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J. Mater. Sci. Technol., 2013, 29(3), 287e290
Surface Properties of Fe4N Compounds Layer on AISI 4340 Steel Modiﬁed by Pulsed Plasma Nitriding J.C. Díaz-Guillén1,2)*, G. Vargas-Gutiérrez3), E.E. Granda-Gutiérrez2), J.S. Zamarripa-Piña2), S.I. Pérez-Aguilar2), J. Candelas-Ramírez2), L. Álvarez-Contreras1) 1) Centro de Investigación en Materiales Avanzados, S. C. Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Chihuahua, Mexico 2) Corporación Mexicana de Investigación en Materiales, S.A. de C.V. Ciencia y Tecnología No 790, Fracc. Saltillo 400, C.P. 25290 Saltillo, Coahuila, Mexico 3) Centro de Investigación y Estudios Avanzados del IPN, Unidad Saltillo, Carretera Saltillo Monterrey km 13, Apdo. Postal 663, C.P. 25000 Saltillo, Coahuila, Mexico [Manuscript received March 29, 2012, in revised form August 30, 2012, Available online 1 February 2013]
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 g-Fe4N for samples processed between 1 and 2.5 mA/cm2. Thickness of nitrided layer increases proportionally to current density (0 mm for 0.5 mA/cm2 to 15 mm 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. KEY WORDS: Plasma nitriding; Current density; Hardness; Corrosion
1. 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. 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. 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
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surface of metallic pieces, for subsequent diffusion process into the crystalline lattice of the material. 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 (100e133 Pa). 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 g-Fe4N and/or ε-Fe2e3N. This layer is considerably hard and has better wear and corrosion resistance than the untreated substrate. It has been reported that there is a preference of monophasic (g-Fe4N or ε-Fe2e3N) layers over biphasic white layers. 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, temperature, duty cycle, gas mixture and component’s geometry, 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 and stainless steels as AISI 316L, there is no enough information about the effect of nitriding current density on the surface
J.C. Díaz-Guillén et al.: J. Mater. Sci. Technol., 2013, 29(3), 287e290
properties of these materials. The current density has been related to ion population in plasmas. 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. 2. Experimental Details Samples of a commercial CreMoeNi 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 mm diamond suspension. The samples were plasma nitrided in a laboratory reactor as previously described. Prior to nitriding the reactor was pumped down to a base pressure of 0.4 Pa (3 103 Torr). Nitriding was carried out using a DC pulsed supply unit for plasma generation, with precise adjustment of discharge current, pulse frequency and duty cycle. Nitriding process was conducted using current density values of 0.5, 1, 1.5 and 2.5 mA/cm2. Current density is deﬁned as the quotient: Current of discharge (A)/Cathode total area (cm2), where current of discharge was measured by means of a sampling resistor in series with the circuit. An auxiliary heating system affords full control over the sample temperature, which was measured by means of a K type thermocouple (2 C) directly fastened on the work piece. The constant process parameters were: temperature 525 C, time 4 h, gas mixture 50% N2e50% H2, pressure 173 Pa (1.3 Torr), frequency 1000 Hz, duty cycle 30% and discharge voltage from 400 to 700 V. Prior to nitriding process, a sputtering cleaning stage was carried out in the same reactor using a gas mixture of 50 wt% Ar and 50 wt% H2. After ion nitriding, the samples were cross-sectioned and metallographically prepared to be observed by scanning electron microscopy. Surface hardness measurements were performed for each sample by using a Vickers indenter under a load of 50 g and test time of 10 s. Analysis by X-ray diffraction was performed with an Empyrean PANalytical instrument, using CuKa (l ¼ 0.15406 nm) radiation, operated at 40 kV and 35 mA. The corrosion performance in chloride ion-rich solutions was evaluated by potentiodynamic polarization technique according to ASTM G5. A calomel electrode was used for reference and graphite as a counter electrode in 3% NaCl. Double distilled water, at room temperature, was used as electrolyte. The electrode potential was set from 1300 mV to 700 mV with the scanning rate of 60 mV/s. Corrosion rate was computed by Tafel extrapolation method.
Fig. 2 Hardness and white layer thickness as a function of nitriding current density.
3. Results and Discussion 3.1. Microstructure and hardness Yildiz et al. obtained white layer of approximately 9 mm 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 from 0 mm at 0.5 mA/cm2 to 15 mm at 2.5 mA/cm2. The increase of nitride layer thickness can be related to plasma ion population. It is known that plasma density, by the effect of an electric ﬁeld under two electrodes in a gaseous environment, increases when the current density rises. Consequently, when nitriding current density increases, the availability of active nitrogen ions on a sample’s surface will be greater (increase in nitrogen chemical potential) and therefore more nitrogen atoms will be incorporated into the surface and subsurface of the sample, if temperature is optimal. Fig. 2 also presents the effect of current density on surface hardness. Ten measurements were carried out for each sample testing. A higher increase of surface hardness was observed between the untreated samples (300 HV50g) and the samples nitrided at 1 mA/cm2 (800 HV50g). Considering the standard deviation of the measurements shown in Fig. 2, further increases of surface hardness were not observed for samples nitrided at current densities higher than 1 mA/cm2. This ﬁnding shows that in order to get the maximum surface hardness of the AISI 4340 steel samples, it is not necessary to increase the applied energy above 1 mA/cm2.
Fig. 1 SEM cross-section view of nitrided samples at (a) 0.5, (b) 1.5 and (c) 2.5 mA/cm2.
J.C. Díaz-Guillén et al.: J. Mater. Sci. Technol., 2013, 29(3), 287e290
Fig. 3 Effect of current density on crystalline phase composition in nitrided samples.
3.2. X-ray diffraction analysis To identify the crystalline phases existing in the surface layer, the samples were characterized by X-ray diffraction carried out within a 2q range from 30 to 100 . The obtained patterns are exhibited in Fig. 3. The XRD analysis shows that samples without white layer (0.5 mA/cm2) exhibit a composition of phases very similar to untreated samples, but incipient g-Fe4N reﬂections appear. In samples treated at current densities of 1, 1.5 and 2.5 mA/cm2, characteristic reﬂections of a-iron practically disappear leading to the emergence of the g-Fe4N peaks. In order to conﬁrm the presence of g-Fe4N phase instead of ε-Fe3N in modiﬁed layer, energy dispersive X-ray spectroscopy (EDS) analysis was carried out for samples nitrided at 1, 1.5 and 2.5 mA/cm2. As shown in Fig. 4, EDS analysis showed nitrogen percentages between 5 and 6 wt% across modiﬁed layers. This value is in good agreement with the stoichiometry of g-Fe4N phase. In the case of ε-Fe3N, the nitrogen percentage should be around 12 wt%. In order to obtain the highest corrosion resistance and the highest mechanical properties, it is well known that the g-Fe4N layer is favored over the ε-Fe3N or the biphasic (g-Fe4N, ε-Fe3N) layers in low alloy steels. For low alloy steels, Corengia et al. and Sule et al. observed biphasic layers g-Fe4N and ε-Fe3N for nitriding time less than 15 h. They obtained g-Fe4N monophasic layer only after 15 h of plasma nitriding at temperatures between 500 and 540 C, respectively. However, results of the present work show
Fig. 4 Cross EDS nitrogen analysis on sample nitrided at 2.5 mA/cm2.
Fig. 5 Potentiodynamic polarization curves for untreated samples, samples without white layer and samples with monophase layer.
that it is possible to obtain monophasic layers g-Fe4N in considerably less time (4 h) through the manipulation of nitriding current density. The development of g-Fe4N or ε-Fe3N monophasic layers can be related to both ion population and sputtering phenomenon. As it was mentioned, the increase of current density promotes the increase of plasma ion population allowing more nitrogen and consequently increasing the incorporation rate of nitrogen atoms on the steel surface. Thus, the ε-Fe3N (phase with higher nitrogen content) will be expected at highest nitriding current densities. However, the presence of ε-Fe3N phase is inhibited by sputtering phenomenon which increases at higher current densities. This phenomenon promotes surface decarbonation in nitrided steels, allowing the stabilization of the g-Fe4N phase instead of the ε-Fe3N phase. 3.3. Electrochemical corrosion susceptibility Potentiodynamic polarization curves for untreated, and nitrided samples at current densities of 0.5 (no white layer) and 2.5 mA/cm2 (mono phase layer) are shown in Fig. 5. The noticeable increase in the rest potential value, from 1060 (untreated samples) to 930 mV (nitrided samples at 2.5 mA/ cm2), indicated a decrease in corrosion susceptibility of nitrided samples. Fig. 6 shows corrosion rate values for all evaluated samples. All nitrided samples, exhibit a decrease in the corrosion rate values in respect to the untreated sample. While the untreated samples exhibited a corrosion rate of 0.153 mm per year,
Fig. 6 Inﬂuence of the increase of nitriding current density on corrosion susceptibility.
J.C. Díaz-Guillén et al.: J. Mater. Sci. Technol., 2013, 29(3), 287e290
the corrosion performance was improved up to 0.03 mm per year at current densities above 1 mA/cm2. There is no noticeable improvement in corrosion resistance beyond 1 mA/cm2. It has been reported that the presence of monophasic layers prevents generation of internal stresses, which are characteristics of biphasic layers as a consequence of the difference in lattice structures. In this sense the highest corrosion resistance will be obtained when layers are constituted of only g-Fe4N. 4. Conclusion Thickness of nitrided layer increases proportionally to current density (0 mm for 0.5 mA/cm2 to 15 mm 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 g-Fe4N for samples processed between 1 and 2.5 mA/cm2 after nitriding for 4 h. While the untreated samples exhibited a corrosion rate of 0.153 mm per year, the corrosion performance improved up to 0.030 mm per year at current densities above 1 mA/cm2 which is about one ﬁfth of the untreated samples’ corrosion rate. Above 1 mA/cm2 there is no noticeable improvement in corrosion resistance. REFERENCES  ASM International, ASM International Handbook, in: Properties and Selection: Irons Steel and High Performance Alloys, tenth ed., vol. 1, USA. (1990).  R.C. Souza, H.J.C. Voorwald, M.O.H. Ciofﬁ, Surf. Coat. Technol. 203 (2008) 191e198.
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