Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding
Introduction
Plasma surface engineering gathers a group of different techniques oriented to modify a material surface. One of these techniques is plasma nitriding, especially when using a DC pulsed power supply. This is a plasma assisted thermochemical process, in which the workpiece acts as a cathode. The ion bombardment heats the workpiece, cleans the surface and provides active nitrogen. As a result, the hardness achieved on the surface is the highest and its value decreases with depth until the core hardness is reached. This surface modification improves its load bearing capacity. Ion nitriding has proved to be a suitable hardening process for stainless steels, improving wear resistance [1], [2], [3], [4]. Providing with a good control of process parameters such as temperature and process time, corrosion resistance of austenitic stainless steels can be unaltered and even improved due to the formation of the so called “S” phase, which is a nitrogen supersaturated expanded austenitic phase [5], [6].
In the case of martensitic stainless steels, however, corrosion resistance is still an issue, because of their low chromium content and the formation of chromium carbides, during heat treatment and chromium nitrides during nitriding. These steels should be nitrided after quenching and tempering, because mechanical properties on the bulk are as important as surface hardening. There are several reports in the literature for AISI 410 and 420 [7], [8], [9], and for precipitation hardening (PH) stainless steels [10], [11], [12] that present some promising results if a low temperature process (around 350 °C–380 °C) is applied. The PH stainless steels could be better candidates for plasma nitriding, because they do not need to go through the quenching and tempering treatment; they only need an aging treatment around 500 °C, to obtain their hardness and mechanical properties, based on the formation of fine precipitates. Besides, they have low carbon content and chromium carbides precipitation could be reduced. On the other hand, high chromium (17 wt.%) martensitic stainless steel could improve its hardness by nitriding and preserve its corrosion resistance as well.
In this work, three high chromium stainless steels designed for corrosive applications which can be hardened by thermal treatment to achieve high hardness and wear resistance were selected to be nitrided and study their corrosion resistance after the process. As they are recommended for corrosive applications, the aim of this work was to study the effect of the nitriding treatment on their corrosion resistance, particularly in relation to their microstructure.
Section snippets
Material and methods
Disc samples 6 mm in height and 24 mm in diameter were sliced from a bar stock for the three steels. Planar faces were ground and polished with successively finer emery paper and diamond paste, down to 1 µm; only one face was nitrided.
M340 is a martensitic stainless steel from Boehler is used in plastic nozzles and pistons, mold inserts, cutting tools and screws, where abrasive wear resistance is necessary. It has good hardenability and high hardness (550–600 HV) after quenching. N695 is also a
Surface aspect and CuSO4 spot
After plasma nitriding, the surfaces of the samples were slightly polished with diamond paste to remove any oxide formed during the cooling phase. It was proved that no change in hardness is produced by this procedure. After that, the first study of corrosion was the CuSO4 spot test. As already mentioned the presence of copper as a deposit on the surface indicates the presence of free iron, indicating incomplete passivation.
All heat treated samples, prior to nitriding, did not show
Discussion
The results of the glancing angle XRD analysis were consistent with several papers that reported that the white layer in nitrided martensitic steels is a stressed structure called “expanded martensite” [19], [20], [21], where nitrogen is present on interstitial sites of the bcc ferrite tetragonally distorted lattice (as it has been indicated above, “expanded ferrite” could be a more appropriate name for this structure). This layer provides high hardness, even if it is only 10–12 μm thick as it
Conclusion
Corrosion resistance of martensitic stainless steels M340 and N695 could not be sustained after nitriding treatment, even carried out at low temperature, 360 °C. Corrax samples showed a better behavior, but nitride samples showed not as good corrosion performance as the non treated ones.
In the case of martensitic stainless steels, more research has to be done to avoid chromium depletion even before plasma nitriding, in the heat treatment condition. In the case of the precipitation hardening
Acknowledgements
This work was financially supported by the National University of Technology (PID 25D/031), the firm IONAR S.A (Buenos Aires) and the National University of Mar del Plata.
The authors want to thank Uddeholm and Boehler for the materials provided for this study, also the collaboration of Eduardo González from Concepción del Uruguay, for the salt spray fog apparatus and tests; Eugenia Dalibón from UTN-FRCU for her collaboration in hardness measurements and XRD analysis, Lisandro Escalada from
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