There have been a number of rate equations proposed in the literature for the growth of grain-boundary cavities during creep. Implicit in all these analyses is the assumption that growth occurs freely and that no constraints exist that would attenuate the predicted rates. It is shown that a constraint on growth rate can occur in polycrystals because creep cavities are inhomogeneously distributed amongst the various grain boundaries. This attenuation of growth rate is predicted to be greatest when high-strength alloys with large cavity populations are deformed slowly and least when pure metals with low cavity populations are deformed quickly. The application of these ideas to multiaxial stress states and extrapolation procedures is discussed in the knowledge that the rate of unconstrained cavity growth is directly proportional to the maximum principal tensile stress, whereas constrained growth is proportional to the nth power of the octahedral shear stress.

Constraints on diffusional cavity growth rates

Metals often fail in service under creep conditions because of the formation of cavities on the grain boundaries which are approximately normal to the applied stress. This phenomenon of creep cavitation is becoming of increasing technological importance. As a result a complete understanding of it is desirable so that alloys with improved cavitation resistance can be designed. This paper reviews the development of our present understanding of the phenomenon which is one of nucleation, growth and linkage leading to failure. Several mechanisms of nucleation, such as at grain-boundary ledges or precipitates, have been postulated and experimental evidence in support of each has been cited. Similarly, deformation- or vacancy-controlled growth mechanisms have been discussed. It is apparent from the literature thatno single mechanism is applicable, indeed, the work discussed here suggests that several mechanisms may operate and each may become dominant at different stages of the creep life. Finally, the status of research into nickel-base superalloys is reviewed with reference being made to such effects as regenerative heat-treatment.

Cavitation in creep

This paper describes part of an investigation into differences in cavitation characteristics in commercial and high-purity Cr-Mo-V steels of the types used in rotors and pipes in steam power plant....

The creep cavitation of commercial and high-purity Cr-Mo-V steels

Remaining creep life estimation by strain assessment on plant

Microstructural instability of 0.5Cr0.5Mo0.25V creep-resistant steel during service at elevated temperatures

The performance of Cr-Mo-V steels, as widely used for steam pipe work, turbine rotors and casings, and associated bolts in steam power generating plant, may be limited by a susceptibility to failure at low ductility, especially when heat treated to high strength levels. To investigate the ductility in creep conditions of such steels, an extensive study of the interrelation of heat treatment, microstructure, creep strength, rupture strength, rupture ductility, and fracture characteristics has been made in commercially produced 0·5Cr-0·5Mo-0·25V and 1Cr-1 Mo-0·25V steels tested at 550°C for times up to >20000h. The results show that the rupture strength and ductility can be varied widely by changing the austenitizing temperature and the rate of cooling therefrom, and that a heat treatment at a high temperature (1300°C), given to simulate the microstructure in the vicinity of a weld, can produce very low creep-rupture ductility (<1%). Low ductility in these steels is associated with an intergranular ...

Effect of heattreatment variations on strength and ductility of Cr-Mo-V steels during creep at 550°C

Examination by transmission electron microscopy of creep cavities retained in thick foils and by scanning electron microscopy of creep cavities exposed by fracture at − 196° C (77 K) has provided direct evidence for the association of certain microstructural features with creep cavitation in a Cr–Mo–V pipe steel heat-treated to give a bainitic structure similar to that obtained on welding. The cavities occurred on prior austenite grain boundaries in proximity to one or more large carbide particles and in areas containing fewer dislocations and much less of the fine precipitate than in the bainitic matrix. Nucleation and growth of cavities was widespread and continuous throughout the creep tests on grain surfaces subjected to both shear and normal stress components. Some coalescence possibly took place but many cavities still retained their identity and appeared in a “honeycomb” network on the creep-fracture surface.

H. R. Tipler

Papers

Effect of heattreatment variations on strength and ductility of Cr-Mo-V steels during creep at 550°C

Some Direct Observations on the Metallography of Creep-Cavitated Grain Boundaries