Low Circle Fatigue Behavior of X80 High-Strain Line pipe

摘要

Symmetrical push-pull low circle fatigue tests have been performed on high-strain X80 line pipe material. Micro-analysis has been performed for deformation mechanism as well. Results show that X80 high-strain line pipe material exhibited a higher ductility and long low-circle fatigue life. Cyclic softening is observed at low strain amplitude of 0.4% and 0.6%, however, cyclic softening occurred after slight cyclic hardening at high strain amplitude of 1.0%, 1.2% and 1.4%. Generally, Cyclic response curves show that the cyclic softening occurred at all strains amplitude (0.4%～1.4%). Fractography analysis suggests that transgranular fracture with well-developed fatigue striations and obvious second crack is the predominant failure mode. The amount of second cracks is much fewer deformed in high-strain X80 line pipe material than that in conventional X80 line pipe material. TEM examination reveals that the primary deformation mode of two kinds of material was dislocation slipping. Abundant dislocation glide bands and dislocation cells are formed at grain boundary as strain amplitude increased. Deformation in ferrite has been suppressed and formation of dislocation cell inside the ferrite has been postponed due to large content of M/A constituent.