Plasma Nitrided 316L Stainless Steel Research Articles

Performance evaluation of plasma nitrided 316L stainless steel during long term high temperature sodium exposure

Enhancement of wear resistance of components used in fast reactors is necessary for long service life of the components. Plasma nitriding is a promising surface modification technology to impart high hardness and improved wear resistance of various steel components. This study discusses the characterization of chrome nitrided SS316L casing ring used in secondary sodium pump of fast breeder reactor and its stability under long term sodium exposure. Microstructural and hardness analysis showed that stress relieved component could be chrome nitrided successfully to a thickness of about 100 μm. Assessment of in-sodium performance of the chrome nitrided casing ring subjected to long term exposure up to 5000h at 550°C, showed retention of chrome nitrided layer with a case depth almost similar to that before sodium exposure. A slight decrease in the hardness was observed due to prolonged high temperature sodium exposure. Tribological studies indicate very low coefficient of friction indicating the retention of good wear resistance of the coating even after long term sodium exposure.

Hemocompatibility of plasma nitrided 316L stainless steel: Effect of processing temperature

The influence of plasma nitriding temperature on the hemocompatibility of 316LSS was evaluated through in vitro thrombogenicity tests. The nitriding processes were carried out using an industrial furnace at a temperature range that does not impair the corrosion resistance. The crystallographic phases, topography, chemical composition, wettability, and thrombogenicity of the modified surfaces were evaluated and compared to untreated 316LSS samples. The results revealed the formation and evolution of the γN phase with increasing the temperature. The γN phase development was assessed with the stacking fault probability, which rose with temperature leading to rougher surfaces. Nitrided samples at 380 ˚C exhibited polar groups at the surface, increasing the surface energy. There was a substantial reduction of these groups in samples nitrided at 400 ˚C and almost negligible at 420 ˚C. Lower amounts and less activation of platelets was observed on the nitrided surface at the lowest temperature, contrary to the results obtained at the highest nitriding temperature. It was correlated with the hydrophobicity and roughness of the plasma nitrided surfaces. Through this investigation it is possible to propose adequate conditions of plasma nitriding to improve the hemocompatibility of 316LSS samples, reinforcing the feasibility of this processing technique for biomaterials on an industrial-scale.

Microstructural characterization of low temperature plasma-nitrided 316L stainless steel surface with prior severe shot peening

Surface nanocrystallization by severe deformation has proven beneficial as pre-treatment to plasma nitriding. It aids in achieving thicker nitride layers at lower temperatures thus making the process more economical. In austenitic stainless steels, severe deformation leads to formation of strain induced martensite on the surface while plasma nitriding alone forms expanded austenite. However, structural characteristics of surface layer of pre-deformed steel after plasma nitriding is still a matter of debate. In present study, 316L stainless steel was subjected to severe shot peening: followed by plasma nitriding at 400°C for 4h. Characteristics of sample surface before and after treatment were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that, this duplex treatment leads to formation of about 45μm thick nitride layer; without CrN precipitation. This is significantly high compared to reported data considering the temperature and duration of nitriding treatment employed. Selected area electron diffraction pattern from topmost surface confirmed the co-existence of austenite and martensite while subsurface layer was predominantly consisting of lath martensite. This indicates that major phase in the nitrided layer is martensitic in nature and nitrogen supersaturation leads to transformation of small fraction of martensite to expanded austenite.

Using of artificial neural network for the prediction of tribological properties of plasma nitrided 316L stainless steel

In the present study, wear rate of the 316L stainless steel was predicted by using artificial neural network (ANN). The influence of plasma nitriding on mechanical and tribological properties of 316L stainless steels were investigated using X-ray diffraction, microhardness tester, scanning electron microscopy and pin-on-disk tribotester. Plasma nitriding treatments were performed for 1, 4 and 8h in 50% H2–50% N2 gas mixture, at the temperatures of 350, 375, 400, 425, 450 and 500°C. The results show that CrN and γ'-Fe4N phases were observed at the higher temperatures than 400°C while s-phase formed at the temperatures under 400°C, Also, modeling results showed that the wear rate of the 316L stainless steel was predicted with high accuracy by ANN.

Structure, hydrophilicity and corrosion resistance of sol–gel derived Ag-containing TiO2 film on plasma nitrided 316L stainless steel

Pure and Ag-containing TiO2 films (Ag/Ti = 3.3 at.%) are coated on plasma nitrided 316L stainless steel by sol–gel method for biomedical applications. The addition of Ag does not cause obvious change in TG–DSC curves of the dried gels. The rough surface generated by plasma nitriding and the addition of Ag improve structural integrity of the TiO2 films. X-ray diffraction reveals N loss and oxidation of the nitride layer during calcination treatment, and peaks of Ag or its oxides are not detected. X-ray photoelectron spectroscopy analysis indicates that Ag presents as metallic state in the film. Water contact angles of the coating samples decrease with UV irradiation treatment. The potentiodynamic polarization tests in a Ca-free Hank’s balanced salt solution show that the TiO2 coated samples have decreased corrosion resistance due to N loss and oxidation of the nitride layer. The methods for crystallization of TiO2 gel layers with minimized or avoided structural changes of the nitride layer will be tried in order to improve corrosion resistance of the duplex treated 316L stainless steel.

Monitoring the near-surface pH to probe the role of nitrogen in corrosion behaviour of low-temperature plasma nitrided 316L stainless steel

Low temperature plasma nitriding of stainless steels improves tribological properties and pitting resistance, but it can activate anodic dissolution in acidic solutions. This work aimed at determining how anodic behaviour can be affected by nitrogen present in the steel. The electrochemical behaviour of AISI 316L steel after nitriding at 415°C (with up to 17at.% N) was examined in solutions of 0.1M Na2SO4+0.4M NaCl acidified down to pH 2.4. An antimony microelectrode was used to measure pH close to the sample surface. It was found that the nitrided layer was resistant to pitting corrosion at all pH's, but at pH below about 3.5 anodic currents were higher than those on untreated steel. For nitrided steel the near-surface pH was increasing when anodic current was rising in the active region, evidently due to binding of protons into NH4+. Nitrided steel underwent reactivation during reverse cathodic sweeps which is characteristic of low content of chromium oxide in surface film. Injection of NH4OH into the solution improved the passivation. Thermodynamic data indicated that the accompanying pH rise enabled the formation of Fe3O4, Fe2O3, Cr2O3, FeCr2O4, NiFe2O4, MoO42− and NO2−. It is suggested that the pitting resistance of nitrided steel results from an easy repassivation of incipient pits due to the formation of the above species.

Sol-gel derived TiO2 coating on plasma nitrided 316L stainless steel

Titania film is coated on plasma nitrided 316L stainless steel by sol-gel method. Crystallization of titania as well as N loss and formation of Fe2O3 occurs during the annealing heat treatment. The titania film has short cracks within the grooves of plasma etched grain boundaries. With the increase of annealing temperature and duration, surface hardness of the samples is increased, but the toughness is decreased due to oxidization of the surface layer. The coating sample heat treated at 350 °C for 10 min and 450 °C for 10 min has better corrosion resistance than the nitrided stainless steel tested by the potentiodynamic polarization in 0.9% NaCl solution. Water contact angle of the titania film on the rough nitrided steel substrate decreases with UV irradiation treatment, reaching 17° after 3 h treatment.

Microstructural characterization of pulsed plasma nitrided 316L stainless steel

Abstract Pulsed plasma nitriding (PPN) treatment is one of the new processes to improve the surface hardness and tribology behavior of austenitic stainless steels. Through low temperature treatment (

Corrosion performance of the plasma nitrided 316L stainless steel

The AISI 316L stainless steel has been widely used in artificial knee or hip joints, as well as internal fixation devices. It is well known that this material has a good corrosion resistance and acceptable biocompatibility properties. Ion nitriding is a well established process for steel hardening that can also be applied to this kind of steels with the aim of enhancing its hardness but without reducing its corrosion resistance. In this work, the effects of ion nitriding on the corrosion performance of a 3l6L stainless steel was evaluated in a 0.9% sodium chloride solution by using electrochemical tests such as potentiodynamic polarization and linear polarization in both nitrided and untreated AISI 316L steel condition. Surface characterization before and after corrosion testing was performed using scanning electron microscopy (SEM) with an energy dispersive X-ray analysis (EDS). It was shown that an ion nitriding treatment in a 25% N2–75% H2 atmosphere performed at a temperature of 410 °C improves the surface hardness of the AISI 316L stainless steel. However, under the experimental conditions carried out in this research, the nitrided steel is as prone to localized corrosion as the untreated one. It is considered that this behavior is mainly due to the presence of CrN, which precipitates during processing, contributing to the depletion of chromium from the adjacent matrix and leading to a galvanic corrosion mechanism.