Hardening and wear protection of hard-alloy coating of the working surface of the rotors of downhole motors (DDM) by high-speed flame spraying (HVOF/HVAF). Restoring wear-resistant coating of DDM rotors.

Hardening and wear protection of hard-alloy coating of the working surface of the rotors of downhole motors (DDM) by high-speed flame spraying (HVOF/HVAF). Restoring wear-resistant coating of DDM rotors.
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Galvanic chromium plating, as a method of protecting the rotors of downhole motors, is outdated about ten years ago. Although some drilling companies still order DDM with chromium plating in the old way, all the world's leading manufacturers and oilfield service companies have already switched to sprayed tungsten carbide coatings for both new and repair DDM rotors. That cermet provides the resource of a rotor of the DDM more than 1000 hours. This article explains the reasons for this solution and the most promising processing technologies.

The causes of wear of the rotors DDM

The efficiency of the DDM is provided by a tight contact of the stator and rotor surfaces. Even minimal wear significantly worsens the characteristics of the downhole motor. The most severe wear occurs when using poorly cleaned or weighted drilling mud or pumping borehole fluid with a high content of abrasive. Thermal sprayed tungsten carbide provides reliable protection of rotors for 1000 and more hours, even in the most aggressive conditions.

Comparison of coatings DDM from tungsten carbide and from galvanic chromium

Galvanic chromium plating has been the de facto standard for many years to protect the DDM rotors from corrosion and wear. Galvanic chromium has a sufficiently high hardness (~60 HRC) to resist the abrasive factor, but the high porosity inherent in electrolytic chromium does not allow the effective use of such coatings in the presence of corrosion factors. Aggressive media (H2S, SO2, etc.) penetrate the coating film through microcracks in the coating, causing sub-film corrosion. This lack of galvanic chromium plating has led to the fact that the use of screw downhole motors for drilling complicated wells led to their premature failure.
The problem of sub-film corrosion was solved by the use of high-speed flame spraying of tungsten carbide. Tungsten carbide coatings have proven to be the best solution for protection against waterjet corrosion in drilling. Their high reliability has been tested on solutions for aviation, then confirmed on wear protection of mandrels of hydraulic shock mechanisms (GUM) and drill balls more than 10 years ago. High-speed tungsten carbide coating has a hardness of 72 HRC, through porosity is absent, adhesion of the coating is more than 80 MPa. Wear resistance of tungsten carbide is due not only to its high hardness, but also to the high viscosity of coatings applied using high-speed spraying equipment.
The most difficult to solve were the problems of dimensional spraying and dimensional polishing of curved surface. The first task was solved by the use of robotic equipment, the second - with the development of JSC "Plakart" automated installation for polishing the rotors of DDM.
The use of tungsten carbide to protect the rotors of DDM began in 2012 and immediately showed a good result. Speakers at conferences on drilling equipment report an increase in the operating time of rotors up to 1000 hours or more. Every year this technology is spreading in Russia and in the world.

Repair of rotor coatings for downhole motors

A significant advantage of tungsten carbide coatings is the possibility of their application for the repair of any rotors — both with coatings of tungsten carbide and with coatings of galvanic chromium. The technology of repair of screw downhole motors by spraying tungsten carbide coatings is used by all oil service companies. JSC "Plakart" carries out repair of rotors DDM on plant surfaces in Naberezhnye Chelny starting in 2015, certified Site of several oil service companies working in Russia.
The most promising technology for rotors is the application of carbides by HVAF . This technology allows to increase the hardness of the surface, while increasing its viscosity and reducing porosity. We are equipped with HVOF and HVAF equipment and apply tungsten carbide technology according to customer's choice.


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