J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
459–466
WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
MEHANIZEM OBRABE ORODIJ ZA IZVEDBO GLOBOKIH VRTIN
Jurij [porin
*
, @eljko Vukeli}
University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geotechnology, Mining and Environment,
A{ker~eva 12, 1000 Ljubljana, Slovenia
Prejem rokopisa – received: 2024-03-15; sprejem za objavo – accepted for publication: 2024-06-07
doi:10.17222/mit.2024.1134
When drilling deep wells with a large diameter drill bit, it is important to ensure optimum progress with the cutting tool, effec-
tively cutting or crushing the rock through which we are penetrating for as long as possible. When drilling a well, we encounter
complex conditions at depth resulting from variables such as the strength of the rock, the load on the bit, the number of revolu-
tions of the tool, significant temperature and pressure fluctuations and the material properties of the tools used in the drilling.
The article shows how mechanisms develop due to the drilling parameters used and the complex drilling conditions, which lead
to wear of the cutting unit and a decrease in efficiency. The influence of the aforementioned drilling conditions on the steel ma-
terials of the roller-cone drill bit, which is the tool we use to penetrate deeper, is reflected in the change in the material proper-
ties of the steels. These changes, which are explained in the article, lead to gradual fatigue and degradation of the materials. The
wear mechanism of the cutting tool is composed of the different material properties of the steel from which the roller-cone drill
bit is formed, the influence of sudden temperature changes on the steel materials and erosion processes on free, newly formed
surfaces, which are the result of the erosion effect.
Keywords: erosion, carbide coating, temperature change, steel recrystallisation, material fatigue
Pri vrtanju globokih vrtin s kotalnimi dleti velikega premera je vodilo, da se z rezalnim orodjem zagotovi optimalen napredek ob
~im dalj{em efektivnem rezanju oziroma drobljenju kamnine skozi katero napredujemo. V toku vrtanja vrtine se v globini
sre~ujemo s kompleksnimi pogoji, ki jih tvorijo spremenljivke kot so trdnost kamnine, obremenitev na kotalno dleto, {tevilo
obratov orodja, izrazito nihanje temperature in tlaka ter materialne lastnosti orodij, ki jih v toku vrtanja uporabljamo. V ~lanku
prikazujemo kako se zaradi uporabljenih parametrov vrtanja in kompleksnih pogojev izvajanja vrtanja razvijajo mehanizmi, ki
privedejo do obrabe rezalnega mehanizma in zmanj{anja u~inkovitosti. Vpliv omenjenih pogojev vrtanja na jeklene materiale
kotalnega dleta, ki predstavlja orodje s katerim napredujemo v globino, se odra`a v spremembi materialnih lastnosti jekel.
Zaradi teh sprememb, ki so pojasnjene v ~lanku, pride do postopnega utrujanja in razpadanja materialov. Mehanizem obrabe
rezalnega mehanizma je skupek razli~nih materialnih lastnosti jekel iz katerega je formirano kotalno dleto, vpliva naglih
sprememb temperature na jeklene materiale in erozijskih procesov na proste novo formirane povr{ine, ki so posledica
erozijskega delovanja.
Klju~ne besede: erozija, karbidna obloga, sprememba temperature, prekristalizacija jekla, utrujanje materiala
1 INTRODUCTION
Carrying out complex deep-drilling operations re-
quires the manufacture of accessories and tools that are
sophisticated from the point of view of both mechanical
engineering and metallurgical alloys. The tools used for
deep drilling work under very unfavourable and variable
conditions for the materials. One of the most stressed
tools and equipment in deep-well drilling is the bit,
which is used to penetrate the rock material. In addition
to the physical stresses acting on the drill bit, drilling is
also associated with an unfavourable and changing envi-
ronment in which the cutting process takes place. There
are strong temperature fluctuations. For this reason, the
quality of the bit design and the materials used are cru-
cial, as we want to drill as long a section of the well as
possible with a single drill bit, because each manoeuvre
to lift the drill bit out of the well and insert a new one is
a time-consuming process, which increases the degree of
instability of the well casing and thus a possible collapse.
To ensure the longest-possible effective service life
of the drill bit, it is important not only to use quality steel
but also to choose the right drilling process to ensure
even wear of the tool’s cutting mechanism. Only if the
cutting mechanism of the tool wears correctly and evenly
can we drill efficiently without putting unnecessary dy-
namic stress on the other parts of the drilling rig.
In the case of drilling parameters or the drilling re-
gime, we restrict ourselves to the direct factors that can
be influenced during the drilling itself, namely: load on
the bit, number of revolutions, torque and flushing of the
drill bit area. We generally have no influence on the fac-
tor of the rock through which we are drilling, as it is de-
termined by the geomechanical and geotechnical proper-
ties of the rock material. An improper choice of drilling
parameters leads to improper wear and thus to failure of
the drill bit in terms of good drilling progress.
Analyses of the changes in the steel materials of the
drill bit after drilling provide detailed information about
the wear mechanisms of the cutting mechanism for the
bit.
Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466 459
UDK 539.375.6:546.261 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(4)459(2024)
*Corresponding author's e-mail:
jurij.sporin@ntf.uni-lj.si (Jurij [porin)

In our study, we performed an in-depth analysis of
the change in material properties of steel using an 8 ½"
(215.9 mm) diameter roller-cone drill bit with IADC
code 117 drilled with the correct parameters and a 6 1/8"
(155.57 mm) diameter roller-cone drill bit with IADC
code 136 drilled with inadequate drilling parameters.
The chosen drilling regime, taking into account the
correct selection of the roller-cone drill bit for drilling
through the rock material predicted by the geological
analysis, represents the selection and adjustment of at
least the following parameters:
• Weight on bit (WOB) [N/m]
• Number of revolutions of the bit [min
–1
]
• Flushing fluid (mud) used
In very simplified terms, propulsion by drilling into
the material can be represented by an axial load and a ro-
tation or moment that is transmitted by the cutting tool to
the material, in this case the rock, which is crushed by
the forces acting in the axial and transverse directions
(Figure 1).
When using the roller-cone drill bit, the mechanism
of penetration into the material is more complex. The ax-
ial force is directed downwards in the vertical direction
and the rotation of the rod is directed transversely to the
vertical axis. This ensures the force on the tool and its
rotation. The roller-cone drill bit, on the other hand, has
a cutting mechanism in the form of rollers that are
mounted slightly transverse to the vertical axis. Under
the influence of the vertical rotation of the tool, the roll-
ers rotate around their axis and ensure that the teeth of
the roller, which form the cutting part of the bit, alter-
nately come into contact with the material – the rock.
The theory of the tooth effect of a roller-cone drill bit
was established by Cheatham,
1
who in his study deter-
mined the load on the tooth (wedge) required to pene-
trate the rock. In his model, he assumed that the rock un-
der the tooth is isotropic and homogeneous. This was
within the limits of the Mohr-Coulomb failure criterion
theory. In 1965, Paul and Sikarskie
2
explained the effect
of a roller-cone drill-bit tooth on the rock by the effect of
a wedge-shaped wedge in the material, which then fails
under load according to the Mohr-Coulomb failure crite-
rion.
Dutta
3
built on the theory of Paul and Sikarskie
2
by
explaining the mechanism of material progression as the
release of energy through crushing and fracturing. It was
shown that the principle of rock fracture can be under-
stood as the breaking of a brittle material, the fractures
of which are the result of the action (load) of the cutting
tool on the rock.
The accelerated wear of the cutting mechanism – the
teeth – has the direct consequence of reducing the rate of
penetration (ROP) of the drilling operation.
Studies have shown that the load on a single tooth of
the bit acting on the rock is a function of the projected
surface area of the tooth in contact with the rock, the
depth of penetration of the tooth into the rock, and the
mechanical properties of the rock.
4
As the bit penetrates the rock, the chosen drilling re-
gime, the mechanical properties of the drill bit and the
mechanical properties of the rock lead to gradual wear of
the bit teeth. The progressive wear is caused by the inter-
action of each tooth with the rock, which breaks and
crumbles under the influence of the force, while at the
same time the temperature in the tooth material increases
due to its resistance and friction.
5–19
Flushing the bottom area of the well and the drill bit
with drilling fluid (mud) serves to effectively remove
rock fragments from the bit area and to lower the tem-
perature of the steel parts of the bit. The poisoning of the
mud, which contains rock fragments around the individ-
ual segments of the bit, influences the secondary grind-
ing of material that has not been effectively removed
from the tooth area of the bit and on the erosive wear of
the materials of the structural elements of the bit (the
rollers) that do not come into contact with the rock.
20–23
As part of our research, we analysed a series of
roller-cone drill bits. We were particularly interested in
the change in material properties in direct contact with
the rock, i.e. the changes that occur when a tooth is
pressed into the rock. In this paper we present the wear
mechanism of a roller-cone drill bit with a cutting part
consisting of teeth.
In our case, the cutting tools (teeth) investigated are
steel teeth that have been protected with tungsten carbide
in the area of the expected more intense wear.
The wear rate and wear mode of a roller-cone drill bit
for drilling in soft rock was investigated:
• When using drilling parameters recommended by the
bit manufacturer
• When using drilling parameters that deviate from the
bit manufacturer’s recommendations
In the first case, a roller-cone drill bit with the IADC
code 117 was used and in the second case a roller-cone
drill bit with the IADC code 136. Both bits are shown in
Figure 2. The IADC code is a system for classifying bits
according to their properties and their applicability in
various soil and rock materials. The IADC classification
is created by the International Association of Drilling
Contractors (IADC).
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
460 Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466
Figure 1: Advance of the tooth into the rock

The wear analysis of the tested roller-cone drill bits
was performed to verify the type and degree of wear of
the cutting mechanism of the teeth and rollers. Using
metallurgical methods of material investigation, a de-
tailed analysis of the damage to the materials was carried
out to determine the causes of the damage and to under-
stand the mechanism of damage expression to potentially
gain new insights into modifications, e.g., to the drilling
regime, tooth arrangement and geometry, flushing, etc.,
that could either increase the ROP or extend the effective
life of the bit.
2 MATERIALS AND METHODS
After the drilling intervals, a comprehensive analysis
of the steel materials of the bit was carried out with the
following tests:
• the chemical composition of the tooth steel by flame
spectroscopy with ICP–OES Agillent 720 and
Agillent 720
• the composition of the carbide steel of the teeth of the
bit using the XRF method (X-ray fluorescence spec-
trometry) with Thermo NITON XL3t
• examination of the cross-section of the teeth by elec-
tron microscopy using the EDS/SEM method (en-
ergy-dispersive X-ray spectroscopy/scanning electron
microscopy) with Jeol JSM 5610
• determination of the deformation properties of the
roller steel and the carbide coating in a dilatometer
with Bähr DIL 801
• measurement of the Vickers hardness of the steel
(100 g load) with a Shimadzu type M microhardness
tester
The drilling parameters were monitored during the
drilling process:
• rotational speed (min
–1
)
• rate of penetration (ROP)
• amount of drilling fluid
The drilling parameters were continuously monitored
during the drilling process using a calibrated set of sen-
sors, known as a drillometer in drilling technology. This
device contains a series of sensors that measure the load
on the drilling tool, the torque, the number of revolu-
tions, etc.
The bit efficiency or bit wear rate was estimated from
the drilling progress.
Figure 3 shows a cross-section through a worn tooth
of a roller-cone drill bit with a reconstruction of the con-
dition before the wear.
3 RESULTS
The chemical composition of the tooth steel is shown
in Table 1.
Table 1: Chemical composition of tooth steel
Element Unit
Result
117 136
C w/% 0.145 0.17
Si w/% 0.25 0.26
Mn w/% 0.59 0.79
P w/% 0.007 0.011
S w/% 0.002 0.016
Cr w/% 0.11 0.60
Ni w/% 3.50 0.74
Cu w/% 0.18 0.27
Mo w/% 0.201 0.50
Fe w/% 95.958 95.385
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466 461
Figure 2: Roller-cone bit IADC 136 (left), IADC 117 (right) – side
and top view
Figure 3: Cross-section of the tooth with reconstruction of the worn
part

Based on the results of the chemical analysis shown
in Table 1, we have established that this is a tool steel
for cold work with increased toughness.
The chemical composition of the tungsten carbide
coating was also analysed using SEM. The carbide coat-
ing layer consists of an alloy with larger round Co-W
grains (19.5–79.54 w/%) and a tungsten crumb embed-
ded in the base (matrix), which consists mainly of Fe,
Ni, W. The lighter colours are the alloys with larger
Co-W grains (19.5–79.54 w/%). The lighter colours are
the materials with the higher density and represent the
tungsten particles in the Fe-Ni matrix (Figure 4).
The hard carbide coating of the tooth is clearly visi-
ble on the left-hand side of the two images (lighter col-
our tones), while the right-hand side of the two images
shows the steel base of the roller-cone drill bit tooth.
Vickers microhardness measurements were carried
out on the tooth cross-sections of the bits and on the car-
bide coating of the teeth of the two roller-cone bits.
It was found that bit 117 has a slightly higher hard-
ness than the tooth itself, with hardnesses averaging be-
tween 380 HV and 328 HV . As expected, the hardnesses
in the carbide coating are much higher, reaching up to
2200 HV .
In bit 136, it was found that the upper part of the
tooth, which is worn, has a slightly higher hardness of
594 HV than the centre part of the tooth, where the hard-
ness averages 433 HV . As expected, the hardnesses in the
carbide coating are higher and reach hardnesses of up to
1436 HV . Figure 5 shows the areas of the Vickers hard-
ness test on the cross-section of the tooth.
Drill bit 117 was used to drill 610.7 m in sandstone
with carbonate binder and a compressive strength of 30
MPa. The 117 drill bit was loaded during drilling in ac-
cordance with the manufacturer’s recommendations. The
drilling regime was as follows:
• RPM: 55 min
–1
• WOB: 40 kN
The rate of penetration (ROP) was 4.68 m/h.
Drill 136 drilled 87.89 m in carbonate mud with
sandstone and limestone pools with compressive
strengths between 25 MPa and 75 MPa. Bit 136 was not
loaded during the drilling operation as per the manufac-
turer’s recommendations due to the variable conditions.
The drilling regime was as follows:
• RPM: 40 min
–1
• WOB: 35 kN
The rate of penetration (ROP) was 0.4 m/h.
When we noticed that the ROP had dropped drasti-
cally, even though we were drilling in the same rock ma-
terial as the effective ROP, we removed the bit from the
borehole and examined it for visual damage.
Examination of bit 117, which had been drilled over
a longer interval and loaded according to the manufac-
turer’s instructions, showed that damage in the form of
wear of the bit-tooth material had occurred wherever the
bit was not protected by a carbide coating. The material
wear is due to the erosive effect of the drill cuttings,
which contain a high proportion of silicate particles. The
damage is reflected in the reduction in the dimensions of
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
462 Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466
Figure 4: Carbide coating. The layer that protects the tooth surface from erosion (a) bit 136, b) bit 117)
Figure 5: Vickers hardness testing areas

the tooth body and the formation of erosion channels on
the top of the tooth (Figure 6).
After extracting and cleaning bit 136, damage or
wear was mainly evident in the wear of the top of the
teeth, which was evident on all the teeth of the bit. Some
teeth were chipped, and some were even broken (Fig-
ure 7).
4 DISCUSSION
The mechanisms that act on the drilling tool during
drilling are complex and can be roughly divided into the
following loads and stresses:
• Stresses caused by the creation of the necessary con-
ditions for rock crushing
• Stresses that act on the drilling tool due to the condi-
tions in the well
The loads caused by creating the necessary condi-
tions for rock crushing include:
• Axial load to generate the force under the influence
of which the cutting mechanism penetrates the rock
• Transverse forces caused by the rotation of the drill-
ing tool
The result of these loads is the generation of stresses
in the rock and in the cutting tool itself. The friction of
the teeth against the rock increases the temperatures and
mechanical fatigue of the materials.
Stresses acting on the drilling tool due to the condi-
tions in the well include the following:
• Abrasive flow of rock particles in the drilling fluid
(mud) created during rock crushing when the cutting
mechanism of the bit is pressed and rotated
• Rapid cooling of the highly heated cutting compo-
nents of the roller-cone drill bit, which have heated
up due to the rotation and turning of the roller
The result of the above mechanisms is the formation
of mechanical material erosion due to the erosive effect
of rock particles in the drilling fluid (mud) and the rapid
heating and cooling of individual surfaces of the teeth
that have been in contact with the rock.
The monitoring of the drilling performance and the
drilling regime shows that the roller-cone drill bit 117
was loaded in accordance with the recommendations,
which stipulate a bit load in the range of 40 kN to
200 kN. The load on the bit during drilling did not ex-
ceed 50 kN.
The failure of the carbide coating on the top of the
teeth of the roller-cone drill bit 117 is due to the differ-
ence in strength and thermal expansion properties be-
tween the carbide material that forms the lining (protec-
tion) of the tooth body and the steel material of the tooth
body. The carbide coating material is welded to the steel
base of the teeth of the bit. During the drilling process,
the teeth of the bit that come into contact with the rock to
be crushed are strongly heated and then cooled by the
flushing medium that flows around the teeth as they ro-
tate around the roller axis. The different expansion or
elasticity properties of the material, as actually observed
during the tests in the low-temperature dilatometer, lead
to the formation and progression of cracks between the
two materials.
The latter led to a failure of the carbide coating. The
failure of the carbide coating led to a new open surface
of the steel tooth material, over which the flushing me-
dium, which contained a large amount of siliceous abra-
sive components, flowed. The abrasive components
eroded the newly opened surfaces at the tooth tips and
caused erosion channels and thus the loss of the steel
tooth material.
Although the carbide coating on the tooth tips and
edges is more resistant to erosion, it is brittle and gradu-
ally disintegrates under the influence of the load on the
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466 463
Figure 6: Overview of the characteristic wear of the teeth of a
roller-cone drill bit 117
Figure 7: Overview of the characteristic wear of the teeth of a
roller-cone drill bit 136

bit and the associated pressure and shear in contact with
the rock, which leads to heating and cooling of the mate-
rials during drilling.
We were unable to determine with certainty the tem-
perature at the tip of the tooth, which just penetrates the
rock and shatters it. However, our experiments have
shown the following:
• the microhardness experiments have shown that the
hardness of the material is constant over the entire
cross-section of the tooth, i.e., there are no
microstructural changes in the tooth steel
• the deformation properties of the steel and the car-
bide coating are characterised at temperatures above
100 °C
• during rapid heating and cooling, microcracks form
at the interface between the tooth steel and the car-
bide coating
• drilling fluid, which contains micron-sized abrasive
rock particles, penetrates the resulting cracks and
spreads them through erosion
• in the area where the carbide coating does not serve
as erosion protection, the drilling fluid containing
crushed rock particles eroded the tooth steel, creating
erosion channels
Figure 8a shows microcracks at the interface be-
tween the carbide coating and the steel base, while Fig-
ure 8b shows an example of a micro-erosion channel in
the steel of a tooth.
When monitoring the execution and the drilling re-
gime, it can be determined that the roller-cone drill bit
136 was not loaded in accordance with the recommenda-
tions, which stipulate a bit load in the range of 15 kN to
27 kN. The load on the bit during drilling was 30–40 kN.
This is due to the very different geological composition
of the area. It can therefore be concluded that the bit was
overloaded and the number of revolutions was too low.
As a result, the drilling progress decreased after 87.89 m
of the drilled interval.
Examination of the macroscopic image showed that
the tooth tips were exposed to high temperatures and
loads.
Figure 9 shows the tooth tip that was in contact with
the rock. The colour of the steel changes from top to bot-
tom and ranges from shades of blue at the point of con-
tact between the rock and steel to shades of brown, the
brightness of which varies from top to bottom.
In the sequence of exploitation, i.e., during drilling,
large thermomechanical alternating and impact stresses
occurred, which caused material hardening at the sur-
face, where the stresses were greatest, due to uneven and
random sudden cooling with drilling fluid. The
microstructure of the material in the alternating zone is
martensite.
To determine the area of temperature influence, the
cross-section of the tooth was examined using a scanning
electron microscope (SEM). The working surface of the
tooth, which was in contact with the rock, is shown in
Figure 10.
The change is noticeable on two levels:
• change up to a thickness of 36 μm
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
464 Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466
Figure 9: Top of the tooth of the roller-cone drill bit 136
Figure 8: a) Microcracks at the interface between the carbide coating and the tooth steel, b) erosion channel

• still noticeable change from 32 to 92 μm
The change in the crystal structure of the steel of the
tooth is due to:
• relatively high temperatures and mechanical stresses
that occurred during drilling, when the tip of the
tooth was in contact with the rock
• the strong cooling of the hot tooth during rotation
around the roller axis and the pouring of cold flush-
ing fluid
• overloading of the teeth during rock crushing due to
excessive weight on the bit and excessive dwell time
of the tooth in contact with the rock due to insuffi-
cient rotational speed
The consequence of such a drilling regime is an in-
crease in the temperature of the material at the tip of the
teeth, which, in combination with the load, leads to a
hardening of the material at the edge of the teeth. As a
result, the hardness of the steel at this point increased.
The measured Vickers hardness was on average approxi-
mately 160 HV higher than the hardness of the steel ma-
terial in the centre of the tooth.
Due to the influence of the intensive cooling with the
drilling fluid, the process of microstructural changes in
the steel only took place up to a depth of 92  m into the
tooth body. Such changes were only observed on the up-
per surface of the teeth, while on the flanks of the teeth,
which were not in contact with the rock, these changes
were not observed. The structure of the material corre-
sponds to the structure of martensite.
Due to the influence of intense temperature fluctua-
tions, there were also differences between the expansion
coefficients of the steel of the tooth and the carbide coat-
ing. For this reason, when these two materials came into
contact due to temperature fluctuations, there was an in-
crease in internal stresses, which led to the formation and
propagation of cracks and consequently to the detach-
ment of the carbide coating from the steel base of the
teeth.
5 CONCLUSION
It can be concluded from the above that the correct
selection of a roller-cone drill bit for an individual rock
type with the correct and recommended drilling regime
(load on the bit, RPM, drilling fluid, etc.) has a major in-
fluence on the wear mechanism of the roller-cone drill
bit and thus on the extension or shortening of the effec-
tive service life of the bit.
Development in terms of material improvement can
be in the direction of developing gradient materials that
make up the more stressed parts of the drill bit.
An ideal roller-cone drill bit should have a tough
tooth core and a very hard and abrasion-resistant surface.
The bond between these two materials must be continu-
ous and coherent so that there are no significant differ-
ences between the material properties. Such a tooth con-
struction could be achieved by welding different
materials from the same family in layers (e.g., different
composition of the carbide coating), which would differ
in toughness and abrasion resistance depending on the
distance from the tooth centre.
J. [PORIN
1*
, @. VUKELI]: WEAR MECHANISM FOR DEEP-WELLS DRILLING TOOLS
Materiali in tehnologije / Materials and technology 58 (2024) 4, 459–466 465
Figure 10: Temperature-influenced area of the tooth of the roller-cone drill bit 136

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