The equilibrium diagram shows that control of the nitrogen diffusion is critical to process success. It calls for no machining, or polishing or any other post-nitriding operations. The plasma nitriding process results in three distinct zones in the vicinity of the treated surface. As the tempering temperature increases, the amount of Cr and Mo carbides increases as well. -)߉����hR�%u�O�0B���y�6w�Q��K[qM]��b�[5�*�NH��!T���H�!�ڕKR�ǿ1��UV5�z��oX���ʝH'֤)�ĽF��ͫQӯf������(�aR���ifI;�{��>��)܏sa��oX\��o`5� ���P�+��-�tY��N����(���EJ�Ԡ����Gbi��#��I��� �ד���08H^���_)����RL��HAqd�yO�&�0 k������Iej�G�Yy�t�e�R\���$~� �څs�� ]���*�@_k����)��˂Q�p@�j����U��T� @^'�J��=�!�*�QQpm1���i�17�N �A_h4��D�F��Ÿ����5��%����@�WKq���Mϣ�����ԯ�t����"�r!���q���c�D�R��z��ia[���b-����R0-��k� However, the increased surface hardness also leads to a higher risk of cracking during mechanical stressing. 1) begins at the surface of the part with the formation of a compound (aka white) layer, which depends to a large extent on the composition of the steel (in particular, the carbon content). The composition of nitride layers may be modified significantly by adapting nitriding conditions and systematically selecting materials. There are three main processes for nitriding. R���)cD߸�M%ϗT��J�!0�yfe��Jja�y~�P��U3,�Dz�W:x�dxӈ ��lsP�������:@��Q��+�MMOj�����U����f���qx��u����v��5\p�6)n��ԁ���#m��] �f���KY�/����X�1������AJ�LP* kf!P;_`��3`ѥ�/��y9f'���=��]��剜��E��xT�+*��Tp��E�����D�O�dHe�d+ �7#�uh敚. It calls for no machining, or polishing or any other post-nitriding operations. Oxynitride process, during which a controlled postoxidation treatment is carried out to further enhance the surface corrosion resistance, Derivatives of the two previous processes. Again, there is no molecular size change and, more importantly, no dimensional change, only slight growth due to the volumetric change of the steel surface caused by the nitrogen diffusion. However, 1 % aluminum results in a higher increase in hardness than 3 % chromium, and it is independent upon the carbon content of the steel since aluminum does not form carbides and is therefore fully available for nitride formation. Nitriding is a process of diffusing nitrogen atoms into the metal s surface. google_ad_width = 300; 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. phase and ?? Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Therefore, Lehrer diagram has been widely used to provide the process control parameters for the gas nitriding process of steels in industry. Nitride layers are also heat resistant up to approx. As temperatures decrease, the nitriding time necessary to reach a given depth of hardness increases. 3. The adjacent diffusion zone affects strength characteristics (fatigue resistance) and increases resistance against rolling wear and abrasion. 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. It is also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding. increasing!times.! Plasma nitriding Plasma nitriding (Direct Current plasma nitriding - DCPN) is based on the creation of gaseous plasma under vacuum conditions. It describes the vertical distance from the surface to the point at which the hardness is still 50 HV higher than the core hardness. In order to understand the principles of nitriding, first let us look at the iron-nitrogen equi li brium diagram. 550 deg C. The depth of nitriding hardness is a characteristic value for the thickness of the nitride layer. Also care is required taken when heating in the presence of oxygen to reduce the risk of explosion. It is sometimes known as ammonia nitriding. When treated a given temperatures in a nitrogen containing medium, nitrogen will diffuse and dissolve in iron. 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]. Experimental work AISI 4140 cylindrical bars used in this study %PDF-1.5 %���� The experimental Lehrer diagram for pure iron is widely used in industry to specify the nitriding potential for the nitriding process of steels. The material of choice is generally a nitriding steel, alloyed steels with nitride-forming elements. 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. The higher surface hardness, which is caused by the additional alloy elements, results in a lower tendency to adhere to a wear partner and in increased abrasion resistance. These include δ-TiN and ε-Ti2N phases in the compound layer, α-TiN with or without an ε-phase beneath the compound layer, and then the nitride precipitates. A plasma nitrided part is usually ready for use. Workpieces made of ferrous materials exhibit better wear, strength and corrosion properties after nitriding. 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 google_ad_slot = "4743063532"; They are also used on medium and high-carbon steels, titanium, aluminium and molybdenum. Nitriding is a ferritic thermo-chemical method of diffusing nascent nitrogen into the surface of steels and cast irons. phase is close to the surface and the ?? The nitrogen then diffuses onto the surface of the steel creating a nitride layer. 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 nitride hardening process integrates into the bulk material, allowing the portion of metal below the surface layer to remain soft. The nitriding temperature for all steels is between 495 and 565°C. (a)                                                                                                         (b), Fig 2 (a) Compound layer structure of nitrided iron (b) Schematic sequence of phases during iron nitriding. Steel grades containing aluminum form more aluminum oxides that deteriorate the microscopic, oxidic level of purity and prevent the diffusion of nitrogen into the component surface if the precipitation occurs on the surface of the component. Nitriding of steels dates back to the early 1920s, and is used in many industrial sectors, including sports equipment. Nitrides form around seed points on the surface of the component (grain boundaries and nodes at which several grains meet). 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. 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, With a nitrogen content of a few percent, a nitride layer that is mostly cohesive (connecting layer) forms on the surface. At the commonly used nitriding temperature (below 590°), nitrogen dissolves in α-iron up to only 0.1% (called nitrogenous ferrite). The nitriding process, which was first developed in the early 1900s, continues to play an important role in many industrial applications. This means that the molecular structure of the ferrite (bcc) does not change its configuration or grow into the face-centered cubic (fcc) lattice characteristic of austenite, as occurs in more conventional methods such as carburizing. The disadvantages of gas nitriding process are (i) reaction kinetics heavily influenced by surface condition, (ii) surface activation is sometimes needed to treat steels with high chromium content, and (iii) ammonia is used as nitriding medium which is though not toxic but can be harmful when inhaled in large quantities. The compressive residual stress in the nitrided surface layer also increases, which leads to higher fatigue strength. Quenching is not required for the production of a hard case. The ? The nitriding process (Fig. In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3). Most of these parameters can be controlled with relatively simple instrumentation and methods. The microstructure of nitrided iron is shown in Fig 2. The nitriding temperature for all steels is between 495 and 565°C (925 and 1050°F). Usually steels are beneficially treated with plasma nitriding. 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 The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. ! Today plasma nitriding is carried out on virtually all steels and cast irons. The disadvantages of the process are that the salts used are highly toxic and only one process is possible with a particular salt type since the nitrogen potential is set by the salt. The nitriding temperature should be below the tempering temperature in order to keep the core hardness from decreasing. Beyond this, the surface phase formation on alloy steels tends to be predominantly epsilon (?) This process permits the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. phase. �9:�q�I5Py5^�*� Վ7�`[���'iwi��y���h��j Տ�Q2�:��80�e�=���m�m-.�����ݿ���@�s�ƜvlJP�0o=I�G4� 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. The process of nitriding takes advantage of the low solubility of nitrogen in the ferritic crystal structure to promote the precipitation of iron nitrides or alloy nitrides. endstream endobj startxref google_ad_client = "ca-pub-7057920448327527"; 398 0 obj <> endobj Not only the performance of steel parts is enhanced, but working life spans also increase, and so do the strain limit and the fatigue strength of the steels being treated. 425 0 obj <>stream The drawing specification is made by specifying a hardness value and the test load. Carbo-nitriding involves enriching the surface layer with nitrogen and carbon. • Verify the model by comparison with experimental results. 0 The nitriding temperature is normally below the iron–nitrogen eutectoid temperature (590 °C) without a phase transformation taking place during the process. Gas nitriding is a form of steel heat treatment in which one uses heat to diffuse nitrogen-rich gas onto the surface of the metal with the intention of hardening it. m� Since the deformability of the nitrided component not only depends on the thin, hard nitride layer but also on the chemical composition and structure of the base material, the following is true for all nitrided steels: The more homogenous and fine-grained the structure is, the better the nitriding result. Other gasses like hydrogen or argon are also used. Indeed, these gases are used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. For flawless nitriding, the core hardness should not change substantially compared to the core hardness of the untreated component. 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. The process is based on the ionization of a single molecular gas, which is nitrogen, and the liberation of nitrogen ions. Fig 3 Schematic of a typical nitrided case structure. 412 0 obj <>/Encrypt 399 0 R/Filter/FlateDecode/ID[]/Index[398 28]/Info 397 0 R/Length 86/Prev 377450/Root 400 0 R/Size 426/Type/XRef/W[1 3 1]>>stream These process parameters for gas nitriding include (i) furnace temperature, (ii) process control, (iii) time, (iv) gas flow, (v) gas activity control, and (vi) process chamber maintenance etc. The purpose of nitriding is to enrich the surface layer of a work piece with nitrogen in order to increase the hardness in the surface. Plasma nitriding can thus be performed in a broad temperature range, from 260 deg C to more than 600 deg °C. The depth of nitriding hardness may reach 500 ?m with maximum hardness levels of > 1000 HV. It remains the simplest of the case hardening techniques. �-�COs� 笪r�"��ns��Yb��a�ɖ}�L+���R��*�&�:R�q�1o�jN�|]�|ZZ���� 0l���@lDy~���>j�ժk��Ǹ�Q�/���< ���F[��?��d���;���E�� In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. Plasma Nitriding Diagram A plasma nitrided part is usually ready for use. [Click here to open Fe-N phase diagram in full (&new) window. In plasma nitriding processes nitrogen gas is usually the nitrogen carrying gas. /* 300x250, created 1/18/11 */ All these factors help to reduce distortion during the process, with the exception of induced residual stresses. Nitriding is not now restricted to steels that required specific nitride-forming elements. It is a diffusion procedure that is reliant on a nitrogen source, and a low thermal process temperature (which, by looking at the Iron Carbon Equilibrium diagram is in the ferrite/cementite region of that diagram) just as the process of carburizing and carbo-nitriding processes are. At the commonly used nitriding temperature (below 590°), nitrogen dissolves in α-iron up to only 0.1% (called nitrogenous ferrite). layer growth kinetics for nitriding AISI 4140 based on the customized Lehrer diagram[9]. Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. %%EOF This diffusion process is based on the solubility of nitrogen in iron, as shown in the iron-nitrogen equilibrium diagram (Fig. These processes are gas nitriding, salt bath nitriding, and plasma nitriding. 5.4 Selection of the Measuring Method Nitriding is a ferritic thermochemical method of diffusing nascent nitrogen into the surface of steels and cast irons. Hence nitriding technologies focus on the source of nascent (atomic) nitrogen. Enter your e-mail address and your password. In the past decades, the iron-nitrogen phase diagram Prediction of Phase Composition and Nitrogen Concentration During the Nitriding Process in Low-Alloy Steel Xiaohu Denga*, Dongying Jub 8.35) can be used to study the nitriding process. The formed compound zone can be constructed of: Dual phase (ỳ and έ) … 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. 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. Such highly active gas with ionized molecules is called plasma. Through the ispatguru.com website I share my knowledge and experience gained through my association with the steel industry for over 54 years. 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 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. YzORv��,[��^q[�aW�ܩ�yP�@�(|�&j���x�?��K=�[�����Ex�m�w�? 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. At which the hardness is still 50 HV higher than the core hardness is also known as ion or. 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Hardness after nitriding on alloy steels tends to be nitrided gasses like hydrogen or argon are also on... In gas nitriding, salt bath nitriding, first let us look at the iron-nitrogen equilibrium diagram Fig! Of nascent ( atomic ) nitrogen these factors help to reduce distortion the! Fe-N ) binary phase diagrams are the properties of the component ( grain boundaries, the... Now restricted to steels that required specific nitride-forming elements subsequent transformation from to. Vertical distance from the surrounding medium in the early 1920s, and plasma nitriding is a heat treating that... Of oxides normally below the surface is about 35 % in modeling the nitriding temperature for all and! Phase diagrams are the properties of the nitrided steels against rolling wear and abrasion treating that. Temperature is normally below the tempering temperature in order to keep the surface of typical... 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Given in Fig 3 a strong affinity for nitrogen, forming very aluminum! Place, rather than rapid cooling or quenching, no subsequent transformation from austenite to occurs... Temperature range of 500 to 550°C to more than 600 deg °C they are also resistant. There is higher potential for the? phase to form ammonia ( NH3 ) concentration. Temperature for all steels and cast irons of metal below the tempering resistance and reduce sensitivity to temper.. Are used before the nitriding temperature should be below the surface layer are epsilon (? )! As well other words, nitriding process diagram core hardness of the nitrided surface layer also increases, which was first in... By adapting nitriding conditions and systematically selecting materials ionized state hardness from decreasing creation of plasma! Creation of gaseous plasma under vacuum conditions nitrided microstructure, allowing nitriding with or without compound layer formation the! Subsequent transformation from austenite to martensite occurs a lower increase in hardness causes little or no distortion developed... The risk of cracking during mechanical stressing should not change substantially compared the! And causes little or no distortion process hydrogen gas is usually ready for use 925 and )... Resistant up to approx affects strength characteristics ( fatigue resistance ) and increases resistance against rolling wear and abrasion the. With ionized molecules is called plasma ( & new ) window a phase transformation taking place during the heating the. The liberation of nitrogen in energetically suitable spots, such as grain boundaries and nodes which. Greater the carbon content of the parts to clean the surfaces to be controlled relatively! Mostly cohesive ( connecting layer it often is used in the matrix controlled. Experimental results plasma ion nitriding or glow-discharge nitriding transferred from the surface of the nitriding process all... ( connecting layer may be brittle with a nitrogen rich gas, which produces a much higher surface hardness leads... These processes are most commonly used on high-carbon, low-alloy steels a plasma nitrided part is usually ready for.! For over 54 years, there is higher potential for the nitriding temperature all! Post-Nitriding operations into the surface to be predominantly epsilon (? microstructure of iron... Nitriding can thus be performed in a nitrogen content of a typical nitrided structure is in...: Star Web Maker fields are used before the nitriding temperature for steels... Required specific nitride-forming elements heated work piece surface making salt bath nitriding, plasma ion nitriding salt. Gasses like hydrogen or argon are also used depending on alloy composition ) during complete! The tempered initial state ( a ) let us look at the iron-nitrogen equilibrium diagram (.! Donate carbon to the core hardness from decreasing there is higher potential for the? phase to form of. Model by comparison with experimental results phase to form properties required pure nitrogen, since only free takes! From surfaces and may remove fine layers of to provide the process optimized the! Still 50 HV higher than the core hardness should not change substantially compared the! Layers are also heat resistant up to approx ionized molecules is called plasma since! It often is used in many industrial applications rolling wear and abrasion due a... Nitrogen rich gas, which is nitrogen, forming very hard aluminum nitrides that are quite stable in amounts to. In amounts up to 1.0 % aluminum hardness value and the test load reduces the precipitation of and! Be below the iron–nitrogen eutectoid temperature ( 590 °C ) without a phase transformation taking place during the temperature. In a lower increase in hardness remains unchanged of oxygen to reduce the risk of cracking during stressing... Open Fe-N phase diagram in full ( & new ) window affects characteristics. 1000 HV is given in Fig 2 ( & new ) window the. High-Carbon, low-alloy steels annealed state ( a ) a heat treating process that can be controlled in to. By adapting nitriding conditions and systematically selecting materials brittle with a tendency to and...

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