For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2-H2 gas mixtures, containing 5, 24, and 76 vol.% N2. Samples were characterized concerning the resulting roughness, microstructure, compound layer’s phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ’-Fe4N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ’-Fe4N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ’-Fe4N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N.

RESUMO Empresas fornecedoras de Zamac reciclam sucatas de peças técnicas provenientes do processo produtivo de zamac injetado sob pressão e revestido por eletrodeposição. No entanto, somente 35% da sucata enviada para beneficiamento é reciclada e retorna para o processo produtivo como matéria-prima. Levando em consideração que a matéria-prima é um dos principais custos das empresas, o reaproveitamento de sucata representa uma oportunidade de economia para o processo fabril. Com isso, o objetivo desse trabalho foi verificar a viabilidade de fundição de sucatas de peças técnicas como matéria-prima para a fabricação de peças para o mercado da moda. A sucata foi analisada quanto à composição química, microestrutura e características térmicas. A sucata foi fundida sob pressão para produção de componentes para calçados e acessórios (elementos estéticos como fivelas e enfeites). Para comparação, enfeites e fivelas foram obtidos com matéria-prima virgem. Os enfeites e fivelas foram analisados quanto à microestrutura, resistência à corrosão e propriedades mecânicas. As amostras fundidas com uso de sucata responderam de forma satisfatória a todos os ensaios e, em alguns casos, de maneira superior às peças de controle, que foram fundidas com matéria-prima virgem. Assim, constatou-se que a sucata oriunda de peças técnicas pode ser usada como matéria-prima para produção de peças e acessórios para o setor de moda e, neste contexto, representa uma possibilidade de redução de custo de fabricação.

ABSTRACT Zamak supplier companies recycle scraps of technical parts from the production process of zamak injected under pressure and coated with electrodeposition. However, only 35% of the scrap sent for processing is recycled and returns to the production process as raw material. Taking into account that raw material is one of the main costs of companies, the reuse of scrap represents an opportunity to save the manufacturing process. With that, the objective of this work was to verify the viability of casting scraps of technical parts as raw material for the manufacture of parts for the fashion market. The scrap was analyzed for chemical composition, microstructure and thermal characteristics. The scrap was melted under pressure to produce components for shoes and accessories (aesthetic elements such as buckles and ornaments). For comparison, ornaments and buckles were obtained from virgin raw material. The ornaments and buckles were analyzed for microstructure, corrosion resistance and mechanical properties. Samples cast using scrap responded satisfactorily to all tests and, in some cases, better than control parts, which were cast with virgin raw material. Thus, it was found that scrap from technical parts can be used as raw material for the production of parts and accessories for the fashion sector and, in this context, it represents a possibility of reducing manufacturing costs.

Abstract Plasma nitriding treatments are applied to achieve excellent surface properties on steel parts and tools. Active screen plasma nitriding (ASPN) does not have the defects presents in direct nitriding plasma treatments (DCPN). However, ASPN require much longer processing times to develop surface layers as deep as those of DCPN. This work presents the development of a biased ASPN treatment system, to investigate the possibility of achieving greater efficiency. The treatments were performed on ASTM M2 steel samples using the same processing parameters in 4 different configurations: DCPN, ASPN and 2 active screen plasma nitriding with biased specimens voltage: one was called "bright bias", because biased voltage is high enough to develop a plasma glow on the samples, and the other "dark bias", where no plasma glow sheath is seen on the samples. The active screen dark biased treatment presented the best results. These results could be understood by the electromagnetic field generated by the bias source promoting the deposition of nitrogen ions on the surface of the part being treated. Therefore, it was demonstrated that the use of dark bias voltage in the active screen treatment in larger reactors is fundamental to achieve good nitriding results in smaller times.

Abstract Thermo-mechanical treatments using continuous cooling after forging are an established method for producing bainitic steels, mainly because of the elimination of energy intensive additional heat treatment processes. The cooling is usually employed in an uncontrolled manner in the industrial sector, which can be detrimental to the resulting microstructural morphology and, consequently, to the final product properties. In this study, a new controlled two-step cooling route based on the principles of bainitic displacive growth was designed and applied in a 0.18C (wt-%) steel. Inverse finite element method was used on the cooling data to obtain the evolution of temperatures for the samples during cooling, allowing to assess point to point cooling rates. Investigations via X-ray diffraction, optical microscopy analysis and hardness testing revealed a variation of bainitic morphology, namely, the transition from granular bainite to lath-like bainite with relatively high hardness and constituents/phase boundaries than the pre-treated microstructure.

The aim of this paper was to determine the influence of machining parameters on the surface layer, and consequently on the electrochemical properties, of ASTM F138 austenitic stainless steels. Eight different surface conditions were generated in cylindrical samples by turning and polishing. The surface was characterized by surface roughness Ra and Rt, Vickers microhardness and microstructural characteristics. Cyclic voltammetric tests were performed to evaluate the potential of repassivation and pitting. The surfaces were analyzed by Scanning Electron Microscopy after the electrochemical tests were performed. The cutting parameters were identified to have a significant influence, and the feed rate was as influential as the depth of cut. The use of polishing alone is not sufficient to ensure that a surface has good surface integrity. Even with the polishing application, usually used to produce prostheses, a significant reduction in corrosion resistance occurs if care is not taken to minimize surface machining damage. Corrosion resistance is lower and pitting nucleation is higher in the samples with no polishing and with more severe machining parameters. The tests indicated that softer machining parameters can contribute to an increase in the life of implantable components, reducing the probability of localized corrosion, as well as improving the mechanical properties of prostheses.

Abstract Due to a combination of advantages, high-performance steel components, especially for automotive manufacturing applications, are generally forged parts. In the forging industry, bainitic steels are being increasingly used, because they can reduce the processing chain and energy consumption. In this case, the bainitic microstructure can be obtained immediately after forging, with controlled cooling, and without any subsequent heat treatment. In the present work, the effect of thermomechanical routes performed in the austenitic and bainitic fields on the final microstructure and final hardness of 18MnCrSiMo6-4 bainitic steel has been discussed. Thermomechanical processing routes were tested and evaluated in a Gleeble® 3800 testing machine with one and two-step deformation. In both cases, the sample had its height reduced by 40% and the strain rate used was 0.1s-1. It could be shown that the plastic deformation promoted in the bainite field induces the bainite transformation. The results also show a strong dependence of bainite morphology concerning the deformation temperature of the steel. Moreover, the knowledge of the hot and warm stress-strain curves is an important result because it allows estimating the necessary stress and the energy consumption per volume unit to deform the material.

In this study, the sliding wear of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel plasma nitrided with a nitrogen rich gas composition was investigated. To evaluate the influence of processing time and temperature on mechanical and microstructural characteristics of nitrided layer, the samples were nitrided at 400, 450, 500 and 550 °C for 3, 6 and 9 h. The produced nitrided layers were characterized concerning the microstructure, phase composition, microhardness and surface roughness. The samples were tested by ball-on-flat reciprocating dry sliding for friction coefficient and wear analysis. The tests were stopped after a given damage criteria involving the rapid growth of the friction coefficients and wear. The correlation of the different treatment parameters and resulting case depths and surface hardness with sliding distance at the time of microcracks formation or delamination of the surface layer was evaluated statistically by the analysis of variance (ANOVA). The plasma nitrided samples at 550 °C showed better wear performances in the ball-on-flat tests than the other groups investigated, since these samples have a thicker compound layer and diffusion zone higher than the other conditions investigated. In general, the beginning wear is slower because of the hardest region of the compound layer.

Abstract The use of continuous cooling bainitic steels can provide a more energy efficient manufacturing route. However, for their use in mechanical components like gears, it is necessary to improve their surface properties without impacting the core properties to guarantee reliable mechanical performance. The effect of temperature and time on the plasma nitriding response of a DIN 18MnCrSiMo6-4 steel was investigated. The plasma nitriding was performed for 3, 6 and 9 hours, at 400, 450, 500 and 550 °C, using a gas mixture composed of 76 vol.% nitrogen and 24 vol.% hydrogen. Samples were characterized before and after plasma nitriding concerning the microstructure, hardness and microhardness, fracture toughness, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different temperatures was determined. Moreover, it could be found that 500 °C gave the best results investigated here, as higher temperature took to core and surface hardness decrease. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples. The diffusion zone presented compressive residual stresses with highest values near the surface.

Abstract It is well recognized the importance of the rheological characterization for the development of the steel in thermomechanical treatments, especially for the mechanical properties improvement of bainitic steels in subsequent hot forging optimization. Therefore, the plastic strain behaviour of a low carbon high silicon bainitic steel was studied through isothermal compression tests using a thermomechanical simulator at temperatures of 1123 K – 1423 K and strain rates of 0.1 – 5 s-1. Arrhenius equation was used to obtain the constitutive constants, which represents the material behaviour of flow stress in high temperature. Besides, work hardening, dynamic recovery, and the JMAK model in the dynamic recrystallization (DRX) of the steel parameters were determined. The second part of this research compared two proposed modified models from the literature, which showed the differences in modelled flow curves behaviour when they are applied for high strain levels. The flow curves were modelled in high strain levels for further implementation in numerical simulation, thus allowing an adjustment of parameters in hot forming processes for this bainitic steel. The proposed models presented an agreement with experimental values. However, only the Avrami equation to DRX showed the dynamic recovery mechanism in high strain levels, which has represented physical behaviour during the thermomechanical process.

RESUMO Os novos aços bainíticos de resfriamento contínuo (CCBS) são uma excelente alternativa quando se requer elevada dureza, tenacidade e resistência à fadiga. Contudo, estima-se que a taxa de desgaste dos CCBS seja insuficiente para aplicação em componentes com alta solicitação mecânica, sendo necessário aprimorar suas propriedades de superfície. O objetivo deste estudo é o de analisar os efeitos da nitretação a plasma sobre à microestrutura e propriedades de desgaste do aço bainítico de resfriamento contínuo DIN18MnCrSiMo6-4. Portanto, tratamentos com duração de 6 horas foram conduzidos com pressão de 3 mbar, mistura gasosa composta por 76% N2 + 24% H2, e temperaturas de 400, 450, 500 e 550 °C, respectivamente. As amostras foram analisadas através de microscopia eletrônica de varredura, difração de raios-X, microdureza e ensaios de desgaste por deslizamento recíproco. Nas condições investigadas, foi possível obter uma camada de compostos e atingir um aumento de até 393% na dureza superficial, entretanto, o uso de temperaturas mais elevadas durante os tratamentos favoreceu a formação de camadas mais profundas. Em todas as amostras nitretadas, constatou-se a formação predominante da fase de nitretos ε-Fe2-3N e, em menor quantidade, da fase de nitretos γ’-Fe4N. Nos ensaios de deslizamento recíproco houve diminuição do desgaste das amostras nitretadas na temperatura de 550°C.

ABSTRACT The new continuous cooling bainitic steels (CCBS) are an excellent alternative when high hardness, toughness and fatigue resistance are required. However, it is estimated that the wear rate of the CCBS is insufficient for application in highly mechanical loaded components, being necessary to improve the surface properties. The objective of this study is to analyze the effects of plasma nitriding on the microstructure and wear properties of DIN18MnCrSiMo6-4 continuous cooling bainitic steel. Therefore, treatments lasting 6 hours were conducted with a pressure of 3 mbar, a gas mixture composed of 76% N2 + 24% H2, and a temperature of 400, 450, 500 and 550 °C, respectively. The samples were analyzed by scanning electron microscopy, X-ray diffraction, microhardness and reciprocal sliding wear tests. Under the conditions investigated, it was possible to obtain a compound layer and to achieve an increase of up to 393% in surface hardness, however, the use of higher processing temperatures favored the extension of the depth layer. In all the nitrided samples, the major presence of ε-Fe2-3N nitrides phase was observed, and the γ'-Fe4N phase content was smaller. In the reciprocal sliding tests, there was a decrease in the wear of nitrided samples at temperature of 550 °C.

RESUMO A correta seleção dos óleos lubrificantes e das superfícies de contato melhoram o desempenho e reduzem o consumo de energia das máquinas Por isso, a engenharia de superfícies propõe novos revestimentos aos quais seu comportamento tribológico combinado com o uso de óleos biodegradáveis em componentes de sistema hidráulicos, não são conhecidos. O objetivo deste trabalho foi avaliar o comportamento do óleo mineral - HLP e do biodegradável sintético - HEES, submetido ao deslizamento de um pino de liga Cu-35Zn, contra uma superfície plana revestida com WC-10Co4Cr por alta velocidade térmica. pulverização - HVOF. A combinação de óleos biodegradáveis com superfícies de menor impacto ambiental é importante para a sustentabilidade e crescimento da indústria moderna. O desempenho dos lubrificantes foi avaliado por meio de testes de deslizamento em um tribômetro pino sobre disco, modificando as cargas, para identificar os regimes de lubrificação, fricção e desgaste. Os testes indicaram que os valores do coeficiente de atrito médio foi de µHEES = 0,054 e µHLP = 0,044, assim mesmo o limite do regime de lubrificação foram próximos. A espessura mínima do filme (hmín) foi 23 % maior para o HLP e os valores obtidos de parâmetro do filme (λ) foi diferente a resultados de outras superfícies. Na avaliação do desgaste do pino e disco, percebeu-se que as maiores concentrações dos aditivos de Zn e P, encontradas no HLP, reduz o dano na superfície. Portanto, há a necessidade de se avaliar o tipo de superfície lubrificada com óleo do tipo HEES, bem como, fazer alterações das concentrações de aditivos.

ABSTRACT The correct selection of lubricating oils and contact surfaces improves performance and reduces the energy consumption of the machines. Therefore, surface engineering proposes new coatings to which their tribological behavior combined with the use of biodegradable oils in hydraulic system components are unknown. The aim of this study was to evaluate the behavior of the mineral oil - HLP and the synthetic biodegradable - HEES, subjected to sliding of a pin made of Cu-35Zn alloy, against a flat surface coated with WC-10Co4Cr by high-speed thermal spraying - HVOF. The combination of biodegradable oils with surfaces of lesser environmental impact is important for the sustainability and growth of the modern industry. The performance of the lubricants was evaluated by means of sliding tests on a pin-on-disc tribometer, modifying the loads, to identify the lubrication, friction and wear regimes. The tests indicated that the average coefficient of friction was μHEES = 0,054 and μHLP = 0,044, so the limit of the lubrication regimes was close. The minimum film thickness hmín was 23 % higher for HLP and the obtained values of the film parameter (λ) were different from the results of other surfaces. In the evaluation of the wear of the pin and disk, it was found to that the higher concentrations of the additives of Zn and P, found in the HLP, reduces the surface damage. Therefore, it is necessary to evaluate the type of surfaces lubricated with oils of the HEES type, as well as changes in additive concentrations should be made.

Yield strength and toughness in steels are directly associated with hot forging processes, especially by controlling austenitic grain size and cooling conditions. The phenomenological JMAK model in macroscale has been applied in different material classes to predict grain size after hot forming. However, on an industrial application, there is still a lack of understanding concerning the synergic effects of strain rate and temperature on recrystallization. This preliminary study aimed at investigating the applicability of coupled semi-empirical JMAK and visco-elastoplastic models in numerical simulation to predict austenitic grain size (PAGS). Hot forging of cylindrical samples of a ferritic-perlitic DIN 20MnCr5 steel was performed followed by water quenching. The main influences, such as temperature, strain and strain rate fields following the recrystallization model were investigated using the subroutine of FORGE NxT 2.1 software. The results were evaluated by comparing experimentally measured and simulated PAGS at process end. The forging process generates different strain and strain rate fields in the workpiece, which in turn lead to a variation in the PAGS and recrystallization fractions. The simulation was able to detect the PAGS variation showing a good agreement between the experimental forging results and the applied model.

RESUMO Neste trabalho foram investigadas as influências de alguns dos principais fatores do torneamento sobre o acabamento da superfície e a rugosidade de uma peça torneada. Foram analisadas barras de aço SAE 1045 torneadas em um torno controle numérico computadorizado (CNC), com auxílio da técnica de Planejamento de experimentos (DOE) para formar trinta e duas diferentes combinações de fatores, analisar suas interações e sua influência no acabamento de cada amostra. Os fatores de usinagem investigados foram: (1) avanço, (2) velocidade de corte, (3) raio de quina, (4) ângulo de saída e (5) região de medição da rugosidade. Por meio do DOE foram realizadas duas análises fatoriais: a primeira avaliou a influência dos cinco fatores na rugosidade média, representada por Ra, e a segunda avaliou a eficácia de cada conjunto de fatores em obter o acabamento alvo. Para isso, foi criado um índice que relaciona a rugosidade teórica de cada processo com a rugosidade medida pelo rugosímetro. Os resultados mostraram que o avanço e o raio de quina possuem elevado efeito padronizado em ambas as análises. Porém, enquanto a aplicação de um menor avanço (ou maior raio de quina) produziu superfícies com baixa Ra, por outro lado também prejudicou a eficácia do torneamento, pois contribuiu para obtenção de uma rugosidade significativamente maior que a rugosidade teórica. Já o fator velocidade de corte foi benéfico ao processo em ambas as análises. Não foi encontrada relevância da região de medição da rugosidade sobre a rugosidade média, Ra.

ABSTRACT This paper investigated the influence of some of the main turning parameters in surface finish and roughness of turning samples. The 0.45%C (SAE 1045) steel bars samples, turned on a CNC turning machine, were analyzed using Design of experiments (DOE) approach to produce a total of thirty-two parameters combination, analyze its interactions and influences in each sample’s surface finish. Machining parameters investigated were: (1) feed rate, (2) cutting speed, (3) nose radius, (4) rake angle and (5) region of roughness measurement. Two factorial analyses were conducted through the DOE data. The first one evaluated the influence of those five parameters in the average roughness, represented by Ra, and the second one evaluated the accuracy of each parameters combination to achieve the expected surface finish. For this measurement, an index was created to compare the theoretical surface roughness of each process with the surface roughness measured by rugosimeter. Results showed that parameters as feed rate and nose radius have strong standardized effect in both factorial analysis. Results shown that reduction of the feed rate (or increase in nose radius) was able to produce lower roughness average (Ra) but, on the other hand, it also contribute to generate a surface roughness significant higher than theoretical roughness. The increase in cutting speed was beneficial to both, achieving a lower roughness average and improving process accuracy. The region of roughness measurement was found not significant to influence the roughness average, Ra.

This paper aims at analyzing the effects of the roller pre-straightening of wire-rods on residual stress distributions in SAE 1045 steel bars. The combined drawing process is used in industrial production of bars in order to obtain a good surface quality and improved mechanical properties complying with specifications of the final products. In this process, prior to the drawing step, a roller straightening of the steel wire-rod is essential, because it provides the minimum straightness necessary for drawing. Metallographic analysis and hardness test were done for selected samples after different processing steps. Also, residual stress analysis of pre-straightened wire-rods by X-ray diffraction and neutron diffraction were carried out. The hardness tests show higher values near the surface and lower in the center of the wire-rod. Besides, the residual stresses results show a big inhomogeneity from one peripheral position to another and also in the evaluated cross section.

RESUMO Nitretação está entre os tratamentos superficiais mais efetivos. Os principais benefícios da nitretação aos aços são o aumento da dureza, resistência ao desgaste, resistência à fadiga e resistência à corrosão. Com isso, este trabalho tem por objetivo avaliar a resposta do AISI 4140 a diferentes condições de nitretação, quanto às propriedades químicas, microestruturais, de dureza e propriedades tribológicas. Para isso, foram feitas análises metalográficas, espectroscopia de emissão óptica de descarga incandescente (GDOES), difração de raios-x, microdureza e ensaio pino-sobre-disco. Nitretações gasosas foram executas em duas diferentes condições, com potenciais de nitretação de 0,6 e 2,0. A duração dos tratamentos foi de duas, quatro e seis horas. Adicionalmente, foi realizado um tratamento de nitrocarbonetação de seis horas, com a adição de carbono via injeção de gás de CO2, com potencial de carbonetação de 1,0. Os resultados mostraram a formação de camada branca seguida de uma camada de difusão com gradiente de propriedades como dureza e composição química. Um aumento significativo de dureza de 400 HV é observado na camada de difusão. Os diferentes tratamentos mostraram que o potencial de nitretação tem maior influência sobre a espessura do que o tempo de tratamento. A nitrocarbonetação formou uma camada nitretada 21% menos espessa do que nas amostras nitretadas de mesmo potencial de nitretação, enquanto que a camada de poros foi semelhante. Nos testes tribológicos realizados, houve uma diminuição do desgaste de 61% com a presença da camada nitretada e o modo de desgaste mudou de abrasivo/adesivo para abrasivo.

ABSTRACT Nitriding is among the most effective surface treatments. The main benefits of nitriding on steels are the increasing in hardness, wear resistance, fatigue resistance and corrosion resistance. Therefore, this study aims to evaluate the response of AISI 4140 to different nitriding conditions, regarding chemical, microstructure, hardness and tribological properties. To accomplish this, metallographic analyzes, optical emission spectroscopy glow discharge (GDOES), x-ray diffraction, hardness and pin-on-disc test were performed. Gaseous nitriding were performed in two different conditions, with nitriding potential of 0.6 and 2.0. The process time was two, four and six hours. Additionally, a nitrocarburizing treatment was carried out with the addition of carbon via CO2 gas injection with carburizing potential of 1.0. The results showed the formation of white layer followed by a diffusion layer with gradient properties as hardness and chemical composition. A significant increase in hardness about 400 HV is observed in the diffusion. The different treatments showed that the nitriding potential have more influence on the thickness than the treatment time. The nitrocarburized sample formed a nitrided layer 21% less thick than the nitrided sample with same nitriding potential, while the pore layer was similar. In performed tribological tests, there was a 61% decrease in wear due to the presence of the nitrided and the wear mode changed from abrasive / adhesive to abrasive mode.