Grade 91 is a CSEF (creep strength enhanced ferritic) alloy introduced in the late 20th century for high temperature service in power boilers. ASME BPVC-II develops material properties based on time-to-rupture data after 100,000 hrs, with a safety factor of 3.5.
While this adds a reasonable factor of safety for designing power boilers to BPVC-I, no specific guidance is given in BPVC-I for pipe stress and more specifically life assessment analysis for creep and fatigue. Creep damage is indicated by voids aligned along the crack boundary in the accompanying image.
Creep damage is indicated by voids aligned along the crack boundary
Gr. 91 material is prone to creep deformation over time and temperature. The creep strain of greatest concern in power boilers is secondary creep, which occurs generally at constant stress and temperature over a period of time. Secondary creep is an energy balance between strain hardening and stress relaxation of the material. Creep gradually softens the material, where the material loses ductility and voids begin to form in the microstructure, leading to the formation of cracks.
ASME introduced an enhanced Gr. 91 Type 2 material with Code Case 2864-2, first approved April 23, 2020. The old Grade 91 allowable stress values (pre-2019) were reduced and called Type 1, while the Type 2 version of Gr. 91 includes additional trace elements to improve performance of the material. Type 1 and Type 2 versions of Gr. 91 and the specific chemical compositions can be found beginning in the 2019 edition of BPVC-II Part A, under material spec SA335.
In many instances in the power industry, creep cracking in Grade 91 pressure parts begins near the flaw or weakness left behind from a weld. Grade 91 has very specific requirements for pre-weld heat treatment (HT), weld interpass temperature, post-weld HT and careful cooling. If these requirements are not carefully followed, premature cracking will occur in the life of the weld near the heat affected zone (HAZ). Field welds are more problematic as the weld conditions are less controlled than in a shop environment.
BPVC-I is the ‘mother code’ of power boilers, but it does not give guidance for life assessment of Gr. 91 materials. Therefore, careful consideration must be taken to understand the operation of the power boiler and to develop a comprehensive plan to identify and inspect critical welds and specific unwelded geometries. Recent creep-related failures of large bore tees and header end caps, point to a confluence of material issues, fabrication issues, operational issues, and lack of inspection.
Tetra Engineering Group has decades of experience with the engineering and inspection of power boilers. With former power boiler OEM engineers, a metallurgist, and ASME Code experts on staff, Tetra is well-versed in the various boiler OEM designs and the specific metallurgy and physics of Gr. 91 material and creep physics.
Contact Tetra to discuss preparing an inspection program for your facility.
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Higher demands on outlet steam conditions has led to increased use of modified low alloy materials (9-12% Cr), such as Grade 91. The superior high temperature material properties of these types of steels (strength, fatigue resistance, creep resistance, improved steam oxidation resistance) gives the designer advantages with respect to boiler reliability and thermal performance. The advantages of using this type of materials are many; however, the long-term integrity has been a concern to the industry ever since the first installations.
Since 2007, the ASME B31.1 Power Piping Code has included mandatory requirements on the Operation and Maintenance (O&M) of Covered Piping Systems (CPS), previously called High-Energy Piping. The main concern is that the safety of these systems is very dependent on good O&M, which justifies its inclusion in the Construction Code. There is room for flexibility depending on individual circumstances but the responsibility is squarely laid on the Operating Company.
Benefits of extended thermocouple installations on HRSG tubes
If your plant suffers from frequent failures, installing additional thermocouples can prove to be invaluable when assessing the actual root cause of the failures.
