Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime

The time of effective drilling operations depends on the properties of the materials from which the roller cone bit components are manufactured. The roller bit will wear out due to the consequences of effects of the rock material it is drilling through and the drilling regime. The penetration rate decreases due to the wear of the bit.

The following factors among all have a significant influence on the effective operation of the roller cone bit are the steel material from which the rollers and teeth of the bit are made, the drilling regime (load on the bit during drilling, torque, number of bit revolutions and the quantity and properties of the drilling fluid) and the properties of rock drilled.

We analysed the mechanical properties of the steel material of the roller cone bit IADC 136, 155.57 mm (6 1/8″), which was drilling through carbonatic siltstone where sandstone plates and rare thin layers of clay and limestone exist in a total length of 87.89 m.

Our research focused on the resistance of the steel material of the roller cone bit to the rock material using a selected drilling regime.

To accomplish this, it was necessary to use scientific approaches to determine the causes that lead to wear of the bit and identify the weak points on the bit that need to be technologically modified in order to enhance the effective drilling time.

The roller cone bit contains cutting elements, i.e. teeth or inserts, which are mounted on the rollers. The rollers, which are inserted into the bearings of the bit, rotate around their axis. They are driven by rotating drilling rods that drives the body of the drill bit on which the roller is mounted. Nozzles (outlet) for the drilling fluid are attached to the bit body to effectively remove the rock particles from the borehole bottom of the borehole and to cool the rollers and teeth of the bit. The drilling fluid flows through the nozzles from the inside of the drill pipes into the area of the bit. When the orifice is reduced, which is the ratio of the cross-section of the drill bit to the cross-section of the nozzles, the velocity of the drilling fluid flow is increased significantly. The increasing velocity of the drilling fluid has a positive effect on the flow of the drilling fluid around the rollers, which effectively removes the drilled rock particles from the bottom of the borehole and contributes to the fracture of the borehole, especially in the softer, poorly bonded rocks [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20].

The roller cone bit with steel teeth, which is investigated here, is used in softer, poorly bonded rock formations. The teeth of the rollers are large and sharp so that they can penetrate deep into the soft rock structures where the rock is crushed and removed from the crushed area. The teeth are protected with a carbide coating to improve durability, which increases the effective operating time. The axes of the rollers do not intersect at the point of the vertical axis of the bit, but do have an offset corresponding to the point of the vertical axis of the bit. The offset of the roller axis is normally in the range of 2°–5°.

The load on tooth F, which causes the tooth to penetrate the rock (h), is a linear combination of the force acting against the surface at the tip of the tooth and the force acting against the surface formed by the inclined surfaces of the tooth.

When rollers and bits with teeth on rollers are made of the same material, we could state the following [21]: (1) F=(C1⋅w+C2⋅h)⋅l⋅σp, F = ({C_1} \cdot w + {C_2} \cdot h) \cdot l \cdot {\sigma_{p},} where F is the load on a single tooth of the bit; C₁ is the coefficient of friction on the surface at the tooth tip (-); w is the tooth width (m); C₂ is the coefficient of friction on the tooth surface in contact with the rock (-); h is the penetration depth of the tooth (m); l is the tooth length (m) and σ_p is the compressive strength of the rock (Pa).

The result of the reaction force F on the tooth of the bit in contact with the rock is equal to the load generated on the bit [22] and is given by: (2) WOB=nt⋅F, {\rm{WOB}} = {n_t} \cdot F, where WOB is the weight on the bit (N) and n_t is the number of teeth in contact with the rock. The volume of the crater of the crushed rock V_crat (m³) created by one tooth, assuming that the crater has a conical shape, is stated by the following equation [22]: (3) Vcrat=13⋅π⋅rcrat2⋅h, {V_{crat}} = {1 \over 3} \cdot \pi \cdot r_{crat}^2 \cdot h, where r_crat is the crater radius (m) and h is the crater depth (m).

By determining the volume of a single crater formed by a single tooth, the penetration rate can be evaluated, which is defined by the volume of the rock craters on the entire cutting surface of the roller cone bit in number of revolutions [22] and can be stated as follows. (4) ROP=Vcrat⋅nt⋅RPMAbit, {\rm{ROP}} = {{{V_{crat}} \cdot {n_t} \cdot {\rm{RPM}}} \over {{A_{bit}}}}, where ROP is the rate of penetration (m/s); V_crat is the volume of the crater of the removed rock (m³); RPM is the number of rotations of the bit (s⁻¹) and A_bit is the cutting area of the bit (m²). As the bit advances through the rock, the teeth of the roller cone bit wear out. Further, the parameters that influence the penetration rate (ROP) of the roller cone bit are focused here to determine the wear of the bit. Bourgoyne and Young [23, 24] defined these as influential parameters as stated in Eq. (5). (5) ROP=f1⋅f2⋅f3⋅f4⋅f5⋅f6⋅f7⋅f8, {\rm{ROP}} = {f_1} \cdot {f_2} \cdot {f_3} \cdot {f_4} \cdot {f_5} \cdot {f_6} \cdot {f_7} \cdot {f_8}, where f₁ is the effect of formation strength or rock durability; f₂ is the effect of formation depth; f₃ is the effect of formation compaction or pore pressure; f₄ is the effect of differential pressure; f₅ is the effect of bit diameter and bit weight; f₆ is the effect of rotary speed; f₇ is the effect of tooth wear and f₈ is the effect of bit hydraulics.

We investigated the wear of the roller cone bit, the occurrence of damage and the change in the properties of the bit materials under the conditions of the properties of the material drilled through, the drilling regime (weight on the bit, speed, etc.) and the hydraulics of the drilling fluid in the area of the bit.

When determining the characterisation of the wear of the roller cone bit at the time of the investigations, detailed analyses were made against the drilling regime, the materials from which the bit was made and the rock through which the bit was drilled. To achieve this purpose, in the first phase, we carried out a survey of the bit, which included an overview of the condition of the bit after drilling according to the IADC bit dull standard. In this part, we investigated the condition of the teeth and rollers of the bit after drilling through known rock.

While drilling through the known rock, the following drilling parameters of drilling regime were monitored: –

The penetration rate;

The results of our analysis show the way and the mechanisms that cause wear of the bit material at fixed or established rock properties in connection with the operating parameters of the drilling.

Roller Cone Bit Investigation

After drilling and cleaning, the drill bit was examined using the IADC dull grading system. We found that the teeth of the drill bit were evenly worn out and several individual teeth were broken. There are no obvious erosion effects due to the rock particles in the drilling fluid that would result in erosion of the steel material of the bit. The condition of roller cone bit after the length of drilled interval of 87.89 m is shown by Figure 1.

Figure 1

Investigations of Tooth Steel Material

We examined the steel material and the carbide coating of the teeth.

The investigation of the chemical composition of the teeth material was carried out with an optical emission spectrometer (ICP). The results of the chemical composition of the examined steel (body) are shown in Table 5.

Table 5

Chemical analysis of the tooth material – body

Elements	Mas. %
Mn	0.79
Ni	0.74
Cr	0.60
Mo	0.50
Cu	0.27
Si	0.26
C	0.17
S	0.016
P	0.011

The results of the chemical analysis (Table 5) show that the steel material of the tooth body in this case is chromium, molybdenum, nickel low alloy steel, which is often used for case-hardened parts.

At SEM, we analysed metallographically the steel base of the tooth (chromium, molybdenum, nickel low alloy steel), the contact area between the tooth body and the carbide coating and the carbide coating itself. We have determined the chemical composition of the steel of each component of the tooth through the analysis of EDS. Figure 2 (SEM) shows the areas where metallographic tests are performed on the teeth. Tables 6–9 show the results of the EDS analyses for each component analysed.

Figure 2

Table 6

The elemental composition of carbide coating material (Figure 2 – Position 1)

Elements	Concentration
	at.%	wt.%
	W	55.415	79.539
Co	42.380	19.500
Fe	0.807	0.962

Table 7

The elemental composition of the carbide coating matrix (Figure 2 – Position 2)

Elements	Concentration
	at.%	wt.%
	Fe	86.096	68.599
W	1.709	29.094
Ni	1.921	1.608
Mn	0.892	0.699

Table 8

The elemental composition of the carbide coating matrix (Figure 2 – Position 3)

Elements	Concentration
	at.%	wt.%
	Fe	86.117	68.616
W	11.103	29.124
Ni	1.511	1.265
Mn	1.269	0.995

Table 9

The elemental composition of the tooth body (Figure 2 – Position 4)

Elements	Concentration
	at.%	wt.%
	Fe	95.427	95.385
Ni	3.164	3.323
Mn	0.660	0.649
Mo	0.218	0.375
Si	0.531	0.267

The elemental composition of the carbide coating material (Figure 2 – Position 1), which is determined by EDS, is shown in Table 6.

The elemental composition of the carbide coating matrix (Figure 2 – Position 2), which is determined by EDS, is shown in Table 7.

The elemental composition of the carbide coating matrix (Figure 2 – Position 3), which is determined by EDS, is shown in Table 8.

The elemental composition of the tooth body (Figure 2 – Position 4), which is determined by EDS, is shown in Table 9.

We carried out a simultaneous thermal analysis (STA) (Figure 3) of the steel sample of the tooth body. The curve shows that eutectoid transformation in the solid state starts at a temperature of 693.9°C and also there is an endothermic peak. The process ends at 800°C when the metal matrix (iron) is transformed into a gamma-phase iron structure – austenite (g-Fe). The temperature 1407.2°C is the low temperature at which the eutectic structure or alloy begins to melt. The remaining material of the metal matrix (austenite) begins to melt at 1474.5°C. The temperature of 1474.5°C is also the solidus temperature of the steel from which the tooth body is made.

Figure 3

The DSC (Differential scanning calorimetry) heating curve of the carbide coating of the teeth in the roller cone bit is shown in Figure 4. The carbide coating contains a thin layer of the mixing zone. The mixing zone is the area formed during the welding of the carbide coating to the tooth. It contains the properties of the steel of the tooth body and the carbide coating. At the temperature 692.5°C, the starting of the eutectoid transformation was detected. This transformation is related to the layer of the mixed zone. Therefore, this curve is not typical for carbide alloys. At the temperature of 1467.3°C, the starting of melting process was detected.

Figure 4

Figure 5 shows the dilatometric curve of the steel of the carbide coating and the steel of the tooth body. It can be seen from the heating curve of the steel of the tooth body that the temperature of the eutectoid transformation increases linearly. The slope of the curve corresponds to the linear expansion coefficient for chromium, molybdenum and nickel low alloy steel, while the slope of the heating curve of the sample of the carbide coating containing a thin layer of the mixing zone is small. A lower curve angle of the carbide coating sample containing a thin layer of the mixing zone is the result of a lower coefficient of thermal expansion.

Figure 5

In view of the fact that the temperatures can locally reach up to 500°C during drilling despite intensive cooling, the curves show that the difference between the expansion properties is 0.05%. Because of this difference, the mixing zone is the preferred area for the formation of internal stresses and hence suitable for the formation and propagation of micro-fractures. The diagram shows the beginning of the eutectoid transformation of the tooth body steel sample, which starts at 680°C (A_c1) and ends at a temperature of 800°C (A_c3). For the carbide coating sample containing a thin layer of the mixed zone, eutectoid transformation starts at 750°C. On the curve of the carbide coating sample containing a thin layer of the mixing zone, a deviation is observed which can be associated with the eutectoid transformation of the thin layer of the mixing zone, but the temperature is slightly higher. The reason for this higher temperature is due to the chemical composition of the mixing zone.

A Vickers hardness test was performed to determine the changes in the hardness properties of the tooth due to the effects of drilling. The measurement points are shown in Figure 6 and the results of the tests are shown in Table 10. The average hardness of the steel of the tooth body is 433.5184 HV. The average hardness of the steel at the tip of the teeth is much higher and is 594.5670 HV. The hardness of the carbide coating is, as expected, much higher and reaches up to 1471,137 HV.

Figure 6

Table 10

Results of the Vickers hardness test

Diagonals (mm)		Hardness (HV0.5)	Average	Note
d1	d2
37.23	46.24				532.3198	594.5670	Tip of the tooth
36.22	37.55	681.5125
36.4	40.81	622.1374
37.99	41.33	589.4786
37.54	41.66	591.2662
37.43	39.54	626.0232
37.57	39.86	618.6071
36.33	42.41	598.1947
38.87	42.45	560.8396
38.48	42.45	566.2580
39.01	42.84	553.6000
44.99	47.95	429.3664	433.5184	Tooth body
44.44	48.37	430.5701
44.61	47.78	434.4937
44.09	47.10	446.0042
45.14	48.04	427.1575
33.28	37.13	748.1087	784.1543	Carbide coating—matrix
31.73	34.04	857.3887
33.64	36.53	753.2349
34.58	36.51	733.8654
30.41	36.51	828.1739	1458.984	Carbide coating—base
24.67	25.96	1446.830
24.43	25.78	1471.137

Monitoring of the operating parameters during drilling revealed that the roller cone bit was not loaded in accordance with the manufacturer's recommendations, which recommends that the bit load should be in the range of 15–27 kN. The load on the bit during drilling was between 30 kN and 40 kN. In addition, the number of bit rotations was too low and they were between 35 rpm and 45 rpm. The recommended number of bit rotations for this type of tool is between 60 rpm and 100 rpm.

It can be noted that the bit was overloaded with weight and the number of bit rotations was too low.

Examination of the microscopic image showed that the tips of the teeth were exposed to high temperatures and stresses. The influence of high temperatures and loads is shown in Figure 7.

Figure 7

Figure 7 shows the tip of the tooth that was in contact with the rock. We can see changes in the colour of the steel, which changes from blue tones (at the point of contact between the rock and the steel) to brown tones that change the light from top to bottom.

Because of the apparent colour change previously observed under the optical microscope, the sample was examined with a scanning electron microscope (SEM). A characteristic view of the microstructure of the area with a SEM is shown in Figure 8.

Figure 8

Figure 8 shows the condition of the tooth tip that was in contact with the rock during drilling. A change in the microstructure of the steel on the surface was observed. The change in the steel structure can be observed in two layers. The first layer, where the change is noticeable, is up to a thickness of 36 μm. The second layer, in which the change in microstructure is less pronounced but still noticeable, moves to a depth of up to 92 μm.

The change in the steel microstructure of the tooth was due to the high temperatures and mechanical stresses that occurred during drilling when the tip of the tooth was in contact with the rock. When the roller cone bit is rotated around its axis, the teeth got cooled. The teeth that were not in contact with the rock were cooled by the drilling fluid flowing out from the nozzles of the bit. In this situation, a temperature change occurs on the surface of the teeth when their temperature is very high and they are washed with drilling fluid at a lower temperature. The other reason for microstructural changes in the steel is that the bit was overloaded during drilling and the number of revolutions was too low when compared to the load. The consequence of such a drilling regime is the increase in temperature of the material at the tooth tip, which in combination with the load results in the hardening of the material at the tooth tip. Because of this reason the hardness of the steel material increased. The measured hardness (Vickers hardness) at that time was on average about 160 HV higher than the hardness of the steel material in the middle of the tooth.

Using differential scanning calorimetry (DSC), we found that the temperature of the eutectoid point is 699.2°C and its completion occurs at a temperature of 800°C. Due to the intensive cooling of the teeth under the influence of the drilling fluid, the microstructural change process occurred only in the steel layer down to a depth of 92 μm. Microstructural changes are only noticeable on the upper side of the teeth, while no changes were observed on the sides. The changes in the steel structure occurred mainly due to excessive operating temperatures and loads on the selected steel. As demonstrated by DSC and dilatometric analysis, the temperature at the beginning of the transformation from ferrite to austenite is about 680°C for the selected steel. This means that the temperature of 680°C (A_c1) is a point at which recrystallisation of the steel is rapidly accelerated. The recrystallisation temperature for steel, when calculated for theoretical information, is somewhere around 0.4 × T_L (T_L – liquidus temperature, °C) [25]. Using the software Thermo-Calc for thermodynamic modelling of the equilibrium phase, we calculated the liquidus temperature, which was 1511°C for the selected steel. Therefore, the recrystallisation temperature for the analysed steel is about 604°C. We also calculated the equilibrium eutectoid temperature A_e1 and the transition temperature A_e3 with the same software. The temperature of t6he former was 685°C and the latter was 811°C. The equilibrium eutectoid temperature A_e1 and the transition temperature A_e3 were also calculated. The calculated values are similar to the values obtained from the dilatometric analysis of the steel (680°C and 800°C), assuming that the temperatures obtained from the calculation are in equilibrium. This confirmed the results of the dilatometric analysis.

Due to the influence of high temperatures and rapid cooling, the carbide coating on the upper surface of the tooth gets decayed. The difference between the temperature-expansibility coefficients of the steel material and the carbide form was found by a dilatometric test at low temperature. This difference is significant as it causes increase in internal stress in the mixed zone. The mixed zone is formed by welding the carbide coating onto the tooth steel and represents a mixture of dissolved steel material of the tooth and the carbide coating (Figure 9). In the mixed zone, during the heating and cooling processes, internal stresses begin to increase due to the different temperature coefficients of the materials, which lead to initiation and propagation of cracks.

Figure 9

Cracks were observed on the edges of the teeth, which progressed through the carbide coating to the steel of the tooth (body) (Figure 10). The occurrence of such cracks can be attributed to the excessive load on the bit during drilling and the stress accumulation due to the weight load of the bit. The carbide coating is tough and erosion resistant, but brittle, which causes it to crack under excessive load and lateral forces. Due to the formation of such cracks at the tooth edges, the carbide coating is sheared or eroded, which resulted in a chipped tooth.

Figure 10

The erosion effect caused by aggressive particles in the drilling fluid could not be expressed comprehensively so far, as only a short length of 87.89 m was drilled with this drill bit. Microscopic images show smaller erosion microchannels, which are not largely pronounced (Figure 11). Erosion microchannels occur only in the area of the tooth tip. There are no erosion channels along the tooth edges that are not protected by carbide coatings, which is partly due to the geometry of the teeth. Here, the teeth sides are quite steep, which means that the drilling fluid with abrasive particles leaves the tooth area within a short time.

Figure 11

In this article, we have described the wear of the IADC 136 roller cone bit drilled into a known standard rock material. The bit wear was characterised by the analysis of the different metallurgical properties of the steel and the carbide coating of the teeth of the bit, which are related to the drilling regime and the properties of the rock material. During drilling, the teeth of the bit are heated when are in contact with the rock and then cooled by the influence of the drilling fluid. At this stage, there is a compressive load on both the steel of the tooth and its carbide coating. Internal stresses developed in the materials of the bit teeth due to the load on the bit can expand through the elastic zone and reach the plastic zone, which results in local dissolution of the carbide coating and a local increase in the hardness of the steel material.

Using an IADC 136 roller cone bit, the length of interval 87.89 m was drilled into a carbonatic siltstone with sandstone plates and thin layers of clay and limestone. The wear of the teeth of the roller cone bit was mainly observed at the tooth tip. During the systematic investigation of the steel structure of the teeth and the carbide coating, it was found that the structure of the steel material at the tip of the teeth was partially changed by the influence of high deformations. This change, which was reflected in the increase in the hardness of the steel material, was higher at the tip of the tooth which is in contact with the rock by 160 HV than at the tooth body. The change in the hardness of the steel material, expressed as a layer parallel to the tooth tip, extends to a depth of about 36 μm. The formation of this layer with a higher degree of hardness can be explained by the implementation of the weight on the drill bit (WOB), which was higher than the recommended and relatively low rotation (RPM), and it was lower than the recommended. The increase in the temperature of the tooth material during drilling was due to excessive loads and inefficient cooling, resulting in micro-fatigue reaching the plastic zone of the steel material at the tip of the tooth. The tip of the tooth was worn out by a modified microstructure due to friction along the rock, and underneath it, a new layer with a modified microstructure formed periodically. From this, it can be concluded that the correct choice of roller cone bit can be recommended for a specified type of rock and operating parameters (WOB, RPM, drilling fluid) since it has a significant influence on the wear of the roller cone bit and thus on the extension or reduction of the effective operating time of the roller cone bit.

The results of our investigation of the materials of the roller cone bits can provide a base and guidelines for the development of new steel alloys that can withstand higher temperatures and allow effective drilling without structural changes in the steel material.