Cavitation-induced damage on a ship propeller’s Kort nozzle made of AISI 316L and investigation of the near-surface microstructural changes

A piece of a Kort nozzle, a ring-shaped component enclosing a ship’s propeller severely damaged by cavitation was examined. The nozzle was made of stainless steel grade AISI 316L. The piece of steel examined exhibits a damage gradient which can be explained by cavitation of regionally varying severity. Areas severely stressed by cavitation are characterized by an increased material removal and high surface roughness. Since cavitation-induced material removal can be attributed to a mechanical damage to the surface, the aim was to investigate the microstructural changes in the steel. Samples were taken along the gradient and cross-sections were prepared. The etched metallographic section revealed clear evidence of deformation. Toward the lower load, a decreasing amount of deformation twins can be observed along the cavitation gradient and also from the surface toward the base material. It can also be assumed that severe deformation leads to the formation of deformation martensite. Against the background of a significant increase in magnetizability from the unaffected material toward the damage zone identified through qualitative analysis, magnetic measurements were performed and confirmed a higher magnetizability in the damaged near-surface areas. The steel’s base metal has a uniform, austenitic microstructure.