Failure of a valve rod caused by improper heat treatment and metallurgy

摘要

Fractography analysis was used to investigate a structural component which failed during its normal service life. It has been found that imperfections in the metallurgy and heat treatment were the major causes of failure. Some detrimental impurities were found in the material. A number of cracks were initiated at the interface between the impurities and the base material. These cracks were the main reason for the failure. Also several kinds of metallography defects contributing to crack initiation have been found. The metallographic structure of the material is bainite + ferrite; the amount of ferrite increases gradually from the outside to the inside of the part. This was the wrong metallographic structure for this kind of material; it should be sorbite. Cracks initiated and propagated under a relatively low stress level by stress corrosion cracking.