When joining cemented carbide with other materials its special properties must be taken into account:
- Carbide is a brittle material
- Edges have to be protected (use radii, chamfers etc.)
- Fits and guides need to be adapted to the material properties (angularity, clearances, centring etc.)
- The combination of different materials almost always causes stress close to the joint because of the variation in rigidity
Brazing cemented carbide
Brazing cemented carbide to steel requires temperatures >450°C. Brazing bonds are characterised by high thermal and mechanical stability. Fluxing agents are used for brazing in oxidising atmospheres. These agents dissolve oxide layers on the carbide surface and improve wettability. In general, carbide and steel surfaces should be clean and free of oil and grease in order to ensure optimum wettability of the brazing metal. Due to the different heat expansion coefficients of steel and cemented carbide, internal stress can cause warping or cracks. Internal stress after brazing can be partially reduced by plastic deformation of the brazing joint.
Screws in cemented carbide
Through holes in carbide for screws present no problem, but threads are very expensive to produce. Threaded bushings made of steel are a very good alternative – these are brazed or bonded to the carbide surface. CERATIZIT studies have shown that the maximum tensile load of screws in cemented carbide internal threads (M4 to M10) is limited by the tensile strength of the screw material. External threads made of solid carbide are notch sensitive and can break off at high torsional loads with excessive torque.
Techniques for joining cemented carbide with other materials
Adhesive bonding is an uncomplicated, fast and inexpensive process. For adhesive bonding the surface treatment is crucial for the quality of the bond and its stability under load:
- Surfaces must be clean, dry and free of oil and grease
- A normal degree of surface roughness is preferable (do not grind)
There are different types of adhesives.
- One- and two-component adhesives
- Cold and warm-hardening adhesives
Adhesive bonds can be used with a fracture resistance of <25–35 MPa and operating temperatures of <150–250°C. Furthermore, liquids can wash out the adhesive, so the bonded joint needs to be protected.
Shrink fit with cemented carbide
Shrink fit is a method for inserting a carbide die into a steel part. The carbide core is clamped by shrinking and hot or cold pressing with the steel fitting. After shrinking into the heated steel casing, the oversize allowance produces a clamping force, together with the cooling of the steel. The maximum bonding temperature must not be exceeded during heating, in order not to have a negative influence on the hardness properties of the steel. On the other hand, the temperature must be high enough that the inner diameter of the steel fitting expands sufficiently.