The present investigation is dealing with an exhaust valve failure of a main diesel engine in a vessel for serving marine petroleum sites. The failure of the valve occurred after 6490 h in service at 380 °C ambient temperature. Fracture is located at a cross-section of the spindle near to the connection between spindle length and neck of the valve dish, while the spindle was subjected to bending. It was found that the upper part of the spindle was subjected to partial dissociation of the martensite structure coexisting with carbides embedded between the martensite laths. The lower part of the spindle, near the fracture surface, was containing coagulated carbide formations (clusters). The precipitates mainly contain Cr-carbides in addition to some few Mo-carbides due to the effect of inadequate Mo-alloying. The carbides clusters represent a suitable location for micro-crack initiation during valve service. Running time and mechanical cyclic loads in addition to hot temperature environment was leading to exaggeration of micro-cracks to become an easy pass for fracture.
The spindle fracture surface showed facets of ferrite grains embedded with deep clusters of carbides due to creep rupture, which is leading to strength collapse due to thermal yielding. It is recommended to use steel-alloy containing higher C- and Mo-contents to raise martensite start temperature and impedes coagulation of carbides during servicing respectively. |
