Untersuchungen zur Kriechermüdungswechselwirkung in der hochwarmfesten Legierung Alloy 800H

The present paper describes investigations on the creep-fatigue interaction in Alloy 800H at 650 oc under two different loading conditions. First, low cycle fatigue tests with strain rates in the range from 10-3 s-1 down to 5·10-6 s-1 are performed. Next, fatigue tests with strain controlled dwell times in tension from 180 s to 1800 s are carried out. Furthermore, the influence of different initial microstructural conditions, viz., solution annealed, stabilized and long term aged, is investigated. The evolution of the microstructure as well as the damage mechanisms are characterized by scanning electron microscopy. Under both loading conditions the solution annealed material shows a pronounced cyclic hardening due to the strain-induced precipitation of small M23C6 chromium carbides. With increasing dwell time, or decreasing strain rate, the maximum peak stress is lowered. As a result of the heat treatment or long term aging at 650 oc, chromium carbides precipitate at grain boundaries as well as inside the grains. Hence, the potential for strain induced precipitation is limited and the steady state condition is reached after a few cycles. Lifetime is only affected by the initial microstructure under slow low cycle fatigue loading. This is a result of two different damage mechanisms.