The Influence of S Addition and Aging Heat-Treatment on the Machinability in Austenitic Lightweight Steels

Machining processes are crucial in industrial applications for shaping steel into specific structural products, and represent a significant portion of the product cost. However, the machinability of lightweight steel is poor, and it exhibits the characteristics of difficult-to-machine materials. This study investigated the effect of adding sulfur on the machinability of austenitic lightweight steel. Additionally, research was also conducted on aging heat treatment, to determine its effect on the steel’s mechanical and machinability properties, to enhance its usability as a structural material. S was added in the range of 0.0048 to 0.0494 wt.%, and the microstructure was observed, and mechanical and machinability properties were evaluated. After aging at 550°C for 6 hours, changes in the steel microstructure, including precipitates, were analyzed, and the same tests were performed. The added S was found to combine with the abundant Mn in the matrix, forming MnS inclusions. These inclusions did not affect the tensile properties but reduced impact energy while significantly improving machinability. However, after aging, κ-carbides formed within the grains and along grain boundaries, resulting in increased yield and tensile strength but decreased impact energy. Notably, machinability considerably deteriorated after aging, likely due to the interference of κ-carbides with dislocation movement. Consequently, the machinability of austenitic lightweight steel can be improved by the addition of S, and it is recommended that the machining process be conducted before aging during product manufacturing, considering the significant reduction in machinability after aging.