Investigation On Fatigue Properties And Size Effect Of WC-Co Cemented Carbides Under Static And Impact Loading

The unique combination of high hardness,excellent strength and good fracture toughness exhibited by WC-Co cemented carbides has made them become acknowledged forefront materials for metal cutting tools,mining bits,structural components.These tools and components in service often suffer from reiterative impact load and lasting static load.And fatigue fracture is the main reason for the failure of this kind of cemented carbides.Therefore,it is very important to study corresponding fatigue properties of cemented carbides and then improve service lifetime of tools.In this paper,impact fatigue tests and static fatigue tests for six kinds of cemented carbides with different combinations of WC grain size and Co content were carried out to study the effect of microstructure parameters on impact fatigue properties and effect of specimen thickness on the static fatigue lifetime,respectively.The fracture surfaces of relevant rupture specimens were examined by scanning electron microscopy to investigate the impact and static fatigue micro-mechanisms.The following conclusions can be drawn:(1)With the increase of binder mean free path thickness,fracture toughness increases but hardness decreases.And hardness is inversely proportional to fracture toughness.When the Co content for cemented carbides is not more than 20%,the more the Co content,the higher the transverse rupture strength is.Meanwhile,cemented carbides with larger WC grain size possess greater transverse rupture strength.(2)The impact toughness of cemented carbides go up with the increase of Co content or the WC grain size.The Cobalt in hardmetals transforms from fcc to hcp during impact loading.Cracks initiate from the fcc/hcp interfaces and powder metallurgy defects.Short cracks gradually grow up into long cracks with impact cycles increasing and long cracks propagate through binder cobalt and WC grains,leading to the fracture failure of hardmetals.WC-Co cemented carbides become more fatigue sensitive as Co content increases.So,cemented carbides with higher Co content have longer impact fatigue lifetime.At the same time,impact fatigue lifetime is also longer for cemented carbides with larger WC grain size.(3)When a cemented carbide specimen is subjected to static stress which is more than 80%of the bending strength,the defects on the tensile surface or subsurface,especially pores,microcracks and abnormally large carbides,lead to a great stress concentration near the imperfections.The maximum stress near the circular pores on the surface or subsurface of specimens is 4 times higher than the external applied stress during the 3-point bending test.And then the crack in cemented carbides is initiated from surface or subsurface defects.The connection of previous cracks and pores creates the main crack.The subcritical growth of main crack leads to the fatigue and fracture of cemented carbides.After being loaded constantly for a period of time,the bending strength of cemented carbides and corresponding Weibull modulus both decrease.(4)The extension of static fatigue lifetime with the increase of specimen thickness is due to smaller plastic zone size near the crack tip,the decrease of elastic strain energy as well as the increase of surface energy and plastic deformation energy.The effect of specimen thickness on static fatigue lifetime is much greater for cemented carbides with larger WC grain size or higher cobalt content,which is attributed to operative ductile ligament bridging and crack deflection toughening mechanisms,respectively.