Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks

doi:10.4236/msa.2012.34034

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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.

Email: *[email protected], *[email protected]

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

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

Radius/mm

Hardness/Hv

(a)

Center

Central

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

Surface

Central plane

Radius

Thickness

Radius/mm

Hardness/Hv

(a)

CenterSurface

Central plane

Radius

Thickness

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

eq Nr



 (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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