Identification of Weak Thick Section Modified 9Cr Forgings in Service

摘要

Investigations of a number of early failures of thick section modified 9Cr (grade 91) forgings in the UK showed that, while they met ASME requirements for chemical composition and room temperature mechanical properties, they nevertheless possessed poor creep properties. Common features of the casualty materials were low hardness and low nitrogen to aluminium ratio. Following these failures Innogy (formerly National Power) carried out a survey of its grade 91 components to establish whether similar problems existed. A number of potentially weak forgings were identified on the basis of their compositions and hardness. The creep strength of the forgings was then investigated using either conventional testing of spare material from the manufacturer, where this could be obtained, or small-scale creep testing of samples obtained in-situ. The forgings, while somewhat stronger than the original casualty materials, were confirmed to have tow creep strength. This paper provides the results of the survey, the basis on which suspect forgings were chosen, and the results of creep testing.