The Improvement of Mechanical Properties of Nodular Cast Iron (NCI) under Two-Step Austempering Heat Treatment

摘要

This research was carried out to studythe mechanical properties of nodular cast iron (NCI), particularly in tensile strength, hardness and toughness after being heat treated by two-step austempering. The result, subsequently, is compared with the single-step austempering one, performed at 900℃ austenite temperature in the holding time of 60 minutes and quenched in the saline solution with various austempered temperatures at 280, 340, and 340℃ in the holding time of 60 and 120 minutes. Meanwhile, for the two-step austempering heat treatment, autenization with temperature as equally given in the single-step one, namely at 900℃ at the holding time of 60 minutes, is performed. In this performance, the step is initiated by quenching at the initial austempered temperature of 260℃ for 60 minutes and followed by gradually increasing the initial temperature into 280, 310, and 340℃, each of which is in the holding time of 60 and 120 minutes. The result of two-step austempering heat treatment towards the mechanical properties show a higher value compared to the single-step one. In this case, the optimum value of the mechanical properties occurred at the tensile strength and hardness at 340℃ of austempered temperature with the holding time of 60 minutes and followed by the lower austempered temperatures, namely at 310℃ dan 280℃. By contrast, the value of impact toughness decreased at 340℃ of austempered temperature with the holding time of 60 minutes.