directly from the still hot carburizing state (direct hardening). With induction hardening, the workpiece is heated by induced eddy currents. Case Hardening. Case hardening is a simple method of hardening steel. Carbon is infused with the metal at a depth specific per application. This carbon accumulation in the surface layer is also called carburisation. The shaft is the same dimensions as example 1 (30 x 500 mm). Since alloying elements generally reduce the critical cooling rate, deeper surface layers can be hardened with high-alloy steels. As a result of development work, carbonization processes are today possible up to a case hardening depth of 9 mm. The workpiece … However, because hardened metal is usually more brittle than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice. At 1450°F the case is only .005" (.13mm) after 1 hour but .016" (.4mm) at 1600°F after the same time. During nitriding, the alloyed steel is exposed to a nitrogenous environment at temperatures of about 500 °C. This austenitized state at about 750 °C is then quenched to achieve the desired martensite formation in the surface layer. Carburising with a carburising medium (e.g. For this purpose, the component is either cooled down to core hardening temperature after carburizing or brought to case hardening temperature. How does a liquid-in-glass thermometer work? The carbon can come from a solid, liquid or gaseous source; if it comes from a solid source the process is called pack carburizing. "Technological Transformations and Long Waves", "MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding", https://en.wikipedia.org/w/index.php?title=Case-hardening&oldid=978413143, Creative Commons Attribution-ShareAlike License, This page was last edited on 14 September 2020, at 19:32. Methods of Measuring Case Depth Hardness. It is suitable for steels that tend to form coarse grains during carburizing or for components that still require intermediate machining before hardening. Carburizing can be carried out in different ways. Other techniques are also used in modern carburizing, such as heating in a carbon-rich atmosphere. In addition, no (toxic) exhaust gases are produced during induction hardening compared to flame hardening. The Case Hardness Depth … 34CrAlMo5). 3. More information about this in the privacy policy. The term case-hardening is derived from the practicalities of the carburization process itself, which is essentially the same as the ancient process. This takes place with pure carbon but too slowly to be workable. In the early days, the steel was practically placed in a “case” of glowing coke. This, together with the hard and durable surface of a strong improvement in fatigue strength. Parts with shallow carbonitrided case, or which are primarily to resist wear may not be tempered such as dowel pins, washers, brackets, etc. With laser hardening, only small surfaces can be hardened economically. The pack is put inside a hot furnace for a variable length of time. This so-called self-quenching eliminates the need for quenching with water. The surface of the specimen to … tech. This is known as surface hardening. This process produces a thin, hard shell (between 0.25 and 0.75 mm, 0.01 and 0.03 inches) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is also important to distinguish between total case depth and effective case depth. 1117 case hardens to a depth or .045" at 8 hours of carburizing at 1700 deg F. Back off to 4 hours and … What are the advantages of induction hardening compared to flame hardening? Toothed wheels are typical cases where surface hardening is used. The material is then quenched to adjust the core properties. This has the advantage that the scaling is relatively low and the post-processing effort is reduced accordingly. Flame or induction hardening are processes in which the surface of the steel is heated very rapidly to high temperatures (by direct application of an oxy-gas flame, or by induction heating) then cooled rapidly, generally using water; this creates a "case" of martensite on the surface. The carburized section will have a carbon content high enough that it can be hardened again through flame or induction hardening. This techniques is used for steels with a low carbon content. The steel darkens significantly, and shows a mottled pattern of black, blue, and purple caused by the various compounds formed from impurities in the bone and charcoal. In these cases, the surfaces requiring the hardness may be hardened selectively, leaving the bulk of the part in its original tough state. Case hardening consists of carburizing hardening and tempering. It is also one of the case hardening processes in which the metal is heated in the temperature range of 871 to 954 º C. Here, the metal part is heated in the presence of the sodium cyanide. Many translated example sentences containing "case hardening depth" – German-English dictionary and search engine for German translations. Many translated example sentences containing "case hardening depth" – French-English dictionary and search engine for French translations. The hardened layer is called the case. Pinion was heat treated vertically one of two … With induction hardening, the austenitized surface is usually quenched by means of downstream water showers, which are pulled evenly over the workpiece together with the inductor. The steel work piece is placed inside a case packed tight with a carbon-based case-hardening compound. WHAT WHY HOW 2. case hardening depth Einsatzhärtungstiefe {f}tech. The reheating during single and double quench hardening makes these processes relatively energy- and time-intensive and therefore expensive. Cyaniding results in the formation of layer of thickness ranging from 0.25 mm to 0.75 mm. With flame hardening, burner flames are moved over the workpiece and quenched with water nozzles! At the same time, however, the hardenability of the material decreases due to the low carbon content, since the forced-dissolved carbon in the lattice in particular leads to the necessary formation of martensite. This means that the locally heated area is quickly quenched by the cooler surrounding areas. However, components such as toothed wheels must combine both contradictory properties: For such applications case hardening is suitable, which is generally structured as follows: In case hardening, a low-carbon steel (case hardening steel) with a maximum of 0.2 % carbon is first exposed to a carbon-containing environment. Thus, as a case-hardening process, nitriding is more expensive as compared to carburising or carbonitriding. Expensive alloy steels can only be nitrided and are used. chrome-molybdenum steels), it is therefore economically more sensible to quench the steel directly after carburizing from the already heated state. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. Case hardening depth is defined as the depth below the surface where the hardness decreased to 550 HV. Larger surface layers have to be scanned line by line with the laser. Although oxygen is required for this process it is re-circulated through the CO cycle and so can be carried out inside a sealed box. Common surface hardness after hardening and tempering is 58-62 HRC. Nitriding is used in particular to improve the fatigue strength of dynamically stressed components! A carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Surface hardening heat treatments are popular in the manufacture of steel products as a means of significantly improving strength and fatigue resistance and mitigating wear [1]. The hardened layer is called the case. This also results in relatively simple control of the hardening depth. Laser hardening is particularly suitable for areas that are very difficult to access, such as after a slow cooling from the reheated state (single and double quench hardening) or. Subsequently, the material is reheated to surface hardening temperature and then quenched in order to obtain optimum surface properties. It forms a thin layer of hardened alloy called a case. The case-hardening depth is approximately 2 mm and the hardness is 57-62 HRC by single-shot hardening. However, such a transformation could become a problem if a workpiece has to be dimensionally accurate, since the microstructure transformation generally leads to hardening distortion. Scaling and hardening distortion are less than with flame hardening! The surface hardening processes explained so far all have in common that the hard surface layer is achieved by a martensitic microstructure. There are several methods of case hardening for gears, including vacuum carburizing, atmosphere carburizing, and induction hardening. The quenching required for this can be either done. The major drawback of cyaniding is that cyanide salts are poisonous. Direct hardening is a special case hardening process. In the case of the nitriding process, this hardness depth is 1.2 mm. Parts that are subject to high pressures and sharp impacts are still commonly case-hardened. The advantage of this process is that it causes little distortion, so the part can be case-hardened after being quenched, tempered and machined. This techniques is used for steels with a low carbon content. For which steels is single or double quench hardening used in comparison to direct hardening? The hardness is achieved by the formation of nitrides. However, due to the permanent change in temperature, the hardness distortion in this double quench hardening is relatively high. This is why the process is also referred to as core hardening or single quench hardening from core hardening temperature. Above all, the increase in fatigue strength makes case hardening very interesting for dynamically stressed components such as gears or drive shafts. This method is generally used on metal alloys that have a low carbon content. These very large eddy currents of up to several thousand amperes per square millimeter lead to heating of the workpiece. Another common application of case-hardening is on screws, particularly self-drilling screws. For which steels is case hardening suitable and what are the mechanical properties of case hardened components? Since the heat input is limited only to the local focal spot of the laser, unnecessary heating of unwanted areas is avoided. A high-frequency alternating current is generated in a copper tool electrode (“primary coil”) which is adapted to the shape of the workpiece to be hardened. Depending on the application, different surface hardening methods have developed. Surface hardening is used to produce a hard and wear-resistant surface layer on steel workpieces, while the toughness in the core is largely retained. Case-hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. Another advantage of induction hardening is the more even heating of the surface, provided the inductor is optimally adapted to the workpiece. Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers. Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. Case depth is the thickness of the hardened layer on a specimen. Single quench hardening is a special case hardening process. The critical quench rate is thereby reduced, which in turn improves the surface hardenability of the steel. How is the depth of the hardening layer controlled during flame hardening? Both carbon and alloy steels are suitable for case-hardening; typically mild steels are used, with low carbon content, usually less than 0.3% (see plain-carbon steel for more information). After quenching, the hardened components are always tempered and thus obtain their final service properties. This generally also applies to the other surface hardening processes, as the unhardened core provides sufficient toughness. In such circumstances, case-hardening can produce a component that will not fracture (because of the soft core that can absorb stresses without cracking), but also provides adequate wear resistance on the hardened surface. The nitride layer also improves corrosion resistance. The advantage, however, is the grain refinement that occurs through the \(\gamma\)-\(\alpha\)-transformations. In the high-frequency range of up to several megahertz, even hardening depth of only a few tenths of a millimeter can be achieved. For this reason, it may only make sense to harden the surface of a workpiece so that the component core still retains its toughness (partial hardening). Single quench hardening specifically influences the properties of the surface (surface hardening) or the core (core hardening)! 19,000 Pound double helical pinion shaft carburized to an effective case depth of 0.250" being quenched into 20,000 gallons of agitated, warm oil. The slower the speed, the deeper the heat can penetrate and austenitize the microstructure and the thicker the hardened surface layer will be after quenching. After carburizing, the workpiece is first cooled slowly and then reheated to core hardening temperature or cooled to core hardening temperature immediately after carburizing. The thickness of the hardened layer is referred to as the case depth. 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