part to a temperature above the upper critical temperature in a medium

Part to a temperature above the upper critical

This preview shows page 46 - 49 out of 73 pages.

part to a temperature above the upper critical temperature in a medium, containing an element capable to saturate the surface layer of the part through diffusion; o Heat treatment of the part in order to obtain the desired combination of mechanical properties of the hard outer “case” and the ductile “core”. As a medium for the case hardening solid, liquid and gaseous substances may be used. Carburizing Carburizing is the process of diffusion enrichment of the surface layer of a part with carbon followed by heat treatment of the part. As carburizing medium the following substances are used: o Charcoal or other carbon-containing solids mixed with sodium carbonate and barium carbonate accelerating the process of dissolving the carbon in steel. The process is carried out in steel or cast iron boxes placed into a furnace at the temperature 900 °C – 950 °C (1650 °F – 1750 °F) resulting in formation of hard case of the thickness 0.5 mm – 2 mm (0.02”- 0.08”) and containing 0.8-1% of carbon. o Kerosene or benzene – liquid carbonizing mediums, which are usually used in dispersed form; o Methane (CH 4 ), propane (C 3 H 8 ) – gaseous carbonizing mediums. The process is carried out in a furnace (batch or continuous) at the temperature 900 °C – 950 °C (1650 °F – 1750 °F) for 3-4 hrs. Thickness of the hard layer formed in the gaseous carburizing may reach 4 mm (0.15”). Heat treatment after carburizing involves hardening - tempering treatments with purpose of controlling structure and properties of both the hard layer and the ductile core.
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~ Page 47 of 73 ~ Carbonitriding Dr. Dmitri Kopeliovich Carbonitriding is the process of diffusion enrichment of the surface layer of a part with carbon and nitrogen. Gaseous carbonitriding is carried out in gaseous medium, consisting of carburizing gas (methane, propane) with addition of 3-8% of ammonia . There are two principal methods of the gaseous carbonitriding: o Low temperature gaseous carbonitriding, conducted at 500 °C – 600 °C (930 °F – 1110 °F). The steel surface is enriched mostly with nitrogen in this process. o High temperature gaseous carbonitriding, conducted at 800 °C – 950 °C (1470 °F – 1750 °F). The steel surface is enriched mostly with carbon in this process. This process is followed by heat treatment. Cyaniding is the carbonitriding process, conducted in molten salt, containing 20-25% of sodium cyanide (an extremely toxic substance). The process is carried out at 820 °C – 860 °C (1500 °F – 1580 °F) for 1 hour. Carbonitrided parts possess better wear resi stance than carburized parts .
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~ Page 48 of 73 ~ Nitriding Dr. Dmitri Kopeliovich Nitriding is the process of diffusion enrichment of the surface layer of a part with Nitrogen . Gas nitriding is carried out at 500 °C – 600 °C (930 °F – 1110 °F) for 40-100 hours in the atmosphere of ammonia , which dissociates to hydrogen and nitrogen.
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