Surface Properties of Fe4N Compounds Layer on AISI 4340 Steel Modified by Pulsed Plasma Nitriding

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In this work, the effect of nitriding current density on hardness, crystalline phase composition, layer thickness and corrosion rate of AISI 4340 steel has been studied. X-ray diffraction analysis shows that thin layers formed during nitriding process are constituted of γ-Fe4N for samples processed between 1 and 2.5 mA/cm2. Thickness of nitrided layer increases proportionally to current density (0 μm for 0.5 mA/cm2 to 15 μm for 2.5 mA/cm2). Plasma nitriding increased the surface hardness from 300 HV50g for untreated sample, to around 800HV50g for nitrided samples at 1 mA/cm2. While the untreated samples exhibited a corrosion rate of 0.153 mm per year, the corrosion performance was improved up to 0.03 mm per year at current densities above 1 mA/cm2, which is about one fifth of the corrosion rate of the untreated sample.

Introduction

AISI 4340 steel is a nickel-chromium-molybdenum alloy which is commonly used in the manufacturing of various parts that must perform in high wear and chemically aggressive conditions[1]. Despite its good strength, ductility and toughness, its life time is not long enough, because of its low corrosion resistance and poor tribological properties[2], [3]. Ion nitriding, a plasma-activated thermo-chemical surface modification technique, has been used successfully to increase the fatigue strength, hardness, wear resistance and, in some cases, corrosion resistance of alloy steels, tool steels and stainless steel alloy[4]. Additionally, this method has been used to improve properties such as load bearing capacity of dynamic loaded components[5], [6].

Plasma nitriding technique uses the direct current (DC) glow discharge phenomenon to introduce elemental nitrogen into the surface of metallic pieces, for subsequent diffusion process into the crystalline lattice of the material[7]. The process is carried out in a vacuum chamber where the sample is directly connected to a cathode. High voltage is applied between the cathode and the anode (the vessel walls acts as the anode) to generate a plasma in a gas mixture, usually in a rough vacuum (100–133 Pa)[8].

The nitriding reaction takes place at the surface as well as in the subsurface by the diffusion of nitrogen atoms from the surface toward the core. The surface layer, also called “white layer”, is a thin layer of iron nitride constituted of γ-Fe4N and/or ε-Fe2–3N. This layer is considerably hard and has better wear and corrosion resistance than the untreated substrate[9]. It has been reported that there is a preference of monophasic (γ-Fe4N or ε-Fe2–3N) layers over biphasic white layers[10]. The diffusion zone, softer than the white layer, consists of very fine nitride particles dispersed in the matrix.

Some researchers evaluated the effect of plasma nitriding variables, such as frequency[11], temperature[12], duty cycle[13], gas mixture[14] and component's geometry[15], on surface properties of AISI 4340 steel. However, although that the effects of nitriding current density on the material properties in nitriding processes have been studied for low alloy steels as AISI 4140[16] and stainless steels as AISI 316L[17], there is no enough information about the effect of nitriding current density on the surface properties of these materials. The current density has been related to ion population in plasmas[18].

In this paper, the role of the nitriding current density has been related to surface properties of AISI 4340 steel. The nitrided region has been characterized by surface hardness testing, scanning electron microscopy (SEM), X-ray diffraction (XRD) and potentiodynamic polarization.

Section snippets

Experimental Details

Samples of a commercial Cr–Mo–Ni low alloy AISI 4340 steel (40 mm × 40 mm × 5 mm), presenting a typical microstructure of tempered martensitic phase, were used in this study. Chemical composition (wt%) was: C 0.38%, S 0.017%, Mn 0.63%, P 0.008%, Si 0.19%, Cr 0.80%, Ni 1.64%, Mo 0.20%, Cu 0.137%. The surface of the samples was ground and polished with a 1 μm diamond suspension. The samples were plasma nitrided in a laboratory reactor as previously described[13]. Prior to nitriding the reactor

Microstructure and hardness

Yildiz et al.[12] obtained white layer of approximately 9 μm thick, after processing at 540 °C for 16 h. In this study the nitriding current density was not reported. Fig. 1 shows SEM images of the cross-section obtained in the present work from nitrided samples at different current densities. Fig. 2 shows the variations of hardness and white layer thickness as a function of nitriding current density. It is seen from both Figs. 1 and 2 that, the thickness of nitrided layer evidently increases

Conclusion

Thickness of nitrided layer increases proportionally to current density (0 μm for 0.5 mA/cm2 to 15 μm for 2.5 mA/cm2). The surface hardness increased up to 800 HV50 at 1 mA/cm2. Considering the standard deviation of the measurements, further increases of surface hardness were not observed for samples nitrided at current densities higher than 1 mA/cm2. The thin layer formed during the ion nitriding of AISI 4340 steel is constituted of monophasic layer γ-Fe4N for samples processed between 1 and

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