EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Fatigue properties of tool steels and ductile irons are a concern in many of its applications, and the two materials may in some applications be competitor selections. In many cases are long life endurance a requirement, and it may even be as high life lengths as ranging up in the Very High Cycle Fatigue (VHCF) regime, i.e. above 107 load cycles. Tool steels are mostly used with a martensitic microstructure in the hardened and tempered condition. Similarly for ductile irons where one class is a bainitic/martensitic structure but with the graphite nodules dispersed in it. For both materials, high static and fatigue strengths are often an engineering requirement.

In the present study, the fatigue strength has been tested in a wide life range for both tool steel and ductile irons. At lower life a servohydraulic machine was used for fatigue testing, while at higher life a 20 kHz ultrasound equipment was used to manage reasonable test times. Hour-glass specimens were loaded in uniaxial fatigue at R=0.1 load ratio, and the specimens were designed to operate in resonance at 20 kHz load frequency. SN-curves were obtained with fatigue lives from 105 to 109 load cycles. Microstructure was characterized and fractography by SEM was performed. The life limiting defect distributions were mapped. Fatigue strength and life was discussed using different model equations, and a comparative perspective of the materials was laid out involving strength, microstructure and defect distribution.