base superalloys are age hardening. Similarly, cast irons, although by no means advanced materials, are relatively complex they are considered in Section 1.3 where graphitisation is discussed. 

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