Grade 91 Steels

Background

Early failures of components fabricated with Creep Strength Enhanced Ferritic (CSEF) Steels in conventional and HRSGs inEarly failures of components fabricated with Creep Strength Enhanced Ferritic (CSEF) Steels in conventional and HRSGs incombined cycle plants have gained widespread attention within the industry. Affected HRSG components included boiler tubing and interconnecting piping as well as non-boiler external piping. CSEF steels such as SA335 Gr. P91 in piping and T91 in boiler tubing (often referred to collectively as “Grade 91”) have been used at many new large combined cycle power plants. Welding of this material requires careful attention; the correct welding technique, weld filler materials and strict adherence to pre- and post-weld heat treatment requirements must be applied [1]. Proper heat treatment is essential in order to obtain the desired enhanced creep resistance. Special problems can arise when Grade 91 components are welded to dissimilar materials. Dissimilar metal welds (or DMW) between P91 and P22 or P91 and stainless steel have been particularly problematic at some plants. 

Increased Creep-Strength

Use of Grade 91 steel in high temperature steam components including pressure vessels is attractive because they provide a superior level of creep (and fatigue) strength and oxidation resistance at elevated temperatures. The steel is a martensitic Cr-Mo steel that has been microalloyed with vanadium and niobium and has a controlled nitrogen content. After welding, brittle Mo steel that has been microalloyed with vanadium and niobium and has a controlled nitrogen content. After welding, brittle martensite with unfavorable material properties is formed in the weld metal. Thus, heat treatment is required in order to produce tempered martensite with precipitated carbides and vanadium/niobium-rich carbo-nitrides which provide for acceptable material properties. Industry-recognized standards for PWHT parameters of Grade 91 steel currently dictates an ideal temperature of 760 °C for 2 hours [1,2], maintaining these precise conditions during actual fabrication requires careful control. Therefore, an understanding of the allowances for variation in the PWHT parameters must be developed in order to ensure that the final mechanical properties of the steam pressure vessel will meet design requirements.

Failure modes

The most common type of Grade 91 creep failure is the so called type IV failure, where cracks develop and progress in the fine grained Heat Affected Zone (FGHAZ). This part of theweld has reduced creep strength properties, hence creep is likely to originate here. Despite superior material properties, in-service failures have been reported in literature. One of the most well-known casesoccurred at the 4 × 500 MW coal-fired power station West Burton in the UK [3,4], where failure was reported after as little as 20,000 to 36,000 h of service. Failures were attributed to degraded material properties and increased stress levels due to the geometry of the failed component. Additional in-service experience of Grade 91 failures have recently been reported by EPRI [5]. The root cause of more than half of the investigated failures could be related to either poor design or poor fabrication.

References

[1] Guidelines and specifications for high-reliability fossil power plants, 2nd Edition — Best Practice Guideline for Manufacturing and Construction of Grade 91 SteelComponents. Electric Power Research Institute (EPRI). (Final Report), June 2015.

[2] ASME Boiler & Pressure Vessel Code - Section I: Rules for Construction of Power Boilers. The American Society of Mechanical Engineers, Edition 2013.

[3] I.A. Shibli, Performance of P91 Thick Section Welds under Steady and Cyclic Loading Conditions: Power Plant and Research Experience, Vol. Volume 1, Issue3European Technology Development, UK. OMMI, December 2002.

[4] S.J. Brett, J.L. Oates, C. Johnston, In-service type IV cracking in a modified 9Cr (Grade 91) header. RWE npower, UK. Creep and Fracture in High Temperature Components:Design and Life Assessment Issues, ECCC Creep Conference, Sept 12–14, 2005 (London, UK).

[5] Service Experience with Creep Enhanced Ferritic Steels in Power Plants in the Asia-Pacific Region, Electric Power Research Institute (EPRI), Palo Alto, CA, 2015.

[6] Fabricius, A. Jackson, P. Premature Grade 91 failures - worldwide plant operational experiences. Engineering Failure Analysis, page 386-406, 2016.

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Established in 1988, Tetra Engineering has more than 25 years experience providing solutions to the power industry. We specialize in solutions for HRSGs, conventional boilers & steam-cycle balance of plant.

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