The effect of surface conditions on the fatigue strength of cemented carbides

Cemented carbide hardmetals have been produced since 1910 and have gradually taken over from tool steels as the major material for high speed metal cutting and forming. One such forming operation is that of cold heading used in the production of nail and screw fasteners. This operation subjects a cemented carbide die to cyclic compressive and tensile stresses and may be considered as a fatigue process. Cold heading manufacturers have always been aware that some dies will produce millions of components and yet others will fail after a much smaller number and, in view of this, have reasoned that the surface finish of the die may be limiting factor in die life. This research has been carried out to investigate how the surface finish .affects die life and to find if there is any numerical correlation between surface finish and life. Commercially manufactured cemented carbides were subject to fatigue trials using standard miniature Wohler specimens. The grades of materials used were B, N and TT with a cobalt content of 6, 6 and 25% respectively. Grades N and TT were commercial grades with a grain size of 1.5-3µm, whereas B grade was an experimental grade with a grain size of 3.0 – 5.0µm. The fatigue tests were carried out on Wöhler rotating bending fatigue machines with stress levels on the specimens of between 700 and 1400 MNm². Initial testing was carried out with specimens in the as received ground condition. In order to demonstrate the effect of surface treatment on the fatigue performance of the test grades, the test specimens were subject to various mechanical and thermal treatments. [...continued]