Implementation of an effective time-saving two-stage methodology for microstructural characterization of cemented carbides

Linear intercept on scanning electron microscopy micrographs is the most commonly used measurement method to determine carbide grain size and contiguity in WC–Co cemented carbides (hardmetals). However, it involves manual time-consuming measurements and is critically dependent on the quality of the micrographs as well as on the identification and definition of grain boundaries. In this study a two-stage methodology for microstructural characterization of hardmetals is presented. First, a digital semi-automatic image analysis procedure for grain size determination of the carbide phase is presented. It involves an experimental assessment of grain size on processed images corresponding to a series of WC–Co and WC–Ni cemented carbide grades with different microstructural characteristics. Obtained results are then compared to the values obtained by means of the linear intercept technique. A good correlation between the mean grain sizes determined following both measurement techniques was attained. Based on experimental findings, a series of empirical relations were found to correlate grain size distributions obtained following both methods. Second, an empirical relation for estimating carbide contiguity in WC–Co cemented carbides is proposed. This relation considers simultaneously the influence of the binder content and the experimentally determined mean grain size on contiguity. The proposed equation for contiguity estimation is based on extensive data collection from open literature. An excellent agreement was attained between contiguity values estimated from such equation and those obtained using the linear intercept technique. This validates the two-stage procedure as an effective time-saving methodology for microstructural characterization of WC–Co cemented carbides.Peer ReviewedPostprint (author's final draft

Residual strength of WC-Co cemented carbides after being subjected to abrupt temperature changes

Thermal shock and thermal fatigue are recognized as common failure modes for WC - Co cemented carbides (hardmetals) in several applications in volving service temperature changes. However, information on microstructure - performance for these materials when subjected to abrupt changes in temper ature is rather limited. In this investigation, the thermal shock resistance of two WC - Co cemented carbides is studied on the basis of their residual strength after being subjected to temperature changes. The materials studied correspond to grades with dif ferent grain size (medium and ultrafine) but similar binder content. Thermal shock variables include two temperature difference ranges (400ºC and 550ºC) as well as number of abrupt changes (1, 3 and 10). Residual strength results were related to parameters extracted from Hasselman’s theory . It is found that medium - sized hardmetal exhibits a higher strength loss in the first quenching cycle but a greater damage tolerance to repeated thermal shocks than the ultrafine - sized. The assessed residual strength tren ds are in agreement with those expected from evaluation of Hasselman’s parameters for quantifying resistance to either crack initiation or crack propagation induced by thermal shockPostprint (published version

Strength degradation of cemented carbides due to thermal shock

Despite the recognition of thermal shock and thermal fatigue as common failure modes in cemented carbide applications, the information on the influence of the microstructure on the resistance of hardmetals to abrupt temperature changes is rather scarce. In this paper, the strength behaviour of cemented carbides after severe thermal shock damage is investigated. In doing so, cemented carbides were subjected to thermal shock at two temperature ranges (¿T of 400ºC and 550ºC) and their retained strength evaluated as a measure of their thermal shock resistance. Residual strength results are then related to crack initiation (R) and propagation (R’’’’) Hasselman parameters. Results indicate that the finer the microstructure, the better the resistance to the nucleation of thermal shock damage of hardmetals. This strength trend is in accordance with higher R and lower R’’’’ parameters for the studied materials.Peer ReviewedPostprint (author's final draft

Corrosion damage in WC-Co cemented carbides: Residual strength assessment and 3D FIB-FESEM tomography characterization

The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterization. Experimental results point out a strong strength decrease associated with localized corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenized and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.Postprint (published version

Microstructural influence on the fatigue behaviour of cemented carbides

Postprint (published version

Residual strength of WC-Co cemented carbides after being subjected to abrupt temperature changes

Thermal shock and thermal fatigue are recognized as common failure modes for WC - Co cemented carbides (hardmetals) in several applications in volving service temperature changes. However, information on microstructure - performance for these materials when subjected to abrupt changes in temper ature is rather limited. In this investigation, the thermal shock resistance of two WC - Co cemented carbides is studied on the basis of their residual strength after being subjected to temperature changes. The materials studied correspond to grades with dif ferent grain size (medium and ultrafine) but similar binder content. Thermal shock variables include two temperature difference ranges (400ºC and 550ºC) as well as number of abrupt changes (1, 3 and 10). Residual strength results were related to parameters extracted from Hasselman’s theory . It is found that medium - sized hardmetal exhibits a higher strength loss in the first quenching cycle but a greater damage tolerance to repeated thermal shocks than the ultrafine - sized. The assessed residual strength tren ds are in agreement with those expected from evaluation of Hasselman’s parameters for quantifying resistance to either crack initiation or crack propagation induced by thermal shoc

Microstructural influence on the fatigue behaviour of cemented carbides

Postprint (published version