Porosity in Hot Formed Cast Steel

Abstract

In order to reduce the number of process steps of steel forging processes. and thus also tool wear and process costs, tailored preform geometries can be produced by casting. By only one subsequent forming operation it is possible to improve the casting microstructure, eliminate possible defects and achieve the required mechanical properties. To evaluate the properties of the cast microstructure and the closure of possible casting defects during warm/hot forming, cylindrical steel billets (C45/1.0503) were produced by sand casting and then upset in a hydraulic press. Global plastic strain φ was varied between 0.3 and 0.7 while forging temperatures of 600 °C and 1200 °C were applied to detect possible temperature effects. Conventional rolled bar material formed under the same conditions was used as a reference. After forming, the specimens were tempered and the mechanical properties were determined by tensile tests (ISO 6892-1) and notch impact tests (similar to ISO 148-1). The microstructures were examined by metallographic analysis while defects were characterised using an optical wide-area 3D measurement system and digital image processing. It could be observed that the ultimate tensile strength of the cast-forged specimens depends on the forming temperature and is about 15 % lower in comparison to the reference material at a forming temperature of 600 °C and 5 % lower at 1200 °C, respectively. The impact energies show a strong dependence on plastic strain for both, the castforged and reference specimens. The values of the mechanical properties of the reference specimens were higher than those of the cast-forged specimens. These results allow a deeper understanding of the cast-forging of steel and will contribute to the cast-forging design of more complex steel parts.