A study of reactions occurring during the carbonitriding process part II

Carbon and nitrogen effects on microstructure and kinetics associated with bainitic transformation in a low-alloyed steel

“…The total duration of stay at 900°C ranged between 5 and 6 h. The achieved control of C and/or N concentration in solid solution was based on a thermodynamic analysis of the gas/solid reaction, where the variables are the partial pressures of the gases in the atmosphere. Such method had already been developed for C enrichment and for N enrichment, but only for simple pure Fe or Fe-C systems [13,14] in the latter case. For N enrichment in !…”

Thermodynamics and kinetics of gas and gas–solid reactions

“…The consequence of this is that the ammonia decomposition on the furnace interior (often relatively large) non-iron surfaces tends to dominate over the decomposition on the surface of the iron(-based) load, particularly at high nitriding potentials. This presumption has been shown to be valid for carbonitriding conditions in a small sealed quench furnace of industrial design (Slycke and Ericsson, 1981a); see the example discussed in Section 1.4.6.…”

“…A more elaborate version of this model for assessment of the partial pressure of the residual ammonia content of the furnace atmosphere can be found in Slycke and Ericsson (1981a) and Slycke and Sproge (1992).…”

“…the formation of an adsorbed nitrogen molecule from two adsorbed nitrogen atoms, to be rate controlling. Lacking detailed observations for non-ferrous surfaces, it is assumed (Slycke and Ericsson, 1981a) that this reaction step is rate controlling also on typical surfaces available for the ammonia decomposition in furnace chambers. With reaction step (V) being the distinctly slowest, all other reaction steps are faster and are therefore treated as having established equilibrium.…”

“…The charge (or furnace load) indicated as 'small' consisted of one basket containing only the specimens (two iron foils for determination of the carbon and nitrogen potentials and one unalloyed steel cylinder (25 mm diameter and length 100 mm) for carbon and nitrogen depth profi le analyses); the charge indicated with 'normal' was a full charge for this furnace type (i.e., charge of mass approximately 100 kg) consisting of two baskets loaded with, besides the specimens as indicated for the 'small' load, also dummy steel components. C pot denotes the carbon potential, a property of the carburising atmosphere, defi ned as the mass percentage carbon taken up by a thin foil of pure iron in equilibrium with this atmosphere (see Section 1.3.3) (Slycke and Ericsson, 1981a). This approximate result for p NH 3 shows that the partial pressure of the residual ammonia content in the atmosphere increases with:…”