Abstract: The cyclic plastic deformation of the weldment of superaustenitic stainless steel with 6% molybdenum (UNS S31254) demonstrates that the strain rate does not affect the strength, but affects the ductility significantly. The response of tensile peak stress versus number of cycles for the weldment shows the cyclic hardening followed by the cyclic softening. A secondary hardening that appears at 1·5% strain amplitude is related to the strain induced martensite formation. The dislocations piled-up at the δ-ferrite grain boundary due to the essential rigid property of ferrite have a higher content of chromium and molybdenum. With a high fatigue strain rate, the austenite matrix of the weld metal exhibits persistent slip bands after fatigue at 0·6% strain amplitude and evolves to dislocation cell structures at 1·5% strain amplitude. The martensite band or the martensite sheath is formed at 1·5% strain, depending on the magnitude of the strain rate.