Growth of massive cementite layers; thermodynamic parameters and kinetics

Abstract

Massive, pure cementite layers were grown on ferrite substrates by nitrocarburising in a dedicated NH 3 / H 2 /CO/N 2 containing gas atmosphere at temperatures in the range of 783-843 K. From the parabolic layer-growth constants, an ''apparent'' activation energy for cementitelayer growth of 109 ± 12 kJ/mol was obtained. This ''apparent'' activation energy can be subdivided into a positive contribution due to the activation energy for (tracer) diffusion of carbon in cementite and a negative contribution due to the temperature dependence of the difference of the carbon activity in cementite at the surface and at the interface cementite/ferrite.

Nitrogen diffusion through cementite layers

“…Fig. 1) [8,10]. The cementite layers exhibit a characteristic microstructure (partially rough ferrite-cementite interface) and a In order to transform hardness-depth profiles into nitrogen concentration-depth profiles (cf.…”

“…The experiments were performed at T = 783 K, 823 K and 843 K, for treatment times of 0.5 h, 2 h, 6 h, 14 h, 24 h and 48 h, leading always to massive cementite layers on ferrite substrates [8,10] (cf. Fig.…”

“…In fact, the 783 K and 843 K specimens are among those used in Ref. [10] to analyse the cementite-layer growth kinetics at the corresponding temperatures.…”

“…All specimens (nitriding and nitrocarburising experiments) were embedded using a Struers LaboPress 3. Embedding was performed with 15 of NH 3 /H 2 /CO/N 2 is (hypothetically) infinite [8] although an "effective" carburising potential acts at the specimen surface [10]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 [21].…”

“…These cementite layers have been investigated in detail with respect to their homogeneous cementite-phase microstructure, with respect to conditions for occurrence of additional (carbo)nitride phases and the corresponding phase equilibria as well as with respect to selected mechanical properties [8,10,11,12,13,14].…”