M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
433–440
MICROSTRUCTURE EVOLUTION AND PHASE
TRANSFORMATIONS IN MICROALLOYED ARMOX 500T STEEL
DURING A DILATATION PROCESS
RAZVOJ MIKROSTRUKTURE IN FAZNE TRANSFORMACIJE
MIKROLEGIRANEGA JEKLA VRSTE ARMOX 500T MED
PROCESOM [IRJENJA
Mervat Youssef
1,2*
, Eman El-Shenawy
2
, Wael Khair-Eldeen
1
, Tadaharu Adachi
3
,
Adel Nofal
2
, Mohsen A. Hassan
1
1
Egypt-Japan University of Science and Technology (E-JUST), Alexandria, Egypt
2
Central Metallurgical Research and Development Institute (CMRDI), Cairo, Egypt
3
Toyohashi University of Technology (TUT), Aichi, Japan
Prejem rokopisa – received: 2023-02-25; sprejem za objavo – accepted for publication: 2023-07-12
doi:10.17222/mit.2023.804
With this work we successfully developed two modified Armox 500T alloys using microalloying with different amounts of nio-
bium (Nb) and boron (B) to obtain a finer grain size, which in return enhances the remnant properties. Furthermore, different
heat-treatment cycles were designed and performed using a quenching dilatometer to study the combined effect of thermal cy-
cling and microalloying with Nb and B on the microstructure and transformation temperatures including the start and finish tem-
perature of the austenite transformation (Ac1, Ac3) and the martensite start and finish temperature (Ms, Mf) of the investigated
alloys. Dilatometry results show that increasing the content of Nb from 0.07 w/% to 0.13 w/% and B from 0.0035 to 0.0046 w/%
increases the temperature range between Ac1 and Ac3 by 55 °C, indicating a broader range for changing heat-treatment temper-
atures. In addition, the Ms temperature is reduced by 13 °C due to austenite refinement caused by the microalloying of Nb and
B. The effect of the annealing temperature at a constant heating rate showed a significant impact on the austenite grain size and
hardness. Furthermore, the kinetics of phase transformations were theoretically studied using Thermo-Calc, and the numerical
predictions were confirmed experimentally with dilatometry results. Metallography investigations using a scanning electron mi-
croscope (SEM) and an optical microscope (OM) were conducted to evaluate the microstructure evolution of the developed al-
loys. Hardness tests were performed to evaluate the effect of the grain refinement of martensite lathes caused by microalloying
with Nb, B, and heat-treatment thermal cycling. It is found that the hardness of the modified Armox alloys in this research was
improved by 14 % in comparison with the conventional Armox 500T.
Keywords: Armox 500T steel, niobium microalloying, heat treatment, grain refinement
V ~lanku avtorji opisujejo raziskavo s katero so uspe{no razvili dve modificirani zlitini vrste Armox 500T. Pri tem so uporabili
postopek mikrolegiranja razli~nih vsebnosti (Nb) in bora (B). Nadalje so oblikovali in izvedli razli~ne postopke toplotne
obdelave ter z uporabo kalilnega dilatometra {tudirali kombinirani u~inek toplotnega cikla in mikrolegiranja z Nb in B na
mikrostrukturo in temperature transformacij. Tako so dolo~ili za~etek in konec temperature austenitne transformacije (Ac1,
Ac3), kot tudi za~etno in kon~no temperaturo tvorbe martenzita (Ms, Mf) preiskovanih zlitin oziroma jekel. Rezultati
dilatometrije so pokazali, da pove~anje vsebnosti Nb z 0,07 w/% na 0,13 w/% in B z 0,0035 w/% na 0,0046 w/% pove~a
temperaturno obmo~je med Ac1 in Ac3 za 55 °C, kar pomeni {ir{e obmo~je temperatur toplotne obdelave. Dodatno se je
temperatura Ms zmanj{ala za 13 °C zaradi mikrolegiranja z Nb in B ter mo`nega udrobljenja austenita. Mo~no je na velikost
austenitnih zrn in trdoto vplivala temperatura popu{~anja pri konstantni hitrosti segrevanja. Nadalje so avtorji kinetiko faznih
transformacij teoreti~no {tudirali s pomo~jo programskega orodja Thermo-Calc in numeri~ne napovedi eksperimentalno potrdili
z rezultati dilatometrijskih preizkusov. Metalografske raziskave in ovrednotenje mikrostrukture razvitih zlitin so avtorji izvedli s
pomo~jo svetlobnega (LM) in vrsti~nega elektronskega mikroskopa (SEM). Vpliv udrobljenja martenzitnih lamel zaradi
mikrolegiranja z Nb in B in toplotne obdelave so dolo~ili z meritvami trdote. Ugotovili so, da se je trdota modificiranih zlitin
vrste Armox izbolj{ala (povi{ala) za 14 % v primerjavi s konvencionalnim Armox 500T.
Klju~ne besede: jeklo vrste Armox 500T, mikrolegiranje z niobijem in borom, toplotna obdelava, udrobljenje kristalnih zrn.
1 INTRODUCTION
Armox 500T is high-strength armour steel whose me-
chanical properties can be controlled with chemical com-
position, heat treatment and deformation processes. It is
used for protection as a shielding material in automotive
vehicles and buildings. Moreover, this martensitic
ARMOX steel is characterized by high performance and
remarkable properties such as high tensile stress ranging
from 1450 to 1750 MPa and nominal hardness of
500 HBW coupled with exceptional toughness values of
32 J at –40 °C.
1–3
One of the fundamental characteristics influencing
the steel’s mechanical properties and increasing its
hardenability during heat treatment is the austenite grain
size. The high strength and toughness are hard to be ob-
tained together except in grain refinement.
4–8
The austen-
ite refinement of steels can be achieved through the for-
mation of carbides and nitride precipitates that act as
obstacles inhibiting the grain growth.
6
It has been re-
Materiali in tehnologije / Materials and technology 57 (2023) 5, 433–440 433
UDK 539.4.016:621.785:669.1.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)433(2023)
*Corresponding author's e-mail:
mervat.youssef@ejust.edu.eg (Mervat Youssef)

ported that percentages of microalloying with B and Nb
should not exceed 0.15 % due to solubility and formation
of secondary phases.
6,7
Gao et al.
8
found that Nb causes a
grain refinement with subsequent heat treatments; fine
spherical precipitates of niobium carbides are formed,
which have a strengthening effect that causes an en-
hancement in the mechanical properties such as hard-
ness, strength and impact toughness. These properties
mainly exist in steels containing tempered martensite.
9
However, dynamic loading failure is due to retained aus-
tenite and coarse martensite, which decrease the mechan-
ical properties.
7,9
Therefore, finer martensite with high
hardness is desired, which can be achieved by Nb
microalloying with heat treatment to retard the softening
effect at higher tempering temperatures.
Carola et al.
10
studied the influence of the prior aus-
tenite grain size (PAGS) on the formed martensite. More-
over, they studied the effect of the austenitization condi-
tions on low-carbon steel, determining the PAGS from
which the martensite would be formed. It was found that
grain refinement lowers the start temperature of
martensite (Ms) and increases the rate of the initial
stages of the transformation as a result of a more promi-
nent grain boundary area that increases the density of nu-
cleation sites.
In the literature, there are no investigations on the in-
fluence of microalloying of Nb accompanied by thermal
cycling on the microstructure evolution of Armox 500T
alloys. Therefore, this work aims at studying the com-
bined effect of Nb and B microalloying and heat treat-
ment on the grain refinement and hardness of Armox
500T. Additionally, the kinetics of phase transformation
is studied theoretically using Thermo-Calc whose out-
come is confirmed by dilatometry results.
2 EXPERIMENTAL PART
2.1 Melting and casting
Two different steel compositions were used in this
current investigation. The melting process was per-
formed in a medium-frequency 300-kg induction furnace
with a quartzite lining. The main raw and alloy additions,
used to achieve the final chemical compositions shown
in Table 1, were high-purity steel scrap, carburizer, Fe-Si
(85.0 w/% Si), Fe-Cr (70.0 w/% Cr), Fe-Mo (70.0 w/%
Mo), Fe-B (60.0 w/% B), Fe-Nb (70.0 w/% Nb), pure Mn
and Pure Ni. It had been reported that the alloying of
molten steel with FeNb requires specific control parame-
ters to avoid segregation inside the molten steel as well
as increase its recovery in the solidified castings.
11
In ad-
dition, FeNb and FeB exhibit high affinity to the carbon
element at high temperatures and can easily form strong
Nb and B carbides, which can segregate at the grain
boundaries. Accordingly, FeNb and FeB were charged
into the final step of the melting process of steel in the
form of fine particles (below 50 mm). After completed
melting and alloying additions, the liquid steel tempera-
ture was measured using a digital handheld pyrometer
and then poured at 1650 °C. The molten steel was
poured into green sand moulds, and the cast shape was a
keel block with dimensions of (200 × 150 × 25) mm. The
green sand moulds were then left for free cooling to
room temperature. Table 1 shows the chemical composi-
tions of the modified Armox alloys in this research. The
apparatus used for the analysis of the chemical composi-
tion was an optical emission spectrometer (OES), Oxford
Foundry-Master Pro (Hitachi Ltd, Tokyo Japan).
M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
434 Materiali in tehnologije / Materials and technology 57 (2023) 5, 433–440
Table 1: Chemical compositions in w/% of the modified Armox alloys
Alloy No. C Si Mn Cr Mo Ni Nb B P S Fe
Armox-1 0.25 0.22 0.87 1.22 0.7 1.69 0.07 0.0035 0.013 0.008 Bal.
Armox-2 0.28 0.18 0.70 1.21 0.64 1.64 0.13 0.0046 0.013 0.008 Bal.
Figure 1: Hot forging and heat treatment process conducted on the designed Armox alloys: (a) hot forging process, (b) austenite-refinement heat
treatment cycles

2.2 Thermodynamic calculations
Thermodynamic calculations were conducted to sim-
ulate and determine the critical transformation tempera-
tures as well as predict the phase compositions under
equilibrium that help design the heat treatment and forg-
ing processes. These thermodynamic calculations were
conducted using the ThermoCalc (TC) software with the
TCFE11 database (Thermo-Calc Software AB, Solna,
Sweden).
2.3 Hot forging process
The as-cast samples were annealed in a heat-resis-
tance furnace to 950 °C and held at this temperature for
60 mins. The annealed samples were then multi-pass
freehand hot forged to round bars with a diameter of
25 mm. The free hot forging process was performed us-
ing a 250-kg capacity hammer machine. The forged bars
were then water-cooled to room temperature. A sche-
matic drawing of the hot forging process is shown in
Figure 1a.
M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 433–440 435
Figure 2: Equilibrium phase fraction as a function of temperature showing the critical temperatures of: a) Armox 500T-R, b) Armox 1-0.07Nb,
c) Armox 2-0.1 Nb, d) equilibrium phase diagram showing Nb solubility in austenite for Armox 500T
Phase definitions: FCC_A1 (Fe, Si, Mn, P, S, Cr, Ni, Mo, Nb)
1
(V A, C, B)
1
; BCC_A2 (Fe, Si, Mn, P, S, Cr, Ni, Mo, Nb)
1
(V A, C, B)1;
CEMENTITE_D011 (Fe, Si, Mn, Cr, Ni, Mo, Nb)
3
(C, B)1; M6C_E93 (Fe, Ni)
2
(Mo, Nb)
2
(Fe, Si, Cr, Ni, Mo)
2
(C)1; KSI-CARBIDE (Fe, Cr,
Mo)
3
(C)1; M23C6_D84 (Fe, Mn, Cr, Ni)
20
(Fe, Mn, Cr, Ni, Mo)
3
(C, B)6; M7C3_D101 (Fe, Si, Mn, Cr, Ni, Mo, Nb)
7
(C, B)3; MC_SHP
(Mo)
1
(C)1, M3C2_D510 (Cr, Mo)
3
(C)
2

2.4 Heat treatment cycles
Several heat treatment strategies have been applied to
the investigated alloys to confirm the decisive role of the
austenite grain size on the final martensite morphology.
The first step of these heat treatment cycles started with
heating the samples above the temperature of Ac
3
at two
austenitizing temperatures, 1000 °C and 1100 °C, and
then held for 240 s. The main objective of selecting sev-
eral austenitization temperatures is to vary the prior aus-
tenite grain size. The second heat treatment step included
three consecutive thermal and rapid quenching processes
(see Figure 1b). It was reported that applying several
rapid heating and quenching processes increases the ten-
dency of austenite nucleation and ultimately results in a
significant refinement in the final microstructure.
10,12
All
heat treatment processes were carried out using a
quenching dilatometer, LINSEIS L78/RITA (Linseis
Messgeraete GmbH, Germany). Cylindrical specimens
with a 3-mm diameter and 10-mm length were machined
from the forged bars. The change in the length was re-
corded against the temperature and time during the com-
puter-controlled test cycle. The heat treatment cycles
were conducted under a vacuum of5×1 0
–2
Pa using a
high-frequency induction-heating generator, while
high-purity helium gas was used for the cooling stages.
2.5 Metallography characterizations
All the samples were prepared for a metallography
analysis including mounting, grinding and polishing. A
3-% nital solution was used for etching the samples. The
microstructure characterization was performed using a
digital inverted metallurgical microscope (Olympus,
GX71). Scanning electron microscopy was also con-
ducted on deep-etched samples using a JEOL
JSM-6010LV .
2.6 Hardness measurements
Brinell hardness tests were done using a ZwickRoell
ZHU 250 universal hardness testing machine using a
load of 100 N and a dwell time of 20 s.
3 RESULTS AND DISCUSSION
3.1 ThermoCalc calculations
Figures 2a to 2c show ThermoCalc diagrams of the
predicted phases and the effect of different Nb contents
on the critical transformation temperature of Armox
500T. Theoretically, under equilibrium, the starting and
finishing austenitization temperatures Ac1 and Ac3 for
the Armox-500T without any Nb additions are 631 °C
and 752 °C, respectively (see Figure 2a). According to
Figures 2b and 2c, increasing the Nb additions to
0.13 w/% has no thermodynamic influence on the aus-
tenite Ac1, but it causes a slight increase in the austenite
end temperatures Ac3 under equilibrium. Furthermore,
under-equilibrium phases such as M
3
C
2
,M C ,M
7
C
3
,
M
23
C
6
and KSI carbides were predicted for the investi-
gated alloys. Nevertheless, Figure 2d indicates that nio-
bium carbides (NbC) might precipitate and segregate at
high temperatures. The same conclusion was also ob-
served and reported by a previous study.
13
However, it
was reported that the precipitation of NbC can reduce
and refine the grain size of austenite during the deforma-
tion and heat treatment processes by impeding the grain
boundary movement; ultimately, this might lead to a
more acceptable martensite morphology of the investi-
gated steel alloys in this work.
14,15
3.2 Phase transformations
Dilatation curves for the heating and cooling stages
of the investigated steel samples are shown in Figure 3.
M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
436 Materiali in tehnologije / Materials and technology 57 (2023) 5, 433–440
Figure 3: Dilatation chart during heating and cooling of the modified Armox alloys

M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
Materiali in tehnologije / Materials and technology 57 (2023) 4, 433–440 437
Figure 4: Dilatometry charts for the cooling stages of the heat treatment process described in Figure 1b: a) at step I for the investigated Armox
alloys, b) at step II for Armox-1 heat treated at 1000 °C, and c) at step II for Armox-1 heat treated at 1100 °C.
Figure 5: Optical and scanning electron micrographs of the investigated Armox alloys: (a, a’) Armox-1 after forging, (b, b’) Armox-1 after the
heat treatment cycles at 1000 °C, (c, c’) Armox-1 after the heat treatment cycles at 1100 °C, (d, d’) Armox-2 after forging, (e, e’) Armox-2 after
the heat treatment cycles at 1000 °C, (f, f’) Armox-2 after the heat treatment cycles at 1100 °C

The investigated samples were heated to 950 °C at
0.2 K/s, held at that temperature for 600 s and then rap-
idly cooled at 50 K/s to room temperature. The critical
transformation temperatures, including the start and fin-
ish temperature of the austenite transformation (Ac
1
,
Ac
3
) and the martensite start and finish temperature (Ms,
Mf), were calculated. It was found that by increasing the
Nb and B content to 0.13 and 0.0048 w/%, respectively,
for sample Armox-2, the austenite finish temperature Ac
3
increased by 50 °C, which resulted in widening the range
between Ac
1
and Ac
3
by 55 °C. Additionally, the
martensite start and finish temperatures shifted down by
13 °C.
3.3 Martensite start temperature
Figure 4 displays parts of dilatation curves for cool-
ing obtained from the heat treatment cycles described in
Figure 1b for the two investigated steel samples. A mi-
nor shift observed in Figure 4a dilatation curves for the
two proposed Armox alloys during cooling at two
austenitization temperatures (1000 °C and 1100 °C),
based on the relation between the thermal expansion and
temperature can be attributed to both the differences in
the alloy compositions and the formation of carbides. It
can be noticed that the martensite transformation contin-
uously shifts to lower temperatures with step II of ther-
mal cycling I, II and III (see Figures 4b and 4c).
Celada-Casero et al.
10
and J. Hidalgo et al.
12
reported that
the thermal expansion related to the martensite transfor-
mation shifts to lower temperatures with a decreasing
austenite prior grain size. This might explain the austen-
ite refinement in both modified Armox steel samples,
even at higher annealing temperatures (see Figure 5).
3.4 Microstructure and grain-size characterization
Figure 5 depicts optical and scanning electron micro-
graphs of the investigated samples in different condi-
tions. A fully martensitic structure can be noticed in all
the samples. However, the martensite lathe sizes differ
with the increasing Nb and B content; for instance, after
hot forging, Armox-2 shows a more refined martensitic
structure than Armox-1. Moreover, after the austenite re-
finement cycles described in Figure 1b, both samples,
Armox-1 and Armox-2, showed a significantly finer
martensitic structure than after forging. Additionally, in-
creasing the annealing temperature from 1000 °C to
1100 °C resulted in a slight difference in the martensitic
morphology. Furthermore, Armox-2 showed more car-
bides and precipitates than Armox-1, as predicted in the
thermodynamic calculations from Figures 2b and 2c.
Figure 6 shows grain-size calculations. It was found
that the average grain length in hot forged Armox-1 was
6.64 μm, which decreased after heat treatment austenite
refinement cycles at 1000 and 1100°C to be 2.25 and
2.64 μm, respectively. Moreover, the increase in the Nb
and B content caused a decrease in the grain size by
nearly 50 % in hot-forged Armox-2, being 3.55 μm.
However, after the heat treatment cycles at 1000 and
1100°C, the grain size was 2.54 and 1.94 μm, respec-
tively, nearly the same as for Armox-1.
3.5 Hardness measurements
Figure 7 shows the Brinell hardness values of the in-
vestigated alloys measured at different stages of the heat
treatment cycle. It can be noticed that the highest hard-
ness values were recorded for hot forged samples, 563
and 577 HBW for Armox-1 and Armox-2, respectively.
M. YOUSSEF et al.: MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T ...
438 Materiali in tehnologije / Materials and technology 57 (2023) 5, 433–440
Figure 6: Grain-size measurements of the investigated Armox alloys: a) Armox-1 after forging, b) Armox-1 after the heat treatment cycles at
1000 °C, c) Armox-1 after the heat treatment cycles at 1100 °C, d) Armox-2 after forging, e) Armox-2 after the heat treatment cycles at 1000 °C,
f) Armox-2 after the heat treatment cycles at 1100 °C

Gradual increases in the hardness were observed until
reaching stage Q-III of the thermal cycling. Moreover,
Armox-2 exhibited higher hardness values than Armox-1
due to the carbide formation and grain refinement, result-
ing from increasing the Nb and B w/%. The hardness
values confirmed the positive impact of Nb, B and heat
treatment on the refinement of the produced martensitic
structure. These results confirm the assumption that a
more refined structure leads to increased hardness val-
ues.
4 CONCLUSIONS
This research successfully developed two Armox
steel alloys modified with two different amounts of Nb
and B. The following points summarize the main find-
ings of this research:
• Thermodynamic calculations predicted the formation
of different carbide phases and precipitates such as
M
3
C
2
, MC, M
7
C
3
,M
23
C
6
, and KSI carbides. Addi-
tionally, NbC could precipitate and segregate at high
temperatures and refine the grain size of the austenite
during the deformation and heat treatment processes.
• The dilatometry measurements revealed that increas-
ing the Nb and B content increased the austenite fin-
ish temperature Ac
3
by 50 °C, which resulted in wid-
ening the range between Ac1 and Ac
3
by 55 °C.
Additionally, the martensite start and finish tempera-
tures shifted down by 13 °C.
• Heat treatment cycles that were performed to obtain a
more refined structure, revealed that the martensite
transformation temperature constantly shifted by
5 °C to lower temperatures in step II, which included
thermal cycles I, II and III for the two investigated al-
loys.
• Optical and SEM micrographs of the Armox-1
heat-treated samples showed fine grain sizes of
2.25 μm and 2.64 μm at 1000 and 1100 °C, respec-
tively. The heat-treated Armox-2 samples achieved
2.54 μm and 1.94 μm grain sizes at 1000 °C and 1100
°C, respectively.
• The Brinell hardness values confirmed a positive im-
pact of Nb, B and heat treatment on the refinement of
the produced martensitic structure. The hardness of
the modified Armox alloys in this research was im-
proved by 14 % compared to the conventional Armox
500T.
• The results obtained with this research may have
promising applications in the protection of ATMs,
VIP vehicles, helmets and personnel bulletproof
vests.
Acknowledgment
This work was conducted in the frame of a project
funded by the Science and Technology Development
Fund (STDF), Egypt Grant No. STDF-33397.
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