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.

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.

Continuous cooling bainitic steels has been widely used in industrial processes owing to its excellent mechanical properties and toughness. Although the surface properties of them are acceptable for many purposes, for their use in mechanical components like gears, it is necessary to improve their surface properties. Plasma nitriding treatments was carried out of a DIN 18MnCrSiMo6-4 steel at 500 °C, with three different nitrogen gas composition: 76, 24 and 5 vol.% nitrogen in hydrogen, for 3, 6 and 9 hours. The surfaces were characterized concerning the microstructure, microhardness, fracture toughness, nitrogen concentration and carbon composition, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different nitrogen gas composition was determined. The carbon profiles of samples indicate that there was decarburization during the plasma nitriding. 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 with 76 and 24 vol.% nitrogen, while the nitrided samples with 5 vol.% nitrogen indicated the formation of monophasic compound layer. The diffusion zone presented compressive residual stresses with highest values near the surface.

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.

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.