These are medium carbon steels containing chromium and molybdenum also. A continuous check of the nitrogen potential of the atmosphere is kept by determining the composition of the exit gases by means of a dissociation pipette. 3. Depth of the hardness zone may be controlled by an adjustment of the flame intensity, heating time, or speed of travel. The case obtained by induction hardening is similar to that obtained by flame hardening, and thinner cases are possible. Depending on the frequency and amperage, the rate of heating as well as the depth of heating can be controlled. In cyaniding, the case is higher in nitrogen and lower in carbon. The first three methods change the chemical composition, carburizing by the addition of carbon, nitriding by the addition of nitrogen and cyaniding/carbonitriding by the addition of both carbon and nitrogen. This process can minimize wear and tear and increase the strength of the steel parts’ surface. Tufftriding is done at a temperature of 570°C in a bath of molten salts (85% shall be consisting of 40% NaCNO, 60% NaCN and 15% Na2CO3) through which dry air is pumped in. The initial investment costs are generally higher than other processes. This process offers a high degree of hardness, but cannot be used with all of the alloy steels. Nitriding may be done in electric-heating furnaces. Compared to this, cyanide cases are seldom to a depth greater than 0.010 in. It has many applications, such as producing cooking utensils, reinforcing metal girders and enhancing the functionality of firearms. In the Cyaniding process, the parts will be heated up to the 871°C-954°C in the presenting of Sodium Cyanide and quenched with the water or oil to remove the residual cyanide. The formation of white layer can be prevented by using a mixture of NH3 and H2. Table 8.10 gives composition of some nitriding baths. 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. The positively-charged-nitrogen-ion then picks up an electron from the part (cathode) and emits a photon. 8.48 shows effect of time and temperature on case depth. The nitriding cycle is quite long depending upon the desired case depth. This process must be carried out in a controlled environment to prevent oxygen from being present in the process which changes the outcome of the metal hardening. This process is also known as dry cyaniding, gas cyaniding and nicarbing. It is a liquid nitriding process in which salts are melted in a titanium crucible. 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. Preheated (350-480°C) parts (to be nitrided) are immersed in it. In this method, it is possible to directly quench the part after carburizing. Heating results from the resistance of the metal to passage of these currents. The methods are:- 1. Oral:Name the 3 methods of case hardening. In the original method, components are heated at about 510°C (500-575°C) in an atmosphere of dissociated ammonia gas. Cyaniding and carbonitriding4. It provides similar advantages as gas nitriding. As tin becomes molten at the nitriding temperature, it is essential that this coating should be so thin (< 0.01 mm) that it does not spread to surfaces that are to be nitrided. Mechanical masking even by mild steel which has indefinite life makes it much cheaper method. Subsequent hardening applications such as these increase strength and wear resistance by diffusing carbon, through a number of different methods, into the surface of steel parts. However, at a given temperature, the constant k’ is orders of magnitude smaller than the constant k of the equation 8.51. Cases that contain both carbon and nitrogen are produced in liquid salt baths (cyaniding) or by use of gas atmosphere (carbonitriding). Gas-Nitriding: Gas nitriding is the most commonly used nitriding method. This region, which varies in thickness up to a maximum of about 0.002 inch, is commonly known as the “white layerâ€. The high-frequency induced currents tend to travel at the surface of the metal. 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. The ε layer is tough, wear and galling resistant. 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. 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. Two stage gas-nitriding processes is used to reduce the thickness of this white layer. Commercial carburizing compounds usually consist of hardwood charcoal, coke and about 20 percent of barium carbonate. 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. Commercial carburizing may be carried out by means of pack carburizing, gas carburizing and liquid carburizing. Work coil of different designs are used to suit different types of heating requirements like external heating, internal heating, etc. 8.37). In this method, selected areas of the surface of a steel are heated into the austenite range and then quenched to form martensite. This improves the rate of nitriding. However, as cyanide salts are poisonous, this method requires careful attention and the parts must be thoroughly washed after heat treatment to prevent rusting. Normally the cyanide salt is melted and aged at the nitriding temperature for 12-24 hours, before the parts are immersed into the bath. Cycle times for liquid cyaniding is much shorter (1 to 4 hours) than gas and pack carburizing processes. It only increases the carbon content to some predetermined depth below the surface to a sufficient level to allow subsequent quench hardening. 5. The resulting nitride case is harder than carburized steels. Carburizing is the best method for low carbon steels. 8.47 illustrates effect of time on case depth for three steels. The higher nitriding temperatures in the second- stage results in- lower case hardness; increased case depth; may lower the core hardness depending on the prior tempering temperature. The following points highlights the four main methods used for nitriding of steels. Carburizing, Nitriding, and Cyaniding. Cyanide-free salt baths have been introduced. In the outer zone, nitride forming elements, including iron, gets converted to nitrides. Hardest cases, approximately RC 70 are obtained with aluminium alloy steels known as Nitralloys. The parts are heated by electrical heaters to heat them to 375° to 650°C. There should not be any trace of rust or mill scale. In practice, nitriding is done in dissociated anhydrous ammonia at a potential when iron-nitride is just formed at the surface, i.e., nitriding is done under the saturated conditions. Nitrided part after gas-nitriding normally exhibits a characteristics matte-grey colour due to some oxygen present in the atmosphere. To control the nitrogen potential in the nitriding furnace, a mixture of gases (NH3, N2, H2) is fed to the furnace. In general, the method is suitable for small and medium size parts as it is difficult to process large parts in a salt bath. Fatigue life is better than that obtained in gas nitriding by about 80% in 30CrMoV1 and 130% in plain carbon steels. Therefore, it is possible to heat a shallow layer of the steel without heating the interior. Equation 8.51 gives the % nitrogen at the surface of the steel, or it is given by the maximum solubility of nitrogen in ferrite at that temperature, whichever is lower. 1. The component that requires hardening is heated to 9000C and exposed to a carbon environment. The case hardening process adds a thin layer of metal alloy to the outer surface of the metal. Skill is required in adjusting and handling manually operated equipment to avoid overheating the work because of high flame temperature. This method is efficient and economical for individual processing of small lots of parts or of large, massive parts. Therefore, very quickly, a surface layer of high carbon (about 1.2 percent) is built up. The process can be automated. In this method, furnace gas in continuously withdrawn to be passed through NaOH-water solution to remove traces of HCN produced. Account Disable 12. The extra strength that comes from case hardening your steel produces a tough outer layer (called the case) while keeping the inner core (case depth) soft enough to absorb shock. 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. In pack carburizing, the work is surrounded by a carburizing compound in a closed container. Generally, steel for flame hardening have 0.3 to 0.6 percent carbon. Barium carbonate promotes the formation of carbon dioxide (CO2). This gas in turn reacts with the excess carbon in the charcoal to produce carbon monoxide, CO. For application where lower hardness is acceptable, medium carbon steels containing chromium and molybdenum (AISI 4100 and 4300 series) are used. To achieve these different properties, two general processes are used: 1) The chemical composition of the surface is altered, prior to or after quenching and tempering; the processes used include carburizing, nitriding, cyaniding, and carbonitriding; and 2) Only the surface layer is hardened by the heating and quenching process; the most common processes used for surface hardening are flame hardening and induction hardening. After completing ion-nitriding, the parts are cooled in inert atmosphere of N2 gas. The equilibrium constant of the reaction 8.49 is-. There are many types of case hardening processes. Low gas and power consumption makes this process a cheap process. The effectiveness of this process depends on the formation of nitrides in the steel by reaction of nitrogen with certain alloying elements. The nitrogen atom, thus formed, strikes the part to heat it. Objects which need to have a hard exterior to endure wear and tear while maintaining a soft interior to withstand shock benefit from this process. 8.44). Atomic nitrogen reacts with the metal to increase its hardness and resistance. Tools and small components are often liquid-nitrided. The relation of time and temperature to case depth is shown in the figure given below. Nitriding is used extensively for aircraft engine parts like cams, cylinder liners, valve stems, shafts and piston rods. The components to be nitrided should be cleaned well and degreased. The hardened zone is generally much deeper than that obtained by carburizing, ranging from 1/8 to 1/4 inch in depth. The hardness of the layer is similar to gas nitriding i.e., 300-350 VPN for plain carbon steels and 600-1100 VPN for alloy steels. Ionitriding (Plasma), or Glow-Discharge Nitriding 4. Most carburizing gases are flammable and controls are needed to keep carburizing gas at 1700oF from contacting air (oxygen). The range of frequencies commonly used is between 10,000 and 500,000 Hz. The equipment can be taken to the job and adjusted to treat only the area which requires hardening. The main advantages of flame hardening are adaptability and portability. Fig. 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. In the original method, components are heated at about […] Hence, it is not practically easy to use nitrogen gas for nitriding.
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