Fatigue testing and properties of hardmetals in the gigacycle range

Abstract

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 20 kHz 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 1010 cycles. 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.Peer ReviewedPostprint (author's final draft

In-Depth Understanding of Fatigue Micromechanisms in Cemented Carbides: Implications for Optimal Microstructural Tailoring

“…From this viewpoint, it is interesting to highlight other studies where different inspection approaches or testing variants are involved. On the one hand, fractographic analysis has been used for invoking correlations between fatigue life and the size of microstructural defects on the basis of Murakami's geometrical parameter of maximum flaw area [17][18][19]. On the other hand, notched specimens have been used for assessing microstructural effects on fatigue response under service-like conditions, e.g., presence of stress concentrators in component geometry as crack starting points and exposure to high temperatures [17,[20][21][22][23].…”

Influence of the Test Configuration and Temperature on the Mechanical Behaviour of WC-Co

“…Moreover, other families of defects can also be generated due to the in-service operating conditions as a result of oxidation, wear, contact damage, etc. [4][5][6]. Therefore, cemented carbides show defect controlling fracture strength, which is associated with the size of the largest (or critical) defect in the material.…”