Response of a DIN 18MnCrSiMo6-4 Continuous Cooling Bainitic Steel to Plasma Nitriding with a Nitrogen Rich Gas Composition

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.

Response of a DIN 18MnCrSiMo6-4 Continuous Cooling Bainitic Steel to Different Plasma Nitriding Gas Mixtures

“…27 . The case depth increases with increasing time, temperature and nitrogen concentration as expected for diffusion-controlled growth 16,17 . On the other hand, Alsaran et al 17 had previously tried to explain the increase in the depth of the nitrided layer by increasing the ratio of hydrogen to nitrogen.…”

“…The present work is a continuation of previous work entitled "Response of a DIN 18MnCrSiMo6-4 Continuous Cooling Bainitic Steel to Plasma Nitriding with a Nitrogen Rich Gas Composition" 16 . For the investigated treatment conditions it was found that the treatment temperature of 500 °C developed the best results; therefore, in this work we opted to use this temperature.…”

“…Palmqvist cracks are clearly developed for nitrided samples with different nitrogen gas composition, however, the appearance of concentric cracking also became apparent, becoming more significant with increasing load. The reason for this is not immediately apparent, although probably related to the increased thickness of the compound layer 16 . As the compound layer thickness decreases and/or the indentation depth increases (also by increasing load), the indentation effects are not restricted to the compound layer, and plastic deformation in the diffusion zone will occur, since the diffusion zone is much more ductile than the compound layer.…”

“…For plasma nitriding, samples were disks with 10 mm of height and 43.2 mm of diameter manufactured from cylindrical bars of a continuous cooling bainitic steel DIN 18MnCrSiMo6-4. The material's chemical composition, microstructure and mechanical properties are shown in previous work 16 . Prior to the surface treatments, samples flat faces were ground with silicon carbide grinding paper in a sequence with increasing mesh (#100, #220, #320, #400, #600, #1200) and then polished with diamond paste with an abrasive size of 3 μm in order to obtain low roughness, almost no plastic deformation and consequently low initial residual stresses in the surface.…”

“…The combination of multiphase microstructures with surface hardening treatments in steel opens new possibilities, by allowing high levels of strength and ductility to be adjusted [13][14][15] . The major influence on the metallurgical properties of plasma nitrided low alloy steels is played by the gas mixture, temperature and treatment time 8,16 . Therefore, the objective of the present work is to evaluate the response of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel to plasma nitriding with different combinations of time and nitrogen gas composition.…”