metals

Article
Mechanical Properties and Microstructure
Characterization of AISI “D2” and “O1” Cold Work
Tool Steels
Mohammed Algarni
Mechanical Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. box 344,
Rabigh 21911, Saudi Arabia; malgarni1@kau.edu.sa




Received: 25 September 2019; Accepted: 28 October 2019; Published: 30 October 2019


Abstract: This research analyzes the mechanical properties and fracture behavior of two cold work
tool steels: AISI “D2” and “O1”. Tool steels are an economical and efficient solution for manufacturers
due to their superior mechanical properties. Demand for tool steels is increasing yearly due to the
growth in transportation production around the world. Nevertheless, AISI “D2” and “O1” (locally
made) tool steels behave differently due to the varying content of their alloying elements. There is also
a lack of information regarding their mechanical properties and behavior. Therefore, this study aimed
to investigate the plasticity and ductile fracture behavior of “D2” and “O1” via several experimental
tests. The tool steels’ behavior under monotonic quasi-static tensile and compression tests was
analyzed. The results of the experimental work showed different plasticity behavior and ductile
fracture among the two tool steels. Before fracture, clear necking appeared on “O1” tool steel,
whereas no signs of necking occurred on “D2” tool steel. In addition, the fracture surface of “O1”
tool steel showed cup–cone fracture mode, and “D2” tool steel showed a flat surface fracture mode.
The dimple-like structures in scanning electron microscope (SEM) images revealed that both tool
steels had a ductile fracture mode.

Keywords: fracture; cold work tool steel; AISI D2; AISI O1; high carbon steel

1. Introduction
In the machining and forming industry, tool steels were invented to increase manufacturing
economic efficiency due to their enhanced mechanical properties, such as high strength, wear resistance,
hardness, and toughness. Metal machining and forming are essential for metal part production in
many industries. The automotive industry, for example, has experienced an increase in car production,
which has led to an increase in the demand for tool steels. The same increase in demand has also
been experienced in other industries, such as aerospace, transport, and precision industries. The vast
increase in production has resulted in substantial growth in the metal forming industry, at a rate of 3%
to the year 2019 [1]. Tool steels are categorized into six classes: cold work, hot work, shock resisting,
mold, high speed, and special-purpose tool steels [2]. The most important class of all is cold work tool
steels. This research investigated two types of cold work tool steel that have a high content of carbon:
“D2” and “O1”. These metals are used for many types of cutting and punching tools and dies and
many other applications. They have high hardness, high wear resistance, and are inexpensive [3–5].
Previous studies have tested the two metals to determine their wearing properties and resistance,
with “O1” tool steel being found to have excellent machinability, whereas “D2” had better wearing
resistance [6–8].

Metals 2019, 9, 1169; doi:10.3390/met9111169 www.mdpi.com/journal/metals

Metals 2019, 9, 1169 2 of 10

The oil-hardening “O1” cold work steel (UNS# T31501) is a low-cost metal that has high hardness
and wear resistance due to the high carbon content along with moderate levels of different elements,
such as chromium (Cr) and silicon (Si). The high content of Si alloy element increases machinability
and die life. In addition, the existence of tungsten (W) alloy element attains high abrasion resistance
and highly sharp cutting edges. Thus, this tool steel is used in surface finishing tools and woodworking
knives [3,9–11]. The high-carbon, high-chromium AISI “D2” cold work steel (UNS# T30402) is
particularly poor in terms of machinability and toughness. In the fabrication process, “D2” is highly
resistant to softening and wearing, with minimal microstructure distortion and high resistance to
cracking during metal formation and fabrication [12]. Therefore, in long-duration fabrication processes,
“D2” is preferable for manufacturers. In addition, “D2” tool steel is heavily used in piercing punches
and dies, forging operations, and trimming tools due to its high wear resistance [9,13–15]. Moreover,
it is generally known that “D2” tool steel is difficult to weld (nonweldable), and it is particularly hard
to attain a high-quality welded joint by conventional welding methods due to its high carbon content
and significant amount of carbides. A recent study [16] proposed a novel thixowelding technology for
joining “D2” steels with different joining temperatures, holding time, and postweld heat treatment
to investigate the joints’ mechanical and microstructural properties. The results demonstrated a
significant improvement in its tensile strength for heat- vs. non-heat-treated joints. Another study [17]
investigated the effect of post-tempering cryogenic treatment on the mechanical properties of “D2”
tool steel. The results showed an improvement in fracture toughness, reduction in residual stresses,
and no change in hardness and modulus values.
In the present study, the characteristics of mechanical properties are reported. Tensile strength,
compression strength, hardness, elongation at fracture, and reduction area at fracture in addition to
the plasticity and ductile fracture behavior of two tool steel metals—AISI “D2” and “O1”—under
monotonic loading conditions were investigated. Furthermore, fracture surfaces, dimensional stability,
and microstructure features were studied. Optical measurements and optical microscopic investigations
were also conducted.

2. Experimental Procedure

2.1. Sample Preparation

Two steel blocks of AISI “D2” and “O1” were purchased from an ASTM-certified local steel shop.
The specimens were cut from the two steel blocks and were subjected to hardening and tempering heat
treatment process. The tempering temperatures for “D2” and “O1” were 200 and 250 ◦ C, respectively.
The fabrication and machining of the specimens were done in two different shapes: (a) smooth round
bar (Figure 1) and (b) cylindrical shape (Figure 2). The smooth round bar was designed for tensile tests,
whereas the cylindrical specimen was designed for compression tests. The shape and dimensions
of both specimens’ geometry are shown in mm in Figures 1 and 2. Note that the smooth round bar
and cylinder specimens of similar metals were machined from one steel block to ensure similarity in
properties. The number of repetitions of each test was two; hence, a total of two tensile tests and two
compression tests for each metal were carried out. The chemical compositions of AISI “D2” and “O1”
are shown in Table 1. In addition, the critical and austenization temperatures are listed in Table 2.

Table 1. Chemical composition of “D2” and “O1” tool steels in mass percent (%).

Metal Type C Mn Si Cr Mo W V Fe
“D2” 1.52 0.34 0.31 12.05 0.76 - 0.92 Balance.
“O1” 0.94 1.2 0.32 0.52 - 0.53 0.19 Balance.
Metals 2019, 9, 1169 3 of 10

Table 2. Critical and austenization temperatures of “D2” and “O1” tool steels.

Metal Type Ac1 Ac3 Ar1 Ar3 Austenization Temperature

“D2” 788 ◦C 845 ◦C 769 ◦C 744 ◦C 1010–1024 ◦ C
Metals 2019, 9, x FOR PEER REVIEW 3 of 11
“O1” 732 ◦ C 760 ◦ C 703 ◦ C 671 ◦ C 802–816 ◦ C
Metals 2019, 9, x FOR PEER REVIEW 3 of 11

Figure 1. Tensile test specimen shape and geometry in mm.

Figure 1. Tensile test specimen shape and geometry in mm.
Figure 1. Tensile test specimen shape and geometry in mm.

Figure 2. Compression
Figure 2. Compression test
test specimen
specimen shape
shape and
and geometry
geometry in
in mm.
mm.
2.2. Experiments Figure 2. Compression test specimen shape and geometry in mm.
Table 1. Chemical composition of “D2” and “O1” tool steels in mass percent (%).
The loadTable
frame used was
1. Chemical a servohydrolic
composition of “D2” andtesting
“O1”machine
tool steelsmanufactured by MTS systems
in mass percent (%).
Metal Type C Mn Si Cr Mo W
corporations®in Eden Prairie, MN, USA with a 100 kN loading cell of tension and compression V Fe force
Metal
limit. The tests Type
were conducted
“D2” C
1.52 Mn
at Si
room 0.31
0.34 temperature Cr with
12.05 Mo a strain
0.76 W‐ rate of V0.005 mm/s.
0.92 Fe The strain
Balance.
reading was “D2”captured and1.52 recorded using an optical measurement ‐ system termed digital imaging
“O1” 0.94 0.34 1.2 0.31
0.32 12.05
0.52 0.76
‐ 0.53 0.92
0.19 Balance.
Balance.
correlation (DIC) [18]. The DIC type was VIC-2D version 5 software made by Correlated Solutions
“O1” 0.94 1.2 0.32 0.52 ‐ 0.53 0.19 Balance.
Inc®in Irmo, SC, USA. DIC requires specific preparation (painting the steel specimen) prior to testing
Table 2. Critical and austenization temperatures of “D2” and “O1” tool steels.
in order to provide sufficient contrast for the camera. The specimens were sprayed in white and
Table 2. Critical and austenization temperatures of “D2” and “O1” tool steels.
Metal
spackled in Type
black to create
Ac1 a fine contrast
Ac3 forArthe1 DIC toAr capture
3 the strain.
Austenization Temperature
Metal Type
“D2” Ac1°C
788 Ac3°C
845 Ar1°C
769 Ar3°C
744 Austenization
1010–1024Temperature
°C
3. Results and Discussion
“D2”
“O1” 788 °C
732 °C 845 °C
760 °C 769 °C
703 °C 744 °C
671 °C 1010–1024
802–816 °C °C
3.1. Tensile and Compression Tests
“O1” 732 °C 760 °C 703 °C 671 °C 802–816 °C
2.2. Experiments
The engineering stress–strain flow performance (total elongation and tensile strength) of “D2”
2.2.
andExperiments
“O1” subjected
The load frameto used
tensilewas
testsa at room temperature
servohydrolic testingaremachine
shown in Figure 3. Theby
manufactured yield
MTS strength of
systems
“D2” and “O1”
corporations® and other
in Eden
The load frame basic
Prairie,
used mechanical
was MN, properties
USA with atesting
a servohydrolic are listed
100 kNmachine in Table 3. The
loading manufactured modulus
cell of tension by andMTSof toughness
compression
systems
(tensile
force toughness),
limit. The fracture
tests were strength,
conducted fracture
at length,
room fracture
temperature strain,
with and
a gauge
strain
corporations® in Eden Prairie, MN, USA with a 100 kN loading cell of tension and compression length
rate of are
0.005 all shown
mm/s. in
The
strainlimit.
force reading
Thewas
testscaptured and recorded
were conducted at room using an opticalwith
temperature measurement system
a strain rate termed
of 0.005 mm/s.digital
The
imaging
strain correlation
reading (DIC) [18].
was captured The
and DIC type
recorded was an
using VIC-2D
opticalversion 5 software
measurement madetermed
system by Correlated
digital
Solutionscorrelation
imaging Inc® in Irmo,
(DIC)SC, USA.
[18]. TheDIC
DICrequires
type wasspecific
VIC-2Dpreparation (paintingmade
version 5 software the steel specimen)
by Correlated
prior to testing
Solutions Inc® in order
Irmo, to provide
SC, USA. sufficient contrast
DIC requires for the
specific camera. The
preparation specimens
(painting the were sprayed in
steel specimen)
whitetoand
prior spackled
testing in black
in order to create
to provide a fine contrast
sufficient contrastfor
forthe
theDIC to capture
camera. the strain.
The specimens were sprayed in
Metals 2019, 9, 1169 4 of 10

Table 4. The differences in content of the alloying elements in “D2” and “O1” tool steels changed the
behavior of the stress–strain flow. For example, the higher ductility and toughness of “D2” over “O1”
tool steels were due to the high content of molybdenum (Mo), vanadium (V), and Cr. On the other
hand, the higher yield tensile strength and ultimate tensile strength (UTS) of “O1” compared to “D2”
tool steels were due to the increased content of tungsten and manganese (Mn). The “D2” steel behavior
under monotonic loading showed particularly high hardening and substantially low softening due
to the high content of Mo and Cr. The range of hardness for “O1” and “D2” steels was 56–58 and
60–62 HRC, respectively. Note that all data reported are the mean value of many testing points for
each specimen.
Metals 2019, 9, x FOR PEER REVIEW 5 of 11

Figure 3. Tensile engineering stress–strain curves to fracture for “D2” and “O1” tool steels.
Figure 3. Tensile engineering stress–strain curves to fracture for “D2” and “O1” tool steels.
Table 3. Basic mechanical properties of "D2" and "O1" tool steels.

Specimen Modulus of Elasticity 0.2% Offset Yield Strength Yield Strength UTS
AISI “D2” 203 GPa 411 MPa 350 MPa 758 MPa
AISI “O1” 211 GPa 829 MPa 758 MPa 846 MPa

Table 4. Experimental tensile tests data summary of "D2" and "O1" tool steels.

Modulus of Fracture Displacement Gauge Fracture Area

Specimen
Toughness Strength at Fracture Length Strain Reduction
AISI “D2” 81 MPa 723 MPa 0.61 mm 30 mm 1.97% 1.3%
AISI “O1” 68 MPa 703 MPa 0.35 mm 30 mm 1.09% 19.7%

In contrast, the “O1” steel behavior under monotonic loading showed a highly narrow strain
range during hardening (before UTS) and a higher range of strain in softening (beyond UTS). The high
amount of metal softening during the tensile strength was seen during the experimental test in the
Figure
form of necking 4. Compression
before engineering
fracture. The stress–strain
compression curves flow
stress–strain for “D2” and “O1”
is shown tool steels.
in Figure 4. The cylinder
specimens were compressed to approximately −90 kN (load cell maximum capacity is ±100 kN) at a
3.2. Fracture
strain rate of of Specimens
0.005 mm/s while the strain flow was captured. The modulus of elasticity and compression
yieldThe
of “O1”
“D2”steel
tool was
steelhigher thansubjected
specimen “D2” steel.to The compression
tensile loadings isplasticity
shown inflow, shown
Figure in Figure
6. The images4,
increased as the stresses increased due to the geometry change in the cylinder specimen.
depict the sequential deformation process as a result of the tensile loading process. The testing
specimen setting was set initially as in Figure 6(1). This image can be used as a reference for
comparison. The maximum elongation is shown in Figure 6(2), where necking prior to fracture can
hardly be seen. Based on the stress–strain curve of “O1” tool steel, shown in Figure 3, this type
showed almost no softening behavior post UTS point, which explains the negligible necking
behavior during the experiment. The crack initiated and instantly propagated toward the outer
radius, similar to the “O1” tool steel specimen (Figure 6(3)). The strain measurements an instant
before fracture are shown in Figure 6(4) with the use of DIC. DIC can also predict the crack initiation
location. The crack initiation location and propagation path prediction by DIC have previously been
Metals 2019,Figure
9, 1169 3. Tensile engineering stress–strain curves to fracture for “D2” and “O1” tool steels. 5 of 10

Metals 2019, 9, x FOR PEER REVIEW 6 of 11

Similarly, the “O1” tool steel specimen subjected to tensile tests is shown in Figure 5. The
collection of images in Figure 5 shows the deformation sequence during the loading process. Figure
5(1) shows the specimen prepared for testing before any loading was applied. This figure can be
used for comparison and reference reasons. Figure 5(2) shows the specimen at its highest elongation
capacity without fracturing. This moment records the maximum necking (localized area reduction)
of the specimen. The necking occurred just before the crack initiated and instantly propagated
toward the outer radius, causing full specimen fracture (Figure 5(3)). The color contour plot in
Figure 5(4) shows the highest accumulation strain by the DIC just before the fracture occurred. The
Figure 4. Compression engineering stress–strain curves for “D2” and “O1” tool steels.
location of Figure
the highest accumulation
4. Compression strainstress–strain
engineering is at the center offor
curves the“D2”
necking area,tool
and “O1” colored
steels. in red. The
crackAnother
initiation and propagation
observation is related tothat appear behavior
the necking in Figure 5(3) tool
of both coincide
steels.with the high necking
The significant strain’s
3.2. Fracture
measurement of Specimens
before fracture (Figure 5) of “O1” steel was represented in the form of softening beyond the crack
location developed due to tensile loading in Figure 5(4). In both metals, the UTS.
location
The prediction
calculated area by DIC was
reduction at in goodwas
fracture agreement
almost with However,
20%. the experiment
“D2” results.
tool steel Finally,
showed the failure
almost no
The “D2” tool steel specimen subjected to tensile loadings is shown in Figure 6. The images
mode of
neckingthe the
prior fracture
to fracturesurface on “O1” tool steel under tensile loading showed a cup–cone-like
(Figure 6). process as a result of the tensile loading process. The testing
depict sequential deformation
fracture pattern (Figure 8).
specimen setting was set initially as in Figure 6(1). This image can be used as a reference for
comparison. The maximum elongation is shown in Figure 6(2), where necking prior to fracture can
hardly be seen. Based on the stress–strain curve of “O1” tool steel, shown in Figure 3, this type
showed almost no softening behavior post UTS point, which explains the negligible necking
behavior during the experiment. The crack initiated and instantly propagated toward the outer
radius, similar to the “O1” tool steel specimen (Figure 6(3)). The strain measurements an instant
before fracture are shown in Figure 6(4) with the use of DIC. DIC can also predict the crack initiation
location. The crack initiation location and propagation path prediction by DIC have previously been
investigated and proven in many studies. [19–22]. In the case of “O1” tool steel, Figure 6(4) shows
high strain concentrations (in red) that resulted in metal cracks and fracture (Figure 6(3)). The
fracture location prediction agreed with the experimental results. Finally, the failure mode showed a
flat fracture surface (Figure 7). It is recommended that the reader refer to [23,24] in order to
understand why the fracture surface shape differs from one metal to another.

Figure 5. “O1” specimen under tension test. (1) (1) Specimen before testing, (2) necking prior to fracture,
(3) specimen
(3) specimenpost
postfracture,
fracture,
andand (4) contour
(4) contour plot plot showing
showing the Lagrange
the Lagrange strain localization
strain localization before
before fracture
fracture
in in red (maximum
red (maximum stain)
stain) and and(minimum
purple purple (minimum
strain). strain).
Metals 2019, 9, 1169 6 of 10
Metals 2019, 9, x FOR PEER REVIEW 7 of 11

Figure 6. “D2” specimen under tension test. (1) Specimen before testing, (2) no necking prior to fracture,
Figure 6. “D2” specimen under tension test. (1) Specimen before testing, (2) no necking prior to
(3) specimen post fracture, and (4) the contour plot showing the Lagrange strain localization before
fracture, (3) specimen post fracture, and (4) the contour plot showing the Lagrange strain localization
fracture in red (maximum stain) and purple (minimum strain).
before fracture in red (maximum stain) and purple (minimum strain).
3.2. Fracture of Specimens
The “D2” tool steel specimen subjected to tensile loadings is shown in Figure 6. The images depict
the sequential deformation process as a result of the tensile loading process. The testing specimen
setting was set initially as in Figure 6(1). This image can be used as a reference for comparison.
The maximum elongation is shown in Figure 6(2), where necking prior to fracture can hardly be
seen. Based on the stress–strain curve of “O1” tool steel, shown in Figure 3, this type showed almost
no softening behavior post UTS point, which explains the negligible necking behavior during the
experiment. The crack initiated and instantly propagated toward the outer radius, similar to the “O1”
tool steel specimen (Figure 6(3)). The strain measurements an instant before fracture are shown in
Figure 6(4) with the use of DIC. DIC can also predict the crack initiation location. The crack initiation
location and propagation path prediction by DIC have previously been investigated and proven in
many studies. [19–22]. In the case of “O1” tool steel, Figure 6(4) shows high strain concentrations (in
Figure
red) that 7. A flat
resulted infracture mode surface
metal cracks for “D2”
and fracture tool steel
(Figure under
6(3)). tension
The (left)
fracture and flattened
location shapeagreed
prediction of
the cylindrical specimen (right) under compression.
with the experimental results. Finally, the failure mode showed a flat fracture surface (Figure 7). It is
recommended that the reader refer to [23,24] in order to understand why the fracture surface shape
differs from one metal to another.
Similarly, the “O1” tool steel specimen subjected to tensile tests is shown in Figure 5. The collection
of images in Figure 5 shows the deformation sequence during the loading process. Figure 5(1) shows
the specimen prepared for testing before any loading was applied. This figure can be used for
comparison and reference reasons. Figure 5(2) shows the specimen at its highest elongation capacity
without fracturing. This moment records the maximum necking (localized area reduction) of the
specimen. The necking occurred just before the crack initiated and instantly propagated toward the
outer radius, causing full specimen fracture (Figure 5(3)). The color contour plot in Figure 5(4) shows
the highest accumulation strain by the DIC just before the fracture occurred. The location of the
highest accumulation strain is at the center of the necking area, colored in red. The crack initiation and
propagation that appear in Figure 5(3) coincide with the high strain’s measurement location developed
Metals 2019, 9, 1169 7 of 10

due to tensile loading in Figure 5(4). In both metals, the crack location prediction by DIC was in good
Figure 6.6. “D2”
Figure “D2” specimenunder
undertension
tensiontest.
test. (1)Specimen
Specimenbefore
before testing,(2)
(2)no
nonecking
neckingprior
priortoto
agreement with the specimen
experiment results. Finally,(1)the failure mode testing,
of the fracture surface on “O1” tool
fracture,
fracture, (3)
(3) specimen
specimen post
post fracture,
fracture, and
and (4)
(4) the
the contour
contour plot
plot showing
showing the
the Lagrange
Lagrange strain
strain localization
localization
steel under tensile loading showed a cup–cone-like fracture pattern (Figure 8).
beforefracture
before fractureininred
red(maximum
(maximumstain)
stain)and
andpurple
purple(minimum
(minimumstrain).
strain).

Figure7.7.7.A
Figure
Figure AAflat
flatfracture
flat fracturemode
fracture modesurface
mode surfacefor
surface for“D2”
for “D2”tool
“D2” toolsteel
tool steelunder
steel undertension
under tension(left)
tension (left)and
(left) andflattened
and flattenedshape
flattened shapeof
shape ofofthe
the cylindrical
the cylindrical
cylindrical specimenspecimen
specimen (right)
(right)
(right) under
under
under compression.
compression.
compression.

Figure 8. A cup–cone mode fracture surface and clear necking for “O1” tool steel under tension (left)
and flattened shape of the cylindrical specimen (right) under compression.

3.3. Microstructure
The microstructure on the fractured surfaces of the “O1” specimens with a cup–cone shape and the
“D2” specimens with a flat shape were analyzed with different magnifications using a scanning electron
microscope (SEM) type JSM-7610FPlus Schottky Field Emission made by JOEL ltd. (Tokyo, Japan).
SEM analysis assists in determining the fracture mode for both tool steels [25]. The existence of the
parabolic dimple-like structures in the SEM images revealed that both tool steels had a ductile fracture
mode (Figure 9). However, for better analysis to assess the fracture mode and failure mechanism,
in-situ X-ray tomography can be performed [26–28].
A careful inspection of both tool steels showed some small differences. The surface fracture
morphology was rougher on the “D2” fractured surface compared to the “O1” fractured surface.
However, the average size of microvoids in the “O1” specimens was smaller compared to the features
in the “D2” specimens (Figure 10). Note that the average microvoid size increased as the hardness
decreased for both steels, as can be seen on the “O1” and “D2” fractured surfaces in Figure 11.
The SEM micrographs showed different microstructures when the two steel metals were compared.
The microvoids were deeper on “O1” and sharper on “D2”. The fracture surfaces of “O1” had smaller
morphology was rougher on the “D2” fractured surface compared to the “O1” fractured surface.
However, the average size of microvoids in the “O1” specimens was smaller compared to the
features in the “D2” specimens (Figure 10). Note that the average microvoid size increased as the
hardness decreased for both steels, as can be seen on the “O1” and “D2” fractured surfaces in Figure
11. The SEM micrographs showed different microstructures when the two steel metals were
Metals 2019, 9, 1169 8 of 10
compared. The microvoids were deeper on “O1” and sharper on “D2”. The fracture surfaces of “O1”
had smaller dimples with fewer cleavage planes compared to “D2” (Figure 11). This observation
reasonably
dimples with explains
fewerthe higher elongation
cleavage in “D2” to
planes compared tool“D2”
steel (Figure
specimens.
11). This observation reasonably
explains the higher elongation in “D2” tool steel specimens.

Figure 9. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×1500
magnification.
Figure 9. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×1500 magnification.
Figure 9. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×1500
magnification.

Metals 2019, 9, x FOR PEER REVIEW 9 of 11

Figure 10. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×3500
magnification.
Figure 10. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×3500 magnification.

Figure 11. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×7500 magnification.
Figure 11. SEM morphologies of fracture surfaces of “D2” (left) and “O1” (right) with ×7500
magnification.

4. Conclusions
In this research study, the plasticity and fracture behavior of two tool steels—AISI “D2” and
“O1”—were investigated. The tool steels were purchased and fabricated locally, and the specimens
were designed for two mechanical tests: tensile and compression. The results demonstrate that AISI
“D2” is recommended for applications that require moderate toughness and dimensional stability.
Metals 2019, 9, 1169 9 of 10

4. Conclusions
In this research study, the plasticity and fracture behavior of two tool steels—AISI “D2” and
“O1”—were investigated. The tool steels were purchased and fabricated locally, and the specimens
were designed for two mechanical tests: tensile and compression. The results demonstrate that AISI
“D2” is recommended for applications that require moderate toughness and dimensional stability.
In contrast, AISI “O1” is more suitable for applications that require better machinability performance
and an excellent combination of high hardness and toughness. The following points conclude the
results of the research:

1. The tensile yield strength of “O1” tool steel was higher than “D2” tool steel.
2. The specimens of “O1” tool steel showed vivid necking prior to fracture with 19.7% area reduction,
whereas the specimens of “D2” tool steel demonstrated no necking throughout the loading
process (1.3% area reduction).
3. The compression yield strength was higher for “O1” than for “D2” tool steel.
4. The surface fracture for “O1” was cup–cone, whereas it was flat for “D2” tool steel.
5. DIC was used to measure surface strains and predict cracks initiation location. The high localized
strains identified in the DIC images pointed out where the cracks initiated. The crack initiation
prediction was in good agreement with the results of the experiments for both tool steel types.
6. The parabolic dimple-like structures in the SEM images revealed that both tool steels had a ductile
fracture mode.
7. The SEM images showed deeper microvoids on “O1” and sharper ones on “D2”. The fracture
surfaces of “O1” had smaller dimples with less cleavage planes compared to “D2”.

Funding: This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University,
Jeddah, under grant No. (DF-309-829-1441). The author, therefore, gratefully acknowledges the DSR technical and
financial support.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Toboła, D.; Brostow, W.; Czechowski, K.; Rusek, P. Improvement of wear resistance of some cold working
tool steels. Wear 2017, 382, 29–39. [CrossRef]
2. Budinski, K.G.; Budinski, M.K. Engineering Materials: Properties and Selection; Prentice Hall: Upper Saddle
River, NJ, USA, 2010; p. 756.
3. Bourithis, L.; Papadimitriou, G.D.; Sideris, J. Comparison of wear properties of tool steels AISI D2 and O1
with the same hardness. Tribol. Inter. 2006, 39, 479–489. [CrossRef]
4. Glaeser, W.A. Characterization of Tribological Materials; Momentum Press: New York, NY, USA, 2012.
5. Kheirandish, S.; Saghafian, H.; Hedjazi, J.; Momeni, M. Effect of heat treatment on microstructure of modified
cast AISI D3 cold work tool steel. J. Iron. Steel Res. Int. 2010, 17, 40–45. [CrossRef]
6. Hutchings, I.; Shipway, P. Tribology: Friction and Wear of Engineering Materials; Butterworth-Heinemann:
Oxford, UK, 2017.
7. Straffelini, G. Materials for Tribology. In Friction and Wear; Springer: Berlin, Germany, 2015; pp. 159–199.
8. Lansdown, A.R.; Price, A.L. Materials to Resist Wear; Pergamon Press: Elmsford, NY, USA, 1986.
9. Roberts, G.A.; Kennedy, R.; Krauss, G. Tool Steels; ASM international: Cleveland, OH, USA, 1998.
10. Kataria, R.; Kumar, J. A comparison of the different multiple response optimization techniques for turning
operation of AISI O1 tool steel. J. Eng. Res. 2014, 2, 1–24. [CrossRef]
11. Camacho, L.D.A.; Miranda, S.G.; Moreno, K.J. Tribological performance of uncoated and TiCN-coated D2,
M2 and M4 steels under lubricated condition. J. Iron. Steel Res. Int. 2017, 24, 823–829. [CrossRef]
12. Wei, M.-x.; Wang, S.-q.; Lan, W.; Cui, X.-h.; Chen, K.-m. Selection of heat treatment process and wear
mechanism of high wear resistant cast hot-forging die steel. J. Iron. Steel Res. Int. 2012, 19, 50–57. [CrossRef]
13. Koshy, P.; Dewes, R.C.; Aspinwall, D.K. High speed end milling of hardened AISI D2 tool steel (~58 HRC).
J. Mater. Process. Technol. 2002, 127, 266–273. [CrossRef]
Metals 2019, 9, 1169 10 of 10

14. Lima, J.G.; Avila, R.F.; Abrao, A.M.; Faustino, M.; Davim, J.P. Hard turning: AISI 4340 high strength low
alloy steel and AISI D2 cold work tool steel. J. Mater. Process. Technol. 2005, 169, 388–395. [CrossRef]
15. Ma, X.; Liu, R.; Li, D.Y. Abrasive wear behavior of D2 tool steel with respect to load and sliding speed under
dry sand/rubber wheel abrasion condition. Wear 2000, 241, 79–85. [CrossRef]
16. Mohammed, M.N.; Omar, M.Z.; Al-Zubaidi, S.; Alhawari, K.S.; Abdelgnei, M.A. Microstructure and
mechanical properties of thixowelded AISI D2 tool steel. Metals 2018, 8, 316. [CrossRef]
17. Sola, R.; Giovanardi, R.; Parigi, G.; Veronesi, P. A novel method for fracture toughness evaluation of tool
steels with post-tempering cryogenic treatment. Metals 2017, 7, 75. [CrossRef]
18. He, W.; Hayatdavoudi, A. A comprehensive analysis of fracture initiation and propagation in sandstones
based on micro-level observation and digital imaging correlation. J. Pet. Sci. Eng. 2018, 164, 75–86. [CrossRef]
19. Caminero, M.A.; Lopez-Pedrosa, M.; Pinna, C.; Soutis, C. Damage monitoring and analysis of composite
laminates with an open hole and adhesively bonded repairs using digital image correlation. Composites Part B
2013, 53, 76–91. [CrossRef]
20. Caminero, M.A.; Pavlopoulou, S.; Lopez-Pedrosa, M.; Nicolaisson, B.G.; Pinna, C.; Soutis, C. Analysis of
adhesively bonded repairs in composites: Damage detection and prognosis. Compos. Struct. 2013, 95,
500–517. [CrossRef]
21. Caminero, M.A.; Lopez-Pedrosa, M.; Pinna, C.; Soutis, C. Damage assessment of composite structures using
digital image correlation. Appl. Compos. Mater. 2014, 21, 91–106. [CrossRef]
22. Romanowicz, P.J. Experimental and numerical estimation of the damage level in multilayered composite
plates. Materialwiss. Werkstofftech. 2018, 49, 591–605. [CrossRef]
23. Algarni, M.; Bai, Y.; Choi, Y. A study of Inconel 718 dependency on stress triaxiality and Lode angle in plastic
deformation and ductile fracture. Eng. Fract. Mech. 2015, 147, 140–157. [CrossRef]
24. Tvergaard, V.; Needleman, A. Analysis of the cup–cone fracture in a round tensile bar. Acta Metall. 1984, 32,
157–169. [CrossRef]
25. Lee, W.-S.; Lin, C.-F.; Liu, T.-J. Strain rate dependence of impact properties of sintered 316L stainless steel.
J. Nucl. Mater. 2006, 359, 247–257. [CrossRef]
26. Landron, C. Ductile Damage Characterization in Dual-Phase steels Using X-Ray Tomography. PhD Thesis,
The National Institute of Applied Sciences of Lyon, Lyon, France, 21 December 2011.
27. Weck, A.; Wilkinson, D.S.; Maire, E.; Toda, H. Visualization by X-ray tomography of void growth and
coalescence leading to fracture in model materials. Acta Mater. 2008, 56, 2919–2928. [CrossRef]
28. Toda, H.; Maire, E.; Yamauchi, S.; Tsuruta, H.; Hiramatsu, T.; Kobayashi, M. In situ observation of ductile
fracture using X-ray tomography technique. Acta Mater. 2011, 59, 1995–2008. [CrossRef]

© 2019 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).