Ultrafine high-cobalt VK40 hard alloy. I. Structure and properties

Abstract

Ultrafine WC-41 wt.% Co powders (VK40) are densified under 1000–1200 MPa at 950–1250 °C and in 0.13 Pa vacuum. It is examined how the pressing temperature and annealing at 1190 °C influence the density, structure, physical and mechanical properties of VK40 alloy. It is shown that high-pressure sintering produces dense samples with ultrafine structure (LWC = 0.35–0.45 µm) and low degree of contact for carbide particles (contiguity CWC = 0.08–0.1) in solid phase. The highest mechanical properties are exhibited by samples densified at 1150–1250 °C or by samples annealed after preliminary pressing at 1050–1150 °C. The alloy has the following properties: transverse rupture strength (TRS) of 3200–3400 MPa, fracture toughness of 25–33 MPa · m1/2, Vickers hardness of 7.5–7.7 GPa under 300 N, compressive strength of 2600–2800 MPa, compressive offset yield stress σ0.2 = 2200–2400 MPa, compressive plastic strain of 6.5–7.0%, and fracture energy of 180–200 MJ/m3. These mechanical properties of ultrafine VK40 alloy differ from those of standard impact-resistant coarse-grained hard metals in higher TRS, fracture toughness, and yield stress.