Failure Analysis of Raceway of Leg for KX84 Series Roller Cone Bit

After working for a period of time, KX84 series cone bit was scrapped due to the failure of raceway. We analyzed the failure mode of the raceway of leg of KX84 series roller cone bit with Hertzman stress analysis, microhardness test, cross-section metallographic observation, and fracture morphology observation. Meanwhile, the depth of crack initiation was also predicted. The result show that contact fatigue is the main form of raceway failure. When the pressure is weaker than 6.895 MPa, both 45° shear stress τ_zy45° and the maximum horizontal shear stress τ_0max are lower than the fatigue limit σ_-1, no fatigue crack is formed. However, fatigue cracks emerge when the pressure exceeds 6.895 MPa. In addition, the depth of cracks can be predicted with half-width of contact face b at 13.79 MPa. For instance, the predicted depths of cracks of KX84-250 and KX84-310 are 0.27, 0.34 mm, respectively, while the actual depths are 0.28, 0.38 mm, accordingly. In order to improve the drilling depth of the cone bit, the occurrence of extreme situations should be avoided during the installation. At the same time, the axial pressure should be controlled to avoid the initiation of fatigue cracks.