Metallurgical response of an AISI 4140 steel to different plasma nitriding gas mixtures

Abstract

Plasma nitriding is a surface modification process that uses glow discharge to diffuse nitrogen atoms into the metallic matrix of different materials. Among the many possible parameters of the process, the gas mixture composition plays an important role, as it impacts directly the formed layer's microstructure. In this work an AISI 4140 steel was plasma nitrided under five different gas compositions. The plasma nitriding samples were characterized using optical and scanning electron microscopy, microhardness test, X-ray diffraction and GDOES. The results showed that there are significant microstructural and morphological differences on the formed layers depending on the quantity of nitrogen and methane added to the plasma nitriding atmosphere. Thicknesses of 10, 5 and 2.5 µm were obtained when the nitrogen content of the gas mixtures were varied. The possibility to obtain a compound layer formed mainly by γ ' -Fe 4 N nitrides was also shown. For all studied plasma nitriding conditions, the presence of a compound layer was recognized as being the responsible to hinder the decarburization on the steel surface. The highest value of surface hardness -1277HV -were measured in the sample which were nitrided with 3vol.% of CH 4 .

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

“…For example, high alloy steels exhibit high hardness due to dispersed alloy nitrides in the matrix and high surface residual stresses are generated in the compound layer and diffusion zone for these steels, which are the result of chemical composition gradients, stress fields around precipitates, volume changes and thermal effects 7 . In the case of low alloy steels, the diffusion zone does not reach hardness values as high as the obtained when nitriding tool steels, for example 6,8 . For low alloy steels, the compound layer plays the most important role against wear, and the diffusion zone has a secondary role, therefore it is important to develop the correct morphology and phase composition of the compound layer 8 .…”

“…In the case of low alloy steels, the diffusion zone does not reach hardness values as high as the obtained when nitriding tool steels, for example 6,8 . For low alloy steels, the compound layer plays the most important role against wear, and the diffusion zone has a secondary role, therefore it is important to develop the correct morphology and phase composition of the compound layer 8 .…”

“…Increasing the nitrogen concentration to 76% vol.% N 2 also could not lead to a monophasic ε-Fe 2-3 (C)N layer. Skonieski et al 8 showed that for nitrocarburized samples with 70 vol.% N 2 and 3 vol.% methane (CH 4 ) the proportion of ε-Fe 2-3 (C)N increased if compared to the nitrided condition with 70 vol.% N 2 and without CH 4 , but the γ'-Fe 4 N phase was still present and, consequently, it was not possible to produce the monophasic ε-Fe 2-3 (C)N on the surface of the samples.…”

“…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.…”

Application of surface modification technologies to improve performance of hot sheet metal forming tools: A review

“…Methods of applying the coatings include chemical vapour deposition, nitriding and physical vapour deposition [12]- [15]. Nitriding involves diffusion of nitrogen atoms on the surface of steels to form iron-carbo-nitrides which exhibit high hardness [16]. Physical vapour deposition (PVD) is the most common coating process developed.…”