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Effect of heat treatment on the grain size, microhardness and corrosion

behavior of the cold-working tool steels AISI D2 and AISI O1

Article in Materials and Technologies · December 2020

DOI: 10.17222/mit.2020.035

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UDK 67.017:621.791.725:669.715 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(6)785(2020)

S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...
785–790

EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE,

MICROHARDNESS AND CORROSION BEHAVIOR OF THE
COLD-WORKING TOOL STEELS AISI D2 AND AISI O1
VPLIV TOPLOTNE OBDELAVE NA VELIKOST KRISTALNIH ZRN,
MIKROTRDOTO IN KOROZIJSKE LASTNOSTI DVEH VRST
(AISI D2 IN AISI O1) HLADNO DEFORMIRANIH ORODNIH JEKEL

Safwan Al-Qawabah1*, Ahmad Mostafa2, Aiman Al-Rawajfeh3,

Ubeidulla Al-Qawabeha1
1Mechanical Engineering Department, Al-Zaytoonah University of Jordan, Amman, Jordan
2Mechanical Engineering Department, Tafila Technical University, Tafila 66110, Jordan
3Chemical Engineering Department, Tafila Technical University, Tafila 66110, Jordan

Prejem rokopisa – received: 2020-03-23; sprejem za objavo – accepted for publication: 2020-07-15

doi:10.17222/mit.2020.035

The current work focuses on the effect of heat treatment on the grain size, microhardness and corrosion behavior of AISI D2 and
O1 tools steels. Samples of the investigated steels were subjected to different heat treatment (quenching and tempering) regimes.
The hardening temperatures for AISI D2 steel were in the range 850–1000 °C with 50 °C step and in the range 780–870 °C with
30 °C step for AISI O1 steel. The tempering temperatures were fixed for AISI D2 and O1 specimens at 550 °C and 450 °C, re-
spectively, to investigate the influence of the hardening temperature only. The results show that the grain size of heat-treated
steels decreased by increasing the hardening temperature and thus the microhardness number increased due to the dense
grain-boundary areas in the fine structures. The corrosion behaviors of the steel specimens were assessed in 0.1-M HCl solution
using a potentiostatic polarization technique. The immersed AISI D2 specimens showed better corrosion resistance than that of
AISI O1 due to the presence of high alloying elements, which may help in forming a protective layer against corrosion. The cor-
rosion rates of the coarse-grained structures were less than that of the fine-grained structures, because the finer the grains, the
greater the anodic areas, which leads to higher corrosion rates.
Keywords: microhardness, heat treatment, grain size, corrosion resistance

V prispevku so se avtorji osredoto~ili na dolo~itev vpliva toplotne obdelave dveh vrst orodnih jekel (AISI D2 in O1) na njuno
velikost kristalnih zrn, mikro trdoto in odpornost proti koroziji. Vzorce jekel so toplotno obdelali pri razli~nih re`imih kaljenja
in popu{~anja. Za jeklo AISI D2 so za temperaturno obmo~je austenitizacije izbrali temperature med 850 in1000 °C v korakih
po 50 °C, medtem ko so za jeklo AISI O1 izbrali obmo~je med 780-870 °C v korakih po 30 °C. Za ugotavljanja vpliva
utrjevanja obeh vrst jekel so izbrali dve temperaturi popu{~anja in sicer 550 °C in 450 °C. Rezultati raziskav so pokazali, da se
velikost zrn toplotno obdelanih jekel zmanj{uje z nara{~ajo~o temperaturo austenitizacije in zato nara{~a tudi mikrotrdota zaradi
ve~je gostote kristalnih mej v drobnozrnati mikrostrukturi jekel. Korozijsko obna{anje vzorcev jekel so analizirali v 0,1 M
raztopini HCl s potenciostati~no polarizacijo. V raztopino potopljeni vzorci jekla AISI D2 so imeli bolj{o odpornost proti
koroziji kot vzorci jekla AISI O1 zaradi ve~je vsebnosti zlitinskih elementov, ki pomagajo pri tvorbi za{~itne plasti. Hitrost
korozije grobo zrnatih mikrostruktur jekel je bila manj{a kot tistih s fino zrnato mikrostrukturo, ker imajo le te ve~ja anodna
podro~ja, kar vodi do vi{jih korozijskih hitrosti.
Klju~ne besede: mikrotrdota, toplotna obdelava, velikost zrn, odpornost proti koroziji

1 INTRODUCTION AISI D2 and O1 steels are designated as cold-work tool

steels and are used in making tools and dies for blanking,
The grain size of structural tool steels significantly punching, forming and other operations requiring high
affects their mechanical properties by the well-known compressive strength and excellent wear resistance.6 De-
Hall-Petch relationship.1,2 without modification of the spite the good mechanical properties of D2 and O1
chemistry of the base alloy.3 Generally, the mechanical steels, the lifetime of parts fabricated from these steels is
performance deteriorates in large-grained structures, be- negatively affected by the increase in the severity of
cause of the dislocation motion that creates the potential working conditions and corrosive operating environ-
for extensive plastic flow.4 However, the large-grained ments.7 Such steels are generally used in different indus-
structures are accompanied by a low volume of grain tries where they come into contact with mineral acids
boundaries and are expected to be less active in corrosive such as HCl, which are used for the cleaning and pick-
environments for pure iron.5 High corrosion resistance ling of metal surfaces.8 Therefore, the need for improv-
results in the long service life of tool-steel parts. The
ing the corrosion performance has increased rapidly in
recent years, opening up a considerable number of op-
*Corresponding author's e-mail: portunities for new technologies to resolve such a prob-
safwan.q@zuj.edu.jo (Safwan Al-Qawabah) lem.9 There are some drawbacks associated with the ef-

Materiali in tehnologije / Materials and technology 54 (2020) 6, 785–790 785

S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...

Table 1: Chemical composition of D2 and O1 alloy steels (w/%)

AISI C Si Mn Cr Mo V W Fe
__ Bal.
D2 1.55 0.3 0.4 11.8 0.8 0.8
O1 0.95 __ 1.1 0.6 __ 0.1 0.6 Bal.

fects of grain size on corrosion resistance, which arise were prepared for metallographic investigation by
largely from the difficulty in isolating grain size effects mounting them in hot-setting epoxy mounts, polished us-
from other microstructural changes caused by grain size ing gradual numbers of sandpapers from 200 to 2000 grit
control processes.3 To the authors’ knowledge, very lim- size, and etched with Nital solution (3 % v/v nitric acid
ited information on the effects of grain size on the corro- in methanol) for 15 s to 30 s.
sion behavior of AISI D2 tool steel can be found in the The etching chemicals were provided by Fisher Sci-
literature, whereas no information related to AISI O1 entific Company. The microstructure of the treated speci-
tool steel can be found. For instance, Yasavol and mens was examined using a Nikon Epiphot 200 metallur-
Jafari.10 observed an improved corrosion resistance of a gical optical microscope (OM) at 200× magnification.
friction-stir-welded AISI D2 steel due to the high vol- Microhardness of the treated specimens was measured
ume fraction of low-angle grain boundaries in the using a Highwood HWDM-3 (TTS Unlimited Inc., Ja-
ultrafine-grain layers. This work aims at studying the ef- pan) Vickers micro-indentation instrument under 500 g
fect of different grain sizes, obtained by systematic hard- of load. An average of three values was taken for each
ening and tempering thermal treatment schemes, on the measurement to ensure data accuracy. The grain size
corrosion behavior of AISI D2 and O1 tool steels. The measurements were carried out according to ASTM
heat-treatment and corrosion-rate results can be of high E112-12.11 (Standard Test Methods for Determining Av-
importance for the use of D2 and O1 steels in industry erage Grain Size) using the intercept method. Interested
under corrosive environment. The controlled grain size, readers could refer to the standard document for detailed
obtained by an optimized heat treatment procedure, test information.
could offer a low-cost corrosion inhibitor for the investi- Corrosion behavior was assessed in 0.1-M HCl solu-
gated steel grades. tion (Fisher Scientific Company) using a potentiostatic
polarization device according to ASTM G31-72.12 stan-
2 EXPERIMENTAL PART dard procedure. A radiometer analytical model PGZ 100
Potentiostat/ Galvanostat with VoltaLab software was
An equivalent to AISI D2 and O1 cold-working tool used to analyze the corrosion results. A standard calomel
steel discs of 20 mm diameter, provided from ASSAB electrode was used as a reference and a platinum wire as
Steels with the chemical composition given in Table 1 the counter-electrode. The treated specimens were used
(in w/%), were subjected to several heat treatment as the working electrode. A scan rate of 1 mV/s starting
schemes. Schematic illustrations of the thermal treatment from 150 mV below to 50 mV above the testing cell in-
cycles are shown in Figure 1. The treated specimens stant potential was operated to run the experiment. The
corrosion potential (Ecorr) and corrosion current density
(Icorr) of each specimen were determined using the Tafel
plot method. All electrochemical experiments were per-
formed at 22±1 °C in 150 mL of solution.

3 RESULTS AND DISCUSSION

3.1 Effect of hardening temperature on the grain size

The effect of hardening temperature on the grain size
was studied and the results are presented in this section.
Figure 2 shows the microstructure of quenched and tem-
pered steels, taking into account the lowest and highest
temperatures for steel samples according to the informa-
tion in Figure 1. Similar microstructures for both tool
steels were reported by Roberts et al.13 It could be seen
that the grain sizes of both AISI D2 and AISI O1 steels
decreased by increasing the hardening temperature. For
instance, the grain size of AISI D2 steel was reduced
gradually from 22.7 μm to 15.8 μm with a 50 °C incre-
Figure 1: Heat-treatment schemes of: a) AISI D2 tool steel and mental increase in the hardening temperature. On the
b) AISI O1 tool steel other hand, the grain size of AISI O1 steel was reduced

786 Materiali in tehnologije / Materials and technology 54 (2020) 6, 785–790

 S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...

density in the fine-grained structure.15,16 The effect of

grain size on the material strength and hardness is known
as the grain-boundary strengthening mechanism 1 and is
defined by the Hall-Pitch 2 relationships as:
sy = s0 + (ky * d –0.5)
H = H0 + (kH * d–0.5)
where sy is the yield stress, d is the average grain diam-
eter, so, ky, Ho and kH are material constants and H is the
hardness number. According to these equations, as the
grain size increases, yield strength sy decreases, and
hardness decreases.
The microhardness could also be increased due to the
formation of the hard martensite structure. The finer
Figure 2: Optical micrographs for heat-treated and tempered steels: structure is known to have a complete martensitic trans-
a) and b) for AISI D2 steel treated at 850 and 1000°C, respectively,
c) and d) for AISI O1 steel treated at 780 °C and 870 °C, respectively
formation. Both AISI D2 and O1 tool steels contain
other alloying elements than carbon, such as manganese
(Mn), chromium (Cr), and vanadium (V) as demon-
from 56.3 μm to 32.6 μm with a 30 °C incremental tem-
strated in Table 1, which are known as precipitate-form-
perature increase from 780 to 870 °C, respectively. The
ing elements.17 Therefore, a secondary hardening effect
calculated average grain sizes for AISI D2 and O1 steels
can occur due to the segregation of the alloying elements
in the investigated temperature ranges are presented by
precipitates.18 Table 2 summarizes the grain size and
bar charts in Figure 3. The bar charts show a typical re-
microhardness of the AISI D2 and O1 tool steel as func-
lationship between the hardening temperature and the
tions of hardening temperature.
grain size, i.e., the grain size decreases by increasing the
It can be concluded from Table 2 that the microhard-
hardening temperature. This relationship is related to the
ness is directly proportional to the hardening temperature
critical temperature of the steels, which is the so-called
and inversely proportional to the grain size. In other
austenitizing temperature.
words, the highest microhardness was recorded for the
The austenitizing temperature is the critical tempera-
finer grain structure, which obtained at the highest hard-
ture necessary for the transformation in steel alloys to
ening temperature.
take place after a long enough time. The fully auste-
nitized alloy can undergo a complete transformation
upon quenching to form the uniform hard martensite 3.5 Corrosion properties
structure shown in Figures 2b and 2d. On the other The influences of grain sizes on the corrosion behav-
hand, when alloys are heated below this critical tempera- ior of both AISI D2 and O1 tool steels were investigated
ture, an incomplete transformation may occur, which re- using potentiodynamic polarization curves. The typical
sults in a non-uniform structure14 as could be seen in potentiodynamic polarization curves for the AISI D2 and
Figures 2a and 2c. For the investigated AISI D2 and O1 O1 steels, of different grain sizes, immersed in 0.1-M
steels, the austenitizing temperatures are 1000°C and HCl solution are presented in Figures 5a and 5b, respec-
820 °C, respectively tively. The corrosion rate in mm/year was calculated us-
ing Equation (1).19
3.3 Grain size vs. microhardness relationship Corrosion rate (mm/year) = 3.28 x Icorr x (M/nr) (1)
The grain size is inversely proportional to the micro- where M is the atomic weight of Fe (55.85 g), n is the
hardness number, as shown in Figure 4. The increase in number of electrons transferred in the corrosion reaction
microhardness could be due to the high grain-boundary (n = 2) and r is the density (7.78 g/cm3 for AISI D2 and

Figure 3: Effect of hardening temperature on grain sizes of: a) AISI Figure 4: Effect of grain size on the microhardness of: a) AISI D2 and
D2 and b) AISI O1 steel specimens b) AISI O1 steel specimens

Materiali in tehnologije / Materials and technology 54 (2020) 6, 785–790 787

S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...

Table 2: Relationships between grain size and microhardness as functions of hardening temperatures for AISI D2 and O1 tools steels

Heat treatment regime

Average Grain Size Average Microhard-
Specimen No. AISI Hardening Tempera- Tempering Tempera- (μm) ness (HV)
ture (°C) ture (°C)
1 850 540 22.7 475.0
2 900 540 18.9 590.0
D2
3 950 540 17.0 610.0
4 1000 540 15.8 624.6
1 780 450 56.3 340.0
2 810 450 46.0 403.0
O1
3 840 450 38.0 491.1
4 870 450 32.6 520.6

Figure 5: Potentiodynamic polarization curves of: a) AISI D2 and b) AISI O1 tool steels in 0.1-M HCl solution

7.85 g/cm3 for AISI O1). All Icorr values were obtained
by extrapolating the Tafel regions.20 The Tafel slopes for
the anodic and cathodic reactions can be obtained from
the linear regions of the polarization curve. Once these
slopes have been established, the anodic and cathodic re-
gions can be extrapolated back to the point where the an-
odic and cathodic reaction rates are equivalent. The cur-
rent density at that point is the corrosion current density
(Icorr) and the potential at which it falls is the corrosion
potential (Ecorr).21
Figures 6a and 6b show the corrosion current density
(Icorr) and corrosion potential (Ecorr) of AISI D2 and O1
steels, respectively, which were obtained by the Tafel ex-
trapolation method. The electrochemical parameters,
Ecorr, Icorr, and corrosion rate (in mm/Y) for both steels,
calculated from Figure 5, are summarized in Table 3.
The corrosion potential for AISI D2 in Figure 5a was in-
creasing with the increase in the hardening temperature.
The small variation in the corrosion rate of AISI D2
treated at 900 °C could be due to that the insignificant
difference in the grain size values with the samples
treated at 950 °C. Similarly, the corrosion potential was
increasing with temperature increase for AISI O1 steel
samples as well. In other words, the corrosion potential Figure 6: Variation of experimental corrosion potential and corrosion
of the fine-grained structures is higher than that of the current density with grain size and temperature for: a) AISI D2 and b)
coarse-grained structures. The increased corrosion poten- AISI O1 steels in 0.1-M HCl solution

788 Materiali in tehnologije / Materials and technology 54 (2020) 6, 785–790

 S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...

tial suggests that fine-grained samples are more suscepti-

ble to corrosion. It can also be seen from Figure 5 and
Figure 6 that the Icorr values increase when the grain size
decreases. The lower current density values indicate that
the corrosion rate (in mm/Y) is decreasing. This can be
attributed to the fact that making the grains finer renders
greater anodic areas than in the coarse grains and thus
leads to higher corrosion rates.22
Corrosion rates of (7.06, 6.82, 7.88, and 19.77)
mm/year were obtained for AISI D2 samples treated at
(850, 900, 950, and 1000) °C, respectively. Whereas cor-
rosion rates of (10.15, 10.50, 13.06, and 14.35) mm/year
were obtained for AISI O1 samples treated at (780, 810,
840, and 870) °C, respectively. It can be concluded that
the computed corrosion rates were increasing with an in-
crease in the hardening temperature at which the grain
size was decreasing. This increase in the corrosion rate
implies that the corrosion resistance decreases when the
Figure 7: Micrographs of oil-quenched specimens for: a) and b) AISI
grain size decreases. Grain boundaries could be anodic D2 treated at 850 °C and 1000 °C, respectively, c) and d) for AISI O1
initiation sites for pit formation. However, since fine treated at 780 °C and 870 °C, respectively
microstructures have more initiation sites, more pits
would grow critical due to the presence of compensating the corrosion effect was very obvious in the AISI O1
cathode area.23 specimens. Furthermore, the depth and size of the pits in
the fine-grained material, in Figure 7b and 7d, were
Table 3: Electrochemical parameters for the AISI D2 and O1 speci-
mens in 0.1 M HCl solution found to be larger than those in the coarse-grained sam-
ples in Figure 7a and 7c due to the presence of the com-
Ecorr Icorr Corrosion rate pensating cathode area in the dense gain-boundary areas.
D2
(mV) (mA/cm2) (mm/Y)
850 -679.8 0.60 7.06
900 -491.8 0.58 6.82 5 CONCLUSIONS
950 -429.5 0.67 7.88
1000 -290.4 1.68 19.77
The current study focused on the effect of the
Ecorr Icorr Corrosion rate heat-treatment regimes on the grain size, microhardness,
O1 (mV) (mA/cm2) (mm/Y) and corrosion behavior of AISI D2 and O1 tool steels. A
780 -784.2 0.87 10.15 typical relationship between hardening temperature and
810 -783.1 0.91 10.50 both grain size and microhardness number was obtained.
840 -411.3 1.12 13.06 The fine-grained structures were obtained at high hard-
870 -342.3 1.23 14.35 ening temperatures, because of complete martensite
transformation. Whereas at lower temperatures, an in-
It is noted from Table 3 that the AISI O1 steel is complete transformation took place and coarse-grained
more susceptible to corrosion in the HCl solution than structures were obtained. The microhardness number in-
the AISI D2 steel. The improved corrosion resistance of creased for the fine-grained structures due to the dense
AISI D2 steel could be due to the presence of chromium grain-boundary areas and the presence of a hard
(Cr), molybdenum (Mo), and vanadium (V) alloying ele- martensitic structure. The immersed AISI D2 specimens
ments, which sacrifice corrosion for the iron. The corro- in HCl showed better corrosion resistance than that of
sion of these alloying elements forms a protective layer AISI O1 due to the presence of high alloying elements,
of reaction products on the martensite surface.23 The which may help in forming a protective layer against
same effect was reported by Revie et al.24 who stated that corrosion. The coarse-grained structures also showed
the corrosion potential increases to the reference state better corrosion resistance, because of the small
when more alloying elements are present due to the reac- grain-boundary areas, which play a major role in initiat-
tion-product layer formation on the alloy’s surface dur- ing anodic sites for pit formation. Furthermore, the oxide
ing corrosion. The corroded surfaces for the AISI D2 and precipitates of other alloying elements accumulate in
O1 steels treated at different temperatures are shown in grain-boundary areas, which could improve the corrosion
Figure 7. behavior of the studied steels.
It can be concluded from the surface examination that
the corrosion has taken place in all samples. However,

Materiali in tehnologije / Materials and technology 54 (2020) 6, 785–790 789

 S. AL-QAWABAH et al: EFFECT OF HEAT TREATMENT ON THE GRAIN SIZE, MICROHARDNESS ...

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