by David A. Osage, P.E., ASME Fellow / Jan 01, 2010

New rules and a Code Case under ASME Section VIII, Division 2 rules permit high strength materials to be used to temperatures approaching and into their respective creep ranges. Very significant reductions in life occur in those regimes of stress, strain-rate, time and temperature where the damage mechanisms due to creep and fatigue interact to influence the microstructure and behavior. Creep-fatigue is of special concern especially when high strength alloys may undergo time-dependent straining including the effects of start-up and shut down procedures. Under the new ASME Section VIII, Division 2 rules, as the respective creep ranges of high strength low alloys steels are approached, allowable stresses are significantly higher than those with which there is service experience that would permit exempting design details from fatigue analysis based on documented “years of relevant experience”. The same limitation exists for the any new alloys and or applications for which there is no relevant service experience. ASME Code Case 2605 provides a template for designing with high strength chrome-moly steels where such exemption is not possible. The Code Case incorporates a novel model that recognizes that straining of high strength ferritic steels accelerates softening at elevated temperatures lowering the tensile strength, hardness and creep resistance. It requires calculation of creep life at discontinuities without cycling and relocation of welds away from locations with high damage rates.

Reference:
Prager, M., and Osage, D.A., “Background to the Development of ASME B&PV Code Case 2605 – Extension of 2.25Cr-1Mo-V Alloy into the Creep Range”, ESOPE Conference, Paris, France , 2010.

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