In-service behavior of creep strength enhanced ferritic steels Grade 91 and Grade 92 – Part 1 parent metal

In creep strength enhanced ferritic steels, such as Grade 91 and Grade 92, control of both composition and heat treatment of the parent steel is necessary to avoid producing components which have creep strength below the minimum expected by applicable ASME and other International Codes. These efforts are required to ensure that the steel develops a homogeneous fully tempered martensitic microstructure, with the appropriate distribution of precipitates and the required dislocation substructure. In-service creep related problems with Grade 91 steel have been reported associated with factors such as incorrect microstructure and heat treatment, welded connections in headers and piping, dissimilar metal welds as well as the manufacture and performance of castings. Difficulties associated with remediation of in-service damage include challenges over detection and removal of damaged material as well as the selection and qualification of appropriate methodologies for repair. Since repeated heat treatment leads to continued tempering, and a potential degradation of properties, specific procedures for performing and then lifing repair welds are a key aspect of Asset Management. This paper presents a summary of in-service experience with Grade 91 steel and outlines approaches for repair welding.

► The steel alloy known as Grade 91 is widely used to fabricate critical pressure part components.
► Designers favor Grade 91 because it provides superior elevated temperature strength at substantially lower cost than the austenitic stainless steels.
► Service experience has confirmed that early failures can occur.
► Life management solutions involved attention to detail at Purchase, during design and all stages of fabrication.