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The duplex stainless steel UNS S32205 was plasma nitrided at 380 °C for 10 h using a gas mixture of 25% N2-75% H2. The thickness of the nitrided layer was 4.5 ± 0.5 μm, composed mainly of nitrogen-expanded austenite and iron nitrides precipitates. There was an increase in surface hardness around 2.6 and 3.8 times in the nitrided layer formed on the austenite and the ferrite phases, respectively, in relation to the untreated samples. The surface texture parameters skewness (Ssk), maximum peak height (Sp) and texture aspect ratio (Str) were the most appropriate parameters for studying the topography changes after treatment. An improvement in the localized corrosion properties after the nitriding treatment was revealed by the cyclic polarization curves. The nitrided samples showed higher pitting corrosion and repassivation potentials compared to the untreated material. The ferritic phases and grain boundaries were more susceptible to corrosion in the nitrided samples. The potentiodynamic curves of the nitrided samples exhibited a hillside on the passive-to-transpassive transition. This feature was already observed by other researchers, but it has not been well investigated. Potentiostatic studies demonstrated that metastable pitting took place on this transition.

Duplex surface treatments involving plasma nitriding (PN) followed by laser hardening (LH) were performed in this work on grade 450 compacted graphite iron (CGI) to investigate the effects of PN duration (1 and 3 h) and LH power (400, 449, and 514 W) on microstructure evolution, nitrogen profile, and hardening depth of CGI. X-ray diffraction (XRD) phase analysis indicated a reduction in and partial decomposition of e and γ’ nitrides corresponding to an increase in laser power. The existence of a new nitrogen-rich austenitic phase—FeN0.076—was observed along with the identification of certain iron oxides. The proposed duplex treatment of CGI was found to facilitate the establishment of duplex layers with higher surface hardness (826–951 HK 0.05) and greater hardening depths (300–750 μm). Additionally, a reduction in the hardness gradient was observed between the surface and substrate. This affords a major advantage in applications involving operations on mechanical components.

The UNS S32750 super duplex stainless steel (SDSS) was low temperature plasma nitrided at 350 °C and 400 °C for 4 h. Pitting and crevice corrosion resistance was evaluated using electrochemical techniques in a NaCl neutral solution. Nitriding treatments resulted in increased in hardness, general corrosion resistance and in the passive layer breakdown potential. Regarding crevice corrosion tests, the nitrided surfaces showed lower current densities during the potentiostatic polarization and lower corrosion depths when compared to the untreated surface. The nitrogen played a fundamental role in the passivation behavior and in the susceptibility to repassivation of localized corrosion.

The properties of the “nitrogen expanded ferrite” or αN phase have been scarcely studied to date. This phase was obtained here on a super ferritic stainless steel (SFSS) by plasma immersion ion implantation (PIII), which is a suitable technique to produce near to surface modifications on the Fe Cr alloys due to the control of energy delivered to the surface. The UNS S44400 SFSS, a special alloy developed for extreme corrosion environments, was nitrided by PIII at temperatures from 300 °C to 400 °C resulting in the αN phase as the main product of nitriding on the modified layer. The surface hardness measured by nanoindentation was investigated in the modified layers, reaching up to 3.5 times (the 400 °C condition) the hardness value presented by the untreated sample. The nitrided surfaces also presented improved corrosion resistance in comparison to the untreated sample, inferred by potentiodynamic polarization tests in NaCl electrolyte. This was evidenced by increased corrosion and pitting potentials (Ecorr and Ep, respectively) and decreased corrosion and passive current densities (icorr and ip, respectively). Moreover, through of the analysis in the polarization curves of the nitrided samples, a greater stability of the passive layer and repassivation processes were identified. The best corrosion resistance was obtained for the sample nitrided at 300 °C, which presented the greatest reduction in icorr and ip (6.6 μA/cm2 and 43 μA/cm2, respectively) and increase in Ecorr and Ep (108 mV and 214 mV, respectively), when compared to the untreated sample. In both untreated and nitrided samples, the addition of Nb and Ti in this alloy to prevent intergranular corrosion led to the formation of N- Ti-Nb-Mo precipitates that acted as initiators of pitting corrosion.

This work presents the application of Solution Heat Treatment after Plasma Nitriding (SHTPN) followed by requenching and tempering to the AISI 409 stainless steel aiming to increase hardness with no compromise of corrosion resistance. The SHTPN process consists of applying solution treatment to a previously plasma nitrided surface, promoting the dissolution of nitrides and the formation of a nitrogen-rich layer, in which nitrogen is in solid solution. When applied to ferritic stainless steels, SHTPN allows the obtention of a high‑nitrogen martensitic layer. It is presented an evaluation of the effects of requenching (from 950 and 1050 °C) and tempering (at 250, 450 and 650 °C) processes on the microstructure, hardness, and corrosion resistance of the nitrided layer of samples previously treated by SHTPN. Corrosion resistance was assessed through cyclic potentiodynamic polarization tests carried out in NaCl 0.5 mol/L, according to ASTM G150 . Optical and scanning electron microscopy, wavelength dispersive spectroscopy, and X-ray diffraction were applied to microstructural and phase characterization. Surface hardening was evaluated by Vickers microhardness tests. Results have shown that the requenching of SHTPN samples promoted a deepening of the nitrided layer and microstructure refinement. The process with requenching from 1050 °C and tempering at 250 °C led to increasing both hardness and corrosion resistance. These results confirmed that nitrogen in solid solution increases the hardness and the potential for pit nucleation in the AISI 409 stainless steel.

Compacted graphite iron is an interesting material class for automotive applications. CGI-450 grade cast iron was subjected to various surface treatments: plasma nitriding (PN), laser hardening (LH), and PN followed by LH (PN+LH). The resulting tribological behaviors were evaluated by means of linear scratch testing under progressive loading (40–70 and 70–95 N). The PN+LH treatment yielded the lowest friction coefficient and shallowest scratching. In this case, the outermost layer was composed of oxide phases (Fe2O3 and Fe3O4), underlain by a compound layer (oxides + nitrides + martensite) and a high-resistance martensitic layer with accompanying nitrides and carbides. These underlying layers provided high load capacity for the external layers, allowing them to function as solid lubricants.

Austenitic stainless steels, although highly resistant to general corrosion, are susceptible to localized pitting or crevice corrosion. Corrosion resistance and mechanical properties could be enhanced by adding nitrogen to the alloy in solid solution. The methods currently in use involve fusion and solidification of the material, which may often be a deterrent, from the cost–benefit analysis stand point. In this article, the solution heat treatment after plasma nitriding (SHTPN) is presented as a new method to increase the surface nitrogen content in the solid state. The results obtained demonstrated the efficiency of this technique, by which the achieved thickness of the nitrided steel reaches up to 200 μm with 0.45 wt% of nitrogen.

The Response Surface Method (RSM) was employed to study the effect of the nitriding parameters on the corrosion resistance of a sintered steel of composition 3.8 Mn - 0.4 C - Fe (wt.%). Two atmospheres were used: 25% H 2 - 75% N 2 , and 1.25% CH 4 - 23.75% H 2 - 75% N 2 . For each atmosphere, samples were nitrided at 783, 813, and 843 K, during 7.2, 14.4, and 21.6 ks. Other significant parameters were left unchanged, such as pressure (400 Pa), and gas flow (4x10 -6 m 3 /s). The samples were characterized according to the thickness and phases present in the nitrided layer, by optical and scanning electronic microscopy (SEM), and X-ray diffraction (XRD). The microhardness profiles were also obtained. Finally, the corrosion rate of the samples was measured using the potentiodynamic polarization method. The contour plots of the resulting response surfaces are presented herein.