Formation of UFG-surface layers on a HSLA steel by a continuous Surface-SPD-Process

Since material failures are mostly initiated from the surface, the combination of ultrafine-grained (UFG) surface layers and coarse-grained materials is an appropriate approach to tailor and improve the properties of a work piece. One approach to generate such ultrafine-grained layers is processes referred to as surface mechanical attrition treatment (SMAT). They are based on grain refinement induced by surface severe plastic deformation (S2PD), but exhibit a disadvantage in terms of high process duration. In contrast, the S2PD-process Linear Bend Splitting (LBS) enables the continuous production of integral bifurcated profiles containing surface layers with ultrafine-grained microstructure. For a lowly alloyed mild steel, the UFG-structure is characterized using EBSD-measurements, hardness indentations and tensile tests. Furthermore, the relation between strength and microstructure, typically described by the Hall–Petch relationship, is examined. The investigations reveal the formation of an ultrafine-grained layer in the profile surface of several hundred microns thickness exhibiting high hardness and strength. The combination of stiffness and strength of these profiles increases the potential for light weight applications in the aircraft or automotive industry.