L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
111–117
CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
KOROZIJSKO OBNA[ANJE @ARJENEGA JEKLA VRSTE 42CrMo4
Lovro Liveri}
1*
, Dario Iljki}
1
, Zoran Jurkovi}
1
, Nik{a ^atipovi}
2
,
Pawe³ Nuckowski
3
, Oktawian Bialas
4
1
University of Rijeka, Faculty of Engineering, Vukovarska 58, 51000 Rijeka, Croatia
2
University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture,
Ru|era Bo{kovi}a 32, 21000 Split, Croatia
3
Materials Research Laboratory, Faculty of Mechanical Engineering, Silesian University of Technology,
ul. Konarskiego 18a, Gliwice 44-100, Poland
4
Department of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian University of Technology,
ul. Konarskiego 18a, Gliwice 44-100, Poland
Prejem rokopisa – received: 2022-09-14; sprejem za objavo – accepted for publication: 2023-01-20
doi:10.17222/mit.2022.624
In this paper, the corrosion behaviour of 42CrMo4 low-alloy steel after normalizing, soft annealing, spheroidizing annealing and
full annealing is investigated. 42CrMo4 is steel for quenching and tempering, and one of the widely used and studied steels due
to its good combination of mechanical properties. Sometimes, it is used in the annealed condition. Nevertheless, the corrosion
properties of 42CrMo4 steel are poorly studied, especially in the annealed condition. The main objective of this work is to in-
crease the knowledge about the corrosion behaviour of the investigated alloy. The mechanical and microstructure properties of
the samples after different annealing processes were characterised with hardness testing, optical and scanning electron micros-
copy (SEM) and X-ray diffraction (XRD). Measurements of the open-circuit potential and potentiodynamic polarisation of the
samples after different annealing processes were carried out in a naturally aerated solution. It was found that the corrosion rate
of the soft annealed samples was higher than that of the spherical and full annealed samples. Moreover, full annealing resulted
in a significant improvement in the corrosion resistance.
Keywords: heat treatment, annealing, corrosion, 42CrMo4 steel, microstructure
V ~lanku je opisano korozijsko obna{anje malo legiranega jekla 42CrMo4 po normalizaciji, mehkem `arjenju, sferoidizacijskem
`arjenju in procesu popolne toplotne obdelave. Jeklo 42CrMo4 se po kaljenju in popu{~anju zelo veliko uporablja zaradi dobre
kombinacije mehanskih lastnosti, v~asih se uporablja tudi v `arjenem stanju. V preteklosti so korozijske lastnosti tega jekla
redko raziskovali, {e posebej v `arjenem stanju. Glavni namen raziskave je bilo izbolj{anje znanja o korozijskem obna{anju
jekla 42CrMo4. Mehanske in mikrostrukturne lastnosti vzorcev preiskovanega jekla so dolo~ili s pomo~jo merilnika trdote,
opti~nega in vrsti~nega elektronskega mikroskopa (SEM) ter z rentgensko difrakcijo (XRD). Po razli~nih postopkih `arjenja
vzorcev so s pomo~jo potenciodinami~ne polarizacije in metode dolo~itve potenciala odprtega tokokroga v naravno prezra~eni
raztopini dolo~ili njihovo odpornost proti koroziji. Na osnovi analiz so ugotovili, da je hitrost korozije mehko `arjenih vzorcev
ve~ja kot tistih, ki so bili sferoidizacijsko ali popolnoma od`arjeni. Poleg tega so ugotovili, da imajo popolnoma od`arjeni
vzorci pomembno izbolj{ano korozijsko odpornost.
Klju~ne besede: toplotna obdelava, `arjenje, korozija, jeklo 42CrMo4, mikrostruktura
1 INTRODUCTION
Low-alloy 42CrMo4 steel (AISI 4140) is a medium
carbon steel alloyed with chromium and molybdenum,
often used for dynamic loads and in impact areas.
1
In a
heat-treated condition, it offers various application possi-
bilities such as high tensile strength and toughness.
Steels with a medium carbon content between
0.30–0.55 % C and 0.60–1.65 % Mn are used when
higher mechanical properties are desired. They are usu-
ally hardened and strengthened with heat treatment or
cold working. This group of materials is widely used for
certain types of cold-formed parts that require annealing,
normalizing or quenching and tempering before use.
1–4
Steel 42CrMo4 is steel for quenching and tempering, and
one of the widely used and studied steels due to its good
combination of mechanical properties.
1
Sometimes, it is
used in the annealed condition.
Normalizing is the heat treatment performed by
austenitizing and air cooling to produce a uniform, fine
ferrite/pearlite microstructure in steel. The higher
austenitizing temperatures used in normalizing ensure
that most carbides are dissolved. Upon cooling, the aus-
tenite transforms into a coarse, non-uniform fer-
rite/pearlite microstructure. This microstructure provides
an excellent starting point for subsequent hardening heat
treatments.
5,6
Heat treatments that produce microstructures consist-
ing of spherical carbide particles, uniformly distributed
in a ferrite matrix are called spheroidizing or spheroidiz-
ing annealing heat treatments. Spheroidized microstruc-
tures are the most stable microstructures found in steels,
formed in any structure heated at sufficiently high tem-
peratures and for sufficiently long times to allow diffu-
sion-dependent nucleation and growth of spherical parti-
Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117 111
UDK 628.483:669-153 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(2)111(2023)
*Corresponding author's e-mail:
lliveric@riteh.hr (Lovro Liveri})

cles. Therefore, there are many different heat treatment
processes for producing spheroidized microstructures.
6,7
There are two stages of the microstructural change asso-
ciated with the formation of a spheroidized micro-
structure. The first stage is the formation of spherical
carbide particles from other microstructures. A common
starting point is pearlite, where cementite is present in
essentially plate-like lamellae with a very large interfa-
cial area per unit volume. The initial spherical micro-
structures consist of highly dense, very fine particles.
Therefore, the second stage of spheroidization begins
when the fine particles with small radii of the curvature
dissolve and coarse particles grow, presenting in turn an
interfacial energy-reduction mechanism. Diffusion-de-
pendent spheroidization can take many hours depending
on the alloy content, spheroidizing temperature and the
degree of coarsening required. The slowest sphero-
idization is associated with coarse pearlitic microstruc-
tures. Many other methods of spheroidization are used to
speed up the process.
6,7
Soft annealing and spheroidize annealing have essen-
tially the same process mechanism for producing sphero-
idized microstructures, but with the main difference be-
ing the cooling time. In the case of soft annealing, the
cooling in air is slow, while in the case of spheroidize
annealing, the cooling in a furnace is much slower.
1–7
Full annealing can be described as a process, in
which steels are heated to just above the A
C3
temperature
for low and medium carbon steels and then slowly
cooled in furnaces after the heating is complete. The
slow cooling causes an austenite transformation into fer-
rite and pearlite near the A
C3
and A
C1
temperatures, re-
spectively, and ensures that coarse-grained equiaxed fer-
rite and pearlite with coarse interlamellar spacing form,
resulting in a microstructure with high ductility and
moderate strength. Once the austenite has transformed
into ferrite and pearlite, the cooling rate can be increased
to reduce the processing time and thus increase produc-
tivity. Although ferrite and pearlite microstructures are
most often formed with full annealing at the above tem-
peratures, microstructures of spheroidized carbide parti-
cles can sometimes also form in ferrite.
6,7
Although the main purpose of heat treatment is to
change the mechanical properties,
8–13
heat treatment also
changes the corrosion properties of a material.
14,15
Electrochemical corrosion is a destructive attack on a
metallic material via an electrochemical reaction with
the environment that negatively affects the critical me-
chanical properties of components. Seawater is a nearly
universal fluid for evaluating the corrosion resistance of
metallic materials.
1–4,7–17
Any material that exhibits good
corrosion resistance in seawater is considered satisfac-
tory for important aqueous applications.
9–17
Corrosion is
affected by external and internal factors. External factors
are corrosive medium parameters such as corrosive me-
dium composition, pH, temperature and velocity, while
internal factors are material parameters.
8–11
Microstructural constituents are the most important
material parameters that can affect the corrosion resis-
tance of steels. A heterogeneous ferrite-pearlite micro-
structure of annealed steel forms anodic and cathodic
sites that lead to corrosion. Alloy additions of less than
5 % have little effect on corrosion by seawater.
13,14,17
In this work, the corrosion behaviour of 42CrMo4
steel subjected to different annealing processes was stud-
ied. The aim of this work was to increase the knowledge
about the corrosion resistance of 42CrMo4 steel after
normalizing (N), soft annealing (A), spheroidizing (AC)
and full annealing (fA). The main objective was to inves-
tigate which process of the previously mentioned heat
treatments exhibits the best corrosion resistance of an-
nealed 42CrMo4 steel.
For this purpose, chemical composition, microstruc-
tural characterization and electrochemical studies were
carried out. The chemical composition was determined
using a glow discharge optical emission spectrometer
(GDOES). The microstructure was studied with optical
and electron microscopy. A phase analysis was per-
formed using an X-ray diffractometer. Measurements of
the open-circuit potential and potentiodynamic polariza-
tion of the annealed samples were performed using a
computer-controlled potentiostat with three electrodes in
a 0.6 M NaCl naturally aerated solution.
2 EXPERIMENTAL PART
2.1 Materials
Investigated samples were prepared by cutting manu-
factured ø16 mm steel bars of normalized 42CrMo4 steel
into smaller pieces measuring 4 mm in length. An LECO
GDS 500 A glow discharge optical emission spectrome-
ter was used to determine the chemical composition of
steel (Table 1).
2.2 Heat treatment
The thermal processes applied in this research are
schematically compared in Figure 1. The heat treatment
parameters are shown in Table 2.
The heat treatment parameters performed on the sam-
ples are shown in Table 2. The A
C3
temperature is
790 °C, while A
C1
is 745°C. The cooling rate and temper-
atures of the processes made the difference in the
L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
112 Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117
Table 1: Chemical composition of 42CrMo4 steel (balance Fe)
Chemical composition, w/%
CS iM nPSN iC rM oC uA lT iV
0.427 0.214 0.846 0.0056 0.0173 0.0407 1.07 0.183 0.167 0.0386 0.0089 0.0106

microstructure that is later observed. Normalization con-
sisted of heating beyond the A
c3
temperature and slow
air-cooling after a similar holding time. A similar treat-
ment with heating beyond the A
c3
temperature is full an-
nealing, using a slightly lower temperature in compari-
son to normalization, and then cooling below the A
c1
temperature. The cooling consisted of very slow furnace
cooling, fallowed by air cooling from a certain point.
With that combination of cooling, the microstructure
changed, resulting in a drop in the hardness. Comparing
the soft and spheroidizing annealing process, we find
that the holding temperature of annealing was nearly the
same, but a big difference was distinguished between the
cooling processes. Soft-annealing cooling included
air-cooling of the samples to room temperature, while in
spheroidising, the cooling process took place in the fur-
nace and was way slower then air cooling. In this pro-
cess, the microstructure change reflected on the mechan-
ical and corrosion properties of the material. However,
the hardness was not the best example for comparing the
two methods.
2.3 Hardness test
The hardness after the heat treatment processes was
measured for the purpose of checking the processes. The
hardness of the specimens was measured using a
ZHU 187.5 universal hardness tester with a Rockwell B
(HRB) hardness 1.588 mm diameter steel carbide ball in-
denter. Before the measurements, the carburized layer
was removed. The hardness results in Table 2 are given
in average values of five measurements with the standard
deviation. The hardness values obtained after the anneal-
ing processes are in accordance with experimental litera-
ture data.
8
The results show that samples 1, 2 and 3 have
similar hardness values, while sample 4 shows a lower
value.
2.4 Microstructure characterization
The importance of microstructure for the properties
of metal materials has long been recognized. Therefore,
a microstructural characterization was done after the an-
nealing processes to understand the connection between
the microstructure and corrosion resistance of 42CrMo4
steel. The microstructure of annealed samples was ob-
served using an Axio Vert A1 light Zeiss microscope
with an AxioCam ERc 5s microscope module and a FEG
QUANTA 250 SEM FEI scanning electron microscope
with an EDS OXFORD detector.
Metallographic samples were prepared by grinding
and polishing them with a STRUERS LABOPOL device,
Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117 113
L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
Figure 1: Thermal processes: a) normalizing (N), b) soft annealing (A), c) spheroidizing (AC) and d) full annealing (fA)
Table 2: Heat treatment parameters
Sample
No:
Heat treatment
Parameters
Hardness
Temperature Time Cooling medium
1 Normalizing (N) 870 °C 120 min Air 90 HRB ±3 HRB
2 Soft annealing (A) 720 °C 120 min Air 90 HRB ±3 HRB
3 Spheroidizing annealing (AC) 735 °C 120 min Furnace cooling 89 HRB ±3 HRB
4 Full annealing (fA) 820 °C 120 min
From 820 °C furnace
cooling to 670°C, then air
75 HRB ±3 HRB

grinding on an MD-Piano 220 resin bonded diamond
disc, followed by using an MD-Allegro composite disc
with different diamond suspensions. The samples were
polished with an MD-Dac polishing cloth with DiaPro
Dac 3- and 1-micron diamond suspension, followed by
an MD-Chem polishing cloth with OP-U NonDry colloi-
dal silica suspension. The samples were etched using
Nital 3 % solution for 10–20 s.
X-ray diffraction was carried out on a PANalytical
X’Pert Pro MPD diffractometer, with the use of filtered
X-ray lamp radiation (Fe filter) with a cobalt anode
(K Co
 = 0.17909 nm), powered by a voltage of 40 kV
and with a filament current intensity of 30 mA. X-ray
diffraction measurements were performed in the
Bragg-Brentano geometry in an angular scope of
20–120° [2 ] with a step of 0.05° and a step count time
of 100 s. The obtained diffractograms were analysed by
means of the X’Pert HighScore Plus software (v. 3.0e)
with a dedicated Inorganic Crystal Structure Database –
ICSD (FIZ, Karlsruhe, Germany).
2.5 Electrochemical corrosion behaviour
Electrochemical investigations were performed using
a computer-controlled Princeton Applied Research
potentiostat, Parstat 2263. A three-electrode set-up was
used: the working electrode with an exposed area of
1cm
2
, a saturated calomel electrode (SCE) placed in a
Luggin capillary as the reference electrode and the
graphite counter electrode. Before the electrochemical
investigations, the samples were ground with the P320,
P600, P800, P1200 and P2400 sandpapers, then
degreased in ethanol and rinsed with deionized water. All
the measurements were conducted at room temperature
(20 ± 2 °C), in a 0.6 M NaCl naturally aerated solution,
prepared from analytical grade NaCl and deionized wa-
ter. Open circuit potential, E
OC
, measurements were per-
formed as a function of time for a period of 2 h. Po-
tentiodynamic polarization was performed at a scan rate
of 0.166 mVs
–1
, over a potential range of E
OC
± 250 mV,
starting from the most negative potential. The corrosion
rate was calculated from polarization curves, based on
the corrosion current density, i
corr
, known equivalent
weight (EW = 28.558 g) and density ( = 7.85 gcm
–3
)of
42CrMo4 steel, in accordance with the expression:
v
iE W
corr
corr
=
⋅⋅ 00033 .
 (1)
3 RESULTS AND DISCUSSION
Figure 2 shows the microstructures of 42CrMo4 steel
after different annealing processes taken with the optical
microscope with 200× magnification lens. The heteroge-
neous microstructure of the normalized 42CrMo4 steel
comprised of ferrite crystals among the colonies of
pearlite is shown in Figure 2a. The microstructures of
L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
114 Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117
Figure 2: Optical-microscope micrographs of annealed samples: a) Sample 1 (N), b) Sample 2 (A), c) Sample 3 (AC), d) Sample 4 (fA)

the samples contain distinguished pearlite and ferrite
phases. The annealing process produced uniformly dis-
tributed pearlite in the matrix of ferrite, with pearlite in
larger fractions as previously reported by Lu et al.
15
Pearlite is dark and ferrite is light. The ratio of the
pearlite/ferrite phases depends mainly on the chemical
composition (% C) and heat treatment parameters where
the cooling rate is critical.
The observed and compared microstructures are gen-
erally similar. They consist of pearlite randomly distrib-
uted in the matrix of ferrite, with differences in the grain
size. Comparing the microstructures of differently
heat-treated samples, we can see the influence of differ-
ent cooling rates on diffusion-dependent nucleation and
growth of spherical cementite particles in pearlite. In
Sample 2 (Figure 2b), we can see the beginning of the
second stage of spheroidization when the fine particles
with small radii of curvature dissolve and coarse parti-
cles grow. While the micrographs from Figure 2 were
obtained with optical microscopy, Figure 3 shows scan-
ning-electron micrographs.
SEM images also show that the samples have similar
grain and boundary structures, but differ in the ratio of
the phases. The largest grain size is found in the normal-
ized sample, Sample 1 (Figure 3a), while the smallest
grain size and the largest number of grains are found in
the spheroidized annealed sample, Sample 3 (Figure 3c).
The fully annealed sample (Figure 3d) shows the biggest
amount of precipitation in the microstructure.
To simplify the estimation of the relative amounts of
the phases present and to identify the phases, the relative
intensity from the XRD results is given, on a scale, in
Figure 4. The following phases are indexed on the fig-
ure: # – iron ( -Fe),
*
– cementite (Fe
3
C). The patterns
are given in the units of reticular distances. All the sam-
ples have the iron alpha phase ( -Fe), known as ferrite,
and cementite (Fe
3
C). As can be seen from the distribu-
tion of the phases in Figure 4, the densities of the phases
in the structure are similar. General differences can be
seen in the distribution of cementite, which is due to dif-
ferent cooling rates of the samples during heat treatment.
Potentiodynamic polarization curves presented with a
semi-logarithmic plot (Figure 5) are composed of cath-
odic and anodic branches, resulting from the electro-
chemical reactions in the system. The anodic branch of a
polarization curve describes the anodic dissolution of
iron in the NaCl solution, while the cathodic part of the
polarization curve represents the oxygen reduction reac-
tion. Compared to normalized steel (Sample 1), whose
state is solid on a large scale, annealed Samples 2 and 3
have an anodic current density that increases much faster
with the potential, indicating active dissolution, while the
cathodic current density increases much slower, being
characteristic of a diffusion-controlled process, which is
L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117 115
Figure 4: XRD patterns of samples: a) Sample 1 (N), b) Sample 2
(A), c) Sample 3 (AC), d) Sample 4 (fA)
Figure 3: SEM, LFD micrographs of annealed-sample microstruc-
tures: a) Sample 1 (N), b) Sample 2 (A), c) Sample 3 (AC), d) Sample
4 (fA)
Figure 5: Potentiodynamic polarization curves for samples in 0.6 M
NaCl solution: a) Sample 1 (N), b) Sample 2 (A), c) Sample 3 (AC),
d) Sample 4 (fA)

reversed compared to Sample 4 as it is similar to the nor-
malized sample.
14,15
It can be seen that the values of the
anodic current, i
a
, are much higher than those of the cath-
odic current, i
c
, at high overvoltage values. This indicates
that the reduction process is slower than the oxidation
process so that the cathodic reaction controls the electro-
chemical corrosion of 42CrMo4 steel.
Corrosion parameters obtained from the polarization
measurements are shown in Table 3. All the tested sam-
ples have a low corrosion current density, i
corr
, indicating
a low corrosion rate, v
corr
. When comparing Sample 1 and
annealed Sample 2, they show similar corrosion rates,
while the corrosion rates of Samples 3 and 4 are up to
twice lower. Similar observations for annealed 42CrMo4
steel was found in the literature.
1–4,9–17
The corrosion re-
sistance of the samples can be explained with the density
ratio of the pearlite phase and with the grain size of the
pearlite/ferrite structure. As seen in Figures 2 and 3, the
grain size and density ratio of the pearlite phase in the
microstructure are similar in Samples 1 and 2, and in
Samples 3 and 4.
Among the annealed samples, Sample 4 had the cor-
rosion rate lower than 0.16 mmpy, which was consis-
tently obtained for full annealing at 820 °C for 120 min,
followed by very slow furnace cooling from 820 °C to
670 °C and air cooling to room temperature.
Moreover, this sample showed a pronounced corro-
sion-potential shift in the positive direction upon heat
treatment. Overall, all the samples have low corrosion
rates, which may indicate the influence of surface
passivation due to the presence of chromium and molyb-
denum in the steel.
1,10
Another reason may be the effect
of the spheroidization process on the coarsening and
spheroidization of cementite that are more pronounced at
a slower cooling time, leading to a smaller cathodic reac-
tivity and to a reduction in the cementite-to-ferrite area.
It can be concluded that the influences of the grain size,
density ratio of the pearlite phase and ratio of the
pearlite/ferrite structure of the annealed 42CrMo4 steel
can influence the corrosion resistance, especially after a
longer cooling time after heat treatment.
5 CONCLUSIONS
The corrosion behaviour of low-alloy 42CrMo4 steel
after different annealing processes was analysed with
potentiodynamic polarization. In addition, the electro-
chemical corrosion behaviour was examined with a
hardness test, optical and electron microscopy and XRD
analyses. The following conclusions can be drawn:
• Annealed 42CrMo4 steel samples have similar hard-
ness values, characteristic for annealed steel.
• The observed morphology of the annealed samples
shows a uniform arrangement of pearlite and ferrite
phases. Soft and spheroidized annealed samples show
different stages of the spheroidization of the pearlite
phases.
• XRD results show that the structures of the annealed
samples consist of the iron alpha phase ( -Fe),
known as ferrite, and the cementite phase (Fe
3
C).
Moreover, XRD results show that the distributions
and densities of the phases in the structures are simi-
lar.
• The parameters of the annealing process influence
the corrosion behaviour of 42CrMo4 steel. The fully
annealed 42CrMo4 steel sample exhibits the best cor-
rosion resistance, while the soft annealed 42CrMo4
steel sample exhibits the lowest corrosion resistance.
• The differences between the sample results can be
explained with the effects on the coarsening and
spheroidization of cementite, which are more pro-
nounced at slower cooling, leading to a smaller cath-
odic reactivity and a reduction in the cementite-to-
ferrite area.
Acknowledgment
This work has been supported in part by University
of Rijeka under the project number uniri-tehnic-18-293.
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L. LIVERI] et al.: CORROSION BEHAVIOUR OF ANNEALED 42CrMo4 STEEL
116 Materiali in tehnologije / Materials and technology 57 (2023) 2, 111–117
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Sample No.
Ecorr
icorr
â
c
[1]
â
a
[2]
vcorr
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(mV) (μA cm
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