Materials Sciences and Applications, 2012, 3, 234-239
http://dx.doi.org/10.4236/msa.2012.34034 Published Online April 2012 (http://www.SciRP.org/journal/msa)
Inhomogeneous Hardness Distribution of High Pressure
Torsion Processed IF Steel Disks
Yuepeng Song1,2,3*, Wenke Wang1,3, Dongsheng Gao3, Hyoung-Seop Kim2*, Eun-Yoo Yoon2,
Dong-Jun Lee2, Chong-Soo Lee2, Jing Guo1,3
1Mechanical and Electronic Engineering College, Shandong Agricultural University, Tai’an, China; 2Department of Materials Sci-
ence and Engineering, Pohang University of Science and Technology, Pohang, Korea; 3Shandong Provincial Key Laboratory of Hor-
ticultural Machineries and Equipments, Shandong Agricultural University, Tai’an, China.
Received November 7th, 2011; revised December 24th, 2011; accepted February 6th, 2012
ABSTRACT
The inhomogeneous hardness distribution of high pressure torsion (HPT) processed IF steel disks along different direc-
tions is investigated. The results indicated that there exists inhomogeneous distribution in HPT processed IF steel disks,
giving lower hardness in the center and higher hardness in the edge regions. However, on the axisymmetrical section
testing plane of the disks’ thickness direction, there is a soft zone near the surface of disks. Further results from radius
testing plane of different depths from the surface of HPT processed disks show that the inhomogeneity rules of hardness
distribution on the radius direction are similar to that on the thickness direction. Compared with the initial state, differ-
ent stages of HPT (compression and compression + torsion) can both remarkably increase the hardness of IF steel disks.
Microstructure investigation results can give a well support to verify the rules of hardness distribution, showing hardly
no change of grains in center and sever plastic deformation in edge. The inhomogeneous distribution of stress and strain
with the huge friction between anvil and disks in the process of HPT play an important role of hardness and micro-
structure distribution.
Keywords: Hardness Distribution; High-Pressure Torsion (HPT); IF Steel; Inhomogeneous
1. Introduction
The processing of bulk metals through the application of
High Pressure Torsion (HPT) is considered to be the
most successful procedure for producing ultrafine-grained
(UFG) materials with exceptionally small grain sizes on
submicrometer or nanometer level, and with a high frac-
tion of boundaries having high angles of misorientation
[1-3]. At present, in a wide range of pure metals, metallic
alloys and intermetallics [1-7], bulk solids with UFG- or
nano-microstructures have been produced by using HPT
technique. Because of high strength, superior fatigue and
a potential super-plastic forming capability at elevated
temperatures, they are attractive candidate materials for
parts in the automotive and aerospace sections [5,8]. In-
terstitial-free (IF) steel is widely used for automotive
deep-drawing applications with micro-alloying elements
(Ti, Nb and V) forming precipitates with carbon and ni-
trogen. Because of the absence of interstitial solutes and
the appropriate texture in IF steel, an excellent deep-
drawability will proceed. The investigation of severe
plastic deformation (SPD) like equal-channel angular
pressing (ECAP) of IF steel has being attracted consid-
erable interest of researchers and demonstrated a practi-
cal applicability of SPD for improving its mechanical
properties [9,10]. However, until now, there is no re-
search on the HPT processed IF steel disks, as far as the
authors know.
There is a significant limitation with HPT because of
the inhomogeneous strain, at least in principle, across the
diameter of the samples during the HPT deformation,
which can lead to the homogeneity of hardness and mi-
crostructure of disks and be verified by materials such as
Al, Ni, Cu and austenitic steel for giving the lower values
of hardness in the centers but higher hardness in the edge
of disks [1,5]. However, recently experiment results of
pure Al material of HPT indicated that there are higher
hardness values in the centers of the disks [11,12]. The
study on inhomogeneity of HPT processed IF steel disks
is particularly concerned by materials researchers.
In this paper, we investigate the inhomogeneity of
hardness and microstructure distribution on the different
direction of IF steel disks processed by HPT.
*Corresponding authors.

Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks 235
2. Experimental Procedures
The investigating IF steel was manufactured by the Po-
hang Steel Company (POSCO, Korea) with the composi-
tion of 0.0026 wt% C, 0.096 wt% Mn, 0.045 wt% Al and
0.041 wt% Ti. The initial state of material is a plate 12
mm in thickness size rolled from casting ingot. After
homogenizing (973 K × 2 h, cooling in furnace), the
grain size is 150 - 200 μm and hardness is about 80 Hv.
For the experiments, application pressure of 6 GPa
was imposed at room temperature on the disks. The time
of compression load is set as 10 seconds. Meanwhile,
two revolutions of 0 and 1 turns are conducted, the sam-
ples namely IF60 (no turn) and IF61 (1 turn), respec-
tively. The axial profiles of HPT disks are ellipse, with
10 mm of major axis and 2 mm of minor axis.
There are two directions of hardness distribution meas-
urement: along the longitudinal direction (namely LD
position) and transversal direction (namely TD position).
On the LD position, the hardness testing plane is the ax-
isymmetrical section of HPT disks. Equal-distance test-
ing points with 0.5 mm on the testing lines follows the
radius direction on the axisymmetrical section plane of
HPT disks. While on TD position, the mounting samples
were grinded and polished layer by layer and the hardness
testing planes were so obtained with different depth from
surface. The schematic drawing of HPT disks and the
measurement of hardness distribution on different direc-
tion is shown as Figure 1.
Hardness was measured using FM-700 Microhardness
Tester and the pressure loading is 100 g, continuous 10 s.
The color-coded contour maps and curves of hardness
distribution of different samples are so obtained. Micro-
structures in different position of disks were observed
using optical microscopy (Olympus U-TV0.5xc) taken
from the discs after HPT.
3. Experimental Results and Discussion
3.1. Inhomogeneous Hardness Distribution on
the Thickness Direction of HPT Disks
Until now, no systematic works of hardness and micro-
structure distribution on the LD position of HPT disks at
the compressive stage has been done, as far as the au-
thors know. Figure 2 shows the hardness distribution on
the axisymmetrical section testing plane of HPT disks.
As the same rules with lots of literatures [5], the fig-
ures clearly indicated lower values of hardness in the
center and higher values in the edge. i.e. as to the central
lines on the testing plane, the hardness in the edge, mid-
dle and center is 190.1 Hv, 155.1 Hv and 153 Hv for
IF60 (only compression) and 379.5 Hv, 330.7 Hv and
249.3 Hv for IF61 (1 turn), respectively. Compared with
the initial state (80 Hv), the HPT processing can increase
the hardness of disks remarkably, especially the effect of
torsion on hardness. For this study, one turn of HPT can
increase the hardness above 3 times in the edge of disks.
Hence, there also exists an inhomogeneity, for IF steel
materials, across the center to edge of HPT disks, not
only in compressive stage but also torsion stage.
The hardness distribution color-coded contour maps of
different samples on the LD position can clearly indicate
this rule as shown in Figure 3. It must be pointed out that
the hardness scale of the two contour maps is different
because of their large difference.
From these figures, almost symmetrical distribution of
hardness exists between top and bottom section of HPT
disks with the central plane. Furthermore, a low hardness
region exits in the center near the surface of disks is also
clearly displayed.
3.2. Hardness Distribution on the TD Position
On the TD position, the testing planes of different dis-
tance from surface of HPT disks were obtained by grind-
ing and polishing the mounting samples layer by layer.
Hardness distribution on testing planes of different depth
from surface is displayed in the Figure 4.
From Figure 4, the inhomogeneous hardness distribu-
tion on the transversal direction is similar to that of on
the longitudinal direction, giving lower hardness in the
center but higher values in the peripheral regions of HPT
disks. Further results indicated that compared with com-
pressive stage, torsion can make the variation of hardness
Central
Center
L1 L2 L3
L1
/
L2
/
L3
/
0.5
Radius
Thickness
Testing line
Testing po ints
0.25
Testing li ne
Testing po in t
IF61 IF60
(a) (b) (c)
Figure 1. The schematic drawing of HPT disks and the measurement method. (a) HPT disks; (b) On LD position; (c) On TD
osition. p

Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks
236
Center
Central
L3
L2
L1
Radius/mm
Hardness/Hv
(a)
Center
Central
L3
L2
L1
hardness/Hv
Radius/mm
(b)
Figure 2. Hardness distribution of IF steel disks processed
by HPT on the thickness direction. (a) Only compression (0
turn); (b) 1 turn.
Figure 3. Hardness distribution color-coded maps of HPT
disks on the thickness direction. (a) IF60 (0 turn); (b) IF61
(1 turn).
Cente
r
Surface
Central plane
Radius
Thickness
P1
P2
P3
Radius/mm
Hardness/Hv
(a)
CenterSurface
Central plane
Radius
Thickness
P1
P2
P3
Radius/mm
Hardness/Hv
(b)
Figure 4. Hardness distribution on testing planes of differ-
ent depth from surface.
from center to edge increasing more sharply as shown in
Figure 5.
3.3. Inhomogeneity of Microstructure
Distribution of IF Steel Disks by HPT
Processed
Difference in hardness is attributed to the microstructure
of different position on HPT processed IF steel disks. As
an example, the microstructure of HPT disks on the LD
position is shown in Figure 6.
Figure 6 clearly indicates that there is a remarkable
inhomogeneity distribution of microstructure on the LD
position, for the HPT disks of not only at the compres-
sive stage but also at torsion stage. From the Figure 6,
the grains of center have no hardly change as the same as
the initial state, however, in the edge, grains occur large
severe plastic deformation and their boundaries become
very obscure. Especially, for the IF60 sample, the flow-
line characteristic microstructure as highly deformed is
clearly displayed. This severe deformed microstructure is

Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks 237
Figure 5. Hardness distribution of HPT IF steel disks on the
radius direction.
similar to that observed in a very wide range of ultrafine-
grained materials produced through processing by HPT.
Further investigation of microstructure on the TD posi-
tion of HPT disks has the same inhomogeneity distribu-
tion rules.
The inhomogeneity distribution of microstructure can
support the ones of hardness, giving lower hardness in
center and higher hardness in edge. The key factors of
inhomogeneity distribution of hardness and microstruc-
ture are attributed to the inhomogeneity deformation of
HPT disks. That is to say, the inhomogeneous distribu-
tion of strain and stress leads to microstructure inho-
mogeneity. As we know, the mechanical properties of the
deformed material are attribution to the amount of plastic
deformation, i.e. the development and distribution of
strain and stress of disks during the HPT processing [13,
14].
The formula of equivalent von Mises strain eq

for
estimating the strain on the HPT disks is [2,5,15],
2π
3
eq Nr
h
  (1)
here, N is the amount of turns, h, r is the thickness and
distance from center of disks, respectively.
From the formula (1), following the radius direction
from center to edge of HPT disks, along the r increasing,
the equivalent von Mises strain is higher to higher which
means the deformation of material more and more severe.
For this study, because of the arc shape of disks for the
lower to lower of thickness h from center to edge, the
equivalent von Mises strain even more sharply variation
and strengthens the effect of hardability on material. So
the arc shape of HPT disks leads to the more inhomoge-
neous strain distribution during deformation. Compared
with the disks only in compressive of HPT, the torsion
can make the strain more inhomogeneous from center to
edge for giving higher strain value in edge and lower
strain in center. Correspondingly, more severe plastic
(b) IF61( 1 tu rn)- C ent er(b) IF61( 1 tu rn)-Edge
Center
100μm100μm100μm100μm
1mm1mm
(a) IF60( 0 tu rn)
Center Edge
(a)
(b) (c)
Figure 6. Microstructure of HPT disks on the LD posit ion. (a) IF60 (0 turn); (b) IF61 (1 tu rn)—cen ter; ( c) I F61 (1 turn)—edge.

Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks
238
deformation occurs in edge while no changes in center,
which is the key reason why the hardness and micro-
structure display so inhomogeneity of HPT disks not
only on the LD position but also on the TD position.
However, because of no turn that means N in formula
(1) is zero, as to the disks only at the compressive stage
of HPT, the inhomogeneity of hardness and microstruc-
ture distribution will not be explained well by formula
(1). In fact, the deformation of disks at the compression
stage of HPT is similar to that of the upsetting process of
forging technology. The friction between the punch and
workpiece during the deformation plays an important
role. Recently, the important effect of friction on the de-
formation of materials during the compressive stage is
paid attention by many researchers [4,16,17]. The results
indicated that the strain will be depended on the degree
of difficulty in rheology deformation of materials. Ac-
cording to the sample IF60 (only compression), the ma-
terial in the edge of disks will be easily to rheologically
deform into the edge flash where displays the character-
istic flow-line microstructure. However, in the center,
especially near the surface because of huge friction with
groove of anvil, the rheological deformation is difficult
to proceed which leads to the lower strain and no hardly
change of grains. The soften region near the surface in
center of disks only compressed will so exist.
4. Conclusion
Inhomogeneous hardness distribution of HPT processed
IF steel disks along different directions is studied. Ac-
cording to the disks of not only at compressive stage but
also at torsion stage, on the longitudinal direction, there
exists an inhomogeneous hardness distribution on the
axisymmetrical section testing plane of HPT disks, for
giving lower hardness value in center and higher hard-
ness in edge. The inhomogeneity rules of hardness dis-
tribution of HPT disks were verified with the testing
planes of different depth from surface on the transversal
direction. Moreover, a soften region near the surface of
disks is investigated. Compared with the initial state,
different stages of HPT (compression and compression +
torsion) can both remarkably increase the hardness of IF
steel disks. For this study, the torsion makes the variation
of hardness from center to edge more sharply. The mi-
crostructure observation was found to correlate well with
the hardness distribution corresponding to the strain of
different position of HPT disks. At the compressive stage
of HPT, the strain will be depend on the degree of diffi-
culty in rheology deformation, however for torsion stage,
on the thickness and distance from center to edge direc-
tion. Especially, the friction between the punch and
workpiece during the deformation plays an important
role.
5. Acknowledgements
Y. P. Song acknowledges the postdoctoral fellowship
supported by the National Research Foundation through
the Korea-China Young Scientists Program, Korea. The
HPT samples were provided by Prof. Hyoungseop Kim
of POSTECH, Korea. Furthermore, this work also was
supported by Key Laboratory of Functional Crystals and
Laser Technology, TIPC, CAS, Graduate Innovative pro-
gram of Shandong Province (SDYY11079), NSFC (5100
1111), Program of “Twelfth Five-Year” National Science
and Technology Support Plan (2011BAD12B02), and the
Fok Ying-Tong Education Foundation for Young Teach-
ers in the Higher Education Institutions of China (Grant
No. 121049).
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