The Effect of Intermediate Annealing on the Fatigue Life of Iron

Abstract

The average life to fracture of vacuum-annealed and polished α-iron specimens fatigued in push/pull with a stress cycle of ± 9 tons in⁻² is 2·6 × 10⁶ cycles. Subcritical annealing of the specimen after 10⁶ cycles produced no significant change in the total number of cycles to fracture. Recrystallization did not occur. When the annealing temperature was raised to 920° C, so that recrystallization was induced by the phase changes, the total life to fracture significantly decreased. Recrystallization at earlier stages of the expected life rendered this decrease more pronounced, and in order to have no effect recrystallization had to be carried out at < 5% of the expected life for an uninterrupted test. Similar results were obtained with higher-stress, shorter-life specimens. Electron and optical micrographic evidence indicated that the surface topography was responsible for the failure of annealing to “renew” the specimen. The behaviour of the yield drop and yield extension provided further evidence of the effectiveness of small fatigue cracks and of the “notch-peak” topography of fatigue slip bands as stress concentrators. An investigation into the effects of subcritical annealing on fatigue specimens showed that recrystallization occurred when the tensile yield stress of the specimen exceeded a critical value. It is suggested that most fatigue specimens fail to recrystallize on annealing because the stored-energy density is not sufficient to nucleate new grains.