F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
295–300
BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
KINETIKA RASTI BORIDNE PLASTI NA JEKLU VRSTE
X90CrMoV-18
Filip @aneti}, Darko Landek, Bo`idar Matijevi}, Jurica Ja~an
University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lu~i}a 5, Zagreb, Croatia
Prejem rokopisa – received: 2023-12-05; sprejem za objavo – accepted for publication: 2024-03-14
doi:10.17222/mit.2023.1064
Boronizing is a type of thermal diffusion with the primary goal of increasing the surface hardness, wear and corrosion resis-
tance. The wear resistance of boronized parts depends on the type of borides that form on the steel surface and their thickness.
This study investigated the properties of boride layers formed on X90CrMoV-18 martensitic stainless steel, using pack
boronizing carried out in a temperature range of 850–1000 °C and in durations of (1, 3 and 5) h. Results indicate a difference in
the boride layer thickness of 3–80 μm and volume share of boride phases depending on the temperature and time of boriding.
From the boriding compound depth, values of the frequency factor and activation energy were determined using the Arrhenius
equation. With these values, the parabolic equation for predicting the growth rate of a boride layer was formulated and validated
for different times and temperatures of boriding.
Keywords: boronizing, martensitic stainless steel X90CrMoV-18, Arrhenius equation
Boriranje je vrsta toplotne obdelave oziroma difuzijskega postopka katerega primarni namen je pove~ati trdoto povr{ine in s tem
tudi pove~ati abrazijsko odpornost in korozijsko obstojnost zlitine. Odpornost proti obrabi boriranih delov je odvisna od vrste
boridov, ki nastanejo na jekleni povr{ini in njeni debelini. V tem ~lanku avtorji opisujejo raziskavo boridne plasti, nastale na
martenzitnem nerjavnem jeklu vrste X90CrMoV-18. Pri tem so uporabili 1, 3 in 5 urno paketno boriranje v temperaturnem
obmo~ju med 850 °C in 1000 °C. Rezultati raziskave so pokazali, da so nastale boridne plasti debeline od 3 μm pa do 80 μm.
Volumski dele` boridnih faz je bil odvisen od temperature in ~asa boriranja. Avtorji so s pomo~jo frekven~nih faktorjev debeline
nastalih plasti in Arrheniusove ena~be dolo~ili aktivacijsko energijo difuzijskega procesa. S temi vrednostmi so oblikovali
paraboli~no ena~bo za napoved hitrosti rasti boridne plasti, ki so jo nato tudi ovrednotili glede na eksperimentalne ~ase in tem-
perature boriranja.
Klju~ne besede: boriranje, martenzitno nerjavno jeklo vrste X90CrMoV-18, Arrhenius-ova ena~ba
1 INTRODUCTION
In general, there are two primary methods for en-
hancing the surface hardness of metals. One method is
metal surface modification, using heat, diffusion of at-
oms, or mechanical modification of a material surface.
The second method is the surface deposition of material
on a metal surface using heat, chemical or mechanical
mechanisms for the formation of a layer. Boronizing,
also known as boriding, is one of the methods of
thermo-chemical surface modification that relies on the
introduction of boron atoms into a metal surface, leading
to the creation of an interstitial solid solution and a hard
boride layer on the metal surface. This layer has the pri-
mary goal of increasing the surface hardness, wear and
corrosion resistance of different types of structural and
tool steel. Pack boronizing an entire surface with pow-
der, or boronizing only a certain segment of a surface
with paste, are the most commonly used methods. These
processes can be carried out at different temperatures
ranging from 850 °C to 1050 °C in solid, liquid or gas
active media with an annealing time of 1–8 h.
1,2
Boronizing with solid media (with powder or paste) is
carried out by covering the workpieces with a layer of
new and previously used powder in metal containers,
which are heated in electric or gas furnaces. If the seal-
ing of a metal container is done correctly, the heating can
be carried out without a protective atmosphere, although
for a successful formation of a boride layer, it is recom-
mended to use a furnace with a protective gas atmo-
sphere.
The boriding process consists of two thermal-chemi-
cal reactions: boride nucleation and boride layer growth.
The first reaction occurs between the boron-rich active
medium and the metal surface through the chemical in-
teraction between the diffused atoms and the base metal.
The rate of boride nucleation on the surface depends on
the duration and temperature of boridation. The second
reaction is controlled by boron diffusion into the surface
layer of the metal and it determines the achieved thick-
ness of the boride layer. Depending on the temperature,
time and type of steel, different boride layers form on the
steel surface. A surface layer obtained by boriding exhib-
its a distinctive saw-toothed morphology and can be
composed of either a single-phase Fe
2
Bo rad o u -
ble-phase layer consisting of an outer phase of FeB and
an inner phase of Fe
2
B. While the FeB layer has a higher
hardness of 1600–2100 HV compared to Fe
2
B
Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300 295
UDK 669.1:546.271 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(3)295(2024)
*Corresponding author's e-mail:
filip.zanetic@fsb.hr (Filip @aneti})

(1400–1600 HV), it is generally regarded as undesirable
due to its brittleness. Additionally, because FeB and
Fe
2
B borides have distinct coefficients of thermal expan-
sion, the interface between FeB and Fe
2
B often experi-
ences crack formation.
3
Along with the type of boride
layer, the thickness of the achieved layer is also very im-
portant. Understanding the growth of a boride layer is
important for predicting the part wear and scheduling
maintenance. As with other thermally activated pro-
cesses, the growth rate of a boride layer can be predicted
by applying the Arrhenius equation, in which the activa-
tion energy and the diffusion frequency factor for the
specific chemical composition of a borated steel should
be experimentally determined. For some types of steel
like C45, the boride layer growth rate and layer thickness
depending on the time and temperature of the boronizing
process are calculated and well presented in the litera-
ture.
1,4
X90CrMoV-18 is a stainless martensitic steel that can
be hardened and strengthened by heat treatment while re-
taining very good corrosion and wear resistance.
Austenitization of this steel is carried out in a tempera-
ture range of 1000–1050 °C, which partially overlaps
with the boronizing temperature range. Cooling after the
austenitization can be carried out in oil, air or inert gas.
These options for choosing heat treatment parameters
make it suitable for the simultaneous application of sur-
face boronizing and hardening of the entire volume. This
work aims to investigate the kinetics of boronization of
X90CrMoV-18 martensitic stainless steel. Powder
boronization experiments, a microstructure analysis of
the surface layer and hardness tests are conducted while
the rate of growth of the layer and the activation energy
of boron diffusion in X90CrMoV-18 steel are deter-
mined.
2 EXPERIMENTAL PART
In the presented study, X90CrMoV-18 steel was
boronized in the Durborid 3 solid agent using a chamber
furnace without an inert atmosphere at temperatures of
850–1000 °C for durations of (1, 3 and 5) h. To achieve a
uniform temperature, samples were heated from room
temperature inside the furnace. After the furnace
achieved the boronizing temperature, one hour was
added to the boronizing time to achieve the exact tem-
perature on a sample surface due to the heat isolating
properties of the boronizing agent. Then the samples
were moved to a different furnace, with a temperature of
300 °C, to achieve a perlite carbide microstructure in the
core of the samples. Due to the 18 % chromium present
in X90CrMoV-18 steel, a martensite microstructure
would have formed if the samples were cooled in air
rather than in furnace. After boronizing, all samples were
longitudinally cut in cross-section and prepared for
metallographic examinations (ground using up to
2000-grit emery paper, alumina polished and etched with
3 % Nital). After metallographic preparation, the thick-
ness of the boride layers was measured using light
microscopy. Cross-section hardness was measured using
a Vickers hardness tester. From the boriding compound
depth, the values of frequency factor (d) and activation
energy were determined using the Arrhenius equation.
From the frequency factor and activation energy, the
equation predicting the growth of the boride layer was
determined. Results indicate the differences in the boride
layer thickness and volume share of boride phases de-
pending on the temperature and time of boriding.
3 RESULTS
3.1 Boriding
The boride layer formed on X90CrMoV-18 steel is
compact, consisting of a dual layer of Fe
2
B and FeB bor-
ides due to the high chromium content which accelerates
the formation of dual boride layers compared to medium
or low alloy steel. Due to the high chromium content, the
boride layer does not exhibit saw-tooth morphology
characteristics on low alloy steel. Although the boride
layer thickens increases with an increase in the
boronizing duration and temperature, the growth rate
slows down with the increase in the duration, which cor-
responds with earlier studies.
5–7
To measure the thickness
of boride layers and carry out other experimental mea-
surements, 36 samples were boronized at temperatures of
(850, 900, 950 and 1000) °C for (1, 3 and 5) h.
3.2 Hardness measurements and light microscopy
Table 1 shows different boride layer thicknesses
(μm) achieved at different durations and temperatures of
boronizing. Standard deviation ( ) is also included to in-
dicate the degree to which the boride layer thickness var-
ies from one temperature and duration to another.
Table 1: Boride layer thickness (μm) after different boriding times and
temperatures
Temper-
ature
t=1h  1h
(μm) t=3h 3h
(μm) t=5h 5h
(μm)
850 °C 3.10 ± 1.61 7.40 ± 1.31 11.40 ± 2.04
900 °C 3.87 ± 0.165 12.34 ± 1.38 16.40 ± 1.77
950 °C 4.61 ± 0.860 24.33 ± 3.42 42.28 ± 1.78
1000 °C 11.20 ± 1.14 48.24 ± 2.17 80.50 ± 1.18
Since the formation of boron layers does not use car-
bon, regions rich in carbon are present below the boride
layer (carbon barrier). According to the metallographic
examination and difference in the nital etching of
FeB-Fe
2
B layers (color difference), accumulation of car-
bon in the diffusion zone slows down the growth of the
boride layer. Boride layers that form in3hormore ap-
pear to be dual layers consisting of FeB-Fe
2
B( Figure 1).
The pictures of the surface-layer microstructures were
captured using an Olympus GX51 light microscope with
a 200:1 magnification.
F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
296 Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300

Figure 2 shows the morphologies of the boride layers
a c h i e v e da f t e r5ho fboronizing at temperatures of
850 °C (Figure 2a) and 1000 °C (Figure 2b). The thick-
ness of the boride layer after5hat850°Cis11.40 μm.
The thickness of the boride layer after5hat1000 °C is
80.5 μm, consisting of Fe
2
B, FeB and other alloying ele-
ments and boron compounds. Following the
metallographic examination, cross-sectional microhard-
ness was assessed at every 20 μm using the Vickers
method (HV0.1), starting from the surface and extending
to the sample’s core. The boronizing hardness depth
(BHD) was calculated as the distance from a sample’s
surface to the average depth of peaks and troughs of the
Fe
2
B compound. All samples exhibit a robust outer layer
with its hardness exceeding 1300 HV0.1, while the core
remains unchanged after boronizing. The hardness of the
base material within the diffusion zone, between the
teeth and boron layer, is higher than that of the core of
samples. This phenomenon arises due to the elevated
carbon content, a result of its suppression beneath the
border layer.
The hardness distribution across boronized samples,
measured with a Tukon 2100B micro-Vickers testing in-
strument, is illustrated in Figure 3. The blue curve repre-
sents the hardness distribution of the sample boronized
for1hat1000 °C, the red curve refers to the hardness
distribution on the sample boronized for3hat1000 °C
and the grey curve refers to the sample boronized for 5 h
at 1000 °C. The thickness and morphology of the boride
layers were influenced by the composition of the sub-
strate material which contained alloying elements:
chrome (Cr), molybdenum (Mo) and vanadium (V).
These alloying elements present in X90CrMoV-18 enter
the iron boride lattice, leading to modifications in the bo-
ron diffusivity. For instance, in the case of martensitic
stainless steel, which has a high chromium (Cr) content,
the boride layer contained less chromium compared to
the substrate due to its limited solubility.
8,9
Consequently,
the low solubility of the Cr element had a detrimental
impact on the boriding process, slowing down the boride
layer growth. Similar diffusion-limiting effects were ob-
served with other elements such as nickel (Ni), alumi-
num (Al), copper (Cu), and so on.
10–12
F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300 297
Figure 1: Color difference in the FeB-Fe
2
B layer after etching in 3 %
Nital (sample boronized at 1000 °C for 3h)
Figure 3: Hardness distribution for the samples boronized at 1000 °C
for 1, 3 and 5 h
Figure 2: Microstructures of the boride layers after etching in 3 %
Nital: a) boronizing for5hat850°C,b)boronizing for5hat1000 °C

3.3 Kinetic studies
In this paper, boride layer growth kinetics is analyzed
with the classical kinetic method based on the Arrhenius
equation.
13–15
Most of diffusion process obeys the para-
bolic law described by:
d
2
=Dt (1)
where:
d: diffusion layer thickness, m
D: boride layer growth rate constant, m/s
t: diffusion time, s
The thickness of diffusion layer linearly increases
with the square root of time as follows:
dDt = (2)
Figure 4 shows a graphical representation of the vari-
ation in the boride layer depth with respect to the time
and temperature of boriding. It was observed that the
depth of the boride layer increased from 3.1 μm to
80.5 μm as the temperature increased from 850 °C to
1000 °C and the time from1hto5h.
Figure 4 confirms the diffusion nature of boronizing
described with the parabolic rule. The graph in Fig-
ure 4b reveals that the thickness of the diffusion layer
linearly increases with the square root of duration.
Growth rate constant D depends on the diffusion temper-
ature and this relationship is expressed by an Arrhenius
Equation (3):
16
DDe
Q
RT
=
−
0
(3)
where:
T: temperature, K
D
0
: frequency factor, m
2
/s
Q: activation energy, kJ/kmol
R: universal gas constant, kJ/(kmol·K)
Taking the natural logarithm of Equation (3), it fol-
lows that
ln ln DD
Q
RT
=−
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 0
1
(4)
F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
298 Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300
Figure 4: Boride layer thickness as a function of: a) time and b) square root of boronizing time
Figure 5: Growth rate constants in relation to the duration of boronizing

Using linear regression, functions for the square of
boriding depth as a function of time were calculated for
temperatures of (900, 950 and 1000) °C.
y
900
= 63.496x – 45.083 ; R
2
= 0.9978 (5)
y
950
= 441.59x – 524.49 ; R
2
= 0.9599 (6)
y
1000
= 1588.8x – 1796.6 ; R
2
= 0.9681 (7)
The growth rate constants, calculated from the gradi-
ents of the linear relationship (Figure 5), are provided in
Table 2 for each temperature.
Table 2: Growth rate constants for X90CrMoV-18 steel
Temperature (°C) Growth rate constant D, m
2
/s
900 1.763 10
–14
950 1.226 10
–14
1000 4.413 10
–13
The results (Table 2) show that the growth rate con-
stant increases with the boronizing temperature for each
temperature relationship between the natural logarithm
of growth rate constants and reciprocal values of the
boronizing temperatures for the boron diffusion for
X90CrMoV-18 steel given in Figure 6.
The plots (Figure 6) reveal linear dependence and
confirm that boronizing follows the Arrhenius equation.
The activation energy was determined from the slope of
the straight line and the frequency factor from the inter-
cept of the extrapolated straight lines and ordinate axis.
The frequency factor is 16342 m
2
/s, and the activation
energy is 402777 kJ/kmol. If considering the used equa-
tions and determined data, the expression for boronizing
X90CrMoV-18 steel in the observed temperature range
and duration is derived as follows:
dt e
RT
=⋅ ⋅
−
16342
402777
(8)
where:
d: thickness of the boronized layer, m
t: boriding duration, s
R: universal gas constant, 8.314472 kJ/kmol
T: temperature, K
3.4 Data representation
Expression 8 can be verified by comparing it to the
values measured on real samples. The results show errors
in the expression ranging from 0.26 % to 6.96 % for dif-
ferent temperatures and times of boronizing. Using the
derived mathematical model, a 3D graph showing the
boride layer depth as a function of time and temperature
of boriding can be constructed (Figure 7).
Using Equation (8), a contour diagram can be con-
structed, which is much more practical for easy and fast
calculation of the boride layer thickness in relation to the
time and temperature. These types of diagrams are
widely used in the nitriding, nitrocarburizing and carbu-
rizing processes. Figure 8 shows a contour diagram of
the boride layer thickness in relation to the time and tem-
perature of boriding.
4 DISCUSSION
Taking into consideration all the results of this study,
the following conclusions can be established: The boride
layers formed on X90CrMoV-18 steel are compact and
they consist of a dual layer of Fe
2
B and FeB boride due
to a high chromium content. For future research, the
presence of FeB-Fe
2
B and the exact shares of different
phases and elements should be confirmed using EDS or
F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300 299
Figure 8: Contour diagram of the boride layer thickness in relation to
the time and temperature of boriding
Figure 6: Natural logarithm of growth rate constant as a function of
reciprocal boronizing temperature for X90CrMoV-18 steel
Figure 7: Layer depth as a function of time and duration of boriding

XRD. The boride layer does not exhibit saw-tooth mor-
phology characteristics for low alloy steel. All the sam-
ples exhibit a robust outer layer with a hardness exceed-
ing 1000 HV0.1, while the core remains unchanged after
boronizing. The hardness of the base material within the
diffusion zone, between the teeth and boron layer, exhib-
its a higher hardness than the core of the samples. The
frequency factor is 16342 m2/s and the activation energy
is 402777 kJ/kmol. Using previous data and the
Arrhenius equation, the expression for boronizing
X90CrMoV-18 steel in the observed temperature range
and duration is derived. This expression can be verified
by comparing it to the values measured on real samples.
The results show errors in the expression of less than
7 % for different temperatures and times. The contour di-
agram derived in this study provides practical, easy and
fast calculations of the boride layer thickness in relation
to time and temperature.
5 CONCLUSIONS
Boronizing is a heat treatment that shows good po-
tential in future steel surface hardening treatment. A
better understanding of the boride-layer growth on dif-
ferent types of steel is important for optimizing the heat
treatment and predicting the thickness of boride layers.
In the presented study, it is shown that X90CrMoV-18
martensitic stainless steel can be successfully boronized.
The practical temperatures of boriding are higher com-
pared to those of low alloy steels because of the differ-
ence in the position of the austenite region of martensitic
stainless steels, ranging from 900 up to 1050 °C. For lon-
ger boriding durations, the boride layer appears to con-
sist of a Fe
2
B-FeB dual phase due to a high chromium
content. From the experimental results and the Arrhenius
equation, a mathematical model for predicting the boride
layer thickness as a function of the treatment time and
temperature was constructed. From this expression, 3D
graphs and contour diagrams were constructed for practi-
cal use in heat-treatment facilities for an easy and fast
calculation of the boride layer thickness in relation to the
time and temperature of boriding X90CrMoV-18
martensitic stainless steel.
6 REFERENCES
1
A. Milinovi}, V. Maru{i}, I. Samard`i}, Research into boride layers
growth kinetics on C45 carbon steel, Metalurgija, 55 (2016)4 ,
671–674
2
I. E. Campos-Silva, G. A. Rodríguez-Castro, Boriding to improve the
mechanical properties and corrosion resistance of steels, Thermo-
chemical Surface Engineering of Steels, (2015), 651–702,
doi:10.1533/9780857096524.5.651
3
A. Motallebzadeh, E. Dilektasli, M. Baydogan, E. Atar, H.
Cimenoglu, Evaluation of the effect of boride layer structure on the
high temperature wear behavior of borided steels, Wear, 328–329
(2015), 110–114, doi:10.1016/j.wear.2015.01.029
4
T. Polat, A. Tuna, A. Özlem, Kinetic Investigation of Boronized
34CrAlNi7 Nitriding Steel, International Journal of Engineering and
Natural Sciences, 2 (2019) 1, 17–22
5
S. T. de Mendonça Ferreira, A. L. Bacco, E. M. do Nascimento, C.
M. Lepienski, Mechanical Characterization and Micro-Wear of
FeB-Fe<sub>2</sub>B Layers on Boriding AISI D2 and AISI 4340
Steels, Materials Sciences and Applications, 12 (2021) 330–344,
doi:10.4236/msa.2021.127022
6
E. Hernández-Sánchez, Y. M. Domínguez-Galicia, C. Orozco-
Álvarez, R. Carrera-Espinoza, H. Herrera-Hernández, J. C.
Velázquez, A study on the effect of the boron potential on the me-
chanical properties of the borided layers obtained by boron diffusion
at the surface of AISI 316L Steel, Advances in Materials Science and
Engineering, (2014) 1–9, doi:10.1155/2014/249174
7
A. P. Krelling, J. C. Milan, C. E. da Costa, Tribological behaviour of
borided H13 steel with different boriding agents, Surface Engi-
neering, 31 (2015) 581–587, doi:10.1179/1743294414y.0000000423
8
R. Carrera-Espinoza, U. Figueroa-López, J. Martínez-Trinidad, I.
Campos-Silva, E. Hernández-Sánchez, A. Motallebzadeh, Tribolo-
gical behavior of borided AISI 1018 steel under linear reciprocating
sliding conditions, Wear, 362–363 (2016), 1–7, doi:10.1016/
j.wear.2016.05.003
9
I. Campos-Silva, M. Flores-Jiménez, D. Bravo-Bárcenas, H.
Balmori-Ramírez, J. Andraca-Adame, J. Martínez-Trinidad, et al.,
Evolution of boride layers during a diffusion annealing process, Sur-
face and Coatings Technology, 309 (2017), 155–163, doi:10.1016/
j.surfcoat.2016.11.054
10
B. Matijevi}, Evaluation of boride layer growth on carbon steel sur-
faces, Metal Science and Heat Treatment, 56 (2014), 269–273,
doi:10.1007/s11041-014-9744-7
11
G. Ibrahim, Y. Ýsmail, Rate of growth of boride layers on stainless
steels, Oxidation Communications, 38 (2015), 2189–2198
12
I. Campos-Silva, M. Ortiz-Domínguez, C. Tapia-Quintero, G.
Rodríguez-Castro, M. Y. Jiménez-Reyes, E. Chávez-Gutiérrez, Ki-
netics and boron diffusion in the FeB/Fe2B layers formed at the sur-
face of borided high-alloy steel, Journal of Materials Engineering
and Performance, 21 (2011), 1714–1723, doi:10.1007/s11665-011-
0088-9
13
A. Milinovi}, D. Krumes, I. Kladari}, Research of boride layers
growth kinetics on C15 carbon steel, International Journal of
Microstructure and Materials Properties, 9 (2014) 38, doi:10.1504/
ijmmp.2014.061051
14
C. Li, B. Shen, G. Li, C. Yang, Effect of boronizing temperature and
time on microstructure and abrasion wear resistance of Cr12Mn2V2
high chromium cast iron, Surface and Coatings Technology, 202
(2008), 5882–5886, doi:10.1016/j.surfcoat.2008.06.170
15
F. E. ªeºen, S. Özgen, M. K. ªeºen, Effect of process parameters on
the formation of boron-titanium and titanium-boron multi-layered
diffusion coatings on interstitial-free steel, Surface Engineering and
Applied Electrochemistry, 53 (2017), 167–178, doi:10.3103/
s1068375517020107
16
G. Sundararajan, Boronizing of 42CrMo4 Steel: Microstructure,
Hardness, and Wear Behaviour, Journal of Thermal Spray Technol-
ogy ,27( 2018) 8, 1327–1339
F. @ANETI] et al.: BORIDE LAYER GROWTH KINETICS ON X90CrMoV-18 STEEL
300 Materiali in tehnologije / Materials and technology 58 (2024) 3, 295–300