Effect of boronising on the cavitation erosion resistance of stainless steel used for hydromachinery applications

Erosive wear due to cavitation severely affects hydromachinery and consequently various sectors of industry, including hydroelectric plants. ASTM A743 CA6NM steel, which is typically used in hydromachinery, was boronised using the packing method at 950 °C for durations of 2, 6, and 8 h Fe2B and FeB phases were identified and characterised using optical microscopy, X-ray diffraction, scanning electron microscopy, and microhardness tests. Under all of the boronising conditions, a surface FeB phase was obtained. Its hardness was 5.22 times that of the base material. The resistance to erosion due to cavitation was evaluated according to the ASTM G32 standard by exposure for 15 h to vibrations that induced cavitation. The boronising time influenced the resistance to cavitation. After boronising for 2, 6, and 8 h, the erosion rates were reduced by 72%, 57%, and 55%, respectively, compared to the erosion rate of untreated ASTM A743 CA6NM steel. According to scanning electron microscopy, the worn surfaces differed for non-boronised steel (ductile behaviour) and boronised steels (brittle behaviour) exhibiting micro-cracks, micro-pores, and detachment of the boride layers.