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Materials Letters 58 (2004) 2335–2339

A new SPD process for spheroidal cast iron

X. Zhao, T.F. Jing*, Y.W. Gao, J.F. Zhou, W. Wang

Key Laboratory of Metastable Materials Science and Technology, Hebei, Qinhuangdao 066004, PR China

Department of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, PR China

Received 28 November 2003; accepted 23 January 2004

Available online 13 March 2004

Abstract

A new severe plastic deformation (SPD) process for cast iron named cylinder covered compression (CCC) is developed by the present

authors. In CCC process, specimens are embedded in a steel cylinder and hot-compressed. Then the compressed material is cut into

pieces, machined out of surface layers, stacked, embedded in a cylinder and hot-compressed again. By CCC process, spheroidal cast iron

has been successfully hot compressed up to 99.2% reduction in height. The shape ratio of deformed graphite, b, increases as the amount

of reduction increases up to 80%, after which b changes with no relationship of the reduction. In case of 80% deformed specimens, a

lamellar structure of graphite and metal matrix forms. Further deformation leads to the decrease of the thickness of graphite and the

fragmentation of graphite.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Severe plastic deformation; Graphite; Iron; Microstructure

1. Introduction

In recent years, materials processed by methods of

severe plastic deformation (SPD) have attracted the grow-

ing interest of specialists in materials science. This

interest is enhanced by unique physical and mechanical

properties of SPD materials. Therefore, some SPD pro-

cesses such as equal channel angular pressing (ECAP)

[1,2], high pressure torsion (HPT) [1,3], multi-axial com-

pression [4,5] and accumulative roll-bonding (ARB) [6,7]

have been developed. However, there were few literatures

concerning SPD of cast iron. The main reason of this lack

is the difficulty in obtaining specimens without crack. In

this paper, the present authors have developed a new

process, named cylinder covered compression (CCC), for

cast iron to realize severe plastic deformation. Up to

99.2% reduction in height has been achieved without

cracking problems. When the reduction is more than

80%, most of graphite spheroids in the core region of

0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2004.01.034

* Corresponding author. Department of Materials Science and

Engineering, Yanshan University, Qinhuangdao 066004, PR China.

E-mail address: Jingtf@ysu.edu.cn (T.F. Jing).

the specimens collapsed and a lamellar structure of

graphite and metal matrix forms. The lamellar structure

is expected to have better tensile strength compared with

cast iron and better damping property compared with

steel. The microstructures of the compressed spheroidal

cast iron are characterized.

2. Experimental

The spheroidal cast iron was obtained from the China

Railway Shanhaiguan Bridge, as 22.6-mm-thick plates with

chemical compositions (mass%): 3.57C, 2.55Si, 0.22Mn,

0.021P, 0.013S and balance Fe. Fig. 1 represents the

microstructure of an as-cast specimen.

A new SPD process, named cylinder covered compres-

sion (CCC), was developed by the present authors. The

schematic illustration of the CCC process is shown in Fig. 2.

Specimens of 8 mm diameter with 20 mm in height were

machined from the cast plates. Cylinders of 8 mm inner

diameter and 10 mm outer diameter were made of low

carbon steel (GB45 steel). Specimens covered by cylinders

were hot-compressed on a Gleeble 3500 Machine. The

details of the thermomechanical treatments are listed in

Table 1

Summary of the thermomechanical treatments

Reduction

(%)

Temperature

(jC)Compressing rate

(s� 1)

Cooling

30 900F 2 1�10� 2 water

50 900F 2 1�10� 2 water

65 900F 2 1�10� 2 water

80 900F 2 1�10� 2 water

94 900F 2 1�10� 2 water

99.2 900F 2 1�10� 2 water

Fig. 1. The microstructure of an as-cast specimen.

X. Zhao et al. / Materials Letters 58 (2004) 2335–23392336

Table 1. After compression, the test material is cut into

pieces, machined out of surface layers, stacked, embedded

in a cylinder and hot-compressed again. After each pass of

compression, specimens were sectioned axially and the

microstructure was characterized using optical microscope

Fig. 2. The schematic illustration of the CCC process.

(OM), image analyzer and scanning electron microscope

(SEM).

3. Results and discussion

All the specimens were hot-compressed on a Gleeble

Machine without lubricant. The deformation in a specimen

is inhomogeneous. It is well known that the deformation in

the core of a hot-compressed specimen is very like that of

hot-rolled one. Therefore, the observation and analysis of

the deformed microstructure were mainly performed on the

core region of the severe strained specimens.

Fig. 3 represents SEM micrographs of hot-compressed

spheroidal cast iron with moderate reduction in height.

Fig. 3a shows that graphite is deformed plastically in the

form of lenses with 50% reduction in height. The increase

in the amount of reduction leads to elongation of graphite

in the direction perpendicular to the compressing force.

After 80% reduction (Fig. 3c), most of graphite spheroids

were collapsed. All collapsed graphite spheroids elongated

along the anvil face and changed into flat flakes. It is

clear that a parallel lamellar microstructure of the graphite

flakes and metal matrix forms at this high amount of

reduction. By further deformation, the graphite flake

density increases but the length of the flakes does not

increase. Fig. 3 also indicates that some of the graphite

flakes were fragmented during severe deformation. In

cases of 94% and 99.2% deformed specimens, five pieces

embedded in a cylinder were bonded (Fig. 3d). To

examine the bonded interface, a piece of tantalum alloy

was added between the centerpiece and the neighbor one.

The whole interface between tantalum alloy and spheroi-

dal iron was bonded well. Fig. 4a shows the well-bonded

interface and it can be observed that part of tantalum

alloy was pressed into a collapsed graphite spheroid in

Fig. 4b.

It has been reported that all graphite spheroids are

collapsed in heavily hot-rolled spheroidal iron sheets [8].

However, some uncollapsed graphite spheroids can be

found in the 94% deformed specimens and even in the

99.2% deformed specimens in the present study. Fig. 5 is

an SEM micrograph, which clearly shows the details

around an uncollapsed graphite spheroid in a 94% de-

formed specimen. The graphite spheroid is prolate as

Fig. 3. SEM micrographs of hot-compressed spheroidal iron with various reduction in height, 50% (a); 65% (b); 80% (c); 99.2% (d).

X. Zhao et al. / Materials Letters 58 (2004) 2335–2339 2337

those in the moderately deformed specimens. In the

vicinity of the graphite spheroid, there are several col-

lapsed graphite spheroids as well.

Fig. 6 shows a qualitative analysis representation of the

uncollapsed graphite spheroid in Fig. 5 by EDS. It is clear

that nearly all the constituents are of carbon, which ensures

that there is no hard inclusion in that graphite spheroid.

Therefore, the phenomenon of the uncollapsed graphite

spheroid should be explained by the difference between

rolling and CCC processes. Some investigations should be

carried out to provide further information about the uncol-

lapsed graphite spheroids in the future (Fig. 7).

The deformation of the spherical graphite in cast iron

used to be measured by the shape ratio, b =D2/D1 [9,10],

where D1 and D2 are minor and major axes of prolate

spheroid. Fig. 8 shows the effect of the amount of

reduction in height on b, D1 and D2. The minor axes

Fig. 4. SEM micrographs of the interfaces between tantalum alloy and iron

matrix (a), and graphite (b) in spheroidal iron.

of prolate spheroid, D1, gradually decreases as the amount

of reduction increases. On the other hand, the major axes

of prolate spheroid, D2, increases as the amount of

reduction increases up to 80%, after which D2 decreases.

The reason is that more and more graphite flakes are

Fig. 5. SEM micrograph of the details around an uncollapsed graphite

spheroid in a 94% deformed specimen.

Fig. 6. Qualitative analysis of the uncollapsed graphite spheroid in Fig. 5 by

EDS.

Fig. 9. A typical lamellar structure of graphite flakes and metal matrix in a

99.2% deformed specimen.

Fig. 7. The effect of the amount of reduction in height on b, D1 and D2.

X. Zhao et al. / Materials Letters 58 (2004) 2335–23392338

fragmented when the amount of reduction is more than

80%. The shape ratio, b, increases with the amount of

reduction up to 80%, after which the value of b changes

with no relationship of the reduction. This suggests that

the shape ratio of prolate spheroid is not suitable to

measure the deformation of spheroid iron when the

amount of reduction is more than 80%. In that case,

most of graphite spheroids are changed into flakes, the

deformation of spheroid iron may be determined by the

decrease of the thickness of graphite flakes and the space

between the centers of graphite flakes.

With the increase of the reduction, the space between

graphite spheroids/flakes decreases. The space was mea-

sured using an image analyzer. Fig. 8 shows the effect of

reduction (%) on the space between graphite spheroids/

flakes. It is clear that the space decreases obviously when

the reduction ranges from 30% to 65%. At the same time,

the lamellar structure of graphite flakes and metal matrix

forms as well. There are two possible reasons to explain

Fig. 8. The effect of the reduction (%) on the space between the centers of

graphite flakes.

this phenomenon. One is that the deformation of graphite

and metal matrix results in the decrease of the space.

Another one is that more and more graphite spheroids are

collapsed and can be seen on the section with the increase

of the reduction as shown in Fig. 9. Fig. 9 shows a

typical lamellar structure of graphite flakes and the

ferrous matrix in a 99.2% deformed specimen. All of

the graphite flakes are nearly parallel to each other and

most of them are fragmented. It is suggested that graphite

spheroids, as a soft inclusion in metal matrix, not only be

collapsed but also flow with the matrix during the

pressing process.

4. Conclusions

(1) By the new SPD process named cylinder covered

compression (CCC), spheroidal cast iron has been

successfully hot-compressed up to 99.2% reduction in

height.

(2) After 80% reduction in height, most of graphite

spheroids in the core of the specimens are collapsed

and a lamellar structure of graphite and metal matrix

forms. Further deformation leads to the fragmentation

of graphite.

(3) The space between the deformed graphites in the core

of the specimens decreases with the increase of the

amount of reduction.

(4) The shape ratio of graphite in the core of the specimens

increases as the amount of reduction increases.

However, it is not true when the amount of reduction

is more than 80%. The decrease of the thickness of

graphite and the space between the centers of deformed

graphite can reflect the deformation of spheroidal cast

iron at that high amount of reduction.

Acknowledgements

The present study was financially supported by the

Nature Science Foundation of Hebei province under Grant

No.503291, and the National Nature Science Foundation

X. Zhao et al. / Materials Letters 58 (2004) 2335–2339 2339

of P.R. China under Grant Nos. 50271061 and

No.50371074.

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