Degradation of Exhaust Valves in Medium-Speed Marine Diesel Engines: An Evidence-Tiered Review of Materials, Surface Engineering, and Qualification

Exhaust valves in medium-speed marine diesel engines experience coupled combustion heating, intermittent seat cooling, repeated impact and microslip, deposit-assisted corrosion, and prolonged service. Their degradation therefore cannot be explained or qualified using a single room-temperature property. This review organizes the evidence within a mechanism–material–process–performance framework and separates marine observations from transferable valve studies and general materials data. A reference-seeded narrative synthesis was supplemented by a documented literature update completed in September 2026. Quantitative comparisons retain test temperature, geometry, exposure, measurement basis, and transfer limitations. The reported 107-cycle strength of 42Cr9Si2 fell from approximately 569 MPa at 400 °C to 300 MPa at 650 °C. Moreover, two marine hardfacings with overlapping post-service hardness ranges exhibited markedly different damage. Together, these findings indicate that hot strength, scale stability, contact evolution, thermal transport, dilution, interface integrity, and repair history must be assessed jointly. Deposit-assisted corrosion, contact degradation, and leakage may form reinforcing feedback, although no universal time-resolved sequence has been validated. The proposed qualification framework links material and surface engineering decisions to staged coupon, component, engine, and field evidence. It also identifies valve-specific datasets, coupled test protocols, repeated repair validation, and uncertainty-aware prognosis as priority research needs.