Beitrag zur Bestimmung des Schädigungszustandes von kriechbeanspruchtem CrMoV‐Stahl

Das Kriechverhalten des warmfesten Stahles 17 MoV84 wird beschrieben. Unterschiede in der Gefugeausbildung bewirken eine Streuung der Zeitstandkennwerte und sind sowohl fur den Kriechvorgang als auch fur die Bestimmung der Kriechschadigung von groser Bedeutung. Durch eine Zwischen-Warmebehandlung von kriechbeanspruchten Proben ist es moglich, die Anteile der reversiblen und irreversiblen Schadigung gegeneinander abzugrenzen. Das in der vorliegenden Untersuchung dargestellte Verfahren sollte die Bestimmung des Schadigungszustandes und der Restlebensdauer von betriebsbeanspruchten Bauteilen ermoglichen. Dazu sind aber noch weitere, vertiefte Untersuchungen notwendig. A Contribution to the Problem of the Determination of the Creep Damage for a CrMoV-Steel0 The lifetime of materials subjected to creep is limited by the accumulated damage. The estimation of the creep damage is difficult, especially in cases where there is uncertainty about the thermal history of a specific component. Creep damage usually consists of a reversible and an irreversible part. The reversible part can be recovered by a heat treatment and is mainly due to changes in the microstructure (dislocation configuration, state of precipitation etc.). Irreversible damage is usually a consequence of necking or of cavitation. In the present investigation a CrMoV-steel (0.18 C/0.6 Mn/0.4 Cr/1.1 Mo/0.4 V) was creep tested at 550 °C and a stress of 245 N/mm2. After certain times or strains the specimens were removed and their microstructural state was determined. From the removed specimens new samples with smaller dimensions were machined and again tested under the same conditions. Between the first and the second creep test a new heat treatment was applied for some of the samples, in order to remove the reversible creep damage and to gain more information about the influence of the irreversible damage on creep. The creep behavior of the investigated CrMoV steel depends very strongly on the initial microstructure. For the same testing conditions the time to fracture can vary from 740 hours (40% upper bainite/60% ferrite) up to almost 10000 hours (100% coarse grained upper bainite). This result indicates that the type and the progress of the creep damage could also be influenced by the microstructure. In fact with the ferritic-bainitic structure cavities mainly develop on ferrite-bainite grain boundaries and are usually not linked together. Within the purely bainitic microstructures, the cavities are found on former austenite grain boundaries and have very often a crack-like appearance. The behavior of newly machined samples after the first creep test without a new heat treatment is dictated by the internal damage of the sample. After the removal of the scale and the macroscopic inhomogeneities the creep curves of the second test can be considered as a continuation of the first creep (curves figure 10 in German text). A new heat treatment ( 1000 °C/l h/air + 725 °C/4 h/air) between te two creep tests leads to a partial recovery of the original properties. For the mixed bainitic-ferritic microstructure, even for fractured samples, a considerale increase in time to fracture is observed, e.g. 1156 hrs for the first test, 6821 hrs for the second test after a new heat treatment. The new heat treatment produced a much more creep resistant microstructure and recovers therefore the influence of any creep damage, especially in the case of the originally ferritic-bainitic microstructure. In order to reveal the effect of creep damage on the second test, the creep properties of the microstructure produced by the intermediate heat treatment has to be considered as a reference (100% upper bainite). This fully bainitic steel exhibited in its new state mean values of time to fracture tf = 8554 hrs, and secondary creep rate = 2 × 10−10 sec−1. The experimentally measured creep data of the second creep test and the mean values form a specific ratio as a measure of the damage during the course of the first creep test. This behavior is shown schematically in Fig. 14. In this figure the creep strain is plotted as a function of the ratio between the mean time to fracture and the time to fracture in the second creep test (or secondary creep rate). The experimental results show that it needs of a minimum creep strain to initiate irreversible creep damage. The amount of strain depends also on the microstructure. In a relatively brittle steel containing e.g. coarse rained upper bainite, creep damage commences early in the secondary creep stage, whereas in more ductile steel the onset of tertiary creep coincides with the begin of irreversible damage.