Measurement of bit-rock interface temperature and wear rate of the tungsten carbide drill bit during rotary drilling

Rock drilling is an essential operation in mining industries. Temperature at the bit-rock interface plays a major role in the wear rate of the drill bit. This paper primarily focuses on the wear rate of tungsten carbide (WC) drill bit and the interrelationship between temperature and wear rate during rotary drilling operations conducted using a computer numerical control (CNC) machine. The interrelationship between the temperature and wear rate was studied with regard to three types of rock samples, i.e., fine-grained sandstone (FG) of uniaxial compressive strength (UCS) that is 17.83 MPa, medium-grained sandstone (MG) of UCS that is 13.70 MPa, and fine-grained sandstone pink (FGP) of UCS that is 51.67 MPa. Wear rate of the drill bit has been measured using controlled parameters, i.e., drill bit diameter (6, 8, 10, 12, and 16 mm), spindle speed (250, 300, 350, 400, and 450 rpm), and penetration rate (2, 4, 6, 8, and 10 mm/min), respectively. Further, a fully instrumented laboratory drilling set-up was utilized. The weight of each bit was measured after the bit reached 30 mm depth in each type of the rock sample. Furthermore, effects of the bit-rock interface temperature and operational parameters on wear rate of the drill bits were examined. The results show that the wear rate of drill bits increased with an increase in temperature for all the bit-rock combinations considered. This is due to the silica content of the rock sample, which leads to an increase in the frictional heat between the bit-rock interfaces. However, in case of medium-grained sandstone, the weight percentage (wt%) of SiO2 is around 7.23 wt%, which presents a very low wear rate coefficient of 6.33×10−2 mg/(N·m). Moreover, the temperature rise during drilling is also minimum, i.e., around 74 °C, in comparison to that of fine-grained sandstone and fine-grained sandstone pink. In addition, this paper develops the relationship between temperature and wear rate characteristics by employing simple linear regression analysis.


Introduction
Drilling of soft and hard rock formations, their drill bit life may be substantially reduced. This is due to the combined effect of temperature produced during drilling and the operational parameters considered [1]. Frictional heat and cooling cause thermal fatigue, which leads to higher formation of a wear flat at the cutter head [2]. Damage of the bit's microstructure shows the non-uniform surface, owing to the roughness of the rock sample as well as the heterogeneities prevalent [3]. The temperature data acquired during drilling demonstrate the functionality of the tool embedded with microthermal sensors that generate rapid response and are more reliable in nature for both fundamental and in-process study [4]. To observe the abrasive characteristics of a tool, Archard's coefficient yields a better result [5]. Thermal stress depicts that plastic deformation of the cutter structure can occur under certain downhole conditions, such as thermal conductivity, rock properties, texture, cooling, bit balling, and transient phases [6]. Wear rate of the | https://mc03.manuscriptcentral.com/friction polycrystalline diamond composite (PDC) bit wear curve has been described in four different phases, i.e., break at entry, wear at layer of diamond, subsequent wear at the carbide layer's substrate and rapid breakdown [7]. Furthermore, thermally stable polycrystalline diamond cutters for drill bits with low permissible brazing temperatures can reduce the thermal stress observed during drilling operations [8]. Measurements of drilling temperatures were essentially used to observe the bottom hole temperature during drilling [9]. During continuous drilling, the temperature gradient of the rock formation can be estimated using the bottom hole temperature. The primary observation in this regard is that the drilling rate increases conduction in the rock sample [10]. The wear rate of the drill bit was caused based on the abrasion of the rock sample. This is due to the silica (SiO2) content of the rock sample. Hence, studying the rock characteristics would help in the selection of the optimum drilling parameters and type of drill bits [11]. In rotary drilling, the bit penetrates into metals, which is similar to that observed in case of rocks. In case of fine materials, low speed provides the best wear resistance, whereas for coarse-grained materials, high speed presents the best wear resistance [12]. Moreover, the tool wear can be measured using the weight difference method [13]. The wear rate of the bit is correlated to the textural properties of the rock samples, such as silica and quartz in the granite samples. Further, a linear relation was established for quartz and silica, with the wear rate of the drill bit [14]. The wear test results can also be compared to the estimation of the wear rate coefficient of the drill bit, which provides good precise observations for the abrasiveness of these materials [15]. For the influence of abrasive size on the cutting performance of concrete by means of an abrasive water jet experiment, it was found that a significant relationship exists between the cutting performance and uniaxial compressive strength [16]. For measuring the wear rates of the bit, the well-known Archard model has been successfully used in PDC and rock combination [17]. The SiO2 content has a greater influence on the wear rate of the drill bit generally, an increase in the SiO2 content of the rock samples leads to an increase in the wear rate of the bit [18].
In the present study, the rock drilling experiment was conducted using an embedded thermocouple technique for three different types of sandstone, taking into consideration their uniaxial compressive strength and the silica content of the rock sample. Moreover, the controlled parameters were considered such as drill bit diameter, spindle speed, and penetration rate in the test. The relation between the temperature produced at the bit-rock interface and the wear rate of the drill bit was examined. Wear rate of the drill bit were estimated using the mass loss method, and the wear rate coefficient of the drill bit was calculated for each type of the rock samples. The relationship between the bit-rock interface temperature and wear rate of the drill bit has also been discussed.

Experimental procedure
The rock drilling experiments were performed on a cylindrical sandstone samples of NX size (as per International Society of Rock Mechanics of diameter 54 mm and length 135 mm), which represents one of the sedimentary rock present in the rock formation. These experiments involved drilling the sandstone core sample on a CNC vertical drilling machine. Furthermore, three types of sandstones were used in these experiments: medium-grained sandstone (UCS 13.70 MPa), fine-grained sandstone (UCS 17.83 MPa), and fine-grained sandstone pink (UCS 51.67 MPa). All three types of these sandstones were 54 mm in diameter and 135 mm length (NX size). The experimental set-up comprised a chuck with controlled parameters such as spindle speed and penetration rate ( Fig. 1). Moreover, a cutting tool dynamometer mounted on the CNC bed, along with a specially designed and fabricated grounded K-type thermocouple (0-1,200 °C ), was fixed into the rock samples to measure the bit-rock interface temperature at a depth of 30 mm. This was done in such a way that the head of the thermocouple was placed 0.5 mm away from the bit-rock interface. In addition, the thermocouple wire was partially connected to the digital temperature indicator to acquire the temperature data [19]. Wear rate was measured for a few experimental combinations by varying tungsten carbide drill bit diameter (6,8,10,12, and 16 mm), spindle speed (250, 300, 350, 400, and 450 rpm), and penetration rate (2, 4, 6, 8 and 10 mm/min), the drilling has been carried out to measure the temperature [20,21]. Under each drilling condition (bit diameter, spindle speed, and penetration rate), the drilling has been carried out for different drilling time (900, 450, 300, 225, and 180 s, respectively) to find out the average wear rate of a particular type of rock sample and corresponding interface temperature was measured [22,23].

Measurement of the thrust
A cutting tool dynamometer was utilized to measure the thrust force. The dynamometer was fixed on the worktable, above which the vice is fixed. Between the jaws of the vice cylindrical rock sample was placed. The dynamometer, vice, and the rock sample lay on the same axis in such a way that the rock drilling experiment was performed ( Fig. 1). This comprises full bridge strain gauge for force measurement. The outputs of this strain gauge bridges are available via a five-pin connector sockets present in the sensor body. The dynamometer was designed to force up to 5,000 N. The drilling force was measured at different drill bit diameters, as shown in Table 1.

Measurement of drilling temperature
Four K-type grounded thermocouples were situated vertically, one below the other, in the rock sample, as depicted in Fig. 2(a). The head of the thermocouple was placed 0.5 mm away from the bit-rock interface, as shown in Fig. 2(b). Moreover, the individual depths of the thermocouple were 6, 14, 22, and 30 mm, respectively, out of which the bit-rock interface temperature at 30 mm depth was measured to correlate with the wear rate of the drill bit.

Wear rate measurements
A series of drilling experiments was performed, during which the wear rate was measured for the regular intervals of known location. The mass loss method was employed to measure the wear rate of the drill bit by weighing it before and after the drill bit reached depth of 30 mm (Eq. (1)). To measure the bit weight, a digital weighing balance was used, with a resolution of 0.1 gm [5]. The majority of the losses occurred at the bit matrix and cutter head, because both of them were exposed to the rock [18]. Wear rate is calculated using the following Eq. (1).

Mass loss Wear rate
(mg/s) Drilling time (1) Archard equation is a model used to observe the abrasive characteristics of a tool based on the theory pertaining to the unevenness of surface contact. As per the Archard model, the following equations can be used to calculate the wear rate coefficient (k) in mm 3 /(N·m). Hence, the equation can be written as k = W/(L·C), where W is the wear rate in mm 3 , L is sliding length (m), C is the normal load (N) [23]. Finally 3 Results and analysis

Microstructure evolution and energy-dispersive X-ray spectroscopy (EDS) analysis
Scanning electron microscope (SEM) micrograph analysis was conducted for three types of rock samples, i.e., medium-grained sandstone, fine-grained sandstone, and fine-grained sandstone pink, as explained in Fig. 3. Furthermore, EDS analysis was carried out to determine the weight percentage of the silica content in the rock samples, as shown in Table 2.
The silica content of the rock sample and wear rate summary of the bits and rock combinations are shown in Table 3.

Effect of UCS and SiO2 of the rock on wear rate of the drill bit
Knowledge pertaining to rock properties and textural characteristics is important for bit design and operational conditions. In this experiment, uniaxial compressive strength and the SiO2 present in the rock samples are identified to correlate with the wear rate of the tungsten carbide (WC) drill bit. Moreover, the strength of the rock samples mainly depends on the Moh's hardness of the mineral present in the rock samples. Abrasive wear is primarily composed of hard mineral. The Moh's hardness of a mineral that is more than 5.5 can be classified as abrasive (causing a high level of abrasive wear) [24]. Hence, SiO2 has the second highest Moh's hardness with a value of 7 [18]. Sandstones with high quartz content possess higher strength properties in comparison to those with lower quartz content [25]. Three different SiO2 of varying wt% were observed in the sandstone using EDS analysis, and the strength of the rock samples in terms of UCS has been shown in Table 4. This occurs owing to the friction between the rock and the drill bit, which depends on the silica content of the rock sample; so does the hardness of the rock sample increases [26], which has been demonstrated in Fig. 4.   Figure 5 shows the effect of thrust on the wear rate of the drill bit. It is ascertained that the thrust increases with an increase in the wear rate of the drill bit. This is because the resistance offered by the rock samples as well as the force required by the drill bit to drill the rocks increase [26].
Subsequently, Fig. 6 shows the effect of temperature on the wear rate of the drill bit. It is found that mediumgrained sandstone contains 7.30 wt% of silica observed at 91 °C , whereas fine-grained sandstone contains 16.45 wt% of silica observed at 124 °C , and fine-grained sandstone pink contains 30.22 wt% of silica observed at 236 °C . Furthermore, SiO2 has a high Moh's hardness value of 7 [18]. In addition, SiO2 results in a ploughing effect that can increase the mechanical force and friction coefficient [26]. Hence, it can be clearly understood that as the silica content of the rock samples increases, heat is transferred between the bit-rock interfaces increases [6]. Moreover, as the drill bit diameter increases from 6 to 16 mm, the volume of rock removal also increases. Thus, the contact area of the bit-rock interface increases, which leads to an increase in the frictional force. The effect of drill bit diameter on its wear rate has been shown in Fig. 7.
Rock formation characteristics have a significant increase in wear temperature [6]. Strength of a rock is defined by means of a SiO2 percentage and generally has a high Moh's hardness value. However, wear rate measurement of the WC bit involves material loss   under fixed speed for constant distance. The Archard model is applied for tribological studies owing to its correlative linear equation between material loss (mg) and the load (N) acting per unit distance (mm). Further, its units are expressed in mg/(N·m) and is shown in Fig. 8.

Statistical analysis
Simple linear regression analyses were conducted for the three types of sandstone. The results of the relationship between bit-rock interface temperature and the wear rate have been presented in Eqs. (3)-(5). Moreover, the statistically significant mathematical models have been tested through the ANOVA and R 2 , and the value of R 2 that is near to 100% can lead one to assert that the obtained model is adequate, as shown in Table 5. Statistical equations (Eqs. (3)-(5)) clearly show that the independent variables are statistically significant, with P-values that are less than 0.05, as depicted in Table 6. The regression analysis further shows that the error between the experimental and predicted values is less than 10%, which has been shown in Table 7. Hence, the predicted models are reasonably accepted within the expected range.

Conclusions
From the aforementioned experimental work, the bitrock interface temperatures of three types of sandstone are investigated, in order to identify the wear rate of the WC drill bit using the mass loss method, and the following conclusions are drawn: 1) A significant relationship exists between wear rate and bit-rock interface temperature, which is essential to understand the thermal environment of rock samples.
2) The average bit-rock interface temperature for MG, FG, and FGP is 74, 88, and 152 °C , respectively. Further, the corresponding average wear rate is 9.111, 15.554, and 22.001 mg/s, respectively. | https://mc03.manuscriptcentral.com/friction 3) The wear rate of the drill bit is primarily influenced by bit-rock interface temperature. From the ANOVA analysis of the three types of rock samples, it is observed that the temperatures are significant, and P-value is lest than 0.05. 4) For the wear rate coefficient determined from the value obtained through experimental analysis using the Archard model, it is observed that the wear rate coefficient increased with an increase in the bitrock interface temperature. This is due to the increase in SiO2 (wt%) and the uniaxial compressive strength of the rock samples. 5) From the SEM and EDS analysis, it is clear that MG sandstone contains large grain particles that are weakly bonded and have less SiO2 content of 7.85 wt%, which gives a less wear rate coefficient of 6.33×10 -2 mg/(N·m) and has a minimum bit-rock interface temperature of 74 °C . Similarly, FG contains 16.45 wt% silica content gives wear rate coefficient of 8.36×10 -2 mg/(N·m), and FGP contains 30.22 wt% silica content gives wear rate coefficient of 10.08×10 -2 mg/(N·m), respectively.
6) The proposed simple linear regression models for the wear rate of the WC drill bit correlate reasonably well with the experimental data.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International Li-cense, which permits use, sharing, adaptation, distribution and reproduction in any medium or for-mat, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not in-cluded in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.