Specimen: Cast iron injection housing
Material: ASTM A 536 Grade 65-45-12 ductile iron
Background: The cast iron injection piston housing had been subject to approximately 1 million cycles in a plastic injection moulding system. Seven identical failures of localized damage to the ring seal on identical cast iron injection piston housings had occurred.
Service Life: Approximately 1.5 years, or 1 million cycles
Findings: Visual examination of the region of localized damage revealed areas of large "craters" surrounded by smaller craters or micro-pits. Such damage is typical of cavitation damage. Cavitation damage is defined as the wearing away of metal through the formation and collapse of cavities in a liquid, subject to rapid and intense pressure changes. In this situation, the liquid was the oil used to lubricate the piston and housing. The very rapid movement of the piston through the housing caused the rapid and intense pressure changes in the oil, thus causing cavitation damage.
a) Region of cavitation damage on a cast iron injection housing, b) Scanning electron microscopy image of a typical "cavity" caused by cavitation damage