Influence of the modes of operation and structural factors on the crack propagation in 12Cr–2Ni–Mo steel

Abstract

We study the influence of temperature, specimen sizes, loading frequency, and the time of holding under long-term static loading on the crack growth rate in 12Cr-2Ni-Mo steel. It is shown that the influence of these factors on the crack growth rate is insignificant. A numerical-experimental model is proposed for the evaluation of the time of subcritical crack growth with regard for the duration and cyclic character of loading.Keywords: crack growth rate, stress intensity factor, cyclic and long-term static loading.Introduction. The designed service life power-generating gas-turbine plants (GTP) is fairly long (of about 100,000 h, i.e., more than ten years). The most heavily loaded structural elements of GTP restricting their service life are compressor and turbine blades and disks. The events of fracture of blades have, as a rule, local consequences and lead to failures of the plant. At the same time, the events of fracture of the disks cannot be localized inside the GTP, which may lead to its complete destruction.The realization of full-scale or accelerated tests of a GTP aimed at the assessment of its life is complicated by their duration and high cost. Moreover, in view of the inhomogeneity of the properties of materials of the disks (regulated by technical specifications), the spread of values of the service life of the disk may be quite large.The problem of exclusion of unexpected failures of the disks can be solved by using the concept of operation of GTP according to their current serviceability. The application of this concept would allow one to prolong the initially specified design life till the exhaustion of the service life of the product, i.e., till the time when crack initiation becomes possible. To do this, it is necessary to specify the volume and terms of scheduled maintenance guaranteeing, on the one hand, the exclusion of the possibility of growth of cracks in a disk to the critical size and, on the other hand, the acceptable level of cost efficiency of operation of the plant (i.e., the cost of the works should be not too high).The character of loading of power-generating GTP depends on the class of their operation: peak, basic, or semipeak. In accordance with these classes, the operating cycles of GTP can be represented in the form of a set of modes reduced, as a final result, to two generalized factors corresponding to the character of damage, namely, to the duration of loading and its cyclic character. In Fig. 1, we present the typical limiting cyclograms of loading of the GTP depending on the class of operation.In analyzing the changes in the stressed state of the disks of GTP depending on the character of operating cycles, the behavior of turbine disks characterized by the maximum level of stresses and temperature is of primary interest. The displacement cooling systems used in the design of power-generating GTP exclude the possibility of access of hot air to the surfaces of the disks and, hence, heat is delivered as a result of heat conduction in the lock joints.