Ageing tests of cemented carbide powders : An investigation for increased quality of metal cutting inserts

Abstract: In this study, the ageing effects on powder used for cemented carbide insert production are examined. Ageing is throughout this study, defined as the time dependent change of the magnetic properties: coercive field strength and saturation magnetization. Testing is done using eight different powder compositions stored in both air and in an argon cabinet for 10 weeks, where sampling is done at specific intervals. Samples are stored in vacuum sealed bags for a combined sintering at the last phase of the test. Magnetic properties are assumed to be dependent on the amount of oxides needed to be reduced by taking carbon from the material itself during the vacuum stage of the sintering. To achieve interpretive results, this study also tested available sintering furnaces (DMK and DEK) by sintering trays with patterns of test pieces. This shows that DEK furnaces are much better for the ageing tests performed in this study, since less variation of the magnetic properties are measured because of the symmetrical heat gradient over each tray. Ageing tests strongly suggest that the cause of ageing comes from water absorbed by the PEG in the powder composition. Changing the molecular weight of the PEG seems to have an effect on the powder's ageing sensitivity. Measurements performed in this study show less ageing for Cr-rich DA-powders than for cubic carbide rich DQ-powders.