Microstructure and mechanical properties of nitrided steels
As shown by hardness, lattice parameter and electron microscopy observations, constant activity nitriding of iron-titanium alloys in the temperature range 400-650°C leads to the formation of mixed substitutional-interstitial clusters. A model for the zone structure of Fe-Ti alloys containing < 0.3 a/o Ti, deduced from internal friction and weight-change measurements, is proposed in which nitrogen atoms occupy four kinds of octahedral site, two of which give rise to internal friction. The distribution of nitrogen atoms between these sites depends on nitriding temperature, rate of quench, titanium content and the nitrogen potential. Low-temperature aging of nitrided Fe-Ti alloys leads to transfer of nitrogen between these octahedral sites with nitrogen atoms condensing onto the Ti-N zones formed at nitriding temperature. The transformations which take place are analogous to those in quench-aged nitrogen-ferrite except the stabilities of the "Fe-N" clusters and the a"-Fe16N2 are enhanced by the Ti-N clusters on which they form. High increases in strength result on nitriding but the product is brittle, with further strength increases occurring on aging at temperatures >5800C. These increases in strength on aging are correlated with changes in zone structure and distribution inferred from internal friction measurements and electron microscopy observations. The maximum strength increases are proportional to the square root of the titanium concentration and are highly temperature dependent, suggesting chemical strengthening as the major hardening mechanism in nitrided re-Ti alloys. A grain size analysis shows that clustering increases the temperature dependent component of the friction stress and also the Hall-fetch slope. Aging at high temperatures causes the substitutional-interstitial zones gradually to transform to the equilibrium precipitate, TiN, with deformation by glide dislocations changing from a cutting to a looping or Orowan mechanism with a corresponding increase in workhardening rate and reductions in friction stress and Hall- Petch slope. By cold-working Fe-Ti alloys prior to nitriding, large strength increases are produced and the product is more ductile than annealed and nitrided material. This is attributed to replacement of grain boundaries, onto which titanium nitride precipitates causing embrittlement, by a fine dislocation sub-cell structure. However, nitriding cold-worked iron-titanium alloys at temperatures where only recovery processes occur results in a brittle product, the origins of which are discussed. Although the mechanism of strengthening in annealed and nitrided alloys has been established the complex microstructure of cold-worked and nitrided material prevents detailed analysis of the strengthening process.