{"id":6422,"date":"2023-12-17T18:58:23","date_gmt":"2023-12-17T18:58:23","guid":{"rendered":"https:\/\/www.jstconsultancy.cz\/?p=6422"},"modified":"2026-08-01T06:39:48","modified_gmt":"2026-08-01T06:39:48","slug":"douhliceni-ve-vakuovych-pecich-s-lpc","status":"publish","type":"post","link":"https:\/\/intelligent-kalam.80-240-27-133.plesk.page\/en\/douhliceni-ve-vakuovych-pecich-s-lpc\/","title":{"rendered":"Carbon restoration in vacuum furnaces with LPC"},"content":{"rendered":"

As a philosopher would say, everything good is bad for something… Carburizing under reduced pressure, or vacuum carburizing or low presssure carburizing has the advantage of being very fast. On the one hand, because unlike carburizing in the ENDO atmosphere, we work with a clean environment where carbon transfer from the atmosphere has a minimal amount of barriers, and on the other hand, because thanks to the vacuum, we can go to higher process temperatures, which will speed up the carburizing process even more. But if we only need to restore the carbon on the surface, i.e. to repair the decarburized layer created during casting or forging, then this speed is detrimental, because we only move in small layers with CHD from 50 to 100 \u00b5m with times only in the order of minutes (Fig. 1).<\/p>\n

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Fig. 1<\/u><\/em><\/strong> \u2013 Dependence of CHD on time and temperature for short diffusion depths<\/em><\/p>\n

LPC is based on the fact that we inject a certain amount of C2<\/sub>H2<\/sub> into the vacuum chamber in such a way as to achieve saturation of the environment for the formation of Fe3<\/sub>C iron carbides. In the next phase, only in the presence of nitrogen, the layer is diffused and the carbon content decreases. These both segments are repeated several times, while the carburizing time, the saturating period, is in minutes and changes only minimally, the diffusion period gradually increases with each subsequent segment. The final period of carburizing is then set so that we get the carbon content to the eutectoid amount, roughly 0.75 wt% C (Fig. 2). The total time of the carburizing process is then expressed by the following formula:<\/p>\n

Time LPC = \u2211(t saturation period_C2H2<\/sub>) +\u00a0 k<\/strong><\/span>\u00a0 *\u00a0 \u2211(t diffusion period_N2<\/sub>)<\/p>\n

where k<\/strong><\/span> is approximately 5.<\/p>\n

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Fig. 2 <\/u><\/em><\/strong>\u2013 Schematic illustration of the vacuum carburizing process from the point of view of saturation and diffusion periods [ECM]<\/em><\/p>\n

The carburizing process is shown schematically in the next picture<\/p>\n

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Fig. 3 <\/u><\/em><\/strong>\u2013 Schematic illustration of the vacuum carburizing process on Fe-Fe3C diagram <\/em><\/p>\n

\u00a0<\/em>For carbon restoration, the process should proceed similarly, however, due to the speed of carbon diffusion, we usually only have one attempt to achieve the optimal result. One saturation period and one diffusion period. During the diffusion period, we do not reach a carbon content corresponding to the eutectoid amount, but corresponding to the chemical composition of the steel, e.g. 0.3 to 0.4 wt% C.<\/p>\n

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Fig. 4 <\/u><\/em><\/strong>\u2013 Schematic illustration of the carbon restoration on Fe-Fe3<\/sub>C diagram \u00a0<\/em><\/p>\n

\u00a0<\/em>What does this imply? Unlike carburizing in multi-purpose furnaces, where we work with an ENDO atmosphere, and where we simply set the Cp potential to a value corresponding to the carbon content in the steel, with LPC we have to test the process. It depends on what happens next. Usual processes are<\/p>\n