Beschreibung des Zeitdehnverhaltens warmfester Stähle unter zyklisch veränderter Zeitstandbeanspruch

FKM 2001
Issue No. 236
Project No. 182

The creep rupture behavior of a number of important high-temperature power plant steels under rectangular-cyclic loading was investigated in a large parameter range in continuation of earlier work. The tests were carried out under altered stress, altered temperature or both. The previous tests leading up to fracture and higher elongations of up to 2 % were now supplemented by tests up to a low elongation of 0.2 % with the usual number of cycles of around 15. The tests under altered stress extended to intermittent tests with a stress ratio of zero. As a further addition, the tests were extended to a steel of the type X 12 CrMoWVNbN 10 1 1. Furthermore, tests with an additional compression phase were carried out or continued. All tests have now been largely completed and evaluated. The results are fracture times and yield times that can be expected under the cyclic changes between a basic and a peak load. These times can be predicted on the basis of the fracture and yield times for constant base and peak loads with the aid of the modified service life fraction rule. The characteristic constants of this rule are a relative fracture life and strain-dependent values of a relative elongation life. These constants can be described by a factor concept, which has now been revised and extended with the addition of some results from a parallel project (AiF No. 10396). The factor concept contains a material factor, two cycle factors, a temperature factor, an elongation factor and a pressure factor. The material factor depends on the type of steel. One cycle factor is intended for changing stress, one for changing temperature. Both factors depend on the magnitude and temporal extent of the corresponding stress change and can be combined for simultaneous stress and temperature changes. The temperature factor depends on the extent to which a steel grade-specific temperature near the lower application temperature limit is exceeded. The elongation factor depends on the elongation achieved and assumes the value one for the relative fracture life. The pressure factor takes into account the effect of additional pressure phases of limited extent inserted into the rectangular cycle. In a transition area from variable creep stress to constant normal creep stress, the product of material factor, temperature factor and, if applicable, strain factor requires interpolation towards the value one. A previously developed program LARA 2 for the application of a modified service life proportion rule with the factor concept was further developed into a modular program LARA 3, whose calculation core can be used as a subprogram for a service life counter. For LARA 3, the elongation limit curves and fracture curves required from the normal creep test for all nine steel grades covered by the factor concept were determined in extensive scatter band evaluations where they were not previously available. Where necessary, these data were created not only in a version adapted to standard values but also in an optimized version. In addition, a separate accumulation of this strain was set up in LARA 3 for steels with high initial plastic strain in accordance with the strain hardening rule, whereby the creep strain is accumulated unchanged using the modified service life fraction rule. The documents required for this were provided for the respective steel grades in the form of mean yield curves and mean creep limit curves. The relative creep life is included in the factor concept for these cases. Together with an extension of LARA 3 to multi-stage creep loading, which was carried out in a parallel project (AiF No. 8972), a program is now available with which yield and fracture times under cyclic creep loading can be predicted in a wide range of stresses relevant to power plant components.