Mechanisms and morphologies of cavitation damage of NN 70 steel

Abstract

Broken test tubes for low-cycle fatigue testing of Nionicral 70 (NN-70) parent material (PM) steel and simulated heat-affected zones (SHAZ) were used to produce samples for cavitation resistance testing. Ultrasonic vibrational cavitation method (stationary sample method) was applied for testing in laboratory conditions. The test conditions and procedure, sample preparation and interpretation of results are defined by ASTM G32. The surfaces of the NN-70 PM and SHAZ steel samples were exposed to cavitation and damage monitoring over time. Measuring the weight loss of samples on the analytical balance after a certain time allowed us to determine the cavitation velocity as a measure of the material's resistance to the effect of cavitation. Scanning electron microscopy (SEM) was applied to monitor variations in surface morphology with changing test time. On the basis of the results of the cavitation resistance test, the morphologies of the surface damage for different exposure times of the cavitation of PM and SHAZ steel NN-70 samples were analyzed, as well as the mechanisms that led to the damage of the sample surfaces.