Investigation of the changes in microstructure and hardness of nodular cast irons used in fitting manufacturing depending on wall thickness with the effect of cooling rate

Fittings are used as pipe connector in sanitation system. Cooling becomes quickly in fittings products because their wall thickness is thin. Because of the high cooling rate, cementit forms in products. This cementit in microstructure causes the increase in hardness up to 340 HB. This hardness causes some damage tapping tools in time and also causes breaking in these tools. As tapping tools are special fabrication, their cost are too high. While and after the fittings are mounted, they are easily broken due to their high hardness, which causes problems such as inundation. Due to these problems in these parts, they are softened by austempering in furnice. The furnice produces electricity of 250 kW during 6 hours and the fittings are hold in it for 6 hours at 800°C. After the processes of austempering, 100% ferritic structure is obtained and the hardness is decreased to 160 HB. However, this process increases the costs. In this thesis study, microstructure and hardness change in the fittings materials are investigated depending on wall thickness, and the optimum cooling rate was depermined.