This paper reports the results of a collaborative investigation of an ex-service grade 91 bend carried out by the UK generating companies Centrica, SSE, Engie and RWE. As part of the handover exercise for Centrica’s Langage power station in 2009 a number of routine checks were carried out on the main steam and hot reheat grade 91 steam pipework. In some cases low hardness readings were found with subsequent metallurgical replication showing the presence of an aberrant non martensitic microstructure. This led to a more extensive inspection programme on the steam lines and the discovery of other areas of suspect material. A review of the operating capability of the plant, including detailed pipework stress analysis and a pipework peaking assessment, along with the assumption that lower strength grade 91 material was present, led to the steam lines being down rated and returning to service under these revised conditions. At the first C inspection in December 2012, after the HRSG and associated pipework had operated for 18720 hours, a bend with a soft weld, along with a section of the straight pipe on either side, was removed from service. An investigation was undertaken to establish how long this component would have survived, had it been left in service, and to consider the implications for the future operation of the plant.

This paper reports the results of a collaborative small scale creep testing exercise carried out by the UK generating companies Centrica, SSE, Engie and RWE as part of an investigation of an ex-service grade 91 bend.

The impression creep test method using a rectangular indenter has been well established and the applicability of the technique has been supported by the test data for a number of metallic materials at different temperatures and stresses. The technique has proved to be particularly useful in providing material data for on-site creep strength assessments of power plant components operating in the creep regime. Due to these reasons, “standard” assessment procedures using the impression testing method are needed in order for the technique to be more widely used. This paper will first address some key issues related to the use of the impression creep test method, involving the data conversion method, typical test types and validity of the test technique etc. Then some recommendations on a number of practical aspects, such as the basic requirements of test rigs, “standard” specimen geometry, indenter dimensions, sampling procedures for scoop samples, specimen preparation, temperature and loading control, and displacement measurement, are briefly addressed. Finally, applications of the test data to assist with the risk management and life assessment programme of power plant components, particularly those with service-exposed materials, using data obtained from scoop samples, are described. Proposals for future exploitation and for improvement of the technique are addressed.

Grade 91 steel, while increasingly popular in high-temperature power plants for both retrofit and new construction applications, faces significant challenges with Type IV cracking at the outer parent side edge of the weld heat affected zone. This structural integrity issue has led to extensive weld inspection requirements and, in severe cases, the premature replacement of grade 91 retrofit headers before their intended design life. This paper presents a method for estimating Type IV cracking timelines in operating grade 91 components by analyzing crossweld Type IV data to determine when Type IV life deviates from parent life. By combining test results from various temperatures, the method generates a generalized prediction of Type IV life that can be extrapolated to any temperature of interest, providing a practical lower bound estimate for service life of the weakest grade 91 material. This approach, which can be applied to service operating conditions to establish realistic inspection timelines for plant components, has already successfully identified early-stage Type IV cracking in two retrofit headers and is being expanded to additional grade 91 components.

This paper investigates the cause of premature failures in certain Grade 91 steel components used in UK power plants. The failures were linked to both low material hardness and specific chemical compositions that fell within ASTM specifications but had a low nitrogen-to-aluminum ratio (N:Al). The investigators examined eight material batches, including those involved in failures, new stock, and in-service components with similar properties. Testing confirmed these materials had lower creep resistance compared to standard Grade 91 steel. Microscopic analysis revealed the presence of large aluminum nitride precipitates, which limited the formation of beneficial vanadium nitride precipitates, leading to reduced creep strength. These findings suggest that even within the ASTM specification limits, a low N:Al ratio can negatively impact the performance of Grade 91 steel.