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.
RESUMONitretação 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 CO 2 , 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 quími-ca. 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. Palavras-chave: Nitretação gasosa, aço AISI 4140, pino-sobre-disco, difração de raios-x, GDOS. ABSTRACTNitriding 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 sim...
ResumoAs T R C (tensões residuais compressivas) induzidas por shot-peening (SP) podem ser relaxadas e redistribuídas sob cargas cíclicas mecânicas, térmicas e termomecânicas. O objetivo desta pesquisa é avaliar a redução da T R C por SP através da difração de raios-X fast in-situ (D RX in-situ) na superfície do aço SAE 4140 durante tempo (t) de patamar de t [s] = 54000 em três diferentes temperaturas (T) de T [K] = 423,16; 523,16 e 623,16. É investigada a relaxação de T R C por cargas térmicas, mantendo as amostras, previamente tratadas por SP, em um forno com diferentes temperaturas de patamar, enquanto são medidos os valores de T R ao longo do t. Foi construída uma câmara de vácuo acoplada a um difratômetro de raios-X. Obteve-se 100 % cobertura por SP, as T R C aliviaram com o aumento da temperatura de patamar e o tempo. A maior variação de T R C ocorreu nos primeiros minutos de ensaio, no restante do tempo a T R permaneceu constante. A macrodureza superficial e FWHM das amostras diminuíram com o aumento da temperatura de patamar ao longo do tempo. A macrodureza superficial das amostras e FWHM são maiores após o tratamento de SP, antes do aquecimento. O FWHM reduziu ao longo do tempo com mesmo comportamento da queda da T R C, correspondente e proporcional à redução da T R C. A energia de ativação do processo, calculado pela função Zener-Wert-Avrami não foi coerente com a autodifusão do Fe-α. Logo, é recomendada a repetição para verificar o resultado. Com base nos dados obtidos, se pode concluir que esta pesquisa possui relevância industrial. Palavras-chave: Shot-peening; Tensões residuais; DRX fast in-situ; SAE 4140. THERMAL RESIDUAL STRESS RELAXATION ANALYSIS OF SAE 4140 STEEL BY FAST IN-SITU X-RAY DIFFRACTION AbstractCompressive residual stresses (T R C) induced by shot-peening (SP) can be relaxed and redistributed under mechanical, thermal and thermomechanical cyclic loads. This research's aim is to to evaluate the T R C reduction by SP through fast in-situ X-ray diffraction (D RX in-situ) on the surface of SAE 4140 steel until the holding time (t) of t [s] = 54000 in three temperatures (T): T [K] = 423,16; 523,16 e 623,16. It was investigated the T R C relaxation via thermal loadings, maintaining the samples, previously treated from SP, inside a furnace with the different T, while were measured the T R values over the t. It was built a vacum chamber to couple in a X-ray diffractometer. Results showed 100 % covering by SP, the T R C relaxed with the holding temperature and time increasing. The T R C most variation was found in the first minutes test, at the remainder time the T R remained constant. Samples surface macrohardness and FWHM decreased with the holding T over time and are larger after the SP without heating. FWHM decreased over time with the same T R C descreasing, proportional and corresponding to the T R C reduction. Activation energy of the process was computed using Zener-Wert-Avrami function didn't result in the Fe-α self-diffusion energy. So, is necessary new avaliation. Based on the obtain...
The distribution of residual stresses (RS) in repairs generated by Friction Hydro-Pillar Processing (FHPP) in AISI H13 was investigated. Three axial force levels, with consequent different deposition rates, were applied to replicate possible repaired conditions. The contour (CM) and X-ray diffraction (XRD) methods were employed for RS analysis in samples that were also evaluated through metallography, microhardness analysis, micro-tensile and Charpy testing. CM produced 2D maps of the RS in the joints, showing symmetrical distributions around the welded rod for all welded conditions. Other common features for all conditions were the maximum level of compressive RS, which was found in the TMAZ of the rod, and the maximum tensile residual stress, which was found near the HAZ of the base block. There was good agreement between the RS measurement techniques. Mechanical tests show similar tensile resistance for all conditions and an apparent increase in toughness at higher force levels.
Residual stresses (RS) in pipes welded by a fully automated friction welding process, called FRIEX, are investigated using the contour method (CM) and the X-ray diffraction method (XRD). Samples were produced using a self-developed equipment, which is based on the interaction between a consumable intermediate rotary ring and two stationary pipe sections, by varying the loading rate applied during the welding of API 5L B pipes with an API X65 ring. Mechanical and metallurgical properties were locally analyzed through metallography and microflat tensile tests. RS measured by XRD and CM showed comparable values; tensile zones were found in the pipe HAZ and compressive stresses were found in its adjacent areas. Tensile RS of approximately half the local yield strength of the components were also found, mostly on the internal surface, and their values decrease with increasing welding loads. Microflat tensile tests showed an increase in yield strength of pipe TMAZ and HAZ, with a clear influence of loading rates.
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