The creep strength of aberrant grade 91 steel

ABSTRACT

The high-temperature steel grade 91 is intended to operate in service with a fully martensitic microstructure, achieved by a well-defined heat treatment. Unfortunately, lack of control, either during the initial material manufacturing or later during post-weld heat treatment, can lead to components entering service in a substandard condition. In the worst cases, this can be a fully ferritic microstructure with a complete absence of martensite. In this aberrant condition, the steel can have a creep strength significantly lower than that of normal martensitic grade 91 with an associated risk of early failure in plant life. This paper reports on investigations into the creep strength of aberrant grade 91,,using both conventional uniaxial rupture testing and small-scale impression creep testing. The creep strength has been found to fall into the range mean-35% to mean-50% in terms of stress relative to the most recent (2019) ECCC rupture life assessment of grade 91.

KEYWORDS:

• Grade 91 steel
• p91 steel
• aberrant non-martensitic microstructure
• uniaxial creep testing
• impression creep testing

Acknowledgments

The uniaxial testing referred to in this paper was commissioned by EPRI and the UKHTPP Forum. The impression creep testing was either carried out by EPRI or commissioned by the UK generating companies Centrica, SSE, Engie and RWE Generation. Impression testing was carried out by EPRI, Nottingham University and Jacobs.

Disclosure statement

No potential conflict of interest was reported by the author(s).