Metallurgy Failure Analysis for an Early Fracture Engine Intake Valve Made of 85Cr18Mo2V

By means of analysis technique such as scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS),and transmission electron microscopy(TEM),deep metallurgy failure analysis for an early fracture engine intake valve made of the martensite material of 85Cr18Mo2V was performed through macroscopical and microcosmic as well as chemical composition angles.According to the result of SEM,this valve belonged to fatigue fracture.The crack sources of dense micro-holes near the surface of intake valve and SiO2 impurity are the direct reasons to cause the valve failure.The EDS analysis for the fatigue source region approved that superfluous Si,O and Al impurity existed.The TEM observation of the slice cut from the region closing to the fracture surface showed that deep areas near the surface of valve were oxidized severely.The material structure got brittle due to the generation of Fe3O4.Cracks will be produced in these oxidation areas under high cycle fatigue loading.Consequently,the synthetical action of various factors finally caused the fracture of valve.