The energizer makes up to 40 of the total composition 2 FUSED SALTS Fused salts

The energizer makes up to 40 of the total composition

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The energizer makes up to 40% of the total composition. 2. FUSED SALTS Fused salts such as molten sodium cyanide, together with sodium carbonate and varying amounts of sodium and barium chloride are used in special salt-bath furnaces. Since cyanide is a deadly poison and represents from 20-50% of the total content of the molten salts, stringent safety precautions must be taken in its use. The components to be carburized are immersed in the molten salts.
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~ Page 44 of 73 ~ 3. GASEOUS MEDIA Gaseous media are increasingly used now that ‘natural’ gas (methane) is widely available. Methane is a hydrocarbon gas containing organic compounds of carbon which are readily absorbed into steel. The methane gas is often enriched by the vapours given off when oil is ‘cracked’ by heating in contact with platinum which acts as a catalyst. It is a fallacy to suppose that carburizing hardens steel. Carburizing merely adds carbon to the outer layers and leaves the steel in a fully annealed condition with a coarse grain structure. Therefore, additional heat-treatment processes are required to harden and refine the case and to refine and toughen the core. 2.0 HEAT TREATMENT Heat-treatment processes that follow carburizing are as follows: 1. REFINING THE CORE Since the core has a content of less than 0.3% carbon, the correct annealing temperature is approximately 870 °C which is well below the carburizing temperature of 950 °C which caused grain growth. After raising the component to 870 °C it is water quenched to ensure a fine grain. Although the temperature of 870 °C is correct for the low-carbon core of the component, (temperature A Fig. 2) it is excessively high for the high-carbon case of the component (temperature B Fig.2). 2. REFINING AND HARDENING THE CASE Since the case has a carbon content of approximately 1.0% carbon its correct hardening temperature is 760 °C. Therefore the component is reheated to this temperature (temperature C Fig. 2) and again quenched. This hardens the case and ensures that it has a fine grain. The temperature of 760 °C is too low to cause grain growth in the hyper-eutectic core providing the component is heated rapidly through the range 650-760 °C during the reheating and quenched without soaking at the hardening temperature. 3. TEMPERING Ideally it is advisable to relieve any quenching stresses present in the component by tempering it at about 200-220 °C.
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~ Page 45 of 73 ~ The above heat-treatment sequence is used to give ideal results in stressed components. However, in the interests of speed and economy the process is often simplified where components are lightly stressed or where alloy steels are used having less critical grain growth and quenching characteristics. In such circumstances the tempering process is left out. Sometimes heat-treatment is limited to simply quenching immediately after carburizing whilst the components are still at the carburizing temperature.
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~ Page 46 of 73 ~ Case hardening Dr. Dmitri Kopeliovich Case hardening is the diffusion heat treatment operation which involves two stages: o Heating a steel
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