Effect of impurity content on creep crack growth resistance in 1Cr1Mo0.25V ferritic steels
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I.
INTRODUCTION
THE reliability
of structural components operating at high temperatures is normally evaluated on the basis of service-simulating creep tests. They lead to useful information, provided stresses and strains are uniformly distributed throughout the whole resisting section. However, they cannot give indications on the material behavior when local damage is present, since the stress distribution in the surrounding area can be modified. Practical experience shows that small cracks can be present in some structural components even before starting service, as in the case of very thick ones t~,2,31 or in the heat-affected zones (HAZs) of welded parts. I2-61 It has also been observed that creep damage, sometimes combined with fatigue stresses, may cause very precocious nucleation of a defect. The interest for a new approach based on the fail-safe design philosophy is rapidly increasing. Safety criteria rely on the assumption that these cracks can propagate, but they do not lead to sudden, unexpected failure of the structural component within the envisaged service life. Much theoretical and experimental work was carried out in order to define parameters related to subcritical fracture propagation and depending on both the material ductility and the stress state at the crack tip. Classical fracture mechanics parameters were considered, such as the stress intensity factor, t6'7,81 crack opening displacement (COD), tg-I1,191and J integral, albeit in the modified form (C* or dJ/dt), tn'131 Others were more closely linked to the creep fracture mechanism, such as the net section stress is'91 and the reference stress. [14A5,16] At the present
state of knowledge, however, C* appears to be the most promising parameter, particularly when considerable creep strains are involved. ~ The stress intensity factor, Kt, is also promising when very marked plane strain conditions affect the crack growth. On the other hand, recent studies revealed that creep brittleness of ferritic steels is closely associated with damage accumulated as a consequence of intergranular cavitation. 09-231 This tendency is influenced by both the precipitation state t22,231 and the segregation level of certain impurities on both grain boundary (P) and the internal surfaces of cavities (Sn, Sb, and Cu). t24j Further studies clarified the effect of a triaxial stress state on the development of intergranular cavitation, t231 Some indications were also obtained on the effect of both applied stresses ~24j and the evolution of the precipitate composition ~251on impurity segregation to grain boundaries and free surfaces. Finally, it was found that various types of intergranular precipitates can be active in cavity nucleation: manganese microsulfides, for instance, occurring in the HAZ t22,231 and carbides such as M23C6, M7C3, and M3C if their size exceeds a critical value depending on the amount of segregated impurities, t22j According to Needham, t231 Mo2C favors cavity nucleation too, while M6C is not very active. Moreover, molybdenum-rich carbides such
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