Microstructure and wear of Ni-WC hardfacing used for steel-body PDC bits

Highlights

• Oil & gas drill bits were subjected to severe high-stress abrasive wear while rubbing against the rock formations.

• ASTM B611 test was more fit than ASTM G65 test to evaluate the wear behaviors of Ni-WC hardfacing used for drill bits.

• The carbide toughness was a predominant factor in determining the high-stress wear resistance of Ni-WC hardfacing.

• The comprehensive hardness of Ni-WC composite could be well represented using a linearly additive approach.

Abstract

Superior hardfacing materials, such as Ni-WC blends, are commonly applied to rock-engaging regions of oil & gas drill bits and other downhole tools to provide sufficient wear protection. In this work, it was found that drill bits were subjected to severe high-stress abrasive wear while rubbing against rock formations. The wear behavior of drill bits could be appropriately evaluated by the wear test in accordance with ASTM standard B611 rather than ASTM standard G65. This work also found that the toughness of tungsten carbides was a predominant factor determining high-stress abrasion resistance of Ni-WC hardfacing. Any choices from various types, sizes, and shapes of tungsten carbides would contribute to the improvements of wear resistance as long as this choice delivered better toughness, such as cemented tungsten carbide and spherical tungsten carbide. Large particle size was also beneficial for reinforcing wear resistance, but played a secondary role as compared to carbide toughness. Therefore, large-sized cemented tungsten carbide pellets provided the best high-stress abrasion resistance for drill bits, with large-sized spherical tungsten carbide being a close second. In addition, the comprehensive hardness of Ni-WC composite could be well represented using a linearly additive approach. The calculated hardness established a strong positive correlation with the wear behavior of Ni-WC hardfacing.

Introduction

Polycrystalline diamond compact (PDC) bits are one of the most important drilling tools in the oil & gas industry. They are employed to drill the wellbores through earthen formations. PDC bits have been accomplishing most of oil & gas field footage around the world. While engaged with rock formations under downhole, PDC bits are subjected to highly abrasive wear. Without appropriate protective methods, the rock-engaging regions of PDC bits will encounter substantial material loss, consequently resulting in bits' premature failure. Typically, the integral bit body of PDC bits is fabricated by milling and turning high-strength steel bars. Advantageously, the steel body exhibits excellent toughness and ductility, enabling the bit to avoid blade breakage caused by the shearing and impact forces generated while the bit cuts the rocks. However, the steel body of PDC bits is more susceptible to the abrasive wear due to direct rubbing with rocks under large compressive load and high rotary speed (Ref 1, 2). In general, metal matrix composite (MMC) materials are deposited on the bit surface as hardfacing overlays to protect critical regions of the bit body in service, as shown in Fig. 1. MMC hardfacing is tailored with the purpose of guaranteeing the bit to sustain as long as possible under extremely harsh downhole conditions.

Steel-body PDC bits commonly require the thickness of hardfacing overlays to exceed 2 mm to insure a long-term downhole service. To fulfill this thickness requirement, the thin-film coatings like physical vapor deposition and diamond-like carbon, as well as the thick-film coatings like high velocity oxygen fuel and high-speed laser cladding, are outside of the scope of this work. Viable options for the oil & gas downhole drilling tools include laser cladding and plasma transferred arc (PTA), as shown in Fig. 2. Both processes are operated automatically and have repeatable and reliable hardfacing quality. However, these two processes are not suggested for the hardfacing of steel-body PDC bits owing to the complex geometry of the bit body. In terms of the bit hardfacing, manual spray & fuse and oxy-acetylene welding are widely adopted. Both processes are the brazing process with advantages, such as little or no dilution of hardfacing materials into the bit substrate, little or no dissolution of tungsten carbides, and less operation cost. A comprehensive introduction of these four welding processes is given in the literatures (Ref 3, 4). Due to the powder feeding design, laser cladding, PTA, and Spray & Fuse all have their own recommended powder size range as shown in Table 1. Out of the recommended size range, the powder blend will either not be fed into the molten pool or cause distinct defects. In the case of large particle size, the powder blends will be made into the form of rod or rope and deposited by oxy-acetylene welding. With compared to laser cladding, PTA, and Spray & Fuse, however, some drawbacks have to be considered while using oxy-acetylene welding, such as low deposition rate, more likely to overheat, and uneven distribution of particles with various densities and sizes.

As the exterior surfaces of drill bits rub against the borehole wall, the bit materials are highly vulnerable to wear away. To provide the desired wear protection, a wide variety of MMC materials have been developed and tried in both lab and field. Up to now, only nickel‑tungsten carbide (Ni-WC) MMC system exhibits the best performance. It has dominated the hardfacing application in oil & gas industry (Ref [5], [6], [7]). In the Ni-WC system, hard tungsten carbide particles provide the desired wear resistance while nickel alloys have relatively high toughness to neutralize the embrittlement of tungsten carbides. There are four types of tungsten carbides commonly used in Ni-WC hardfacing: irregularly shaped fused tungsten carbide (CTC), spherical fused tungsten carbide (CTCS), macrocrystalline tungsten carbide (MTC), and cemented tungsten carbide (WC-Co) particles. CTC is the eutectic tungsten carbide with a typical composition of 78–80 wt% W₂C and 20–22 wt% WC. Its carbon content is 3.8–4.2 wt%. With the CTC as the feedstock, CTC-S goes through an extra melting-solidification cycle to obtain its spherical morphology by techniques such as the plasma spheroidization process. Apart from the morphological difference, CTC-S and CTC share the same W₂C/WC eutectic composition and carbon content. MTC is fabricated by fully carburizing the tungsten powders at elevated temperature. It is composed of only WC phase with a carbon content of approximately 6.1 wt%. WC-Co particles are manufactured by agglomeration and sintering, in which the fine WC grains are mixed together with Co alloys to improve the sub-optimal impact toughness of tungsten carbides. The particles have irregular and spherical shapes. Fig. 3 shows the cross section of typical WC-Co pellets. As shown, WC-Co pellets possess typical characteristics of cermet with a dense and consistent microstructure, in which WC grains and Co matrix are bonded well.

Besides tungsten carbides, researchers are also interested in combining iron-based or Ni-based alloys with alternative carbides, such as titanium carbide (TiC), chromium carbide (Cr₂C₃), and vanadium carbide (VC) (8). Although these carbides do have attractive attributes outperforming tungsten carbides, such as higher thermal stability, better corrosion resistance, and potential cost savings thanks to low density, successfully commercial applications have not been reported yet. A key barrier is that current manufacturing techniques cannot mass-produce TiC, Cr₂C₃, and VC with the same size and quality as tungsten carbides on an industrial scale.

Although the melting temperature of W₂C and WC is 2730 °C and 2870 °C, respectively, the tungsten carbide has poor thermal stability and is extremely sensitive to high temperature, in particular W₂C. To avoid the dissolution and degradation of tungsten carbide, the self-fluxing (also known as self-fusing) nickel alloys are preferred for Ni-WC hardfacing. Alloy elements, Boron (B) and Silicon (Si), are commonly added to Ni-based alloys to lower the welding heat input. Of various self-fluxing nickel alloys, Ni-Cr-Si-B and Ni-Si-B are most commonly used. Researchers have found that Chromium (Cr) plays a detrimental role in Ni-WC hardfacing in terms of facilitating the carbide dissolution, embrittling the matrix, and being prone to cracks (Ref 9–11). Therefore, the Cr-free alloys are recommended for laser cladding to avoid or at least alleviate cracks in the Ni-WC hardfacing. On the other hand, the bit hardfacing deposited by the manual spray & fuse and oxy-acetylene welding prefers Ni-Cr-Si-B alloys. These two processes have less heat input and slow solidification rate as compared to laser cladding and PTA, making the detrimental effects of Cr less harmful. The advantages of Cr instantly become more appealing, such as hardening the matrix, increasing the corrosion resistance, and preventing porosity by capturing carbon. Besides the self-fluxing Ni alloys, the researchers are also interested in the combination of other types of Ni alloys with tungsten carbides. For example, Inconel 625 alloys have excellent corrosion resistance, but poor wear resistance. Researchers tried to add tungsten carbide into Inconel 625 matrix to improve the wear resistance (Ref 12). However, the high melting point of Inconel 625 inhibits the successfully commercial applications of this combination.

Due to the high cost of the Ni-WC system, researchers are constantly interested in the combination of iron-based alloys with tungsten carbides. However, the appreciable solubility of carbon in iron results in unacceptable dissolution and degradation of all types of tungsten carbides. Take WC for example. Although WC exhibits good thermal stability and sufficient resistance to dissolution in self-fluxing Ni alloys, it dissolves to unacceptable level in the iron matrix. Up to now, there is no promising options for iron-based alloys to address this pain point. In terms of carbides, however, the alloyed carbide W(Ti)C presents a highly attractive dissolution resistance in the iron matrix. In the carbide W(Ti)C, Ti as an active compounding element of carbon can lock the carbon in the carbide, resisting the carbide dissolution. This type of alloyed carbides retains intact in the iron matrixes, such as 316 L and 420SS, during the PTA process (Ref 13). Due to the usage of Ti, though, W(Ti)C is highly sensitive to oxygen and fails in the spray & fuse process as well as in the infiltration. Both processes don't have the inert gas protection as good as PTA and laser cladding.

Cobalt (Co) and Ni are adjacent to each other in the periodic table and share similar chemical and physical characteristics. Although some publications found that Co and Ni as the hardfacing matrix would result in differences in terms of embrittlement, corrosion resistance, etc. (Ref 14), Co-based and Ni-based alloys can be used interchangeably to combine with tungsten carbide. Thanks to the high concentration of its supply as well as its use in lithium-ion batteries, however, Co experienced more pronounced volatility in its price over the past few years, greatly limiting its application in hardfacing. Copper (Cu) based alloys are capable of providing excellent corrosion resistance which has always been appealing to some industrial applications, such as in Canada oil sand. In addition, Cu is relatively inert to carbon, making the dissolution of tungsten carbides not an issue anymore. The wetting behavior of Cu on tungsten carbide surfaces is also excellent (Ref 15). Although Cu is not the carbide-forming element, the strong intermetallic bonding between Cu based matrix and tungsten carbides can be achieved with the addition of carbide forming elements, such as Ti and Cr. However, the hardness of Cu alloys is relatively low. It is also difficult to achieve densely packed tungsten carbide distribution in the Cu matrix probably due to the viscosity. These two drawbacks weaken the wear resistance of Cu-WC system, making it difficult to be used commercially as hardfacing overlays. It has to be pointed out that Cu-based alloys are commonly used to infiltrate Cu-WC matrix body of PDC bits (Ref 16). In this case, a high volume fraction of tungsten carbides is achieved, which is high enough to make up for the low hardness of Cu alloys to survive in the harsh wear conditions.

ASTM standard G65 is a widely used three-body abrasive wear test method to rank various hardfacing materials no matter what types of applications, even for oil & gas drill bits. Although there is a huge mismatch between ASTM G65 results and field feedbacks, the drawbacks using ASTM G65 to evaluate hardfacing materials of oil & gas drill bits were rarely published. Additionally, most publications studied the abrasion resistance mainly based on ASTM G65 test, which may reach incorrect conclusions. Therefore, it is meaningful to re-evaluate these conclusions. Besides ASTM G65 test, ASTM standard B611 is the other test method developed to evaluate the three-body abrasive wear resistance. The conflicts between ASTM G65 and B611 test results have been reported in literatures (Ref 17, 18). However, the investigations into these conflicts were rarely conducted either. As a result, the main objective of this work was to find a reliable wear test method for oil & gas drilling applications. Based on this method, the influences of hardfacing factors on the abrasion resistance were explored, such as amount, type, shape, and size of tungsten carbides, as well as the matrix hardness. Finally, new Ni-WC blends were developed to attain superior service performance.