Hot deformation and damage of Free Cutting Steel

Hot rolling as a manufacturing process of Free Cutting Steel (FCS) can produce products with enhanced properties through controlled thermo-mechanical processing. However, like any other manufacturing process, imperfections at various scales can appear during hot rolling therefore affecting product quality and cost effectiveness of the industrial process. Hot rolled FCS in particular suffers mainly from edge cracking taking place during the rolling process at high homologous temperature. Research studies have been conducted in order to understand the cause of such problem. Stress triaxiality and local microstructural inhomogeneities leading to damage formation are of particular interest in the current research. High temperature laboratory tests with specimen geometries specifically designed to generate stress triaxiality levels comparable to those experienced by the material during the hot rolling process have been conducted to initiate damage at the surface of these specimens. Particular attention has been paid to the influence of Manganese-Sulfide (MnS) inclusions on damage formation during these tests. A new experimental procedure has been developed to measure strain distributions at the scale of the microstructure in order to study the effect of strain localisation on damage formation and how this localisation is affected by the presence of MnS inclusions. A microgrid technique has been used to measure intra-granular strains in specimens deformed at temperatures up to 1200⁰C typical of a hot rolling process. Strain maps have been produced over relatively large areas of the microstructure in non-transformable austenitic stainless steels representative of the behaviour of FCS at high temperature and produced specifically for this research. Tests on FCS have also been conducted at lower temperatures including tensile tests inside a scanning electron microscope with full-field strain measurements obtained using Digital Image Correlation (DIC) to study damage development over a range of temperatures and scales.
Results have shown that local deformation and local damage at high temperature are highly influenced by the presence of grain boundaries and inclusions. Strain distribution maps highlight such influence on strain localisation. Moreover, the deformation temperature has a significant effect on the mechanism of local deformation and damage in FCS with damage nucleation along grain boundaries observed at the surface of the specimens deformed 1000⁰C as opposed to intra-granular cracking involving inclusions observed at the subsurface.