Metallurgical investigation on cracked plunger-sleeves

The axial cracking through oil–hole took place on diesel engine plunger-sleeves used in a truck in service. The internal cavity and local external circle surfaces of the plunger-sleeve were carburized as the specification. The numerous elongated MnS inclusions along the axial of the plunger-sleeves were found in the matrix material by the detailed metallurgical and fracture observations. A lot of the axial spear-like openings were found at the fringe of the oil–hole of the plunger-sleeve by visual and SEM. MnS inclusions reacted with oxygen and pulled out from the matrix to form the axial spear-like openings in unsuitable gas-carburizing operation due to excessive oxygen content in the carburization atmosphere. Under the action of the alternative circumferential stress at the oil–hole, the fatigue cracks initiated from the ends of the spear-like openings at the upper and low edges of oil–hole and propagated oppositely along the axial so that axial cracking occurred on the plunger-sleeves.