Cavitation Erosion Mechanisms in Cast Irons
Abstract: The research presented in this thesis investigated the mechanisms by which cavitation erosion damage develops in lamellar graphite iron (LGI) and austempered ductile irons (ADIs). This has been achieved by image sequences of surface erosion on test samples in tandem with weight change measurements. Cavitation erosion is caused by the appearance and collapse of bubbles in a liquid which undergoes rapid pressure oscillations. Imploding bubbles release heat, shockwaves and high-speed microjets which may strike nearby solid walls and damage them.The heavy-duty automotive industry encounters this problem in the engine cooling system. The combustion chamber requires precise temperature control for optimal operation and excess heat must be removed by a liquid coolant. In trucks, the coolant liquid achieves this by circulating around the cylinder liner, a hollow cylindrical part that encloses the combustion chamber and prevents its gases from escaping. However, the engine’s intense vibrations create repeated pressure variations in the coolant, and bubbling ensues. With prolonged operation, the cylinder liner’s wet outer wall may be severely worn, resulting in surface roughening, eroded patches and pits. Cavitation is responsible for great losses due to vehicle downtime and maintenance costs. The present work aims, therefore, at analyzing the behavior under cavitation exposure of cast irons that are currently used, or being considered for use, in the cooling system.Cylinder liners are currently made of lamellar graphite iron with a matrix structure consisting of pearlite and a network of steadite, and the analysis for this material has been presented in Paper 1. Austempered ductile irons are candidate materials for pumps and other components of the cooling system due to their very good mechanical properties; three ADIs of increasing hardness, obtained from different heat treatments of a spheroidal graphite iron, have been analyzed in Paper 2. Experiments consisted of an ultrasonic vibratory probe to which material samples were attached and subsequently immersed in a beaker containing engine coolant. The samples were weighed and photographed in an SEM after several predetermined time intervals. This produced a detailed sequence of images which, in combination with mass loss data, can explain the mechanisms by which cavitation damage initiates and develops in these materials. The text of this thesis summarizes the findings presented in the appended articles and compares the behavior of LGI and ADI.
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