Effects of DC plasma nitriding parameters on properties of DIN low alloy steel

Transcription

1 2nd National Iranian Conference on Gas Hydrate (NICGH) S Semnan University Effects of DC plasma nitriding parameters on properties of DIN low alloy steel H. Khorsand a*, H. Aghajani b, N. Mohamadian a, A. Akrami c a- Material Division, Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran b-department of Materials Engineering, Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran c-school of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran *Corresponding Author s hkhorsand@kntu.ac.ir Abstract The aim of this paper is optimizing of parameters can be affected to depth of nitrided layer for improving the corrosion resistance of DIN low alloy steel by plasma nitriding.in this study, DIN low alloy steel samples were plasma nitrided at 3 level temperatures (450, 500 and 550 ), 3 level time (2, 4, 6 hour) and in 2 level gas compositions (25%N 2-75%H 2 and 75%N 2-25%H 2 ). Diffusion zone was determined by optical microscopy. Phases formed in these layers were studied by X-ray diffraction method. The XRD pattern reveals that all specimens consist Fe 3 N, Fe 2 N and Cr 2 N phases.results have shown that depth of diffusion zone increases byincreasing gas compositions from 25% N 2 to 75% N 2. In addition, thickness of this layer have increased by increasing temperature from 450 to 550, but It seems that by increasing nitriding time of 2 to 4 hour the depth of the white layer increased but by increasing to 6 hour the depth of this layer decreased sligthly. The optimun condition for plasma nitriding of DIN is 550, 4h and 75% N 2. Keywords: DIN , Microhardness, Diffusion zone, Optimum. Research Highlights (Times New Roman 10 pt. Bold) A brief summary of your manuscript's novelties: It must be written in 3 bullets and between words. The bullets must be similar to this example. The first page of the manuscript should be like this page. 1.Introduction One of the most important problems of transportation and reservoir of crude oil and gas liquids is corrosion of pipelines. The corrosion of pipeline has been caused deposition and contamination of groundwater and will be irreparable damage to environmental. Different methods are used for increasing the corrosion resistance of steel pipes.nitriding is a surface

2 Effects of DC plasma nitriding parameters treatment technique used to introduce nitrogen into metallic materials to improve their surface hardness, mechanical properties, as well as wear and corrosion resistance [1], [2] and [3].In the plasma-nitriding process, by means of a glow-discharge using a mixture of N 2 and H 2, with the steel at the High constant temperature, nitrogen can penetrate the surface and diffuse into the steel [4]and wear and corrosion resistant layer will be formed.this layer, which is called white or compound layer can consist of iron nitride phases, gamma prime, Fe 4 N, or epsilon, Fe2 3N [5].The diffusion zone with its induced compressive residual stresses, enhances fatigue strength of the work piece. Detail of plasma nitriding process and its parameters have been described by Kovacs and Russell [6]. The response of steel to plasma nitriding mainly depends on its composition [7]. Further to this, though plasma nitriding is influenced by process factors such as time, temperature, chemical composition of the treatment gas, working pressure, width of the glow sheath, the amount of electrical power and the geometry of the chamber. The purpose of this work is to contribute towards understanding the effect of temperature, time on properties of the nitride layer of DIN 2344 low-alloy steel. 2. Experimental procedures The specimens were in the form of 20mm diameter and 10 mm thick discs made from DIN low alloy steel. Table 1 shows the chemical composition of the steel, which was obtained by spark emission spectroscopy. The surface of the samples exposed to plasma was grounded and polished using SiC emery papers and alumina powder with 0.3 μm grain size. The samples were then cleaned with ethanol and placed in a 1 to 2.2 kw pulsed DC discharge plasma nitriding unit. Fig. 1 shows a schematic diagram of the equipment used in this work. Plasma nitriding was carried out 75% N 2 +25% H 2 and 25% N 2 +75% H 2 atmosphere at a pressure of 500Pa. A range of processing temperatures from 450 to 550 C and times ranging from 2 to 6 h were used. After the nitriding process, all samples were slowly cooled down in chamber under constant pressure of flow to room temperature. The specimens were sectioned before mounting. The mounted samples were grounded, polished and then etched in 2% Nital. The microstructures of nitrided and layers were observed by optical microscope.in order to characterize the phases in the nitrided specimens, X-ray diffractometry (XRD) by the use of PHILIPS X-ray diffractometer was accomplished. Table. 1 Chemical composition of DIN 2344 low alloy steel. Element C Si Mn Ni Cr Mo V Cu Wt% Results and discussion Phase evolution in plasma nitriding was characterized by X-ray diffraction (XRD) analysis. Fig. 2 shows the XRD profiles of as received and plasma nitrided sample treated at 450 for 6 h in 75% N 2. While the as received sample shows only α-fe peak, XRD patterns of plasma nitrided samples record a number of peaks due to different nitrides. In this sample, it seems that Fe 2 N and Fe 3 N peaks accompany the strong α-fe peaks.moreover Chromium nitride (Cr 2 N) precipitate was detected in this specimen. The chromium can combine with nitrogen to form a dense and continuous protective oxide film on the surface, and mitigate the corrosion attack in comparison with unnitrited specimens. In earlier studies, it has been reported that Once Chromium nitride forms, the nitrogen and chromium in the solid solution of the

3 2nd National Iranian Conference on Gas Hydrate (NICGH) S Semnan University nitridedlayer will be depleted, which prevents the formation of passivating films on the surface and then deteriorates the corrosion resistance of the stainless steel [8].

4 Effects of DC plasma nitriding parameters Because nitride is a noble phase, formation of more nitride helps to protect the surface from corrosion attack.for this reason, increasing of thickness of white layer due to increase of the nitriding time or temperature, corrosion resistance improves.this result agrees quite well with the plasma-nitriding study mentioned earlier [9,10], which showed that the nitrided layer with the nitritedphase has excellent corrosion resistance. Nitrided layer can be detected by etching samples and creating of Color distinctions. The average of depth of nitride layer can be measured by Computer and these result can be seen in Fig.3 and 4.It can be seen that the thickness of white layer of specimens that nitrited at 75%N 2 are bigger than 25%N 2. The thickness is approximately linear with temperature. Growth of the compound layer is controlled by diffusion of nitrogen through this layer [12]. At low treatment temperature (450 ) the diffusion coefficient of nitrogen atoms is low and therefore the compound layer and diffusion layer are very thin. Increasing treatment temperature results in an increase in the nitrogen diffusivity, leading to the formation of thicker layers. Also, it can be seen that by increasing the time of nitriding from 2 to 4 hour, the value of thickness increased, but by increasing to 6 hour, this value decreased in both atmosphers. For acheivment to the maximum value of the thickness of white layer is 4 hour but for determinating of the reason of this decreasing, it must be studyaccurately.

5 2nd National Iranian Conference on Gas Hydrate (NICGH) S Semnan University 4. Conclusions 1-Phases of Fe 3 N, Fe 2 N and Cr 2 N have been created in all of specimens and have been shown by XRD pattern 2-Depth of nitrided layer has been increased by increasing of temperature, time and N 2 concentration in all of specimens and corrosion resistance improved. References [1] Q.F. Peng., Improving abrasive wear by surface treatment, Wear, Vol. 129, No , [2] U.Huchel, S.Bramers, J.Crummenauer, S.Dressler, S.Kinkel., Single cycle combination layers with plasma assistance, Surf. Coat. Technol., Vol. 95, No , [3] C.V.Franco, F.G.Mittelstadt, J.L.R.Muzart, A.R.Souza, C.P. cardoso., Plasma surface treatment of AISI 4140 steel for improved corrosion resistance, Mater. Sci, Vol. 31, No , [4] K.T.Rie., Recent advance in plasma diffusion process, Surf. Coat. Technol., Vol. 112, No , [5] Edenhofer B., Heat treatment of metals, part 2,pp , [6] Kovacs.W, Russell.W, Ion Nitriding, Metals Park, pp. 9 17, [7] Bell T., Heat treating, ASM International, ASM handbook vol. 4, pp , [8] Yun-taoXi, Dao-xin Liu, Dong Han., Improvement of corrosion and wear resistances of AISI 420 martensiticstainless steel using plasma nitriding at low temperature, Surf. Coat. Technol., Vol. 202, No , [9] D.C.Wen., Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding, Appli. Surf. Sci., Vol. 256, No , 2009.

6 Effects of DC plasma nitriding parameters [10] T. Michler, M. Grischke, K. Bewilogua, H. Dimigen., Properties of duplex coatings prepared by plasma nitriding and PVD Ti C:H deposition on X20Cr13 ferriticstainless steel, Thin Solid Films, Vol.322, No , [12] Edenhofer.B, Bewley.T.J., Heat Treatment, The Metals Society, Vol. 76, pp. 7 13, 1976.