Comparison and Analysis on Low-cycle Fatigue Behavior of Two X80 Linepipe Materials

Symmetrical push-pull low circle fatigue tests were carried out with conventional and high-strain X80 linepipe material respectively. Fractography was performed to identify failure mode. Microstructure analysis was conducted to determine deformation mechanism. Results showed that high-strain X80 linepipe material exhibited a lower cyclic softening rate and a longer low-circle fatigue life than those of conventional X80 linepipe material. Cyclic response curves showed that the cyclic softening occurred at all strains amplitude (0.4%~1.4%) on two X80 line pipe materials. For conventional X80 linepipe material, cyclic softening was found after slight cyclic stress saturation at high strain amplitude of 1.0% ~1.4%. However, for high-strain X80 linepipe material, cyclic softening was observed after slight cyclic hardening at high strain amplitude of 1.0% ~1.4%. Fractography analysis suggested that transgranular fracture with well-developed fatigue striations and obvious secondary cracks is the predominant failure mode. The detected amount of secondary cracking is fewer in the high-strain X80 material than the conventional. TEM examination reveals that the primary deformation mechanism in the investigated materials is dislocation slipping. Abundant dislocation glide bands and dislocation cells were formed at grain boundary as strain amplitude increased. There are more blocked dislocations and thicker dislocation cells in the conventional X80 than the high strain one. The better cyclic deformation resistance of high-strain X80 material is attributed to the presence of more M/A islands in its microstructure.