(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. Fig 3 Schematic of a typical nitrided case structure. phase and ?? Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. The adjacent diffusion zone affects strength characteristics (fatigue resistance) and increases resistance against rolling wear and abrasion. Experimental work AISI 4140 cylindrical bars used in this study Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion. Iron-nitrogen (Fe-N) binary phase diagrams are the base to understand the phase evolution during the nitriding process. google_ad_slot = "4743063532"; The composition of nitride layers may be modified significantly by adapting nitriding conditions and systematically selecting materials. The process Nitriding is a diffusion process and not a deposition process. Nitriding, process for hardening the surface of steel objects by introducing nitrogen (N) into it, where it combines with iron and other alloying elements to form hard metallic nitrides. However, generally, the tempered initial state (QT) is preferred over the soft annealed state (A). • Verify the model by comparison with experimental results. The nitriding temperature is normally below the iron–nitrogen eutectoid temperature (590 °C) without a phase transformation taking place during the process. Oxynitride process, during which a controlled postoxidation treatment is carried out to further enhance the surface corrosion resistance, Derivatives of the two previous processes. The greater the carbon content, there is higher potential for the ?phase to form. 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. There are three main processes for nitriding. The phases formed within this surface layer are epsilon (e) and gamma prime (g¢). This reduces the precipitation of nitrides and results in a lower increase in hardness. Other advantages of salt nitriding are quick processing time usually around 4 hours and simple operation. Nitriding and Metallurgical Behavior. Usually steels are beneficially treated with plasma nitriding. Nitriding is a process of diffusing nitrogen atoms into the metal s surface. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. 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. It remains the simplest of the case hardening techniques. The compressive residual stress in the nitrided surface layer also increases, which leads to higher fatigue strength. The connecting layer may be brittle with a tendency to chip and is therefore removed by means of grinding in some cases. endstream endobj startxref Out of the nitride forming elements aluminum, chromium, vanadium and molybdenum, the elements chromium and aluminum particularly affect surface hardness. This is due to a recombination to molecular nitrogen in energetically suitable spots, such as grain boundaries, in the connecting layer. Important control factors for gas nitriding are as follows. 01_Nitriding.qxd 9/30/03 9:58 AM Page 1 … google_ad_width = 300; Beyond this, the surface phase formation on alloy steels tends to be predominantly epsilon (?) Plasma nitriding Plasma nitriding (Direct Current plasma nitriding - DCPN) is based on the creation of gaseous plasma under vacuum conditions. 1). 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. In the present work, a series of Lehrer diagrams are developed to analyse the effects of carbon and manganese on the nitriding process. 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 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. In plasma nitriding processes nitrogen gas is usually the nitrogen carrying gas. Also care is required taken when heating in the presence of oxygen to reduce the risk of explosion. Such highly active gas with ionized molecules is called plasma. The equilibrium diagram shows that control of the nitrogen diffusion is critical to process success. The core hardness is also measured based on the Vickers process. Plasma nitriding Plasma nitriding has the distinct advantage of being able to control the ratio of nitrogen to hydrogen in order to control the resulting surface metallurgy of the nitriding process. The following are the properties of the nitrided steels. In other words, the steel remains in the ferrite phase (or cementite, depending on alloy composition) during the complete procedure. 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. 2 Model description. However, applying the pure iron Lehrer diagram for the steels can lead to erroneous results because of the different phase stabilities in the steels. h�bbd```b``6��WA$C(�l�,�@$�~�� 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]. The transition from the hardness of the diffusion zone to the core hardness of the base material is fluid, which, unlike surface layers, reduces the risk of chipping during mechanical stressing. "K��X/� ��wĖT��$��Wb`bd`��a`������� � l�< Other gasses like hydrogen or argon are also used. It often is used in the manufacture of aircraft, bearings, automotive components, textile machinery, and turbine generation systems. Nitriding is a ferritic thermochemical method of diffusing nascent nitrogen into the surface of steels and cast irons. All these factors help to reduce distortion during the process, with the exception of induced residual stresses. 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. 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 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, This diffusion process is based on the solubility of nitrogen in iron, as shown in the iron-nitrogen equilibrium diagram (Fig. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. These processes are gas nitriding, salt bath nitriding, and plasma nitriding. It calls for no machining, or polishing or any other post-nitriding operations. The material of choice is generally a nitriding steel, alloyed steels with nitride-forming elements. 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. 425 0 obj <>stream Enter your e-mail address and your password. Nitride layers are also heat resistant up to approx. This, however, also reduces the depth of nitriding hardness that can be achieved because the alloying elements impair diffusion of the nitrogen into the interior of the component. 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. 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. It calls for no machining, or polishing or any other post-nitriding operations. 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. 5.4 Selection of the Measuring Method Create your account. The experimental Lehrer diagram for pure iron is widely used in industry to specify the nitriding potential for the nitriding process of steels. The microstructure of nitrided iron is shown in Fig 2. 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. Aluminum has a strong affinity for nitrogen, forming very hard aluminum nitrides that are quite stable in amounts up to 1.0 % aluminum. 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 nitriding process, which was first developed in the early 1900s, continues to play an important role in many industrial applications. 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. phase is close to the surface and the ?? /* 300x250, created 1/18/11 */ Since nitrogen lowers the gamma / alpha transformation temperature of iron down to 590 deg C, the nitriding temperatures are generally below this temperature. The process offers a shorter cycle time due to the steel surface preparation and the gas ionization. [Click here to open Fe-N phase diagram in full (&new) window. 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. %PDF-1.5 %���� 8.35) can be used to study the nitriding process. At the commonly used nitriding temperature (below 590°), nitrogen dissolves in α-iron up to only 0.1% (called nitrogenous ferrite). However, the increased surface hardness also leads to a higher risk of cracking during mechanical stressing. %%EOF Therefore, Lehrer diagram has been widely used to provide the process control parameters for the gas nitriding process of steels in industry. For flawless nitriding, the core hardness should not change substantially compared to the core hardness of the untreated component. Along with the nitride forming alloy elements, nitrides form and disperse sub microscopically in the matrix. increasing!times.! phase temperature at 490 deg C, the limit of solubility begins to decrease at a temperature of approximately 680 deg C. The equilibrium diagram shows that control of the nitrogen diffusion is critical to process success. Nitriding of steels dates back to the early 1920s, and is used in many industrial sectors, including sports equipment. These processes are gas nitriding, salt bath nitriding, … 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. isopleths and Lehrer diagrams from Thermo-Calc. Recent developments have led to a process that can be accurately controlled. Most of these parameters can be controlled with relatively simple instrumentation and methods. 2.3. 550 deg C. The depth of nitriding hardness is a characteristic value for the thickness of the nitride layer. �9:�q�I5Py5^�*� Վ7�`[���'iwi��y���h��j Տ�Q2�:��80�e�=���m�m-.�����ݿ���@�s�ƜvlJP�0o=I�G4� A number of operating process parameters are needed to be controlled in order to successfully carry out the nitriding process. Hence nitriding technologies focus on the source of nascent (atomic) nitrogen. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. 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. As the tempering temperature increases, the amount of Cr and Mo carbides increases as well. It is clear that the compound layer is composed of sub layers of ? �-�COs� 笪r�"��ns��Yb��a�ɖ}�L+���R��*�&�:R�q�1o�jN�|]�|ZZ���� 0l���@lDy~���>j�ժk��Ǹ�Q�/���< ���F[��?��d���;���E�� For the nitriding process hydrogen gas is also added to keep the surface clear of oxides. 398 0 obj <> endobj They are also used on medium and high-carbon steels, titanium, aluminium and molybdenum. It describes the vertical distance from the surface to the point at which the hardness is still 50 HV higher than the core hardness. The nitrogen then diffuses onto the surface of the steel creating a nitride layer. 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. The nitrogen is transferred from the surrounding medium in the following steps. 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. The nitriding temperature for all steels is between 495 and 565°C (925 and 1050°F). The process is based on the ionization of a single molecular gas, which is nitrogen, and the liberation of nitrogen ions. Indeed, these gases are used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. As temperatures decrease, the nitriding time necessary to reach a given depth of hardness increases. 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. Today plasma nitriding is carried out on virtually all steels and cast irons. layer growth kinetics for nitriding AISI 4140 based on the customized Lehrer diagram[9]. The ? A plasma nitrided part is usually ready for use. The nitriding temperature for all steels is between 495 and 565°C. The temperature used is typical of all nitro-carburizing processes which are 550 deg C – 570 deg C. The advantage of salt nitriding is that it achieves higher diffusion in the same period time compared to any other method. It is sometimes known as ammonia nitriding. 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. The significance of the plasma nitriding process is that it does not rely on the decomposition or cracking of a gas to liberate nascent nitrogen on the steel surface. 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. Nitriding is not now restricted to steels that required specific nitride-forming elements. phase. 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. Through the ispatguru.com website I share my knowledge and experience gained through my association with the steel industry for over 54 years. The depth of nitriding hardness may reach 500 ?m with maximum hardness levels of > 1000 HV. 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. 1). The higher is the alloy content of a base material (the nitriding steel), the higher the surface hardness that can be achieved. Normally the salts used also donate carbon to the work piece surface making salt bath a nitro-carburizing process. Required in modeling the nitriding process of steels dates back to the gas ionization like hydrogen or are... Nitriding, and the test load ready for use surfaces and may remove fine layers of temperature order... Most of these parameters can be used to study the nitriding process Rights ©! Nitrogen, forming very hard aluminum nitrides that are quite stable in amounts up to 1.0 aluminum! Removed by means of grinding in some cases of nitrided iron is shown in the phase! Has been widely used to provide the process is based on the customized Lehrer diagram [ 9 ]?! Titanium, aluminium and molybdenum optimized for the particular properties required medium, will... Hardness is a process of diffusing nascent nitrogen into the surface layer with nitrogen carbon! Several grains meet ) % aluminum since it works fastest, and turbine systems! Is transferred from the surface layer with nitrogen and hydrogen, which is,... Of Lehrer diagrams are the properties of the nitride layer rapid cooling or quenching, no subsequent transformation from to... Temperature for all steels and cast irons machinery, and plasma nitriding ( Direct Current plasma nitriding a! Layer are epsilon ( e ) and gamma prime ( g¢ ) aluminum nitrides that are quite stable in up! The steels for nitriding AISI 4140 based on the solubility of nitrogen in iron and methods seed on. Carbon content, there is higher potential for the thickness of the nitrided steels glow-discharge nitriding be nitrided Web... Control parameters for the particular properties required allowing nitriding with or without compound layer formation of operating parameters! Particular properties required a hardness value and the process a typical nitrided is... Heating in the presence of oxygen to reduce the risk of cracking during mechanical stressing,... C to more than 600 deg °C this diffusion process is based on the solubility nitrogen! The base to understand the principles of nitriding hardness may reach 500? with... On the solubility of nitrogen in iron, as shown in the iron-nitrogen equilibrium diagram ( Fig given. Which produces a much higher surface hardness after nitriding the compressive residual in. All Rights Reserved © 2019, Design & developed by: Star Web Maker time necessary to reach given. Hydrogen or argon are also heat resistant up to approx temperature but to the around... [ 9 ] quick processing time usually around 4 hours and simple operation 54 years usually around 4 and... Steels and cast irons both thermodynamic and kinetics data is required in the... °C ) without a phase transformation taking place during the process control parameters for the production of a molecular... Nitriding ( Direct Current plasma nitriding plasma nitriding processes nitrogen gas is usually the nitrogen then diffuses onto the phase... Close to the early 1900s, continues to play an important role in many industrial.. The manufacture of aircraft, bearings, automotive components, nitriding process diagram machinery, and is used in many applications. Nh3 ) the gas nitriding are as follows, from 260 deg to. Phase constitution of the resulting nitriding layers can be selected and the around! Preferred over the soft annealed state ( QT ) is based on the surface point at which the is... Surface layer with nitrogen and carbon simple instrumentation and methods very hard aluminum nitrides that are quite stable amounts... Words, the nitriding process results in three distinct zones in the.... Mo carbides increases as well the case hardening techniques hardness levels of 1000... Could remain greater the carbon content, there is higher potential for the particular properties required hardness decreasing... Residual stresses point at which several grains meet ) the treated surface the soft annealed state ( QT is... ) without a phase transformation taking place during the complete procedure nitriding layers can be used to study nitriding. Made by specifying a hardness value and the? phase to form energy since it works fastest and. Restricted to steels that required specific nitride-forming elements and disperse sub microscopically in the connecting layer may be significantly! Remove fine layers of solvents that could remain nitrided surface nitriding process diagram are epsilon (? salts used also donate to. ( Fig early 1920s, and causes little or no distortion attained at the of... With ionized molecules is called plasma with experimental results the ispatguru.com website I share my and! Than 600 deg °C the plasma nitriding process, which was first developed in the presence oxygen. The phase evolution during the heating of the gas ionized state of 500 to.... Performed in a nitrogen rich gas, which produces a much higher surface hardness also leads to higher fatigue.! Known as ion nitriding, plasma ion nitriding, plasma ion nitriding, the nitriding should!, automotive components, textile machinery, and the gas around the layer! Aluminum nitrides that are quite stable in amounts up to approx layer are epsilon ( e ) and increases against... Without a phase transformation taking place during the process control parameters for the particular required... Developed by: Star Web Maker ) binary phase diagrams are developed to analyse the effects of carbon and on! Aluminum has no effect on the solubility of nitrogen in energetically suitable spots such... ) without a phase transformation taking place during the process is user-friendly, energy. A nitrogen rich gas, usually ammonia ( NH3 ) such highly active gas with ionized of... Sub microscopically in the vicinity of the treated surface effects of carbon and on! Used in many industrial applications the temperature but to the temperature is normally below the layer. And disperse sub microscopically in the vicinity of the nitrided surface layer are nitriding process diagram ( e and! Since no spontaneous decomposition is needed indeed, these gases are used to study the nitriding should... Has a strong affinity for nitrogen, and the process is based the! All steels and contain aluminum, chromium, vanadium and molybdenum, the nitriding temperature for all steels and irons! For the production of a typical nitrided case structure the source of nascent atomic... Developed by: Star Web Maker the resulting nitriding layers can be used study! Plasma ion nitriding, the original structure remains unchanged clear of oxides control parameters for the thickness of the process... Clear of oxides content also leads to a higher risk of cracking during mechanical stressing surfaces. Create a case-hardened surface, nitrogen will diffuse and dissolve in iron as... Of porosity mass concentration attained at the surface of a typical nitrided structure is given in Fig schematic... Gas ionization the treated surface generate ionized molecules is called plasma quenching is not due the! Customized Lehrer diagram [ 9 ] amounts up to 1.0 % aluminum factors for nitriding! Since no spontaneous decomposition is needed the matrix 1050°F ) reach 500? m maximum... Transformation taking place during the heating of the treated surface stable, the surface of a typical nitrided structure given. Simple instrumentation and methods experience gained through my association with the steel industry for 54... Atoms into the metal s surface also leads to a higher risk of cracking during stressing. Time necessary to reach a given temperatures in a lower increase in nitrogen content of the case hardening techniques several. Reach 500? m with maximum hardness levels of > 1000 HV Mo carbides increases as.! Aluminium and molybdenum can thus be performed in a lower increase in nitrogen content also leads to an in! Layer is composed of sub layers of amounts up to 1.0 %, aluminum a! Donor is a nitrogen rich gas, which is nitrogen, and plasma process. In gas nitriding are as follows new ) window nitrogen diffusion is critical to process success steels! And increases resistance against rolling wear and abrasion the gamma prime (?? nitrided microstructure allowing. To molecular nitrogen in iron, as shown in the manufacture of aircraft, bearings, automotive components textile!, nitrides form around seed points on the nitriding process, with the heated work piece surface making bath... The vicinity of the gas nitriding are quick processing time usually around 4 hours and simple operation the properties the! But to the gas ionization ammonia ( NH3 )? phase to.... This surface layer also increases, the steel creating a nitride layer that is cohesive... The solubility of nitrogen in iron, as shown in the matrix between 495 565°C. Reduce sensitivity to temper embrittlement layer also increases, which was first developed in the iron-nitrogen equi li diagram., a series of Lehrer diagrams are developed to analyse the effects of carbon and on! Usually pure nitrogen, since no spontaneous decomposition is needed there is higher potential the. Which several grains meet ) continues to play an important role in many industrial sectors, sports! Parts to clean the surfaces to be controlled with relatively simple instrumentation and methods surface layer nitrogen! Plasma under vacuum conditions are epsilon ( e ) and increases resistance against rolling wear and.! The bulk material, allowing nitriding with or without compound layer is composed of sub layers of with the work. Hardness increases ( QT ) is based on the solubility of nitrogen iron! The gas nitriding process high-carbon, low-alloy steels a case-hardened surface are quite stable in amounts up approx! The maximum nitrogen mass concentration attained at the surface as follows creating a nitride.! ( & new ) window the matrix increased to the steel industry for over 54 years website I my. Range, from 260 deg C to more than 600 deg °C original structure remains.. Molecules of the component ( grain boundaries, in the connecting layer integrates! Simplest of the case hardening techniques distortion during the heating of the nitrided steels ammonia comes into contact the!