Keywords: ion-nitriding , low-temperature nitriding , iron nitride , surface hardening , hydrogen , crystal structure , nitrogen concentration , austenitic stainless steel. Published: received: July 02, Released: May 23, accepted: - [Advance Publication] Released: - corrected: -. Article overview. References Related articles 0. Figures 0. Information related to the author. Supplementary material 0. Result List. Previous article Next article.
Related articles. Share this page. Six samples for each process were produced. After nitriding, samples were cooled down to K by forced convection in the same gas mixture, and then cooled to room temperature under vaccum. Samples treated by both processes were longitudinally cut and mounted in resin. They were then prepared using the adequate metallographic process, and the morphology, thickness, and microhardness profiles of the resulting layers were evaluated. The morphology evaluation of the nitrided layer was carried out by means of optical microscopy OM and scanning electron microscopy SEM.
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Microhardness measurements Vickers indenter were obtained with the use of a microhardness tester Schimadzu model HMV 2. Quantitative measurements of the nitrogen concentrations were conducted for the solubilized samples SHTPN by means of wavelength dispersive spectrometry WDS microanalysis. Such measurements were made in cross-sections of the samples, from the surface to the core. Further details are described elsewhere 7.
The corrosion behavior and the effect of nitrogen in solid solution of treated samples were evaluated by electrochemical tests potentiodynamic anodic polarization test and open circuit corrosion potential — Ecorr vs time. Prior to electrochemical testing and after nitriding treatments, the samples were ultrasound cleaned in ethanol during one hour in order to eliminate any contaminant from handling, and then cleaned again with acetone.
Just before setting the corrosion test cell, samples were smoothly ground with mesh emery paper in order to standardize the initial passivation condition of the tested surfaces. For the electrochemical tests, a potentiostat-galvanostat IviumStat Ivium Technologies connected to a microcomputer was used. The electrolyte was a 0.
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Before the start of potentiodynamic tests, samples remained immersed in the electrolyte during one hour, until the corrosion potential was reached Ecorr vs time. The potential scan rate used for the potentiodynamic anodic polarization tests was 0. In order to check the effect the high temperature plasma nitriding process over localized corrosion resistance, a potentiodynamic anodic polarization test was conducted on a PN 1, sample, which is characterized by the presence of chromium-nitride precipitates see Reis, et al.
Thus, they won't be discussed in this work. Figure 1 shows the microstructure of a specimen solubilized after nitriding at 1, K. It can be seen from the picture that it's a precipitates-free structure. In order to verify the possible existence of chromium-based precipitates, a long lasting electrolytic etching was made onto the surface. As can be observed from Figure 1 , there are no signs of attack in the grain boundary regions. According to the values of the Tables 1 and 3 , considering the Equation 2, it appears that the PREN value was raised from A thin layer on the surface was formed on the surface of samples for all processing temperatures.
The obtained diffractograms are show in Figure 3. However, the observation of the sample micrograph Figure 2c , reveals that the area around grain boundaries at the surface layer region was attacked dark aspect. Even though no chromium compounds were identified by XRD, such attack at grain boundaries is consistent with the presence of chromium precipitates Cr x N y or Cr w C z in those areas.
Table 4 shows the thickness of the nitrided cases, measured in cross-sections of the samples. The case thickness increased with process temperature, with lower values than those reported by Gontijo et. These differences can be explained due to the lower diffusion coefficient and greater solubility of nitrogen because of the bigger molybdenum content in the ISO 14 steel.
Due to the small thickness of the formed layers, it wasn't possible to obtain the microhardness profile. So, the surface microhardness was measured see Table 4 , even though the layer thickness wouldn't be enough to meet the requirements of the ASTM standard Since the layer thickness affects the hardness and the measured values on the treated samples reflects the interaction between the layer hardness and part of the substrate, higher hardness values are obtained when the portion of the substrate, which contributes to the measurement, decreases with increasing thickness of the modified layer.
However, for the present work, it is considered that the portion contributing to this effect is smaller when the increased hardness of the material is taken into account. This hardness increase is caused by the solubilization of nitrogen in the S-phase and, for samples nitrided at K, by the presence of chromium compounds.
For samples nitrided at and K, the nitrogen concentration in the hardened layer can be estimated form Equation 1, provided that the hardness increase is caused by nitrogen in solid solution SS. These values, as well as the main results obtained from low temperature nitriding regarding the production of the S-phase layer area shown in Table 4.
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From the results obtained for the low temperature nitriding process, the condition that led to the formation of a thick, chromium precipitate-free S-phase layer was PN nitrided at K. So, this sample was chosen for the corrosion tests. Based on the steel chemical composition Table 1 , the measured nitrogen content Table 4 , and Equation 2, it can be stated that the low temperature plasma nitriding led to an increase in PREN, as well as the SHTPN process.
PREN changed from Again, there is in an indicator of the corrosion resistance improvement for this sample. Obtained results are shown in Figure 4. Except for the sample PN 1,, the results indicate that the open circuit potential is less noble at the beginning of the test, and increases with time, stabilizing in a more noble potential. This shows that the material is passivating in this medium, i.
Measured open circuit potentials show that there is a little variation among tested conditions — starting condition, low temperature nitriding, and SHTPN. The greatest potential was measured for the sample PN , which is consistent with the grater surface nitrogen concentration in solid solution for this condition. A major discrepancy on the results was verified for the sample nitrided at 1, K PN 1, , where the open circuit potential decreases to values far below the others. This decrease in the potential can be explained by the formation of chromium-nitrides precipitates in the compound layer see Figure 4 of the article Reis et.
The potentiodynamic anodic polarization test allow comparing the susceptibility to localized corrosion as a function of the nitrogen surface enrichment caused by the different treatments that the samples were submitted.
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The electrochemical behavior verified for the starting condition SC is consistent with materials that have a defined critical pitting corrosion potential Ec. This potential corresponds to the value where the breakdown of the passive layer occurs, and localized corrosion starts to happen. This potential is characterized by a sharp increase in the corrosion current density. When this reaction occurs at the sample surface, it is impossible to say if the increase in current density is due to passivity breakdown or the reaction itself, which limits the test to potentials of this magnitude.
Table 5 summarizes the results obtained in the potentiodynamic anodic polarizations tests. Differently from what was observed in the previous test Ecorr vs time , where a better behavior in terms of uniform corrosion was observed for the PN condition, it was impossible to distinguish the behavior in terms of localized corrosion critical pitting corrosion potential of the low temperature plasma nitriding PN and SHTPN conditions.
Both treatments resulted in significant improvement of the localized corrosion resistance, as observed by the increase in the critical pitting corrosion potential. The main difference between studied conditions is the thickness of the nitrogen-rich layer.
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For the SHTPN condition, this thickness is about times greater than those produced by low temperature plasma nitriding see Tables 3 and 4. Because of this significant difference, an increase in the lifespan of treated components submitted to wear is expected. Further investigations regarding the cavitation erosion resistance of samples treated under the same conditions of this work are being conducted.
As expected, the localized corrosion resistance of the samples nitrided at high temperature PN 1, decreased significantly, since the critical pitting corrosion potential dropped from about This reduction is due to the precipitation of chromium-nitride compounds, which reduces the chromium content in solid solution in the steel matrix close to the precipitates, impairing the localized corrosion resistance 7.