Low pressure carburising, often combined with high pressure gas quenching, has become a competing technology to the conventional carburising processes, which have been well known for decades. However, from the user's point of view, the lack of detailed background information about low pressure carburising introduces a certain amount of risk. Because this technology is still in a development stage the influence of the process parameters on material properties and life-cycle time has not yet been completely investigated. One phenomenon that causes concern among users of carburising processes is hydrogen embrittlement-induced cracking, which often leads to total failure of the part. The aim of our investigation was to determine the amount of hydrogen pick up taking place during low pressure carburising, compared to that which occurs in traditional gas carburising processes. For this purpose 4 different case hardening steels (20MnCr5, 20MnCrS5, 18CrNiMo7-6 and 25MoCr4) were first tempered to bring all samples into the same initial state, and after that low pressure carburised. The process parameters were defined to ensure a surface carbon content of 0.8 Conc.-% at a carburising depth of 0.6mm. The influence of steel grade, carburising gas (propane and acetylene), pressure and process-time was evaluated and the resulting hydrogen content of the treated samples, measured by the hot-gas extraction method, was compared with typical values for the conventional gas carburising process achieved during treatment in endothermic-generated carrier gas outside the furnace in an external generator and in-situe generated nitrogen-methanol mixtures.
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