Plasma nitriding of steels with severely plastic deformed surfaces

摘要

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 pancakeshaped 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 UFGmicrostructure.