8620 Çeliğine Uygulanan İyon Nitrürasyonu

Abstract

Plazma ile nitrürasyon ilk kez 1930' larda A.B.D'de uygulanmıştır. Bu metodun endüstrideki kullanımı, işlemin bilinen nitrürasyona göre daha karmaşık olması, hassas elektronik kontrol ünitelerini ve büyük kapasitelerde vakum cihazlarını gerektirmesi nedeni ile yaygınlık kazanamamıştır. 1960'dan sonra teknolojideki gelişim (özellikle elektronik ve vakum teknolojisi) iyon nitrürasyonu işleminin işletmeler tarafindan daha iyi anlaşılmasını sağlamış ve kullanım hızla yayılmaya başlamıştır. İyon nitrürasyonu metallerin geniş bir kullanım alanına sahip olmaları için yüzeylerinin sertleştirildiği işlemdir. İsminden de anlaşılacağı üzere bu işlem iyonize olmuş azotun yalnız veya diğer gazlarla reaksiyona girerek yüzeyi kaplamasıdır. İyonize olan azot parça yüzeyinde karakteristik erguvani akkor renk olarak parça yüzeyinde gözlenir. Bu çalışmada 8620 çeliğine uygulanan iyon nitrürasyonu prosesi anlatılmış ve yüzey sertlikleri incelenmiştir. Uygulamalarda %80 H2 + %20 N2 içeren karışım gaz kullanılmış ve mikroyapı fotoğrafları çekilmiştir.

Among the thermochemical processes for the improvement of surface the properties of machine components, nitriding holds an important position in industry today. Together with conventional gas and bath nitriding there is a new process called ion nitriding which has been developed to complete industrial application in the last few years. Today, ion nitriding is used on a large scale in industries concerned with iron, steel, cast iron, and sintered- iron products. Because of its special basic technique -making use of the plasma of a glow discharge- ion nitriding has a large variety of properties to offer and therefore opens up to new perspective in the field of nitriding. The main equipment of ion nitriding is vacuum furnace, the gas distribution system and the electric unit. The work pieces are placed in the vacuum furnace in such away that electric isolation is provided. Together with the vacuum pump, the gas distribution system enables to be evacuated filled with appropriate treatment gas and maintained at the required vacuum, usually between 0.1-10 mbar. A D.C. voltage, which can be set any value from about 100 volts to approximately 1500 Volts, is applied between the work piece to be ion nitriding and the wall of the fumace-the former being connected as the cathode and the latter as the anode. Under this potential difference, the molecules and atoms of the treatment gas are excited and ionised, producing the typical luminous phenomenon known as glow discharge. The positive ions of the treatment gas are accelerated towards the negatively connected work piece and hit its surface with tremendous kinetic energy. The release of this energy heats the work piece up and the ions are occluded into its surface, because of this reason the furnace does not require a separate heating system. Ion nitriding can be carried out using either pure nitrogen or mixtures of nitrogen with hydrogen or suitable hydrocarbons. In my study I worked with %80H2-%20N2 to the work piece of 8620(2 lNiCrMo2) steel. The temperature of the work piece is measured with the aid of thermocouples and regulated by means of a controller varying the power output of the electric unit, the treatment temperature is, in most cases, set between 400°C and 600°C,depending on the composition and structure of the material to be treated as well as the stress which the work piece will have to resist. A particular advantage of ion nitriding is the fact that even at lower temperatures of between 350°C and 450°C nitrogen saturation of the surface is possible. Treatment times vary from ten minutes to a maximum about twenty hours, depending on the grade of steel and required depth of hardening. A glow discharge is produced by the shock excitation of gas atoms and molecules in an electrical field accordance with physical laws. The appearance of a glow discharge are the presence of a low-pressure ( gas medium-high vacuum) and the application of voltage of about 300 V minimum depending upon the gas pressure, between two electrodes in the discharge vessel. As the ion nitriding uses this plasma stage and could also be called "PLASMA NİTREDİNG". Plasma consists of electrically charged particles i. e ions and electrons. In a purely thermal process, this state can only be reached by heating the gas to a temperature at least some hundred degrees. If, however electricity is employed, the state can be reached as in the glow discharge. The small numbers of charge carriers, which are present in any gas, are accelerated in the voltage drop between cathode and anode. The reaction of the plasma with the surface of the work piece contains four processes.. Sputtering. Heating. Condensation. Diffusion of Nitrogen (Implantation) Sputtering: Sputtering may be considered as a vaporisation process since the very "Hot" ion hitting the surface will intensely heat a localised area of the work piece causing atoms in that particular region to vaporise. Heating: The work pieces to be treated do not require any external heating. Because when the nitrogen ions hit and penetrate the top most atom layers of work piece surface, the remaining energy is converted into heat to heat the work piece. Condensation: Iron atoms which are detached from the surface can combine with the highly reactive nitrogen atoms in the plasma near the surface of the work piece and will then, owing to adsorption, be deposited as iron nitride (FeN) on the work piece surface. The FeN nitride which is condensed on uncooled cathodic work piece surfaces (400-600 °C) is unstable and decomposes into the lower nitrides Fe2N, Fe3N, Fe4N. Diffusion of Nitrogen: In particular, attempts have been made again and again to reduce the extremely long treatment times associated with conventional gas nitriding by accelerating the process of nitrogen occlusion. Ion nitriding considerably reduced the time of nitrogen occlusion. After ion nitriding, the surface layers have essentially the same macroscopic structure as conventially nitrided surfaces. The outermost layer, which is very thin and consists mainly of iron nitrides, is known as white layer. With regard to the structure and the special character of this layer, the more recent term "compound layer" is seen. The compound layer is also called as diffusion zone where the nitrogen has mainly incorporated into the existing iron lattice as interstitial atoms or a finely dispersed alloy nitride precipitate. The thickness of the diffusion zone depends on the temperature, time of treatment and the alloy steel. The different structures of ion nitrided surface layers are usually achieved by means of the sputtering rate, which can be controlled by varying the voltage or the pressure and composition of the gas mixture. The mechanical properties of the materials, which are ductility, wear, resistance to rolling friction, are depended on two factors.. The homogeneity of the nitride structure. The thickness of the layer The ductility of the layer decreases with increasing layer thicknesses. The thickness of the compound layer is produced by ion nitriding is preset to the optimum is done by controlling the rate of sputtering. Good hardening of the surface of the alloy is obtained by means of a very intensive, finely dispersed precipitate of alloy nitrides. The treatment time, the alloy content and the nitriding temperature are therefore responsible for the depth and hardness of the diffusion zone. Ion nitriding is used in plastic and automotive industry.. Plastic Industry. Chrome coated moulds. The screws that are surface hardened. Packaged moulds Automotive Industry. Valves . Crankshafts. Gears. Forging moulds As I wrote before ion nitriding, is a process used to import surface hardening to metals for a wide variety of applications. As the name infers, the process uses ionised nitrogen alone, or in combination with other gases to react with the work surface. The ionised nitrogen also provides the characteristic purple glow around the pieces being treated. In this study the ion nitriding process is done to the 8620 steel and observed the hardness of the surface of the test pieces. We used %80 H2 + %20 N2 gases, and take the photographs of the surfaces of the test pieces.