Influence of nodule count on residual stresses and distortion in thin wall ductile iron plates of different matrices

Surface characteristics of thin wall ductile iron (TWDI) parts, such as residual stresses, distortion and dimensional changes produced during casting and heat treatment, are relevant variables when it comes to production processes design. This work focuses on the effect of nodule count on residual stresses (RS) and of TWDI plates distortion. As-cast, ferritised and austempered samples were employed. The role played by two typical austempering temperatures (280 and 360 °C) and three significantly different nodule counts (265, 1200 and 1700 nodules (nod)/mm2) are discussed by establishing microstructural changes, i.e., microstructure fineness, retained austenite volume fraction (Vγ%), and austenite carbon content (Cγ%). Besides, residual stresses profiles below the surface and dimensional changes are determined. Results show that all samples display compressive RS on the surface and neighbouring layers. Samples with high nodule count austempered at 280 °C lead to higher compressive RS and distortion as well as to an increase in the RS field below the surface. As-cast and ferritised plates exhibit noticeably lower compressive RS values.