The Influence of 20Kh13 Steel Specimen Surface Pitting Damage on the Growth of Fatigue Cracks

Abstract

The damage to the surface of steam turbine rotor blades by pitting reduces significantly their fatigue resistance and can lead to fracture resulted from the development of fatigue cracks. To determine the service life of blades or the probability of their breakage, it is necessary to have data that allow one to assess how the degree and nature of damage to their surface affects their fatigue resistance. If the rotor blade has passed, in an off-design operation mode, the zone of increased stress amplitudes, then, given the known number of cycles, the measure of fatigue damage to the blade material can be estimated. If the stress amplitudes did not exceed the endurance limit (taking into account the safety factor and the loading cycle asymmetry value), the fatigue cracks which could lead to blade failure did not appear in the blade material. If the stress amplitudes and the corresponding number of cycles are in the zone to the left of the French line, irreversible fatigue damage did not occur in the blade material. Determination of the lengths of fatigue cracks appearing at the pit bottom as a function of stress amplitude and number of operating cycles is a topical issue. This will make it possible to evaluate the size of the damaged layer and to restore the blade fatigue resistance by removing this layer. The results from studies of crack resistance under fatigue loading of specimens made of 20Kh13 blade steel with artificially created pitting damage to the surface are presented. The possibility of using the calculated dependences for evaluating the French line for samples with initial pitting damage has been confirmed, and an assessment of the damaged metal layer at the pit bottom depending on the stress amplitude and number of operating cycles has been obtained. The length with which the crack must be removed to restore the blade system service life has been determined.