Thermodynamics and kinetics of gas and gas–solid reactions J T Slycke1, E J Mittemeijer2, M A J Somers3 Consultant, The Netherlands; 2Max Planck Institute for Intelligent Systems and Institute for Materials Science, University of Stuttgart, Stuttgart, Germany; Technical University of Denmark (DTU), Kongens Lyngby, Denmark 1.1 Introduction Thermochemical surface treatments of steel components are of great importance for industry; the mechanical (hardness, wear and fatigue resistances) and chemical (corrosion resistance) properties of the surface layer of treated parts can be improved considerably as a result of the chemical and structural changes induced by the processes in a surface region of the treated parts Today the most important of these processes are nitriding/nitrocarburising and carburising/carbonitriding, where interstitially dissolved nitrogen and/or carbon diffuse(s) into the surface of the component Nitriding and nitrocarburising of heat treatable steels are carried out at relatively low temperatures where the steel is essentially ferritic, whereas carburising and carbonitriding are carried out at higher temperatures where the steel is essentially austenitic and, after the treatment, usually experiences a desired martensitic transformation upon quenching Besides these nitriding and carburising processes and their variants, also boriding involves the dissolution of interstitials, in this case boron, into the host material and brings about a hard and wear-resistant surface layer onto the steel surfaces Especially the nitriding/nitrocarburising treatment, which in practice can be carried out according to quite a number of process variants (see Part Three: Nitriding, nitrocarburising and carburising), can be considered as the most versatile surface engineering treatment available today, since it enables, by tuned application of this process, improvement, to a great extent, of the fatigue, corrosion, wear and tribological performance of steel components Furthermore, as compared to processes that are based on a martensitic transformation, such as classical heat treatment of carbon and alloyed steels, very small dimensional changes of the component occur Nitriding/nitrocarburising is therefore the surface engineering process that is today most widely applied (see Mittemeijer, 2013) In order to obtain the desired properties, it is necessary to have (i) full process control and (ii) knowledge of the relationship between composition, temperature and pressure of the medium used in the thermochemical treatment and the resulting microstructure of the surface layer produced Although such process control, as determined by freely chosen, specific values of process parameters is perfectly feasible in the laboratory, in (commercial) practice, and particularly as regards nitriding/nitrocarburising, a largely phenomenological approach has been adopted Thermochemical Surface Engineering of Steels http://dx.doi.org/10.1533/9780857096524.1.3 Copyright © 2015 Elsevier Ltd All rights reserved Thermochemical Surface Engineering of Steels until now, i.e one has relied more on experience with individual equipments than on knowledge, steering and understanding of the effect of the fundamental process parameters The possibility of using some ‘probe’ in gaseous media, not only in carburising/carbonitriding but now also in nitriding/nitrocarburising, can change this situation fundamentally The use of the oxygen probe and other sensors in practical heat treatment for control of the nitriding/nitrocarburising process is discussed in Chapter and in Mittemeijer and Somers (1997) In this chapter thermodynamic and kinetic fundamentals are presented for gaseous processing, since the gaseous treatment allows the largest flexibility for controlled thermochemical surface engineering (i.e., in principle the chemical potentials of the components, in the gas atmosphere, can be set at specific values) More important, for basic understanding, is the recognition that gaseous processing can be understood on the basis of elementary chemical reactions in the gas and at the surface of the component Each reaction, whether this is an interchange reaction between different gas species (equilibria of gas exchange reactions are discussed in Section 1.2), or a mass transfer reaction between the gas phase and the solid iron-based substrate at its surface (equilibria of such gas–solid reactions are discussed in Section 1.3), proceeds at its own rate The kinetics of these reactions are discussed in Sections 1.4 (gas-exchange reactions) and 1.5 (gas–solid reactions) In many processing atmospheres more than one mass transferring reaction for a given species is possible Thus a ‘competition’ of the different mass transfer paths occurs, a situation that the researcher and the process engineer have to be aware of Practical heat treatment involves processing in closed chambers or reactors, through which the process gas flows The process gas only resides within the reactor for a finite period of time As a consequence, a given gas phase reaction may, or may not, reach (or closely approach) its equilibrium, depending on the reaction rates between the gas components A fast reaction, for example the water–gas exchange reaction (cf Section 1.2.3), may attain the corresponding equilibrium state in most cases, whereas a reaction that proceeds relatively slowly, for example the decomposition of ammonia, will generally not reach the corresponding equilibrium state (Section 1.4) Yet, and very remarkably, it is in particular this relatively slow progress of the ammonia decomposition reaction that provides the basis for several different industrially important surface engineering processes such as nitriding, nitrocarburising and carbonitriding Control of the process parameters is a required, but not a sufficient prerequisite to realise tuned thermochemical processing The relation between the process parameters and the composition and structure of the surface layer produced must be known as well (cf Section 1.6) On the one hand, it should be simply admitted that, as regards the low temperature processes of nitriding/nitrocarburising, insufficient data is available on layer-growth kinetics to be able to employ in practice model descriptions, as given in Somers (2011) and Woehrle et al (2013) On the other hand, the relations governing the thermodynamics of the equilibria of the (gaseous) media for nitriding/nitrocarburising as well as for carburising/ carbonitriding (gas-exchange reactions; see Section 1.2) and the gas–solid equilibria Thermodynamics and kinetics of gas and gas–solid reactions possibly (closely) realised at the surface of the solid substrate (see Section 1.3) are known The ‘nitridability’ and the ‘carburisability’ of a medium (here ‘outer gas atmosphere’), i.e the ability of that medium to nitride respectively carburise, are thermodynamically characterised by its chemical potential of nitrogen, and that of carbon, respectively Equilibrium with the component requires that the chemical potential concerned must be the same in both the process medium and in the workpiece at the surface Particularly in the initial stage of the treatment, considerable deviations from this equilibrium occur (for the example of nitriding, see the kinetic calculations and experimental data in Rozendaal et al., 1983; Friehling et al., 2001; and Stein et al., 2013) The chemical potentials are not only used to describe the equilibrium, but, although less well founded, also to describe the kinetics of processes involving the components (here nitrogen and carbon) concerned It can therefore be concluded that control of the chemical potentials of nitrogen and carbon is a prerequisite for controlled and targeted nitriding/nitrocarburising and carburising/carbonitriding The chemical potentials of nitrogen and carbon in the iron-based substrate correspond with so-called nitrogen and carbon activities, respectively, which, then as well, prescribe both the phase present and its composition (nitrogen and carbon sC-N phase diagram at 575°C, obtained by interpolation between experimental sections at 550 and 600°C (after Naumann and Langenscheid, 1965b) Several attempts to calculate the iron-rich corner of the Fe-C-N phase diagram have been published over the years These calculations were based on the sub-regular solution descriptions for the binary Fe-N and Fe-C systems and the incorporation of composition data for the Fe-C-N phases in compound layers (Slycke et al., 1988; Kunze, 1990, 1996; Du and Hillert, 1991; Du, 1994) presuming local equilibrium at phase boundaries and at the compound layer/diffusion zone interface These predicted Fe-C-N diagrams permitted the equilibrium coexistence of e-phase and a-phase at 575°C (848K), implying that no e + q+ g¢ three-phase region occurs The results of the modelling of the Fe-C-N diagram by Slycke et al (1988), Kunze (1990) and Du (1994) are shown for the e-phase boundaries with g¢, a and q phases in Figure 1.19 The large spread among the results of the various evaluations indicates the sensitivity of the calculations for the (experimental) input data and the assumptions made Experimental work by Nikolussi et al (2007) on annealing of nitrocarburised samples indicated that, at constant pressure of atm, an invariant transformation q‑Fe3C + g¢‑Fe4N ´ e‑Fe2N1‑z + a‑Fe would occur at a single temperature in the range 560–570°C Below the temperature of the invariant reaction, a three-phase equilibrium a + g¢ + q occurs; above this temperature the three-phase equilibrium a + e + g¢ occurs If this is the case indeed, a two-phase region a + e exists in the FeC-N system (see experimental results presented in Somers and Mittemeijer, 1987a, and Woehrle et al., 2012, 2013) Clearly, the thermodynamic understanding of the Fe-C-N phase diagram is incomplete Rather than representing the thermodynamics of the system Fe-C-N, the experimental data obtained can be influenced by kinetics in the nitrocarburising medium and/or in the solid 108 Thermochemical Surface Engineering of Steels The main reason for the current relatively poor description of the thermodynamics of Fe-C-N phases is thus the lack of data which with certainty represent a (local) equilibrium situation Many of the data used for the evaluation of the calculated Fe-C-N phase diagram, as in a CALPHAD procedure, may refer to a transition state, affected by kinetics, as can occur in a growing layer of carbonitrides during nitrocarburising In this respect, it is recalled that the experimental data in Figure 1.19, which were used for validating calculated Fe-C-N phase diagram calculations, were obtained from as-grown compound layers Moreover, the simplifying assumptions made in the evaluations of thermodynamic data in Slycke et al (1988), Kunze (1990) and Du (1994) are different, which can explain the large differences of the shape and size of, for example, the predicted e phase fields Thus, the g¢ and q phases were considered to be stoichiometric phases in Kunze (1990, 1996) It is questionable whether these assumptions are justified: the homogeneity ranges in terms of the N and C contents of a few Fe-C-N phases may well be narrow, but the corresponding chemical potential ranges wherein such phases are stable are far from narrow (see Section 1.6.2 and the isothermal width of the g¢ region in the Lehrer diagram in Figures 1.12(a) and (b)) Recent research on gaseous nitrocarburising has established with certainty that (i) definition of values of chemical potentials of both nitrogen and carbon in a gaseous nitrocarburising atmosphere is not a trivial task (Leineweber et al., 2012) and (ii) establishment of local equilibrium (or a stationary state) at the gas–solid interface can take a long time after the start of nitrocarburising (Somers and Mittemeijer, 1987a, Slycke et al (1988) Kunze (1990) Du (1994) Somers et al (1990) e+q C-content (at.%) e+a+q e e+a 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