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It has long been established that strain localization effects, particularly at grain boundaries are important during creep of engineering steels. It is therefore reasonable that the loss of creep rupture strength in T91 steel is associated with preferential recovery of the microstructure in the vicinity of a prior austenite grain boundary. The preferential recovery at the boundary promotes an acceleration of the local creep rate and the additional strain leads to further recovery. Hence, the process, once started, is self sustaining and leads to premature rupture.
Formation of intermetallic Laves phase, (Fe,Cr)2(Mo,W), can influence creep behavior in two ways:
- It appears that relatively large amounts of Mo and, in Grade 92 steel W, are incorporated in this phase. The depletion of these elements from solid solution will therefore cause a reduction of the overall creep resistance, and
- The Laves phase appears to nucleate mostly on subgrain boundaries and on prior austenite grain boundaries. This phase will coarsen during creep exposure at temperatures
up to about 650°C. Since Laves is relatively hard, it can provide preferred sites for nucleation of creep voids.Advertisement
Examination of parent metal Grade 92 steel samples which failed with low ductility reveals that, as expected, this brittle behavior is a consequence of high densities of creep cavities. It appears that very high densities of creep cavities are initiated so that the time involved in link–up and crack growth is relatively short. It thus appears that this transition is associated at least in part with the fact that more nucleation sites are available after relatively long time exposure. Evidence thus supports the view that Laves phase formation is associated with creep void development.
The EPRI guidelines are available for purchase at www.epri.com. The authors gratefully acknowledge the contributions of the members of the project collaborative, whose analysis and comments were invaluable to both the organization and content of the guidelines.
Kent Coleman manages EPRI’s Boiler Life and Availability Improvement Program. He has been a member of EPRI’s Generation staff for 12 years after a 17-year utility background and has an extensive background in the materials, life assessment and welding areas, and holds several patents in the areas of boiler materials, welding and repair. He also is a member of several ASME Code committees including SCI, Power Boilers. You may contact him by e-mailing editorial@woodwardbizmedia.com.
Dr. Jonathan Parker is a senior project manager at the Electric Power Research Institute (EPRI). He provides expert technical support for several projects associated with understanding the factors affecting damage in critical components. He received a bachelor’s degree and PhD from Swansea University (UK). He is a Chartered Engineer with the UK Engineering Council and a Fellow of the Institute of Materials, as well as a Fellow of the Institution of Mechanical Engineers. He has been an expert reviewer and external assessor for higher degree projects and research programs and has served on the Editorial Board of Learned Journals and Technical Conferences, is the editor of three books and author or co-author of more than 200 publications. He also was awarded the Charles Hackett Medal for work on alloy steels. You may contact him by e-mailing editorial@woodwardbizmedia.com.
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