1
 Open Access. © 2020 Šporin J., published by Sciendo.  This work is licensed under the Creative Commons 
Attribution-NonCommercial-NoDerivatives 4.0 License. 
Received: Dec 07, 2020 
Accepted: Dec 11, 2020 
DOI: 10.2478/rmzmag-2020-0013
Original scientific article
Abstract
The rapid drop in the penetration rate or failure of the 
drill bit during the drilling process delays the drilling 
process. In our investigation, the ‘in situ’ drilling pa-
rameters were monitored during the drilling process 
along with the roller cone drill bit, which is suitable for 
drilling in soft rock formations (IADC 136). The drill bit 
was thoroughly examined to determine its damage and 
wear occurred during drilling along with decreasing 
penetration rate. The modern and standardised inves-
tigation methods were used to analyse the rock materi-
als and the micro- and macro-structure of the materials 
of the roller cone bit. The analyses were performed by 
means of optical and electron microscopes, simultane-
ous thermal analysis of the steel materials, analysis of 
the chemical composition of the materials of the drill 
bit and determination of the geomechanical param-
eters of the drilled rock. The resulting wear, localised 
fractures and cracks were quantitatively and qualita-
tively defined and the parameters were correlated to 
the drilling regime and the rock material. The results 
of our investigation of the material of the roller cone bit 
can serve as a good basis for the development of new 
steel alloys that can withstand higher temperatures 
and allow effective drilling without structural changes 
of the steel material.
Keywords: drilling regime, roller cone bit, wear, redu-
ced penetration rate, change of the material properties.
Characterisation of the Wear of the Roller Cone Drill Bit 
Caused by Improperly Chosen Drilling Regime
Karakterizacija obrabe kotalnega delta povzročene z nepravilno 
izbiro režima vrtanja
Jurij Šporin*
Faculty of Natural Sciences and Engineering, Department of Geotechnology, Mining and Environment, University of Ljubljana, 
Aškerčeva 12, Ljubljana, Slovenia
* jurij.sporin@ogr.ntf.uni-lj.si
Izvleček
Upad napredka vrtanja ali odpoved dleta med vrta-
njem imata za posledico daljši čas izvajanja vrtalnih 
del. V toku raziskave obrabe kotalnega dleta IADC 136, 
ki je namenjen uporabi pri vrtanju skozi mehkejše ka-
mnine, so se izvajale opazovanja in meritve vrtalnih 
parametrov. Po končanem odseku vrtanja, ko je hitrost 
napredovanja drastično padla, se je kotalno dleto na-
tančno pregledalo za ugotovitev nastalih poškodb med 
vrtanjem. Za ugotavljanje in določanje načina obrabe 
so se uporabile moderne in standardne metode. Izve-
dla se je analiza hribinskega materiala skozi katerega 
se je vrtalo, ter analiza mikro in makro strukture ma-
terialov kotalnega dleta. Analiza materialov se je izve-
dla z uporabo optičnega in elektronskega mikroskopa, 
simultane termalne analize jekla kotalnega dleta, dolo-
čila se je kemijska sestava jekla kotalnega dleta in ge-
omehanske karakteristike hribinskega materiala skozi 
katerega se je vrtalo. Izražena obraba kotalnega dleta 
se je kvantitativno in kvalitativno ovrednotila v poveza-
vi z režimom vrtanja in karakteristikami hribine skozi 
katere se je vrtalo. Rezultati naše raziskave materialov 
kotalnega dleta lahko služijo kot dobra osnova za razvoj 
novih jeklenih zlitin, ki bodo prenašale visoke tempera-
ture, ki nastajajo med vrtanjem, brez strukturnih spre-
memb jeklenih materialov.
Ključne besede: režim vrtanja, kotalno dleto, obraba, 
zmanjšani napredek vrtanja, sprememba lastnosti ma-
teriala.

Šporin J. RMZ – M&G | 2020 | Vol. 67 | pp. 01–12
2
Introduction
The time of effective drilling operations de-
pends on the properties of the materials from 
which the roller cone bit components are man-
ufactured. The roller bit will wear out due to 
the consequences of effects of the rock materi-
al 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 signifi-
cant 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 quan-
tity 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 sci-
entific 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 drill-
ing time.
Drilling Principle of the Roller 
Cone Drill Bit
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 roll-
er 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 noz-
zles, the velocity of the drilling fluid flow is in-
creased 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 contrib-
utes to the fracture of the borehole, especially 
in the softer, poorly bonded rocks [1–20].
The roller cone bit with steel teeth, which is in-
vestigated here, is used in softer, poorly bond-
ed 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 ef-
fective 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]:
F = (C
1
•w + C
2
•h)•l• σ
p
, (1)
where F is the load on a single tooth of the bit; 
C
1
 is the coefficient of friction on the surface at 
the tooth tip (-); w is the tooth width (m); C
2
 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:
WOB = n
t
•F, (2)

Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime
3
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
 (m
3
) created by one tooth, assuming that 
the crater has a conical shape, is stated by the 
following equation [22]:
𝑉𝑉 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
=
1
3
⋅ 𝜋𝜋 ⋅ 𝑟𝑟 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
2
⋅ ℎ ,  (3)
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 vol-
ume of the rock craters on the entire cutting 
surface of the roller cone bit in number of revo-
lutions [22] and can be stated as follows.
ROP =
𝑉𝑉 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
⋅ 𝑛𝑛 𝑐𝑐 ⋅ RPM
𝐴𝐴 𝑏𝑏𝑏𝑏𝑐𝑐
,  (4)
where ROP is the rate of penetration (m/s); 
V
crat 
is the volume of the crater of the removed 
rock (m
3
); RPM is the number of rotations of the 
bit (s
-1
) and A
bit
 is the cutting area of the bit (m
2
).
As the bit advances through the rock, the teeth 
of the roller cone bit wear out. Further, the pa-
rameters 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 pa-
rameters as stated in Eq. (5).
ROP = f
1
•f
2
•f
3
•f
4
•f
5
•f
6
•f
7
•f
8
, (5)
where f
1
 is the effect of formation strength or 
rock durability; f
2
 is the effect of formation 
depth; f
3
 is the effect of formation compaction 
or pore pressure; f
4
 is the effect of differential 
pressure; f
5
 is the effect of bit diameter and bit 
weight; f
6
 is the effect of rotary speed; f
7
 is the 
effect of tooth wear and f
8
 is the effect of bit hy-
draulics.
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 con-
ditions 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.
Materials and Methods
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 pur-
pose, 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 in-
vestigated the condition of the teeth and rollers 
of the bit after drilling through known rock.
While drilling through the known rock, the fol-
lowing drilling parameters of drilling regime 
were monitored:
 ― The penetration rate;
 ― The length of the drilled interval;
 ― The load on the bit;
 ― The bit RPM; and
 ― The amount, pressure and properties of the 
drilling fluid.
We carried out a complete analysis of the steel 
materials of the roller cone bit, which included 
the following investigations:
 ― Analysing the micro- and macrostructure of 
roll cone bit materials with an reflected light 
microscope (Olympus BX61 and Olympus 
SZ61 stereo microscope using the Analysis 
6.0 image analysis system);
 ― The composition of the carbide coating of 
bit teeth using the XRF (X-ray fluorescence) 
method (Thermo NITON XL3t XRF analyser);
 ― A cross-sectional view of bit teeth using a 
scanning electron microscope (SEM) and 
EDS analysis (energy dispersive X-ray spec-
troscopy) (Jeol JSM 5610);
 ― A dilatometric analysis of tooth steel and 
tooth carbide coating with the low tempera-
ture dilatometer (Baehr-Thermoanalyse 
GmbH DIL 801);
 ― Chemical analysis of the components of the 
teeth and rollers with an ICP (inductively 
coupled plasma) analyser (ICP-OES Agil-
lent 720);

Šporin J. RMZ – M&G | 2020 | Vol. 67 | pp. 01–12
4
 ― DSC (Differential Scanning Calorimetry) of 
tooth carbide coating and tooth steel with a 
thermal analyser (NETZSCH STA 449 C Jupi-
ter) and
 ― Vickers hardness tests of the tooth steel with 
a 100 g load using microhardness tester (Shi-
madzu type M).
The properties of the rock obtained by sam-
pling were estimated based on the analysis of 
Rock Lab 1.0.
Results
The results of our analysis show the way and 
the mechanisms that cause wear of the bit ma-
terial at fixed or established rock properties in 
connection with the operating parameters of 
the drilling.
Drilling work
Drilling was carried out with a 1992 N-1000 
drill rig. The interval in which we observed the 
performance of the IADC 136 roller cone drill 
bit included a length of drilling interval from 
1535.15 m to 1623.04 m depth, correspond-
ing to a length of 87.89 m. The penetration 
rate during the observed interval ranged from 
0.2 m/h to 0.4 m/h. The remaining drilling re-
gime parameters are shown in Table 1.
The 40 mass.% bentonite drilling fluid was 
used during drilling. The properties of the drill-
ing fluid are shown in Table 2.
Table 1. Drilling regime parameters
Parameters Units Value Value
Depth m 1535.15–1600.00 1600.00–1623.04
Rotary speed rpm 35 45
Load on drill bit kN 30 40
Pump pressure MPa 5.5 5.5
Pump capacity m
3
/min 0.845 0.845
Table 2. The properties of drilling fluid
Parameters Units Value
Density kg/m
3
1150–1170
Viscosity (Fann Funnel) s 45–48
Plastic viscosity (Fann Viscosimeter) mPas 16–18
pH – 9.5
Filtration mL/30 min 9.8–10
Rock Material Properties
The rock material we drilled through was reg-
ularly sampled from drilling fluid. Each rock 
sample was examined and compared to a pre-
determined lithological column based on the 
results of geophysical and laboratory measure-
ments in the surrounding boreholes.
As far as the lithology of the material of the 
drilled interval concerned, silty claystone is 
dominant and layers of carbonatic siltstone 
with sandstone plates and thin layers of clay 
and limestone occur in the lower and upper 
parts of the sequence.
The geomechanical properties of the drilled 
borehole section were evaluated based on ex-
perience. The strength properties were eval-
uated using the Hoek-Brown’s criterion using 
RockScience software, Rock Lab 1.0.
The estimated rock properties values for the 
carbonate silt are shown in Table 3.
The estimated rock properties values for lime-
stone are shown in Table 4.

Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime
5
Roller Cone Bit Investigation
After drilling and cleaning, the drill bit was ex-
amined using the IADC dull grading system. We 
found that the teeth of the drill bit were evenly 
worn out and several individual teeth were bro-
ken. 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 Fig-
ure 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 re-
sults of the chemical composition of the exam-
ined 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-hard-
ened parts.
At SEM, we analysed metallographically the 
steel base of the tooth (chromium, molybde-
num, nickel low alloy steel), the contact area 
between the tooth body and the carbide coat-
ing and the carbide coating itself. We have de-
termined the chemical composition of the steel 
of each component of the tooth through the 
analysis of EDS. Figure 2 (SEM) shows the ar-
eas where metallographic tests are performed 
on the teeth. Tables 6–9 show the results of the 
EDS analyses for each component analysed.
Table 3. Results of the rock properties – carbonatic siltstone
Parameters Units Value
Cohesion MPa 0.722
Elastic module MPa 9375
Angle of internal friction ° 20.94
Compressive strength MPa 25
Table 4. Results of the rock properties – limestone
Parameters Units Value
Cohesion MPa 5.50
Elastic module MPa 28125
Angle of internal friction ° 38.42
Compressive strength MPa 75
Figure 1. Characteristic dull of the roller cone bit after drilled 
interval.
Figure 2. Scanning electron microscope (SEM) image of tip 
of the bit tooth. Position 1 – carbide material; Positions 2 
and 3 – carbide coating matrix; Position 4 – tooth body and 
Position 5 – mixed zone.

Šporin J. RMZ – M&G | 2020 | Vol. 67 | pp. 01–12
6
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 coat-
ing material (Figure 2 – Position 1), which is de-
termined by EDS, is shown in Table 6.
The elemental composition of the carbide coat-
ing matrix (Figure 2 – Position 2), which is de-
termined by EDS, is shown in Table 7.
The elemental composition of the carbide coat-
ing matrix (Figure 2 – Position 3), which is de-
termined 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 transfor-
mation in the solid state starts at a tempera-
ture of 693.9°C and also there is an endother-
mic peak. The process ends at 800°C when the 
metal matrix (iron) is transformed into a gam-
ma-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.
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 mix-
ing 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 
Figure 3. DSC heating curve of the steel of the tooth body.
Figure 4. DSC heating curve of carbide coating.

Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime
7
temperature 692.5°C, the starting of the eutec-
toid transformation was detected. This trans-
formation is related to the layer of the mixed 
zone. Therefore, this curve is not typical for car-
bide alloys. At the temperature of 1467.3°C, the 
starting of melting process was detected.
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 tempera-
ture 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.
In view of the fact that the temperatures can lo-
cally reach up to 500°C during drilling despite 
intensive cooling, the curves show that the dif-
ference 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 for-
mation 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 tem-
perature of 800°C (A
c3
). For the carbide coat-
ing sample containing a thin layer of the mixed 
zone, eutectoid transformation starts at 750°C. 
On the curve of the carbide coating sample con-
taining a thin layer of the mixing zone, a devia-
tion 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 mix-
ing zone.
A Vickers hardness test was performed to de-
termine 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 
Figure 5. Dilatometric heating curve of the steel of the carbide 
coating and the steel of the tooth: 1 – tooth steel material 
(blue), 2 – carbide coating (black).
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
24.67 25.96 1446.830
1458.984
Carbide 
coating—
base
24.43 25.78 1471.137

Šporin J. RMZ – M&G | 2020 | Vol. 67 | pp. 01–12
8
the steel at the tip of the teeth is much higher 
and is 594.5670 HV. The hardness of the car-
bide coating is, as expected, much higher and 
reaches up to 1471,137 HV.
Discussion
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 num-
ber of bit rotations for this type of tool is be-
tween 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 Fig-
ure 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 previ-
ously observed under the optical microscope, 
the sample was examined with a scanning elec-
tron microscope (SEM). A characteristic view 
of the microstructure of the area with a SEM is 
shown in Figure 8.
Figure 8 shows the condition of the tooth tip 
that was in contact with the rock during drill-
ing. 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 
Figure 6. Display of measurement positions of Vickers 
hardness tests.
Figure 7. Microscopic photo of the tip of the tooth.
Figure 8. A view at the tip of tooth that was in contact with 
the rock.

Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime
9
mechanical stresses that occurred during drill-
ing 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 tem-
perature 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 microstruc-
tural 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 hard-
ness of the steel material increased. The mea-
sured 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 pro-
cess 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 demonstrat-
ed by DSC and dilatometric analysis, the tem-
perature 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 recrystalli-
sation of the steel is rapidly accelerated. The 
recrystallisation temperature for steel, when 
calculated for theoretical information, is some-
where around 0.4 × T
L
 (T
L
 – liquidus tempera-
ture, °C) [25]. Using the software Thermo-Calc 
for thermodynamic modelling of the equilibri-
um phase, we calculated the liquidus tempera-
ture, 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 tempera-
ture 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 calcu-
lated. 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 differ-
ence between the temperature-expansibility 
coefficients of the steel material and the car-
bide 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 weld-
ing 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 cool-
ing processes, internal stresses begin to in-
crease due to the different temperature coeffi-
cients of the materials, which lead to initiation 
and propagation of cracks.
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 
Figure 9. Photography of tooth steel material. Position 1 – 
carbide coating; Position 2 – mixed zone and Position 3 – 
tooth steel (body).

Šporin J. RMZ – M&G | 2020 | Vol. 67 | pp. 01–12
10
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.
The erosion effect caused by aggressive parti-
cles 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. Mi-
croscopic images show smaller erosion micro-
channels, which are not largely pronounced 
(Figure 11). Erosion microchannels occur only 
in the area of the tooth tip. There are no ero-
sion channels along the tooth edges that are not 
protected by carbide coatings, which is part-
ly 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.
Conclusions
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 char-
acterised by the analysis of the different metal-
lurgical 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 ma-
terials 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 dissolu-
tion 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 par-
tially changed by the influence of high defor-
mations. This change, which was reflected in 
the increase in the hardness of the steel mate-
rial, 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 m m. 
The formation of this layer with a higher degree 
of hardness can be explained by the implemen-
tation of the weight on the drill bit (WOB), 
which was higher than the recommended and 
relatively low rotation (RPM), and it was low-
er than the recommended. The increase in the 
temperature of the tooth material during drill-
ing was due to excessive loads and inefficient 
cooling, resulting in micro-fatigue reaching the 
Figure 10. Photography of cracks through carbide coating.
Figure 11. Erosion channels at the top of the tooth.

Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime
11
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 pa-
rameters (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.
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