Abstract: In petroleum/gas drilling processes, down hole tools work in an environment with water based mud. Bearings in such mechanical systems fail fast due to the extreme working condition. Friction and wear causes damage of bearings, leading to the failure of the whole system. Tribological properties of four types of bearing steels, including GCr15, 9Cr18, G20CrNiMo and Cr4Mo4V, are investigated. The materials are made into pins and rings to fit a Falex pin-on-ring tribometer. The variations of coefficient of friction, wear scar diameter and wear mechanisms of bearing specimens with loads and speeds are evaluated. The results show that, in water based drilling fluid, the coefficients of friction of GCr15, G20CrNiMo, Cr4Mo4V stay steady with increasing loads while they reduce with increasing rotation speed. For 9Cr18, however, a remarkable increase of coefficient of friction with load and speed is observed. The primary wear forms of G20CrNiMo and GCr15 are corrosion wear and abrasive wear. The corrosion of GCr15 is more serious. The primary wear forms of Cr4Mo4V are corrosion wear, fatigue wear and abrasive wear. When the rotational speed and load reached 40 N and 100 r/min, the wear form of 9Cr18 changes from abrasive wear to adhesive wear. Results provide helpful information to select bearing steel in water based mud and to increase the performance and service life of down hole tools.

Key words: bearing steel, tribological properties, water based mud, wear