4145H steel is a commonly used material for downhole tools. However, up to now the wear behavior of 4145H drilling tool steel under real drilling fluid environment conditions is still not clear. In this work, this was investigated using a modified ASTM B611 rubber ring wet grinding test system, in which six kinds of abrasives (talc, dolomite or fluorite, as well as their mixed abrasive with quartz) with metal hardness-to-abrasive hardness ratios (H/H_A) ranging from 0.25 to 6.25 were used in the drilling fluid for experiments. The results show that the H/H_A value determined the wear mechanism of 4145H steel. When a single soft abrasive was used (with H/H_A higher than 1.3–1.5), polishing was the dominantly observed mechanism. While mixed abrasives were applied, a microcutting mechanism due to the ploughing of hard abrasive particles on the steel surface was also observed. The increase in mass fraction of the soft abrasives has little effect on the wear rate of 4145H steel, but its wear rate will significantly increase as the mass fraction of hard abrasives increases. Therefore, in order to extend the life of drilling tools and reduce downhole accidents, the mass fraction of hard particles in the drilling fluid should be reduced as much as possible.

In drilling engineering, the wear of tribo-pairs is the primary cause for the rapid failure of rotating seals in pneumatic downhole tools. In order to reduce the wear of tribo-pairs, a new type of rotating seals was designed in this work, which introduced copper alloys between the stator and rotor. To elucidate the wear and failure mechanism of the copper-steel tribo-pair rotating seals in pneumatic downhole motors, pin-on-disc dry sliding tests with Qbe-2 beryllium bronze pin against 38CrMoAlA steel disc under different loads were thus designed to simulate the friction and wear behavior of such tribo-pair. During the dry sliding process, the friction behavior of the copper pin would go through a running-in period and then become stable. As the load increases, the running-in period will be shortened, while the friction coefficient during the stable period decreases. Interestingly, a false stability occurs when the load is low. However, this phenomenon will disappear under heavy loads. The wear mechanism of the copper pins would change from adhesive wear to ploughing wear as the load increases, which is mainly related to the spalling of asperities and the filling of wear debris into the steel disc. The wear debris consists of copper and copper oxide. The surface roughness of the steel disc and copper pin decreased and the size of the wear debris increased with the increase of the load. The material removal mainly occurs on the copper pin, which will present a relatively small value under 45 N. On contrast, due to the filling of wear debris, the volume of the steel disc increased. Therefore, considering the value and stability of friction coefficients, as well as the wear amount of the sample, it would be better that such tribo-pair could work under 45 N. The present work will provide a fundamental understanding and solid support for systematically designing the tribo-pairs in pneumatic downhole tools under practical working conditions.

To improve the service life of the newly designed pneumatic downhole motor, a kind of commercially available lithium complex grease was used to help avoid the rapid wear-induced failure of the dynamic seal of pneumatic downhole tools in operation. The investigation on the tribological behaviors of QBe2 beryllium bronze and 38CrMoAlA steel tribo-pairs in pneumatic downhole motor revealed that under lubrication, the instantaneous friction coefficient and wear volume became very low, showing greatly reduced friction and excellent anti-wear performance. Compared with the case without lubrication, the adhesive wear mechanism and ploughing effect of the tribo-pairs were significantly alleviated. Moreover, Cu and Fe were oxidized to form composite oxides between the frictional surface, further improving the lubrication effect between the QBe2 beryllium bronze and 38CrMoAlA steel. Notably, the test could be operated stably for more than 150 h under lubrication, fully reaching the requirement of pneumatic downhole tools under actual drilling conditions. These results provide a solid support for the practical application of the reported pneumatic downhole tools.

Downhole drilling operations expose all-metal positive displacement motors (AMPDM) to abrasives of different sizes, causing serious abrasive wear behavior. Controlling the entry of large particles into the motor by installing a filter can effectively reduce wear conditions and improve its lifespan. This study investigated the particle size effect on abrasive wear to seek an appropriate filter size for AMPDM. The multi-function tribo-tester was modified according to the actual working conditions of the stator and rotor. The tribological properties of stator material (nitrited 38CrMoAl steel) against rotor material (YL10.2 cemented carbide) were conducted using SiO₂ particles with 1 to 500 μm sizes in the range on a pin-on-disc tribo-tester under wet sliding-vibration conditions. The results showed more complicated friction coefficient evolution characteristics for small particles, existing two stable stages. Two critical-size values of average wear rate and wear mechanism transformation are determined. The wear products of tribochemical reaction were determined. The filter porosity of about 200 μm may be an optimal selection to decrease wear. This work has developed new insights into how AMPDM responds to changing abrasive sizes and provides a proposal for controlling the entry size of abrasive particles.