Evaluation of microstructure and mechanical properties of nitrided steels

Abstract

The present investigation has been conducted in order to develop a rational approach, able to evaluate quite accurately the depth of a nitrided tool steel that should be removed previous to hard PVD coating deposition, and the dependence of such depth on the nitriding conditions, information that cannot be obtained easily by means of simple metallographic observations. The method thus proposed is based on the use of the hardness data corresponding to the hardness depth profile of the nitrided material, in conjunction with Fick's second law. The analytical procedure is described employing the experimental data obtained from a gas nitrided AISI H11 steel (X38CrMoV5.1), and has been validated by computing a rough estimate of the diffusivity parameters of nitrogen in the α-Fe matrix. In the particular case of the H11 steel, it has been determined that nitriding at a temperature of 580 °C for 8 h seems to have important advantages over the process conducted at 510 °C and 48 h, particularly in terms of the characteristics of the hardness profile obtained and the ability of the nitrided material to withstand indentation loads, a measure of its load-carrying capacity, as a feasible substrate for hard PVD coating deposition. The GDOS analysis that were conducted showed that within the first 8–25 μm of the sample depth, depending on the nitriding conditions, a layer of compounds with a high nitrogen content (7–19 wt.%) is formed, which is followed by a different layer with a lower nitrogen content, in the range of ∼ 2 wt.%, presumably constituted by a solid solution of nitrogen dissolved in the octahedral interstitial position of the α-Fe, together with a large volume fraction of sub-microscopic coherent and/or semi-coherent nitride precipitates, which explains the high hardness, in the range of ∼ 9–10 GPa observed. As expected, at a constant temperature, the extent of the compound layer was found to increase with the nitriding potential.