increasing!times.! The experimental Lehrer diagram for pure iron is widely used in industry to specify the nitriding potential for the nitriding process of steels. As the nitrogen concentration and nitriding time increase, the nitrides grow deeper and expand laterally into the grains until a closed layer has been formed. It calls for no machining, or polishing or any other post-nitriding operations. Nitriding is customarily conducted preferably in a temperature range of 500 to 550°C. The salt bath process uses the principle of the decomposition of cyanide to cyanate and the liberation of nitrogen within the salt for diffusion into the steel surface. It is also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding. Principal reasons for nitriding are: Fig 3 Schematic of a typical nitrided case structure. Today plasma nitriding is carried out on virtually all steels and cast irons. It is sometimes known as ammonia nitriding. google_ad_client = "ca-pub-7057920448327527"; There are three main processes for nitriding. The plasma nitriding process results in three distinct zones in the vicinity of the treated surface. Out of the nitride forming elements aluminum, chromium, vanadium and molybdenum, the elements chromium and aluminum particularly affect surface hardness. Create your account. layer growth kinetics for nitriding AISI 4140 based on the customized Lehrer diagram[9]. Lehrer diagram, as modified by L. Maldzinski for nitrogen concentration by Nitriding layers (Epsilon, gamma prime and alpha) These more controlled processes allow the elimination of final grinding, thus reducing the cost of the whole manufacturing process. The drawing specification is made by specifying a hardness value and the test load. This also helps the thermal stability of the plasma unit, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. The phases formed within this surface layer are epsilon (e) and gamma prime (g¢). "K��X/� ��wĖT��$��Wb`bd`��a`������� � l�< Nitriding is a ferritic thermo-chemical method of diffusing nascent nitrogen into the surface of steels and cast irons. PROCESS-ELECTRONIC GmbH, A member of United Process Controls, Heiningen, Germany km.winter@process-electronic.com, phone +49 7161 94 888 0 Abstract Expert systems for gaseous nitriding, be it simulators or controllers, are largely based on the Lehrer Diagram, which shows the correlation between nitrogen-iron phases, phase, which is not shown on the iron – nitrogen equilibrium diagram at Fig 1, exists from 11 % to 11.35 % N at temperatures below approximately 500 deg C. The solubility limit of nitrogen in iron is temperature dependent, and at 450 deg C the iron base alloy absorbs up to 5.7 % to 6.1 % of nitrogen. However, applying the pure iron Lehrer diagram for the steels can lead to erroneous results because of the different phase stabilities in the steels. The nitriding temperature should be below the tempering temperature in order to keep the core hardness from decreasing. When treated a given temperatures in a nitrogen containing medium, nitrogen will diffuse and dissolve in iron. Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating (Ac 1) as defined by the Iron-Carbon Phase Diagram phase is near to the diffusion zone. The nitriding process (Fig. The lower limit for the nitriding temperature is considered to be 350 deg C, because below this temperature, nitrogen diffusion does not occur on a rate that can be exploited technologically or economically. The model can be used to calculate concentration profiles of nitrogen as a function of time and position during the nitriding process and to predict the thickness of the compound layer. phase. 5.4 Selection of the Measuring Method isopleths and Lehrer diagrams from Thermo-Calc. Figure 9: Schematic diagram of the automat ic system of the nitriding process with th evisualization system for the course of the layer growth [3 7]. m� 8.35) can be used to study the nitriding process. In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3). The greater the carbon content, there is higher potential for the ?phase to form. Above 1.0 %, aluminum has no effect on the resultant nitriding hardness. This is greatly influenced by the carbon content of the steel. The advantages of gas nitriding process are (i) precise control of chemical potential of nitrogen in the nitriding atmosphere by controlling gas flow rate of nitrogen and oxygen, (ii) all round nitriding effect, (iii) large batch sizes possible, (iv) with modern computer control of the atmosphere the nitriding results are closely controlled, and (v) relatively low equipment cost. It calls for no machining, or polishing or any other post-nitriding operations. The knowledge of both thermodynamic and kinetics data is required in modeling the nitriding process. 398 0 obj <> endobj %%EOF YzORv��,[��^q[�aW�ܩ�yP�@�(|�&j���x�?��K=�[�����Ex�m�w�? Also care is required taken when heating in the presence of oxygen to reduce the risk of explosion. Beyond this, the surface phase formation on alloy steels tends to be predominantly epsilon (?) Iron-nitrogen (Fe-N) binary phase diagrams are the base to understand the phase evolution during the nitriding process. %PDF-1.5 %���� With a nitrogen content of a few percent, a nitride layer that is mostly cohesive (connecting layer) forms on the surface. 3. All these factors help to reduce distortion during the process, with the exception of induced residual stresses. google_ad_height = 250; According to DIN EN 10052:1994-01, nitriding is defined as the thermo-chemical treatment of a work piece in order to enrich the surface layer with nitrogen. Workpieces made of ferrous materials exhibit better wear, strength and corrosion properties after nitriding. Since nitrogen lowers the gamma / alpha transformation temperature of iron down to 590 deg C, the nitriding temperatures are generally below this temperature. For instance, at moderate temperatures (like 420 deg C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties. With increasing holding times at nitriding temperature there are no changes in the phase composition but the distortion of the crystal lattice, manifest in line broadening, increases. /* 300x250, created 1/18/11 */ • Verify the model by comparison with experimental results. The process offers a shorter cycle time due to the steel surface preparation and the gas ionization. What can (and does) produce distortion are the induced surface stresses being released by the heat of the process, causing movement in the form of twisting and bending. phase is close to the surface and the ?? The compound zone which is developed at a high value of nitriding potential K N = p NH 3 / p H 2 3 / 2 (the nitriding potential is directly proportional to the nitrogen activity in the gas mixture) is in accordance with Lehrer's diagram : at the surface, ε phase (Fe 2,3 N) developed, while phase γ′ (Fe 4 N) lies directly adjacent to the diffusion zone (Figure 1). (adsbygoogle = window.adsbygoogle || []).push({}); The basic of the nitriding process is that it does not require a phase change from ferrite to austenite, nor does it require a further change from austenite to martensite. 2 Model description. At the beginning of the nitriding process (0.5 h) all phases corresponding to the Fe−N phase diagram are observed in Armco iron and the steels tested. Most of the commercially used control systems are based on the Lehrer Diagram … The higher is the alloy content of a base material (the nitriding steel), the higher the surface hardness that can be achieved. Nitride layers are also heat resistant up to approx. This layer is connected to a diffusion zone, in which the precipitated nitrides are evenly distributed in the steel matrix and which results in hardening, particularly for alloyed steels. The following are the properties of the nitrided steels. Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. Nitrogen is plentiful on Earth, however, in nature it exists as a two-atom molecule, chemically inert and too large to penetrate the surface. The nitriding temperature for all steels is between 495 and 565°C. Also, since only free cooling takes place, rather than rapid cooling or quenching, no subsequent transformation from austenite to martensite occurs. Nitriding is a process of diffusing nitrogen atoms into the metal s surface. At the commonly used nitriding temperature (below 590°), nitrogen dissolves in α-iron up to only 0.1% (called nitrogenous ferrite). endstream endobj startxref endstream endobj 399 0 obj <>>>/Filter/Standard/Length 128/O(A�B�4�|r��Hg�#v@~���\\5?h\()/P -1852/R 4/StmF/StdCF/StrF/StdCF/U(��� �f�mb��S� )/V 4>> endobj 400 0 obj <> endobj 401 0 obj <> endobj 402 0 obj <>stream It will take less then a minute, Bulk Material Storage and Storage Yard Machines, Technological and other processes/equipments associated with steel industry, Management in steel plant along with training and development, Raw materials and other materials used in steel plants, Marketing Concepts and Comparison with Selling Concepts, Role of Safety and its Importance in a Steel Organization, Role of Leadership in the Management of Organizations, Process pressure inside the sealed process chamber, Gas delivery pressure system into the sealed process chamber, Exhaust gas system from the sealed process chamber, Control of the preheat treatment procedure prior to nitriding, including stress relief and pre-hardening and tempering, Quality and integrity of the steel surface pre-cleaning prior to nitriding, Consistent steel chemistry to maximize “nitridability”, Adsorption of nitrogen atoms on the surface of the component, Absorption of (nitrogen atoms) by the component surface, Diffusion of the nitrogen atoms along the grain boundaries and within the grains, Improved heat resistance to about 500 deg C. Recent developments have led to a process that can be accurately controlled. Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature in contact with a nitrogenous gas, usually ammonia. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion. The process is based on the ionization of a single molecular gas, which is nitrogen, and the liberation of nitrogen ions. In order to understand the principles of nitriding, first let us look at the iron-nitrogen equi li brium diagram. In the present work, a series of Lehrer diagrams are developed to analyse the effects of carbon and manganese on the nitriding process. Quenching is not required for the production of a hard case. As the temperature is further increased to the gamma prime (??) 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