Degradation of Armco iron caused by cavitation: Part II – Correlation with stress analysis

Abstract

Degradation of Armco iron caused by cavitation was analysed. Cavitation erosion tests were carried out in a cavitation tunnel with a system of barricade exciters at two conditions: inlet pressure of 1000 kPa and 1200 kPa, and outlet pressure of 125 kPa and 130 kPa, respectively. In each test, three adjacent samples were examined. The case with increased inlet pressure caused more than a 3-fold increase (3.75 times) in the mass loss of sample 1 – as placed closest to the barricade exciters during 20 min of testing. Mass losses observed in samples 2 and 3 were equivalent for each test condition. Hardness measurements showed that the increase in surface hardness (i.e. at a depth of 0.02 mm) occurred only in samples 1, while in samples 2 and 3 softening occurred (the decrease in hardness). For sample 1, higher inlet pressure caused over 5 times hardness increase. However, the greater hardening depth was observed at the lower inlet pressure test. Numerical simulation showed that the impact of water micro-jet with the velocity of 100 m/s exceeds the tensile strength of Armco iron, and produces a pit with the depth of 0.7 μm. The increase in impact velocity to 500 m/s increases a pit depth to 4.6 μm. The depth of pits observed in sample 1, which was tested at the inlet pressure of 1000 kPa, indicated that the impact velocity was up to 200 m/s. For tests conducted at 1200 kPa, the depth of an observed pit indicates a micro-jet impact velocity was estimated around 500 m/s.