Carburizing
• Carburizing is the most widely used method of surface hardening. Here, the surface layers of a low carbon steel (<0.25) is enriched with carbon up to 0.8-1.0%. The source of carbon may be a solid medium, a liquid or a gas.
• In all cases, the carbon enters the steel at the surface and diffuses into the steel as a function of time at an elevated temperature. Carburizing is done at 920-950°C. at this temperature the following reaction takes place
• Where Fe(c) represents carbon dissolved in austenite. 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 equation given previously, Fe+2CO Fe(c) + CO2 is reversible and may proceed to the left, removing carbon from the surface layer if the steel is heated in an atmosphere containing carbon dioxide (CO2). This is called decarburization.
• Decarburization may be prevented by using an endothermic gas atmosphere in the furnace to protect the surface of the steel from oxygen, carbon dioxide and water vapor.
• An endothermic gas atmosphere is prepared by reacting relatively rich mixtures of air and hydrocarbon gas (usually natural gas) in an externally heated generator in the presence of a nickel catalyst.
Liquid carburizing
• Liquid carburizing is a method of case hardening steel by placing it in a bath (8% NaCN, 82% BaCl2 and 10% NaCl) of molten cyanide so that carbon will diffuse from the bath into the metal and produce a case comparable to one resulting from pack or gas carburizing.
• Liquid carburizing may be distinguished from cyaniding by the character and composition of the case produced. The cyanide case is higher in nitrogen and lower in carbon; the reverse is true of liquid carburized cases.
Pack carburizing
• In pack carburizing, the article s to be carburized are packed in a box, embedding them in a powdery mixture of 85% charcoal and 15% of energizers such as BaCO3 . The box is sealed with fireclay and loaded into the furnace kept at 930°C. The residual air in the box combines with carbon to produce CO. The energizer decomposes as below :
• The carbon enters the steel through the following reaction:
• If selective carburization is to be done, copper is electroplated to a thickness of ~0.05 mm in regions where carburization is not desired. Alternatively, a refractory paste of fireclay mixed with asbestos can be applied. Control of temperature and penetration depth is less in pack carburizing as compared to liquid and gas carburizing. Also, direct quench from the carburizing temperature to harden the surface is not possible.
Gas Carburizing
• The steel is heated in contact with carbon monoxide and/or a hydrocarbon which is readily decomposed at the carburizing temperature.
• A mixture consisting of 5-15% methane (or propane) in a neutral carrier gas is used. The methane decomposes according to the following reaction:
• The carbon potential of the gas mixture increases with increasing concentration of methane. Too large a concentration or too high a gas velocity releases carbon faster than it can be absorbed and may result in soot formation on the surface.
• Closer control of temperature and case depth is possible in gas carburizing, as compared to pack carburizing. Also, post quenching can be done directly.
Heat treatment after carburizing
• Since steel is carburized in the austenite region, direct quenching from the carburizing temperature will harden both the case and core will harden both the case and core if the cooling rate is greater than the critical cooling rate. Direct quenching of coarse grained steels often leads to brittleness and distortion, so that this treatment should be applied only to fine grained steels. A diagrammatic representation of various hardening treatments for carburized steels shown in next slide.
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