Plasma nitriding of steels with severely plastic deformed surfaces

Linear Flow Splitting (LFS) is a new massive forming process, which enables the continuous production of integral bifurcated profiles with ultrafine-grained surface layers. Owing to the uniaxial material flow during LFS, the grains in the ultrafine-grained (UFG)-layer are highly elongated with minimum grain dimensions perpendicular to the split surface. With increasing distance to the split surface, the UFG-microstructure changes into a conventionally strain-hardened microstructure. The microstructural gradient is accompanied by a gradient in hardness and strength with maximum values at the split surface in the UFG-layer. Further improvement of hardness and wear resistance can be achieved by nitriding. In spite of their low thermal stability, earlier investigations demonstrated that nitriding of UFG-microstructures is feasible even at elevated temperatures like 500 °C. But, as most publications are referred to equiaxed grains, it is unclear whether the results can be transferred to LFS-profiles with their highly elongated grains. Whether pancake UFG-microstructures are still beneficial for plasma nitriding is the subject of this work. For this, the microstructure of a linear flow split micro-alloyed HSLA steel is characterized after nitriding and subsequent heat treatment by EBSD and SEM measurements. Mechanical properties are examined by hardness indentations. It is shown that nitriding of pancake-shaped UFG-microstructures is still beneficial in terms of higher compound layer thickness and hardness compared to a strain-hardened microstructure. Moreover, nitriding reduces the grain growth, i.e. stabilizes the UFG-microstructure.