Understanding The Fracture Toughness Testing Method And Toughness Evaluation Of WC-Co Based Cemented Carbides

Cemented carbides acts as a widely used tool material in industry, in order toenhance its life and quality under complex working conditions, research on itsaccurate fracture toughness determination and reasonable toughness evaluation hasbeen concerned by workers of the cemented carbide field. The fatigue precrackingsingle edge notch beam method based on the basis of the drop K method and thecompliance method was used to determine fracture toughness for differentcombinations of WC grain size and Co phase cemented carbides, and also comparedwith the K_Icmeasured values of the line incision method. the correlation between theconventional mechanical properties of all experiment alloys and the low cycle （<104）impact fatigue performance of some alloys have been studied. Last, macro or microfracture behavior and toughening mechanisms and toughness evaluation methods ofthe experiment alloy were also analysed. Main conclusion are as follows:Compared with the K_Icmeasured values of the line incision method withoutprecrack, the measured values of the SENB method with precrack are highlycorresponding to intrinsic fracture toughness for the experiment alloy. the lineincision method may not be able to accurately and effectively evaluate the fracturetoughness of the experiment alloy due to defective notch root curvature radius. TheSENB method avoid taking place catastrophic fracture in the precracking processand has highly accurate data. It is a more ideal and effective way to evaluate fracturetoughness of cemented carbides.The K_Icresults show that the fracture toughness of all experiment alloysare enhanced with the increase of cobalt content when the WC grain size is certain,the main toughening mechanism are the increased mean free path of Co and a largerrange of the multiligament zone in the fracture path and the more strong plastic flowof Co phase. Dimple pattern with ductile fracture feature visibly reside in thefracture surface of115^#and130^#alloy. The K_Icvalue of208^#alloy with6.5μmWCand108^#alloy with1.8μm WC are19.9MPa·m^1/2and13.1MPa·m^1/2respectively, theformer is higher than the latter. Moreover, the fracture toughness of311^#and315^#alloy with1.3μmWC is weaker than111^#and115^#alloy with1.8μmWC at the samelevel of cobalt content. The toughness of more coarse grain alloy is improved byincreasing the energy consumption of crack deflection and transgranular fracture.The low cycle impact fatigue life of all experiment alloys are also enhanced with the increase of cobalt content in constant dynamic impact loading condition. Inaddition to the mean free path of Co, the bridging toughening effect from Co phase isthe most critical factor in cemented carbides. The low cycle impact fatigueperformance of208^#alloy is better than108^#alloy, apart from crack deflection andWC transgranular fracture, the enhancement of the Co mean free path that indirectlyresult from the increase of cobalt content make contribute to the tough reinforcementfor the alloys. But the low cycle impact fatigue performance of311^#alloy is strongerthan111^#alloy, it is adverse to condition in K_Ic. This phenomenon show thatcemented carbides with heterogeneous structure from different grain size WCactually have more better toughness.It is only use σbb, αk, K_Icto evaluate the toughness of cemented carbides thatseems to be not perfect, the low cycle impact fatigue behavior is sensitive to actualtoughness of coarse-grained low cobalt or dual graincemented carbides. Thuscombining the low cycle impact fatigue behavior with σbb, αk, K_Icindicators, it mayprovide a new way to solve the problem in carbide toughness evaluation.