Resumo: O reparo por soldagem de regiões erodidas por cavitação em rotores de turbinas hidráulicas é muitas vezes realizado utilizando como metal de adição o aço inoxidável austenítico AISI 309L. Ciclos térmicos provenientes dos sobrepasses, bem como a realização de tratamento térmico de alívio de tensão após a soldagem, induzem a formação de fase sigma na microestrutura do material depositado. Neste trabalho são apresentados resultados de ensaios de cavitação realizados conforme a norma ASTM G32-10 obtidos em depósitos de soldas do aço AISI 309L, com e sem fase sigma resultante de tratamentos térmicos. Os resultados do ensaio de cavitação revelaram que a fase sigma ocasionou a redução de 78% do período de incubação e o aumento de 30% da taxa de erosão máxima em comparação ao material sem esta fase. Infere-se que a redução do período de incubação estaria, a princípio, relacionada a diferenças de propriedades mecânicas entre a fase sigma e a matriz austenítica. Por outro lado, constatou-se que a taxa de erosão máxima seria resultante da presença da fase sigma a qual promoveu a redução da tenacidade do material

Abstract: Welding repair of cavitation damage in hydraulic turbine runners can be carried out using 309L austenitic stainless steel. Thermal cycles produced by the subsequent passes as well as the stress relief heat treatment after welding induce sigma phase precipitation in the weld microstructure. In this work results of cavitation tests performed according to the ASTM G32-10 standard obtained in weld deposits of AISI 309L steel, with and without sigma phase resulting from heat treatments, are presented. Cavitation test revealed that sigma phase caused a 78% reduction in the incubation period and a 30% increase in the maximum erosion rate compared to the material without this phase. It is inferred that the reduction in the incubation period would, in principle, be related to differences in mechanical properties between the sigma phase and the austenitic matrix. On the other hand, it was found that the maximum erosion rate would be the result of the presence of the sigma phase, which promoted a reduction in the toughness of the material

Formation of metastable C–, N–, or even Formation of metastable C–, N–, or even C/N– expanded phases can be observed for typical non-equilibrium conditions attained at the plasma assisted thermochemical treatments when temperatures relatively low are used. In present work, kinetics data are considered in a comparative study comprising low-temperature plasma assisted carburizing, nitriding and nitrocarburizing of AISI 420 martensitic stainless steel samples treated at 350, 400, and 450 °C, aiming to put in evidence the main metallurgical differences of the obtained layers. Microstructural characterization and hardness measurement results for untreated and treated sample surfaces show significant difference for the carburized layer growth in relation to that verified for the nitrided and nitrocarburized layers. While the carburized layer is constituted of a thin outer layer and a deep diffusion layer, just the opposite was observed for the other two treatments, id est., formation of thicker outer layers and thinner diffusion layers. Finally, carbide-/nitride-precipitation-free layers were supposedly obtained for samples carburized, nitrided, and nitrocarburized at 350 °C temperature.

This work presents the first results of the plasma nitriding study performed in pure niobium in order to increase its cavitation erosion resistance. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bars. Annealing treatment of the cold-worked niobium samples was carried out in vacuum furnace at 1.33 Pa pressure, in the temperature of 1000 °C, for a time of 60 min. Annealed samples showing hardness of 80 HV were cut to dimensions of 20 × 30 × 4 mm3. Nitriding treatment was conducted at 1080 °C, gas mixture of 90% N2 + 10% H2, flow rate of 5 × 10–6 Nm3s–1, and pressure of 1200 Pa (9 Torr), for a total time of 4 h comprised by two treatment steps of 2 h each. For comparison purpose, results for nitrided and non-nitrided niobium are confronted. Samples were characterized by XRD, nanoindentation, microhardness, SEM, and 2D surface topography and 3D interferometry profile analysis techniques. Cavitation testing was conducted according to ASTM G32-09. Comparatively, promising results based on the formation of niobium nitride phases in treated surfaces are presented and discussed in the present work.

Low-temperature plasma assisted nitriding is a very promising technique to improve surface mechanical properties of stainless steels, keeping unaltered or even improving their surface corrosion resistance. During treatment, nitrogen diffuses into the steel surface, increasing its hardness and wear resistance. In the present work the nitriding process of different martensitic stainless steels was studied. As-quenched AISI 410, 410NiMo, 416 and 420 stainless steel samples were plasma nitrided at 300, 350, 400, 450, and 500°C, for 4 h, at 3 Torr, in a gas mixture of 70% N2 + 20% H2 + 10% Ar, and flow rate of 3.33×10–6 Nm3s−1. The study of the nitrogen-rich layer thickness indicates two different activation energies, one for low (<400°C) and another one for high temperature ( ≥400°C), except for the AISI 410NiMo. The surface hardness increases with temperature for all steels between 300 and 450°C. For 500°C, except for the AISI 410NiMo, the surface hardness decreases if compared to that obtained at 450°C. This hardness decrease is related to chromium nitride precipitation, also associated to the treated surface sensitization. Finally, the CrN precipitation in treated surfaces is dependent on the steel composition, with the AISI 410NiMo steel presenting the lower average chromium nitride precipitation rate.

This work reports experimental results on sintered PIM 316L stainless steel low-temperature plasma nitriding. The effect of treatment temperature and time on process kinetics, microstructure and surface characteristics of the nitrided samples were investigated. Nitriding was carried out at temperatures of 350, 380, 410 and 440 ºC , and times of 4, 8 and 16 h, using a gas mixture composed by 60% N2 + 20% H2 + 20% Ar, at a gas flow rate of 5.00 × 10-6 Nm³s-1, and a pressure of 800 Pa. The treated samples were characterized by scanning electron microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma nitriding is a diffusion controlled process. The calculated activation energy for nitrided layer growth was 111.4 kJmol-1. Apparently precipitation-free layers were produced in this study. It was also observed that the higher the treatment temperature and time the higher is the obtained surface hardness. Hardness up to 1343 HV0.025 was verified for samples nitrided at 440 ºC. Finally, the characterization of the treated surface indicates the formation of cracks, which were observed in regions adjacent to the original pores after the treatment.

This work aims to characterize the phases, thickness, hardness and hardness profiles of the nitride layers formed on the CA-6NM martensitic stainless steel which was plasma nitrided in gas mixtures containing different nitrogen amounts. Nitriding was performed at 500 ºC temperature, and 532 Pa (4 Torr) pressure, for gas mixtures of 5% N2 + 95% H2, 10% N2 + 90% H2, and 20% N2 + 80% H2, and 2 hours nitriding time. A 6 hours nitriding time condition for gas mixture of 5% N2 + 95% H2 was also studied. Nitrided samples results were compared with non-nitrided condition. Thickness and microstructure of the nitrided layers were characterized by optical microscopy (OM), using Villela and Nital etchants, and the phases were identified by X-ray diffraction. Hardness profiles and hardness measured on surface steel were determined using Vickers hardness and nanoindentation tester, respectively. It was verified that nitrided layer produced in CA-6NM martensitc stainless steel is constituted of compound layer, being that formation of the diffusion zone was not observed for the studied conditions. The higher the nitrogen amounts in gas mixture the higher is the thickness of the nitrided layer and the probability to form different nitride phases, in the case γ'-Fe4N, ε-Fe2-3N and CrN phases. Intrinsic hardness of the nitrided layers produced in the CA-6NM stainless steel is about 12-14 GPa (~1200-1400 HV).

The acquirements and potentiality universe of cleaning, heating and/or sputtering effects caused by plasma species bombardment phenomenon on the surface characteristics and finishing of manufactured parts treated in DC abnormal glow discharge opens a new research and development field, called here of Plasma Assisted Parts' Manufacturing (PAP'M). The adequate control of the sputtering mechanism allows the production of different kinds of surface. Therefore, the design of rough or smooth surface, presenting a modified distribution of surface porosity and texturing could be idealized, in accordance with the desired surface characteristics, as the parts are simultaneously sintered and treated. The first part (out of two) of this work performed in hollow cathode discharge (HCD) presents the results of the surface morphology's changes in the pressed iron samples and in the internal surfaces of the external cathodes as a function of the sintering time and temperature. Potential applications include small diameters' cylindrical parts and components like axles, pins, pivots, and tubes and pipes presenting small internal diameters. Sputtering effects were quantified by means of mass loss and Ra and Rz roughness measurements and qualified by means of SEM. The overall work comprises the study of different parameters, including sintering time and temperature (Part I), and inter-cathode distance and pressure (Part II). Results indicate the sputtering mechanism in HCD sintering and surface texturing treatments could be adequately controlled as a function of the plasma parameters, comprising an important contribution to the development of green technologies for surface texturing, surface engineering, and part's manufacturing.

The acquirements and potentiality universe of cleaning, heating and/or sputtering effects caused by plasma species bombardment phenomenon on the surface characteristics and finishing of manufactured parts treated in DC abnormal glow discharge opens a new research and development field, called here of Plasma Assisted Parts' Manufacturing (PAP'M). The adequate control of the sputtering mechanism allows the obtainment of different kinds of surface. Therefore, the design of rough or smooth surface, presenting a modified distribution of surface porosity and texturing could be idealized, in accordance with the desired surface characteristics, as the parts are simultaneously sintered and treated. This is the second part (out of two) of the work performed in hollow cathode discharge (HCD) and it presents the first results of the surface morphology's changes in the pressed iron samples and in the internal surface of the external cathode as a function of the inter-cathode distance and gas pressure. Potential applications include small diameters' cylindrical parts and components like axles, pins, pivots, and tubes and pipes presenting small internal diameters. Sputtering effects were quantified by means of mass loss, and Ra and Rz roughness measurements and qualified by means of SEM. Results indicate the sputtering mechanism is highly dependent of the hollow cathode effect, in HCD sintering and surface texturing treatments, comprising an important contribution to the development of green technologies for surface texturing, surface engineering, and part's manufacturing.

The influence of a previous deposition treatment on the final amount of alloying elements (Cr and Ni) deposited and diffused into the surface of iron parts sintered in hollow cathode discharge (HCD) was studied. Cylindrical pure iron pressed samples, being a central cathode, were placed concentrically in the interior of an AISI 310 steel machine-made outer cathode, resulting in a 6 mm inter-cathode radial spacing. The study was divided in two steps: a) deposition treatment with the outer cathode acting as target and the iron sample acting as substrate (1123K -850 ºC- and 60 minutes deposition temperature and time, respectively); and b) deposition treatment plus HCD sintering (1423K -1150 ºC- and 60 minutes sintering temperature and time, respectively). The electrical discharge was generated using a pulsed voltage power source. The results indicate the presence of 6.5 at.% Cr and 6.9 at.% Ni on the samples surface. The concentration profiles were mathematically treated to quantify the actual amounts of Cr and Ni deposited on and diffused into the samples, and the integration of the fitted equations yielded the calculated areas of 133 (µm × at.% Cr) and 105 (µm × at.% Ni), respectively.