High-chromium white cast iron, because of its superior properties, has been widely used as a wear-resistant material. However, the demand for a longer service life has generated an interest in improving the quality of these wear-resistant alloys.
It has been reported that the addition of strong carbide-forming elements such as vanadium, tungsten, niobium and titanium improves the mechanical properties of high-chromium white irons [1][2][3][4][5]. Reportedly, niobium forms hard niobium carbides that improve the hardness and wear resistance [5][6][7]. However, there is little information available concerning the influence of niobium on the fracture toughness or how much of this element should be combined with high-chromium white iron to obtain optimal fracture toughness and abrasive wear resistance.
In this study the influence of niobium on the microstructural characteristics of high-chromium white iron containing 17% Cr was examined in both as-cast and heat-treated alloys.
An iron alloy comprising 2.94% C, 16.7% Cr, 0.83% Mo, 0.72% Cu and 0.85% Mn, with the usual impurities, was selected. This particular chemical composition is widely used due to its hypoeutectic microstructure in which M7C 3 carbides are present. Therefore, four alloys containing 0, 0.63, 1.0 and 2.06% Nb were melted in a 500 kg high-frequency induction furnace and cast into moulds of a bentonite-sand mixture (10 m m x 20 mm ร 55 mm test blocks for abrasion tests and 13 mm x 13 mm x 55 mm test blocks for fracture toughness). The samples for structural analysis and hardness were selected from abrasion test blocks.
The best combination of abrasion resistance and toughness are in a heat-treated condition to obtain a martensitic matrix. Samples were heat-treated in an electric furnace with no protective atmosphere at 950 ยฐC for 2 h, followed by cooling to room temperature in still air.
The abrasive wear resistance was evaluated by measuring the mass loss per the procedure described in American Society for Testing and Materials (ASTM) Standard Practice G-65, Procedure B (Rubber Wheel Abrasion Test) [8].
The dynamic fracture toughness (Kid) was measured using an impact test machine equipped with an instrumented Charpy tup. The standard Charpy specimen was notched by electrical discharge machining (EDM) a 0.2 mm radius slot 2 mm in depth. The fracture load was used to calculate K l d