No abstract

Cemented carbides coated with CVD multilayers are commonly used in metal cutting turning and milling operations. For many applications, a micro-blasting finishing procedure based on an impact treatment of the surfaces is carried out in order to smooth the coated surface and reduce sharp cutting edges. In this work, micro-blasting with corundum in aqueous solution at pressures between 0.05 and 0.3 MPa was applied to CVD TiN/Ti(C,N)/κ-Al 2 O 3 multilayer coatings deposited onto cemented carbides in order to investigate its influence on the micro-topography, microstructure and residual stresses. The results showed that the microblasting reduces the surface roughness and affects the coating thickness. TEM investigations revealed no significant changes on the microstructure of the κ-Al 2 O 3 top layer. Synchrotron X-ray investigations showed that the residual stress state of the as-deposited κ-Al 2 O 3 top layer is not affected by the micro-blasting treatment under the conditions investigated.

Welding of dissimilar materials such as steel and cemented carbides (hardmetals, cermets) is particularly challenging e.g. because mismatches in their thermal expansion coefficients and thermal conductivities result in residual stress formation and because of the formation of brittle intermetallic phases. Laser beam welding of cemented carbides to steel appears as an attractive complementary technique to conventional brazing processes due to its high precision, high process speed, low heat input and the option of welding without filler.Here a laser welding process including pre-heat treatment and post-heat treatment was applied successfully to joining as-sintered and nitrided hardmetals and cermets to low alloyed steel. The microstructure and mechanical properties of the welds are investigated by microscopy, X-ray diffraction, microhardness measurements, and bending tests. The results reveal that the three-step laser beam welding process produced crack-free and non-porous joints. Nitridation of the cemented carbides results in a significant reduction of the amount of brittle intermetallic phases. The mechanical properties of the joints are competitive to those of the conventional brazed steel-cemented carbide joints.Keywords: hardmetals; cermets; nitridation; laser beam welding; g-phases.

The effect of a nitridation surface treatment on the microstructure and corrosion resistance of Co and Ni-binder (W,Ti)C-based hardmetals with additions of (Ta,Nb)C was investigated. Nitridation treatment produces a surface layer with a higher hard phase (Ti,Ta,Nb,W)(C,N) content. The near-surface layer thickness and microstructure are affected by the choice of binder (Co respectively Ni). The corrosion resistance of the hardmetals was characterized using linear sweep voltammetry. The microstructure of the hardmetals was determined before and after corrosive attack by optical microscopy and SEM. The results of the corrosion tests and microstructure analyses are interpreted in terms of the correlation between the hardmetal morphology and porosity and ion transport through the interface binder-electrolyte. The nitridation treatment of the hardmetals strongly increases the resistance to aggressive corrosion media for hardmetals both with Co and Ni-binder phase and thus may positively influence the hardmetal lifetime in cutting applications in chemically aggressive environments.