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

Abstract

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.

Microstructure and Wear Properties of a Low Carbon Bainitic Steel on Plasma Nitriding at Different N2-H2 Gas Mixtures

“…Similarly, when the drop in hardness was gradual, the reduction in compressive residual stresses was gradual. It is also noteworthy that the residual stresses profiles become tensile in the vicinity of the case/core interface, as already observed by other researchers 18,22,23,36,37 . Compressive residual stresses found in the diffusion zone are the result of structural misfits caused by the precipitation of alloy nitrides and carbonitrides in the iron matrix 17 , and also due to the saturation of the iron network with interstitial nitrogen.…”

“…All the plasma nitrided samples showed diffraction peaks that indicate the formation of a compound layer 22,23 . Figures 3a-c also show the formation of a biphasic compound layer (ε-Fe 2-3 (C)N and γ'-Fe 4 N) on the plasma nitrided samples with 24 and 76 vol.% N 2 , while the plasma nitrided samples with 5 vol.% N 2 indicated a monophasic compound layer (γ'-Fe 4 N).…”

“…The microstructure of the compound layer and of the diffusion zone, as well as the combination with the steel core properties are designed to *e-mail: rldalcin@gmail.com accomplish with the desired final properties [19][20][21] . In previous articles the authors investigated the microstructure and residual stresses of plasma nitrided DIN 18MnCrSiMo6-4 continuous cooling bainitic steel 22,23 , which showed excellent results concerning case depth, surface hardness, and compressive residual stresses in the diffusion zone. The influence of different plasma nitriding temperatures on the surface modifications and reciprocating dry sliding wear properties of the continuous cooling bainitic steel was investigated 21 .…”

Influence of Plasma Nitriding with a Nitrogen Rich Gas Composition on the Reciprocating Sliding Wear of a DIN 18MnCrSiMo6-4 Steel

“…Plasma nitriding leads to an increase in hardness on the surface (Table 2), for the reason it also ends up creating a less ductile region more prone to brittle fracture 36 . The mechanical properties of the diffusion zone influence the fracture properties of the compound layer, since the diffusion zone provides support for the surface compound layer 15,16,36 . Previous work 15 showed that the higher nitriding temperature and time promoted an increase in the fracture toughness of the compound layer.…”

“…The plasma nitriding process can be used to develop outstanding surface properties while carrying treatments using relatively low treatment temperatures 15,16 , thus representing a reliable alternative to conventional thermal and thermochemical treatments. Since long nitriding times can have a detrimental effect on the mechanical properties of steel, the treatment temperature should be carefully selected to avoid overheating of the steel itself and to preserve the bainitic microstructure of these steels, as well as softening of the plasma nitrided layer due to the excess aging of nitrides 15,17 .…”