Effect of Post Heat-treatment on Hardness and Case Depth of 17NiCrMo6-4 Gear Steel after Liquid Carburizing

MUHAMMAD ISHTIAQ, F. Ahmed, Bilal Saleem, L. Ali, A. Inam, T. A. Tabish

Abstract


Materials having high hardness and wear resistance are being increasingly used for gear manufacturing. So it is the need of time either to develop such materials or some treatments to achieve such a combination of said properties. Carburization is a thermo-chemical treatment to attain high surface hardness and wear resistance in steels. A hard case is developed over a soft core in steel by diffusion of carbon from carburizing media (solid, liquid, or gas). The gear steel 17NiCrMo6-4 is difficult to carburize by conventional carburizing techniques because of its high nickel content (̴ 1.35 wt. %). Therefore, in this work different carburizing bath compositions have been used to carburize 17NiCrMo6-4 steel. It was found that a liquid salt bath of composition Na2CO3 (30%), NaCN (40%) and BaCl2 (30%) worked well for the carburising of the 17NiCrMo6-4 steel. Furthermore, an additional post heat-treatment process has been adapted, in which steel was heated to 850°C for a period of 2 hours followed by oil quenching. This additional post heat-treatment process proved beneficial and resulted in case depth >1.5 mm and hardness > 50 HRC.


Keywords


liquid carburizing; gear steel; carburizing media; case depth; carbon diffusion

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