Fatigue of WC-Co Hardmetals under Tension-Compression Loading

The Effects of Co Content and WC Grain Size

Abstract

The effects of Co content and WC grain size on the fatigue strength of WC-Co hardmetals have been investigated. Alternating tension-compression (stress ratio R=-1) fatigue tests and compression fatigue tests (R=-∞) were carried out on WC-Co hardmetals whose Co contents are 6%, 12% and 20%, and their WC grain size ranges from 1.1μm to 3.0μm.
For tension-compression fatigue tests (R=-1), all the materials tested have shown about the same values of fatigue limit, regardless of Co content and WC grain size. In the region of finite life, WC-12%Co (WC grain size 1.4μm), which is the material with medium Co content, has shown the minimum life and WC-6%Co, which is the material with smallest Co content, has shown the longest life. It has been confirmed that the minimum life of the material with 12%Co (1.4μm) is due to the largest size of the initiation crack zone formed by the cleavage of hcp Co and WC grains.
The residual tensile strength of the fatigued specimens under cyclic compression has been investigated as a function of the number of load cycles. The results have shown that the materials with lower Co content (or smaller Co mean free path) show higher residual tensile strength. Though, in the material with 20%Co, fracture by tensile loading was initiated from the shear mode cracks formed during the cyclic compression loading, in all other materials tested, fracture by tensile loading was initiated from the grooves formed by the dropping of the surface layer during the cyclic compression loading, in the same way as in the material with 12%Co 1.4μm WC grain which was reported in the previous reports.