Microstructure and wear of Ni-WC hardfacing used for steel-body PDC bits

Abstract

Superior hardfacing materials, such as Ni-WC blends, are commonly applied to rock-engaging regions of oil & gas drill bits and other downhole tools to provide sufficient wear protection. In this work, it was found that drill bits were subjected to severe high-stress abrasive wear while rubbing against rock formations. The wear behavior of drill bits could be appropriately evaluated by the wear test in accordance with ASTM standard B611 rather than ASTM standard G65. This work also found that the toughness of tungsten carbides was a predominant factor determining high-stress abrasion resistance of Ni-WC hardfacing. Any choices from various types, sizes, and shapes of tungsten carbides would contribute to the improvements of wear resistance as long as this choice delivered better toughness, such as cemented tungsten carbide and spherical tungsten carbide. Large particle size was also beneficial for reinforcing wear resistance, but played a secondary role as compared to carbide toughness. Therefore, large-sized cemented tungsten carbide pellets provided the best high-stress abrasion resistance for drill bits, with large-sized spherical tungsten carbide being a close second. In addition, the comprehensive hardness of Ni-WC composite could be well represented using a linearly additive approach. The calculated hardness established a strong positive correlation with the wear behavior of Ni-WC hardfacing.