Erosion wear characteristics of cemented carbide for mud pulser rotor

Highlights

• The mass loss at low erosion angle is mainly the removal of Co due to plastic deformation.

• The mass loss at high erosion angle is mainly the removal of Co and the pits caused by the removal of WC.

• The erosion rate of cemented carbide increases first and then decreases with the increase of abrasive particle size.

Abstract

Mud pulser is the most commonly used Measurement While Drilling(MWD) instrument for downhole data transmission to the surface. The rotor of the key component of the mud pulser is made of cemented carbide material. Under the action of high speed mud erosion containing solid phase, the rotor will produce erosion wear and reduce the quality of signal transmission. To explore the erosion wear mechanism of rotor carbide in mud containing solid phase particles, the influences of different erosion angles and different erosion abrasive sizes on the erosion wear properties of cemented carbides were studied by taking quartz sand as erosion abrasive. By means of surface profilometer, scanning electron microscope (SEM) and energy spectrum analysis (EDS), the erosion wear mechanism of cemented carbide was revealed. The experimental results show that the erosion wear rate increases gradually with the increase of erosion angle. When the erosion angle is 90°, the maximum average erosion rate of WC-5Co, WC-6Co and WC-10Co cemented carbides is 2.39%, 2.42% and 3.22%, respectively. With the increase of the abrasive particle size, the erosion wear rate of the material increases first and then decreases. When 100–200 um, the maximum average erosion rates of WC-5Co, WC-6Co and WC-10Co are 1.84%, 1.98% and 2.48%, showing a significant “particle size effect”. At low erosion angle, the wear mechanism is furrow and cutting mark, while at high erosion angle, the wear mechanism is pit. The erosion wear mechanism of small particle size abrasives is that a large area of surface layer falls off in the erosion area. The erosion wear mechanism of large particle size abrasive is the material loss in the erosion pit in the erosion area.

Introduction

Mud pulser is a Measurement While Drilling (MWD) instrument that can realize real-time transmission of downhole signals. Its key components are mainly composed of stator and rotor, which can realize real-time uploading of downhole parameters such as inclination, orientation and formation [1]. The continuous and reciprocating rotation of the rotor makes the mud flow area change continuously, so that the pressure in the drill string changes continuously, forming a series of continuous pressure pulse signals, which are coded by the downhole exploration pipe, and decoded and calculated by the modulation system to obtain the measured data [2].

The rotor is generally made of hard alloy materials, hard alloy with refractory metal carbide (WC Tic TaC, etc.) as the hard phase, iron group metal (Fe Co Ni, etc.) as the bonding metal, the material made by powder metallurgy method [[3], [4], [5]]. Mud is a kind of multi-phase flow mixture with high solid content and high flow speed. The rotor will produce erosion wear under the scouring action of the mud containing solid particles, which will decrease the strength of the mud pulse signal, affect the quality of the signal, and lead to the failure of signal decoding. If the rotor breaks due to mud erosion wear, the rotor debris, driven by mud, will damage the drill collar innards. If the rotor is replaced, it will need to lift the drill after a period of work, resulting in a waste of manpower and financial resources. Reducing the solid content in the mud can reduce the erosion wear of the mud on the rotor [[6], [7], [8], [9]]. However, the solid phase in the mud can balance the formation pressure, so there are some limitations to reduce rotor erosion wear by reducing the solid phase content in the mud. Optimization of rotor structure design can reduce erosion wear of rotor [[10], [11], [12]]. However, the working environment of the rotor determines that the rotor should always be in contact with the mud, which can reduce the erosion wear of the rotor in a short time. Ultimately, the rotor failure is caused by the insufficient performance of the cemented carbide material.

To sum up, the current research only reduces the erosion wear of the rotor caused by solid particles in the mud from the perspective of optimizing rotor structure. However, there are few reports on improving the erosion resistance of cemented carbide rotors from the perspective of analyzing the failure mechanism of cemented carbide rotors under the erosion environment of solid particles and changing the chemical composition of rotors. The most widely used material in mud environments is WC-Co cemented carbide because of its high hardness, wear resistance and corrosion resistance. However, under the high speed erosion condition of the slurry carrying solid content, the surface of cemented carbide will still produce obvious erosion pits.

Previous studies have shown that the main failure mechanism of cemented carbides in the process of solid phase particle erosion is WC brittle fracture in cemented carbides. Other studies have shown that the failure mechanism is the removal of binder first, followed by the shedding of WC particles [[13], [14], [15]]. A.J. Ga [16] explored the relationship between slurry erosion wear and microstructure and mechanical properties of cemented carbide, the results show that the erosion wear of cemented carbides is related to the hardness of the alloy, the mean free path of the bonding phase and the grain size of WC, among which the grain size of WC is the main factor. Scanning electron microscopy (SEM) shows that the erosion wear of the larger WC grains is the grain edge chip, while the smaller WC grains are often crushed or pulled out. A.J. Gant [17] found that there is an approximate inverse logarithmic relationship between the hardness of cemented carbide and the amount of slurry erosion wear. Under the same hardness, the erosion wear increases with the increase of erosion angle. Antonov M et al. [18] believe that the intergranular fracture between WC/WC and WC/Co is the main wear form of the erosion wear of WC-Co cemented carbides, and the erosion wear of ductile materials is mainly due to the displacement and removal of surface materials. The erosion wear of brittle materials is mainly caused by the crack and fracture of the surface material. Hussainova [19] used silicon particles as abrasive materials and different binders (Ni, Co and Fe) as variables to test the erosion wear properties of cemented carbides. The results showed that the higher the ratio of material hardness to fracture toughness, the greater the possibility of brittle fracture. The wear of the material is caused by the propagation of transverse and radial cracks in the elastoplastic region near the impact point. Sharma [20] found that when the impact angle between the target and the surface was 30°, the erosion wear of the material was the largest, and the wear gradually decreased with the increase of the impact angle. Scanning electron microscopy (SEM) shows that the erosion failure mode of ferrite-martensite dual phase steel is a mixed failure mode combining cutting and impact. Therefore, it is of great significance to explore the failure modes of cemented carbide under different working conditions and improve the anti-erosion performance of cemented carbide materials for prolonging the service life of equipment and preventing the erosion failure of components.