Fatigue testing and properties of hardmetals in the gigacycle range

• Published in: International journal of refractory metals & hard materials 2017-01, Vol.62, p.183-191
• Main Authors: Kotas, Agnieszka Betzwar, Danninger, Herbert, Weiss, Brigitte, Mingard, Ken, Sanchez, Jose, Llanes, Luis
• Format: Article
• Language: English
• Subjects: Carbides; Crack initiation; Enginyeria dels materials; Fatiga; Fatigue; Fatigue endurance strength; Fracture mechanics; Gigacycle fatigue; Materials; Materials durs; Ultrasonic fatigue testing; WC-Co hardmetals; Àrees temàtiques de la UPC
• ISSN: 0263-4368; 2213-3917
• DOI: 10.1016/j.ijrmhm.2016.07.004

Hardmetal products are frequently fatigue loaded in service, such as e.g. cutting tools for milling or percussion drills. In the present work, the fatigue behaviour of hardmetals was investigated into the gigacycle range using ultrasonic resonance fatigue testing at 20kHz in push-pull mode at R=−1. Liquid cooling was afforded using water with addition of a corrosion inhibitor. Hourglass shaped specimens were prepared, the surface being ground and polished with subsequent stress-relieving anneal to remove the high compressive residual stresses introduced during grinding. S-N curves with fairly low scatter were obtained, which indicates microstructure-controlled and not defect-controlled failure. Low binder content as well as fine WC grains were found to improve the fatigue endurance strength. In no case, however, a horizontal branch of the S-N curve was observed, i.e. there is no fatigue “limit” at least up to 1010cycles. The initiation sites were in part difficult to identify; in such cases when the site was clearly visible, decohesion of the binder from large WC grains seems to have caused crack initiation. This further corroborates that microstructural features and not singular defects as e.g. inclusions are the initiation sites, which underlines the high purity of the hardmetal grades used. Based on fracture mechanical consideration a damage diagram was determined allowing to deduce critical defect sizes. •S-N graphs up to 10E10 cycles are presented for different hardmetal grades.•Ultrasonic resonance testing is shown to be well suited for fatigue testing of hardmetals.•No fatigue “limit” was found but the S-N curves drop consistently.•Crack initiation is microstructure-controlled and not defect-controlled.