Surface damage on the mating interior and exterior pistons used in a marine exhaust valve

Abstract

The wear damage occurred on the two mating pistons, interior piston and exterior piston used for a marine exhaust valve. Visual and SEM observations indicate that the heavy wear marks and wear strips with flaky spalling occurred on the contact surfaces of both pistons. The primary failure mode is the adhesive wear accompanied by little fatigue wear. Metallurgical observations reveal that a white etching layer (WEL) is presented beneath the seriously worn surfaces. XRD analysis indicates that the amount of austenite on the unworn or the lightly worn surfaces of both pistons is in a high level. During operation, the metastable austenite would transform to the martensite under the friction stress, which results in a volume expansion. The clearance between mating pistons would become small to make the friction force on the contact surfaces increase. And the increased friction force induced the concentrated frictional heating. Inadequate lubrication oil cannot bring frictional energy away in time so that the located surface temperature would exceed critical temperature (Ac3). This temperature rise is high enough for austenitization and under such high temperature, the instantaneous hardness of located contact surfaces is low to lead to the occurrence of adhesion wear. Subsequent γ→α′ transformation would occur as the bulk of the piston rapidly conducts heat away from the piston surfaces to form a WEL on the seriously worn surface. The micro-crack formation inside the WEL and subsequent propagating would lead to the occurrence of spalling.