{"id":30174,"date":"2021-06-21T09:31:19","date_gmt":"2021-06-21T09:31:19","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30174"},"modified":"2023-09-14T14:23:42","modified_gmt":"2023-09-14T14:23:42","slug":"surface-hardening-case-hardening","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/metalworking\/surface-hardening-case-hardening\/","title":{"rendered":"Surface Hardening – Case Hardening"},"content":{"rendered":"
In materials science, hardness<\/strong> is the ability to withstand surface indentation<\/strong> (localized plastic deformation<\/strong>) and scratching<\/strong>. Hardness<\/strong> is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, an indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.<\/p>\n Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of the pure metal, we can use different ways, which include:<\/p>\n Case hardening<\/strong> or surface hardening<\/strong> is the process in which the hardness of an object\u2019s surface (case) is enhanced while the inner core of the object remains elastic and tough. After this process surface hardness<\/strong>, wear resistance,<\/b>\u00a0and fatigue life<\/strong> are enhanced. This is accomplished by several processes such as a carburizing or nitriding process by which a component is exposed to a carbonaceous or nitrogenous atmosphere at elevated temperature. As was written, two main material characteristics are influenced:<\/p>\n The case-hardening process involves infusing additional carbon or nitrogen into the surface layer for iron or steel with low carbon content, which has poor to no hardenability. Case hardening is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear and a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over hardening (that is, hardening the metal uniformly throughout the piece) because less expensive low-carbon and medium-carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections. An atomic diffusion from the ga搜索引擎优化us phase introduces a carbon- or nitrogen-rich outer surface layer (or case<\/em>). The case is normally 1 mm deep and harder than the material\u2019s inner core.<\/p>\n Case hardening by surface treatment can be further classified as diffusion or localized heating treatments. Diffusion methods introduce alloying elements that enter the surface by diffusion as solid-solution or hardenability agents that assist martensite formation during subsequent quenching. During this process, the alloying element concentration is increased at a steel component\u2019s surface. Diffusion methods include:<\/p>\n Localized heating methods for case hardening include:<\/p>\n Carburizing<\/strong> is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with an excellent capacity for contact load, good bending fatigue strength, and good resistance to seizure. Carburizing is usually used for low-carbon steels<\/a>, heated to a temperature sufficient to render the steel austenitic, followed by quenching and tempering to form a martensitic microstructure<\/a>. So that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core, in its earliest application, parts were placed in a suitable container and covered with a thick layer of carbon powder (pack carburizing). Today, the steel piece is exposed, at an elevated temperature (usually above 850\u00b0C), to an atmosphere richydrocarbonarbon gas, such as methane (CH4). In gas carburizing, commercially the most important variant of carburizing, the source of carbon is a carbon-rich furnace atmosphere produced either from gashydrocarbonsrbons, for example, methane (CH4<\/sub>), propane (C3<\/sub>H3<\/sub>), and butane (C4<\/sub>H10<\/sub>), or from vaporhydrocarbonrbon liquids. Heat enhances the diffusion of carbon into the steel surface and subsurface regions. The depth of diffusion (case depth) follows a time-temperature dependence such that:<\/p>\n Case depth <\/em>\u221d D .\u00a0<\/em>\u221a<\/em>Time<\/em><\/p>\n \u00a0<\/em>where the diffusivity factor, D<\/em>, depends on temperature, the chemical composition of the steel, and the concentration gradient of carbon at the surface. In terms of temperature, the diffusivity factor increases exponentially as a function of absolute temperature. This diffusion rate increases greatly with increasing temperature; the rate of carbon addition at 925\u00b0C is about 40% greater than at 870\u00b0C. The depth of any carburized case is a function of time and temperature.<\/p>\n Nitriding<\/strong> is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create a case-hardened surface. Nitriding produces hard, highly wear-resistant surface<\/strong> (shallow case depths) of product with a fair capacity for contact load, good bending fatigue strength, and excellent resistance to seizure. In contrast to carburizing, nitriding nitrogen is added into ferrite instead of austenite. Therefore nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite. A temperature is significantly lower, and a range of 500 to 550 \u00b0C is typically used. These processes are most commonly used on low-carbon and low-alloy steels<\/a>,\u00a0and they\u00a0are also used on medium and high-carbon steels, titanium, aluminium, and molybdenum. The most significant hardening is achieved with a class of alloy steels (nitralloy type) that contain approximately 1% Al. Typical applications include the production of machine components, shafts, axles, gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, or forging dies.<\/p>\n Carbonitriding<\/strong> is a hardening heat treatment that introduces carbon and nitrogen in the austenite of steel conducted from 1073 K to 1173 K. This treatment is similar to carburizing in that the austenite composition is changed. High surface hardness is produced by quenching to form martensite. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult-to-work grades of steel without the need for drastic quenching, resulting in less distortion and reducing the danger of cracking the work. The surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Carbonitriding (around 850 \u00b0C \/ 1550 \u00b0F) is carried out at temperatures substantially higher than plain nitriding (around 530 \u00b0C \/ 990 \u00b0F) but slightly lower than those used for carburizing (around 950 \u00b0C \/ 1700 \u00b0F) and for shorter times. It is often performed on power transmission parts, such as gear teeth, cams, shafts, and bearings, submitted to structural and surface fatigue operating conditions.<\/p>\n Boriding<\/strong>, also called boronizing is a thermochemical diffusion process similar to nitrocarburizing in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h) and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys. The resulting surface contains metal borides, such as iron borides, nickel borides, and cobalt borides. As pure materials, these borides have extremely high hardness and wear resistance.<\/p>\n Their favorable properties are manifested even when they are a small fraction of the bulk solid. The properties of boride layers are usually superior to those formed by nitriding and carburizing, particularly in terms of their hardness. Most borided steel surfaces will have iron boride layer hardnesses ranging from 1200-1600 HV<\/strong>. Nickel-based superalloys<\/a> such as Inconel and Hastelloy typically have nickel boride layer hardnesses of 1700-2300 HV. The hardness of the boride layer can be retained at higher temperatures than, for example, that of nitrided cases. On the other hand, both gas carburizing and plasma nitriding have the advantage over boronizing because those two processes offer reduced operating and maintenance costs, require shorter processing times, and are relatively easy to operate. Boriding is typically used for many high-performance applications such as automotive, machine tools, aerospace, hydraulic tools, agricultural and defense industries, etc.<\/p>\n\n
Surface Hardening – Case Hardening<\/h2>\n
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Classification of Case Hardening Methods<\/h2>\n
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Carburizing \u2013 Advantages and Application<\/h3>\n
Nitriding<\/h3>\n
Carbonitriding<\/h3>\n
Boriding<\/h3>\n
Titanium-nitride and Titanium-carbide Coatings<\/h3>\n