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MICROSTRUCTURAL INVESTIGATION OF ICDP IRON DESIGNED FOR WORKING LAYERS

OF COMPOSITE CENTRIFUGALLY CAST ROLLS IN HOT ROLLING MILLS

Lucie STŘÍLKOVÁ1), Tomáš VÁLEK2)

1)MATERIÁLOVÝ A METALRGICKÝ VÝZKUM s.r.o., Pohraniční 693/31, 706 02 Ostrava, Czech Republic,

e-mail : lucie.strilkova@mmvyzkum.cz 2)

Vítkovické slévárny, spol.s.r.o., Halasova 2904/1, 706 02 Ostrava - Vítkovice, Czech Republic,

e-mail : Valek@vitkovickeslevarny.cz

Abstract

Centrifugally cast rolls with a working layer from ICDP (Indefinite Child Double Pour) cast iron have found

place in finishing stands of hot rolling mills. The term "indefinite" is associated with the fact that part of total

carbon content creates eutectic carbides and a small amount is present in the form of graphite (1-5%). ICDP

cast iron provides high mechanical and thermal stability of working layer, good surface quality and

simultaneously then eliminates sticking of rolled material. The core of the composite rolls is poured from

softer gray iron with spheroidal or lamellar graphite. This paper summarizes the results of experimental

studies of new grades of ICDP cast irons inoculated by aluminium. In several cases, the cerium addition was

applied in order to partially neutralize the aluminium effects and to improve graphite nodularity.

Keywords : centrifugal casting, ICDP iron, inoculation

1. INTRODUCTION

Centrifugally cast cylinders with a ICDP working layer are commonly used in finishing stands of hot rolling

mills. The core of composite ICDP rolls consists of a “softer” spheroidal graphite cast iron or grey iron with

lamellar graphite. The shape and distribution of graphite particles in the metal matrix are function of chemical

composition, melting technology, inoculation methods and solidification rate. Many publications describe the

effect of elements like Si, Al, Mg, Ce and Ca which can significantly affect the formation of graphite [1]. The

elements generally present in the alloy can be also divided into strong or weak graphite stabilizers and

neutral elements (1.1). Si and Al are elements supporting graphitisation. On the contrary, Mg and Ce

promotes globular graphite morphology. On the elements delaying graphitisation very often promote

segregation of carbides [2].

(1.1) promoting graphitisation delaying graphitisation

Aluminium is added to inoculants with intention to partial replacement of silicon. Detrimental effects of higher

aluminium contents on quality of cast can be neutralized by cerium addition. Furthermore, the cerium as well

as manganese increases graphite nodularity, stabilizes carbides and promotes their segregation. Cerium is

able to partially replace magnesium in the production of high quality ductile iron by modification. For

example, the gray cast iron modified by 0.015 to 0.020% Mg mixed with 0.02% Ce results in full structure

with nodular graphite [2]. The small amount of rare earth metals added to modifier causes lowering of the

tendency to carbide precipitation and promotes the number of graphite nuclei in cast iron. The presented

contribution deals with assessment of modifier effects based on aluminium and cerium with regards to

changes on microstructure of cast ICDP cast iron. Experimental studies were realised within the framework

of development new grades of ICDP cast irons designed for working layers of centrifugally cast rolls.

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2. EXPERIMENTAL MATERIALS AND RESULTS

Experimental work was performed in cooperation with Vítkovické slévárny, spol.s.r.o.. Specimens were

prepared from test Y-block castings of modified ICDP cast iron (see Table 1, ISO standard 1564-1997).

Cross-section metallographic samples for microstructural analysis were prepared from the bottom part of the

castings (area marked by red box in the following picture). The experimental results were completed by

testing of cast bars with circular cross section of diameter Ø30mm. The same cooling conditions were

applied for all investigated castings.

Table 1: Y-block dimensions according to ISO standard 1564-1997

Dimensions [mm]

U 75

V 125

X 65

Y 175

z 200

The nominal chemical composition of ICDP cast iron is shown in Table 2. The testing material was then

modified by either Al or Al+Ce addition as is shown in Table 3.

Table 2: Nominal chemical compositiion of ICDP cast iron, wt.%.

C Si Mn Smax Pmax Ni Cr

3 – 3.5 0.8 – 1.5 0.6 – 1.5 0.05 0.08 4 – 5 1.2 – 2.0

Table 3: Al and Ce concetrations in the tested samples, wt. %.

Specimen Code

I II III 1 2 3 1/1 1/2 1/3 1/4

Y-Blocks ISO standard 1564-1997 Circular cross section bars

Al 0.017 0.083 0.110 0.035 0.100 0.140 0.050 >0.500 0.351 0.161

Ce - - - 0.010 0.055 0.007 0.0035 0.0033 0.0029 0.0037

The influence of the aluminium and cerium additions on graphite morphology and microstructure was

observed using the optical microscopy.

2.1. ICDP cast iron modified by aluminium

Typical graphite shape and distribution in ICDP alloys modified by Al addition in the range from 0.017 to

0.110% is shown in Figs. 1(a)-(c). The graphite particles were distributed in interdendritic area and had flake

morphology. The highest graphite content was observed in the case of sample III with 0.11%Al, Fig. 1(c).

According to the paper [3] a higher amount of Al causes a reduction in of graphite particles diameter and

increases its dispersion in the metallic matrix, but there was no such as significant effect on graphite size

particles observed in our analyses.

(a) Sample I – 0.017%Al (b) Sample II – 0.081%Al (c) Sample III – 0.110%Al

Fig. 1(a)-(c) - Graphite morphology in ICDP cast iron modified by Al

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Microstructure consisted of eutectic carbides, martensite and retained austenite, see Figs. 2(a)-(c). The

highest amount of retained austenite was observed in sample that had been modified by the lowest

aluminium content, Fig. 2(a). The results of experimental studies of modified ICDP cast iron show only small

differences in amount of segregated eutectic carbides (Table 4).

(a) Sample I (0.017%Al) (b) Sample III (0.110 % Al) (c) Sample III (0.110 % Al)

Fig. 2(a)-(c) Investigations of the Microstructure of modified ICDP cast irons

Several investigations reported the positive effect of aluminium on the graphitization that can be explained as

follows. The addition of aluminium to cast iron causes a boost in the temperature of eutectic transformation

and the distance between stable and unstable solidification lines. This fact promotes the formation of

graphite and retards the formation of eutectic carbides. An increase in the eutectic transformation

temperature increases the rate of carbon diffusion in the melt, which in turn causes the formation of bigger

graphite particles [4]. Aluminium in the range 0-0.1% also acts as inoculant which increases the number of

graphite nuclei. The similar positive effect was observed in the studied ICDP cast iron modified by aluminium

in the range from 0.017 to 0.11 wt. %.

As is evident from Table 4, the lower hardness of ICDP iron with 0.017% Al was probably associated with a

higher amount of retained austenite in the microstructure. This could be also associated with higher amount

of carbon which remained dissolved in the matrix and thus stabilized austenite.

Table 4: Microstructural analysis and hardness HV50 of ICDP cast iron modified with Al

Sample code % G % EC % γret. HV 50 % Al % Ce

I 1.86 27.61 47.1 637 0.017 -

II 3.91 28.65 34.1 731 0.083 -

III 4.93 24.74 19.4 732 0.110 -

where: G – graphite, EC - eutectic carbides and γret. - retained austenite

2.2. ICDP cast iron modified by aluminium and cerium

Investigated ICDP cast iron was modified by combination of Al addition (up to 0.5%) which acted as the

inoculant and Ce (up to 0.05%) to modify graphite morphology. It was reported that the elements including

Mg, Ce and rare earth metals can adsorb on the interface graphite/melt and promote spheroidal growth of

graphite [1]. The optimum content of cerium and rare metals varied according to several studies. For

instance, Onsøinen in 1997 reported that the optimum amount of Ce in low sulphur ductile iron is 0.035% [5].

The effect of both inoculants, cerium and aluminium, on microstructure of ICDP cast iron was experimentally

verified and the results are stated in this paper, as well.

Figs. 3(a)-(e) show graphite morphology and distribution in the metal matrix of the studied castings. In most

cases, the graphite presented in interdendritic areas was of the flake type. Fig. 3(b) demonstrates the

negligible occurrence of graphite in the sample with 0.100%Al-0.55%Ce which had especially granular

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morphology. Interaction of aluminium and cerium is most evident when comparing the ICDP cast iron with

0.140%Al -0.007%Ce and 0.1%Al-0.055%Ce, see Figs. 3(b) and 3(c).

(a) Sample 1 – ICDP with

0.035%Al-0.010%Ce (b) Sample 2 – ICDP with

0.100%Al-0.055%Ce (c) Sample 3 – ICDP with

0.140%Al-0.007%Ce

(d) Sample 1/1 – ICDP with

0.05%Al-0.0035%Ce (e) Sample 1/2 – ICDP with

0.5%Al-0.0033%Ce (f) Sample 1/3 – ICDP with

0.351%Al-0.0029%Ce

Fig. 3 (a)-(f) Graphite morphology in ICDP iron modified by Al + Ce addition The addition of 0.055%Ce to the iron containing 0.14%Al almost completely suppressed graphitisation. In the

studied sample 2 there were locally observed clusters of non-metallic particles, see detail in Fig. 3(b). EDX

microanalysis proved that certain amount of the cerium content in ICDP cast iron with 0.10%Al and

0.055%Ce formed sulphides or complex oxides which often precipitated as the clusters, see EBSD image in

Fig. 4(a).

(a) (b) Fig. 4 Microstructure in EBSD image (Electron Backscatter Diffraction), (a) - clusters of non-metallic inclusion, (b), (c) – inclusions surrounded graphite particles

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Except those inclusions small amount of aluminium nitrides was revealed. Clusters of non-metallic inclusions

can cause local decline of mechanical properties and therefore also their presence is generally undesirable.

Microanalytical investigations of ICDP cast iron inoculated by 0.100%Al-0.055%Ce confirmed that Ce-rich

inclusions were present inside some graphite globules, see arrows in Fig. 4(b). It is commonly agreed that

particles of oxides, sulphides and nitrides can act as graphite nucleation sites. In the study [1] it was

published that complexes of inclusion Ce2O2S.Ce2O3 are formed after addition of cerium to the cast iron.

There is also assumed that heterogeneous nucleation of graphite is associated with parts of these

inclusions.

Figures 5(a)–(f) show typical microstructure of studied casts; the mixture of eutectic carbides and

martensitic matrix with variable amount of retained austenite. As can been seen from Figs. 5(a) and 5(b), the

cerium significantly promotes the segregation of eutectic cementite.

(a) Sample 1 - 0.140%Al-

0.007%Ce (b) Sample 2 - 0.10%Al-

0.055%Ce (c) Sample 2 - 0.10%Al-

0.055%Ce, detail

(d) Sample 1/4 - + 0.161%Al-

0.0037%Ce (e) Sample 1/2 - + 0.5%Al-

0.0033%Ce (f) Sample 1/1- 0.05%Al-

0.0035%Ce, detail

Fig. 5(a)-(f) Microstructure of modified ICDP cast irons

Table 5: Microstructural analysis and hardness HV50 of ICDP cast iron modified with Al and Ce

Sample code % G % EC % γret. HV50 % Al % Ce

1 2.26 26.52 24.1 539 0.035 0.010

3 7.59 19.32 19.4 581 0.140 0.007

2 0.01 35.33 16.1 624 0.100 0.055

1/1 2.03 29.62 51.5 600 0.050 0.0035

1/4 2.09 24.62 41.3 630 0.161 0.0037

1/3 7.34 17.37 37.37 573 0.351 0.0029

1/2 7.64 16.62 35.4 595 0.5 0.0033

where: G – graphite, EC - eutectic carbides and γret. - retained austenite Table 5 summarizes the results of microstructural analysis and hardness HV50 of studied ICDP castings.

There was revealed twice as high amount of retained austenite in cast bars compared to Y-blocks. It can be

attributed to different size of the test pieces and therefore also different cooling rates. This is supported by

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the observation of real centrifugally cast rolls with ICDP work layer. Nevertheless, as far available results are

concerned the direct association with modifiers was not so obvious. As is evident from HV50 measurements,

the highest hardness from the series Y-test samples was found in ICDP cast iron with 0.100%Al - 0.055%Ce,

which probably corresponds to negligible occurrence of graphite and the lower retained austenite content in

the matrix. The hardness is also higher due to the presence of eutectic carbides.

3. CONCLUSIONS

This experimental work was focused on the study of relationship between addition of the Al-based modifier

and microstructure of ICDP alloy. The results showed that there was significant increase in number of

graphite particles with increasing of Al content. Furthermore, the effect of inoculation of ICDP cast iron with

combination of aluminium and cerium additions was investigated. The Influence of cerium which had been

added to modify graphite morphology was not exactly proven. Higher content of cerium completely

suppressed graphitisation, which promoted segregation of eutectic carbides. The results showed that

inoculation of ICDP cast iron by “higher” cerium addition per se was unsuitable. Experimental studies were

realised within the framework of development of new grades of ICDP irons designed for working layers of

centrifugally cast rolls.

Acknowledgement

This paper was created with the financial support from the state budget through the Ministry of

Industry and Trade within the project No. FR – TI2/188.

REFERENCES

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