Numerous industrial applications require a hard wear-resistant surface called the case, and a relatively soft, tough core.
There is no technical limit to the depth of hardening with carburizing techniques, but it is not common to carburize to depths in excess of 0.050 in. All of the carburizing processes (pack, gas, liquid) require quenching from the carburizing temperature or a lower temperature or reheating and quenching.
Commercial carburizing may be carried out by means of pack carburizing, gas carburizing and liquid carburizing. In pack carburizing, the work is surrounded by a carburizing compound in a closed container.
Commercial carburizing compounds usually consist of hardwood charcoal, coke and about 20 percent of barium carbonate. It is difficult to quench the part immediately, as the sealed pack has to be opened and the part removed from the pack.
This method is efficient and economical for individual processing of small lots of parts or of large, massive parts. An endothermic gas atmosphere can be prepared by reaction of relatively rich mixture of air and hydrocarbon gas (usually natural gas) in an externally heated generator in the presence of a nickel catalyst. This method allows the surface carbon to be reduced to any desired value by using a diffusion period, during which the gas is turned off but temperature maintained. Most carburizing gases are flammable and controls are needed to keep carburizing gas at 1700oF from contacting air (oxygen). In this method, the material is placed in a bath of molten cyanide so that carbon will diffuse from the bath into the metal and produce a case comparable with one resulting from pack or gas carburizing.
Cycle times for liquid cyaniding is much shorter (1 to 4 hours) than gas and pack carburizing processes. In general, the method is suitable for small and medium size parts as it is difficult to process large parts in a salt bath. However, as cyanide salts are poisonous, this method requires careful attention and the parts must be thoroughly washed after heat treatment to prevent rusting. The steel is usually hardened and tampered between 1100 and 1300A°F to produce a sorbitic structure of maximum core toughness and then nitrided.
The parts to be nitrided are placed in an airtight container through which the nitriding atmosphere is supplied continuously. The white layer is brittle and tends to chip from the surface if its thickness is more than 0.0005 inch. Hardest cases, approximately RC 70 are obtained with aluminium alloy steels known as Nitralloys. Nitriding is used extensively for aircraft engine parts like cams, cylinder liners, valve stems, shafts and piston rods. Cases that contain both carbon and nitrogen are produced in liquid salt baths (cyaniding) or by use of gas atmosphere (carbonitriding).
Since nitrogen increases the hardenability, carbonitriding the less expensive carbon steels for many applications will provide properties equivalent to those obtained in gas-carburized alloy steels. It has also been found that the resistance of a carbonitrided surface to softening during tempering is markedly superior to that of a carburized surface. In cyaniding, the proportion of nitrogen and carbon in the case produced by a cyanide bath depends on both composition and temperature of the bath, the temperature being the most important. This process is particularly used for parts requiring a very thin hard case, such as screws, small gears, bolts and nuts. Carbonitriding is a case hardening process in which a steel is heated in a gaseous atmosphere of such composition that carbon and nitrogen are absorbed simultaneously.
In this method, selected areas of the surface of a steel are heated into the austenite range and then quenched to form martensite. In flame hardening, heat may be applied by an oxyacetylene torch as shown in the figure given below or it may be a part of an elaborate setup which automatically carries out different tasks like heating, quenching and indexing. Depth of the hardness zone may be controlled by an adjustment of the flame intensity, heating time, or speed of travel.

Induction hardening depends for its operation on localized heating produced by currents induced in a metal placed in a rapidly changing magnetic field.
The operation resembles a transformer in which the primary or work coil is composed of several turns of copper wire and the part to be hardened is made the secondary of a high-frequency induction apparatus. As shown in the figure given above, when high-frequency alternating current passes through the work coil, a high-frequency magnetic field is set up. Depending on the frequency and amperage, the rate of heating as well as the depth of heating can be controlled.
Nitriding is a lower distortion process than carburizing but it can be used for certain type of steels such as chromium-molybdenum alloy steels or Nitralloy-type steels. Flame hardening is preferred for heavy cases or selective hardening of large machine components. Induction hardening works best on parts small enough and suitable in shape to be compatible with the induction coil.
The most basic form of a metal detector is made up of an instrument called an oscillator that sends off an alternating electrical current that moves through a coil which creates an alternating magnetic field. When the metal detector identifies an anomaly in the magnetic field caused by a piece of metal various electronic instruments feed this information to the user.
Sophisticated metal detectors not only indicate the presence of metal but can also indicate the depth of the metal under the ground, the size of the metal detected as well as the type of metal. One can also set the sensitivity of your hobby metal detector so that deeper or shallower items can be found and one can metal detect over very trashy soil.
The last two methods do not change the chemical composition of the steel and are essentially shallow-hardening methods. In this method, low carbon steel, usually 0.20 percent carbon or lower is placed in an atmosphere that contains substantial amount of carbon monoxide.
At this temperature, the maximum amount of carbon that can be dissolved in austenite can be found out from the Acm line of the iron-iron carbide equilibrium diagram. The container is heated to the proper temperature for the required amount of time and then cooled slowly.
Due to this, the entire pack is cooled slowly and the part is subsequently hardened and tempered. However it is not well suited to the production of thin carburized cases that must be controlled to close tolerances. The gas produced consists of 40 percent nitrogen, 40 percent hydrogen and 20 percent carbon monoxide. The effectiveness of this process depends on the formation of nitrides in the steel by reaction of nitrogen with certain alloying elements. The temperatures used are generally lower than those used in carburizing, between 1400 and 1600A°F. The case also contains up to about 0.5 percent nitrogen, therefore, file-hard cases can be obtained on quenching in spite of the relatively low carbon content. Skill is required in adjusting and handling manually operated equipment to avoid overheating the work because of high flame temperature.
The equipment can be taken to the job and adjusted to treat only the area which requires hardening. A phosphate coating is applied over the steel to facilitate absorption of the laser energy. Work coil of different designs are used to suit different types of heating requirements like external heating, internal heating, etc. This magnetic field induces high-frequency eddy currents and hysteresis currents in the metal work piece. When a piece of metal is near by the alternative current being produced by the coil something called eddy currents will be  induced in the  metal.
Different metal give off different types of eddy currents when within a magnetic field and can thus be singled out. For example you can set your detector to pick up only gold or only silver and exclude iron.

In flame and induction hardening the steel must be capable of being hardened and therefore, the carbon content must be about 0.30 percent or higher. This gas in turn reacts with the excess carbon in the charcoal to produce carbon monoxide, CO. Because of inherent variation in case depth, the method is not used on work requiring a case depth of less than 0.030 in. Commercial practice is to use a carrier gas, such as obtained from an endothermic generator and enrich it with one of the hydrocarbon gases.
This region, which varies in thickness up to a maximum of about 0.002 inch, is commonly known as the a€?white layera€?.
For application where lower hardness is acceptable, medium carbon steels containing chromium and molybdenum (AISI 4100 and 4300 series) are used.
Overheating can result in cracking after quenching and excessive grain growth in the region just below the hardened zone.
Parts too large to be placed in a furnace can be handled easily and quickly with the torch. The case obtained by induction hardening is similar to that obtained by flame hardening, and thinner cases are possible. This eddy current coming from the metal produces an alternating magnetic field of its own which is detected by another coil within the device which is called a magnetometer.
Since the core is of low carbon content, the carbon atoms trying to reach equilibrium will begin to diffuse inward. Carbon monoxide reacts with the low-carbon steel surface to form atomic carbon which diffuses into the steel. The reaction of nitrogen with the steel causes the formation of very hard iron and alloy nitrogen compounds.
The atmospheres used in carbonitriding generally comprise a mixture of carrier gas, enriching gas and ammonia. After quenching, the part should be stress-relieved by heating in the range of 350 to 400A°F and air cooled. Another advantage is the ability to treat components after machining since there is little scaling.
The rate of diffusion of carbon in austenite, at a given temperature, is dependent upon the diffusion coefficient and the carbon concentration gradient. The carburizing compound is in the form of coarse particles or lumps so that when the cover of the container is sealed, sufficient air is trapped inside to form carbon monoxide. At the furnace temperature, the added ammonia (NH3) breaks up or dissociates to provide the nitrogen to the surface of the steel.
Under known and standard operating conditions, with the surface at fixed carbon concentration, the form of carbon gradient may be predicted, with reasonable accuracy, as a function of elapsed time. The advantage of this process is that hardness is achieved without the oil, water or air quenching. Therefore, it is possible to heat a shallow layer of the steel without heating the interior. After diffusion has taken place for the required amount of time depending upon the case depth desired, the part is removed from the furnace and cooled. It only increases the carbon content to some predetermined depth below the surface to a sufficient level to allow subsequent quench hardening. As an added advantage, hardening is accomplished in a nitrogen atmosphere which prevents scaling. However, heat applied to the surface tends to flow towards the center by conduction, and so time of heating is an important factor in controlling the depth of the hardened zone.

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