The turbine rotor in steam power-generating plant operates under conditions in which creep-fatigue-oxidation interactions may occur. Generally, fatigue damage is known as damage resulting from the initiation of the fatigue crack at the material surface and its growth. Creep damage also has been attributed to creep cavitation associated with the grain boundary cracking, and oxidation may facilitate the earlier initiation of fatigue cracks and increase crack-growth rate. The interactions among above three damages are known to be responsible for reducing rotor life significantly less than that designed from the single mechanical property data. Although individual effort has been devoted to investigate the mechanism of creep-fatigue-oxidation interactions in rotor materials, it is not fully identified.
The present work investigates the environmental effect on the crack initiation and high temperature low-cycle fatigue life of Cr-Mo-V steel under both continuous cycling and conditions in which creep-fatigue-environment interactions may occur.
All fatigue tests are carried out in pure Ar and air atmosphere at 823K, strain wave forms are symmetrical triangle and strain cycle with hold time either at the maximum strain(tensile hold) or at the minimum strain (compressive hold), or at the both strain values.
From the continuous cycling test results in an Ar atmosphere, it is understood that a large fraction of the fatigue life is consumed in the crack initiation, and decrease in fatigue life of specimen with rough surface relative to that with smooth surface is mainly due to the reduction in the number of cycles for the crack initiation. Also, the fatigue life of specimen tested at air is shorter than that in an Ar atmosphere, which results from the earlier crack initiation and increased initial crack-growth rate in air.
The introduction of hold time during a low-cycle fatigue test has been shown to decrease fatigue life relative to that of continuous cycling, and it is observed that the fatigue life decreases with increasing hold time at both Ar and air. Also, the fracture mode of the specimens tested with hold time is observed to be transgranular regardless of hold modes. The decrease in fatigue life of specimens tested with 1 min hold time in an Ar atmosphere is thought to be due to the reduction in number of cycles necessary for the crack initiation by surface oxidation regardless of hold modes because Cr-Mo-V steel is sensitive to the oxidation even under an Ar atmosphere. On the other hand, from the fatigue test results with 30 min hold time under an Ar atmosphere, the reduction of the fatigue life is mainly attributed to decrease in the number of cycles for the crack initiation due to oxidation regardless of hold modes. When tensile hold time of 30 min is introduced, the increased crack-growth rate relative to that with 30 min compressive hold time partly contributes to the reduction in fatigue life. The increased crack-growth rate with 30 min tensile hold time is believed to be due to oxidation and damage caused by creep deformation in the crack-tip region during tensile hold. And decrease of the fatigue life at air compared to that under an Ar atmosphere is attributed to the earlier crack initiation and increased crack-growth rate due to oxidation.
