Chromium and copper influence on the nodular cast iron with carbides microstructure

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A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 10 Issue 4/2010

47 – 54

10/4

Chromium and copper influence on the

no-dular cast iron with carbides microstructure

G. Gumienny

Department of Materials Engineering and Production Systems, Technical University of Łódź Stefanowskiego 1/15 Street, 90-924 Łódź, Poland

Corresponding author. E-mail address: grzegorz.gumienny@p.lodz.pl

Received 26.07.2010; accepted in revised form 04.09.2010

Abstract

In this paper chromium to 1,00% and copper to 1,50% influence at constant molybdenum content of about 1,50% on the nodular cast iron with carbides microstructure has been presented. It was found, that as a result of synergic addition of above-mentioned elements there is the possibility obtaining an ausferrite in nodular cast iron with carbides castings. Conditions have been given, when in nodular cast iron with carbides at cooling at first in the form, then air-cooling austenite transformation to upper bainite, its mixture with lower bainite, martensite or ausferrite takes place. Transformations proceed during cooling and the crystallization of cast iron have been determined and the casting hardness has been presented.

Keywords: Innovative foundry technologies and materials, Ductile cast iron with carbides, Bainite, Ausferrite, TDA method

1. Introduction

In nodular cast iron microstructure there is the possibility ob-taining bainite without heat treatment [1 ÷ 4]. For that purpose to cast iron a molybdenum and nickel in proper amount are added.

In [4] paper nickel influence at constant molybdenum content on the nodular cast iron with carbides microstructure has been presented. This paper is an explication and continuation of re-searching nodular cast iron with different metal matrix micro-structure. Chromium and copper influence on the microstructure and hardness of nodular cast iron with carbides containing about 1,50% Mo is presented in this paper.

2. Work methodology

Tested cast iron was melted in the 20 kg, 15000 Hz frequency induction furnace. Cast iron was superheated to 1530 C, in order to during reaction with magnesium its temperature was amount to

about 1480 C. It guarantee total magnesium solution in liquid metal and its maximum yield. Nodularization process was made in the mould. The mould scheme and its dimensions are shown in Figure 1. A master alloy in amount of 1,00% of casting mass was inserted into the reaction chamber. This chamber was located in the gating system behind the sprue. Behind this chamber the mixing and the control chambers were located. Inside the control chamber the thermocouple PtRh10-Pt (S type) was placed. It was connected with Cristaldigraph to thermal derivative analysis (TDA) curves recording. After the solidification finish castings were knocking out and free air cooling.

The chemical composition was tested with using SPECTRO-MAXx stationary metal analyzer made by Spectro Analytical Instruments GmbH. It is presented in Table 1 together with an equivalent carbon content Ec and a degree of eutectic saturation Sc.

Equivalent carbon content was calculated according to:

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4 3 2

2

9

5

400

1

0

1

0

A

5 6

B B

B - B A - A

Fig. 1. The scheme of elements spacing inside the mould: 1 – experimental casting, 2 – mixing chamber, 3 – control

cham-ber, 4 – reaction chamber, 5 – sprue, 6 - thermoelement’s shield

Table 1.

The chemical composition of tested cast iron, its equivalent car-bon content Ec and a degree of eutectic saturation Sc

Chemical composition, % Ec,

% Sc C Si Mn Mo Cr Cu Mg

2,88

4,03 2,37

2,61 0,22

0,32 1,44

1,53 0,00

1,00 0,00

1,50 0,04

0,05 3,62

4,82 0,84

1,13

The average concentration of P and S was amount to properly 0,04% and 0,01%.

Cast iron microstructure was tested on metallographic sam-ples etched by nital, magn. ×1000 with use of Eclipse MA200 Nikon microscope. Hardness tests were made by use of HPO hardness testing machine for 2,5/187,5/30 conditions.

3. Results

In Figure 2 (a, b) TDA curves of nodular cast iron with car-bides containing 1,50% Mo (a) and its microstructure (b) are presented.

a)

o

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

C

/s

900 1000 1100 1200 1300

t,

C

t = f( ) dt/d = f`( ) A B D F H K L

E

Point , s t, C dt/d , C/s

A 73 1210 -0,19

B 107 1176 -1,37

D 147 1141 –

E 152 1142 0,14

F 168 1143 –

H 253 1089 -1,47

K 297 1035 -0,78

L 320 1012 -1,16

b)

microstructure: nodular graphite, upper bainite, ferrite, ledeburitic carbides

Fig. 2 (a, b). TDA curves (a) and the microstructure (b) of nodular cast iron with carbides containing: 2,98% C, 2,46% Si, 0,29%

Mn, 1,53% Mo, (Ec = 3,72%)

It is hypoeutectic cast iron (Ec = 3,72%) and its microstructure consists of: nodular graphite, upper bainite, ferrite and small amount of ledeburitic carbides.

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In Figure 3 (a, b) TDA curves of nodular cast iron with car-bides containing 0,25% Cr (a) and its microstructure (b) are pre-sented.

a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) AB DE F H K L

A 112 1149 -0,04

B 129 1147 -0,33

D 154 1144 –

E 165 1144 0,15

F 181 1145 –

H 276 1102 -1,42

K 329 1042 -0,84

L 364 1008 -1,18

b)

microstructure: nodular graphite, ferrite, pearlite, upper and lower bainite, ledeburitic carbides

Mn, 1,45% Mo, 0,25% Cr (Ec = 4,20%)

Cast iron microstructure consists of: nodular graphite, ferrite, pearlite, upper and lower bainite and ledeburitic carbides. It is hypoeutectic cast iron (Ec = 4,20%). Its solidification begins with austenite precipitations in the melt, what causes on the derivative curve AB thermal effect. Next the melt solidifies as an austenite + graphite eutectic mixture (BDEFH thermal effect). Both Mo and Cr are characterized by the straight microsegregation, so the remaining liquid is chromium and molybdenum enriched and crystallizes according to the metastable system and creates com-plex ledeburitic carbides (Fe,Cr,Mo)3C at the temperature of tH = 1102 C ÷ tL = 1008 C (Fig. 3 a). These carbides crystallize on eutectic cells boundaries. 1,45% Mo and 0,25% Cr combina-tion cause obtaining unfavourable metal matrix microstructure consisting large amount of ferrite, pearlite and small amount of upper and lower bainite. A large amount of ferrite is caused, like in previous cast iron, by known, molybdenum ferritizing action [5].

The average hardness of castings made of that kind of cast iron is amount to 303HB.

Increase of Cr concentration to 0,50% caused changes of TDA curves and the microstructure presented in Figure 4 (a, b).

a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) DE F H K L

D 116 1138 –

E 132 1140 0,21

F 148 1142 –

H 290 1094 -1,21

K 342 1044 -0,57

L 381 1010 -1,27

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b)

microstructure: nodular graphite, upper and lower bainite, ferrite, pearlite, ledeburitic carbides

Mn, 1,53% Mo, 0,50% Cr (Ec = 4,26%)

From TDA curves results, that it is eutectic cast iron (Ec = 4,26%), so its solidification begins with the austenite + graphite eutectic mixture forming. Chromium concentration in-crease to 0,50% caused decreasing eutectic transformation tem-perature (Fig. 4 a).

From Fig. 4 b results, that Cr in amount to 0,50% caused de-creasing of ferrite surface fraction compared to cast iron with 0,25% Cr. It testify, that chromium not only is a part of carbides, but in a certain amount dissolves in austenite and has an influence on its solid-state transition. Carbides surface fraction is increased. It is presented in Figure 5 (a, b).

The average hardness of castings made of cast iron containing 1,53% Mo and 0,50% Cr was amount to 360HB and is higher than hardness of cast iron with 0,25% Cr. It is caused by increased carbides surface fraction, too (Fig. 5 b).

a)

microstructure: nodular graphite, ferrite, pearlite, upper and lower bainite, ledeburitic carbides

b)

Fig. 5 (a, b). Nodular cast iron microstructure containing 0,25% Cr (a) and 0,50% Cr (b)

TDA curves and the microstructure of cast iron containing 1,00% Cr are presented in Figure 6 (a, b).

10 m 100 m

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a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) DE F H K L

D 115 1138 –

E 132 1140 0,21

F 160 1143 –

H 277 1101 -1,31

K 317 1062 -0,62

L 344 1035 -1,37

b)

microstructure: nodular graphite, pearlite, ledeburitic carbides, ferrite

Mn, 1,50% Mo, 1,00% Cr (Ec = 4,27%)

From Fig. 6 a results, that chromium content increase to 1,00% did not caused eutectic transformation temperature change. Temperature recalescence was increased of about 1 C compared to cast iron with 0,50% Cr and amount to 5 C. Metal matrix microstructure of cast iron changes essentially. It consists of pearlite, ledeburitic carbides and small amount of ferrite. Results

from it, that in cast iron with 1,00% Cr molybdenum amount equals 1,50% did not caused austenite transformation to upper or lower bainite during permanent cooling. Carbides amount is similar, like in cast iron with 0,50% Cr. It is presented in Figure 7.

microstructure: nodular graphite, pearlite, ledeburitic carbides, ferrite

Fig. 7. Nodular cast iron microstructure containing 1,00% Cr

The average hardness of pearlitic-ferritic cast iron with car-bides is amount to 307HB.

In Figure 8 (a, b) TDA curves of nodular cast iron with car-bides containing 1,49% Mo and 0,50% Cu (a) and its microstruc-ture (b) are presented.

a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) AB D F H K LE

A 78 1223 -1,40

B 91 1200 -2,04

D 134 1159 –

E 148 1160 0,12

F 153 1160 –

10 m

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H 292 1114 -1,86

K 348 1038 -0,90

L 362 1023 -1,23

b)

Mn, 1,49% Mo, 0,50% Cu (Ec = 4,82%)

From TDA curves results, that it is hypereutectic cast iron, so its solidification begins with the nodular graphite precipitation (AB thermal effect). Copper addition caused increase of austenite + graphite eutectic mixture crystallization temperature (BDEFH thermal effect) compared to cast iron with chromium. Molybde-num presence causes, like in previously described cast irons, solidification of the remaining liquid according to the metastable system and (Fe,Mo)3C carbides forming. Compared with cast iron containing 1,53% Mo and 0,50% Cr it has reduced amount of carbides and similar amount of ferrite and pearlite. It is exemplary presented in Figure 9.

The average hardness of cast iron with about 1,50% Mo and 0,50% Cu was amount to 328HB.

Increase of copper content to 1,00% caused in cast iron changes presented in Figure 10 (a, b).

a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) ABD F H K LE

A 139 1156 0,04

B 146 1156 -0,31

D 168 1153 –

E 173 1153 0,09

F 199 1153 –

H 294 1114 -1,61

K 355 1038 -0,77

L 376 1015 -1,24

b)

microstructure: nodular graphite, upper and lower bainite, marten-site, ferrite, ledeburitic carbides, retained austenite Fig. 10 (a, b). TDA curves (a) and the microstructure (b) of

nodu-lar cast iron with carbides containing: 3,50% C, 2,49% Si, 0,32% 10 m

10 m

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In this cast iron eutectic transformation takes place in the temperature of 7 C less, than in cast iron with 0,50% Cu, but it is still higher than in cast iron with chromium. Molybdenum pres-ence caused ledeburitic carbides forming (HKL thermal effect, Fig. 10 a). Copper increase caused small amount of martensite forming, but did not change significantly carbides amount. It is important, that ferrite amount is decreased and pearlite disap-peared in cast iron metal matrix microstructure.

The average hardness of castings made of cast iron containing about 1,50% Mo and 1,00% Cu was amount to 379HB and it is 51HB higher, than hardness of castings made of cast iron contain-ing 0,50% Cu.

TDA curves and the microstructure of cast iron containing 1,50% Mo and 1,50% Cu is presented in Figure 11 (a, b).

a)

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

oC

/s

800 900 1000 1100 1200 1300 1400

t,

oC

t = f( ) dt/d = f`( ) DE F H K L

D 113 1157 –

E 123 1158 0,20

F 140 1159 –

H 274 1113 -1,52

K 334 1038 -0,69

L 351 1021 -1,23

b)

microstructure: nodular graphite, martensite, upper and lower bainite, ledeburitic carbides, retained austenite Fig. 11 (a, b). TDA curves (a) and the microstructure (b) of

nodu-lar cast iron with carbides containing: 3,39% C, 2,54% Si, 0,29% Mn, 1,50% Mo, 1,50% Cu (Ec = 4,26%)

It is eutectic cast iron and its microstructure consists of: nodu-lar graphite, martensite, upper and lower bainite, ledeburitic car-bides and retained austenite. Eutectic transformation temperature in this cast iron is similar to the temperature of this transformation in cast iron with 0,50% Cu (DEFH thermal effect, Fig. 11 a) and it is higher than in cast iron with chromium. Compared with cast iron containing 1,00% Cu this cast iron has got increase marten-site surface fraction and decrease – upper and lower bainite. Re-sults from it, that in cast iron with 1,50% Mo copper causes hardenability increase. Carbides amount is similar to previously described cast irons with Cu.

The average hardness of castings made of cast iron containing about 1,50% Mo and 1,50% Cu was amount to 493HB.

Metal matrix microstructure consisting an ausferritie and car-bides was obtained in cast iron with 0,50% Cr, 1,50% Mo and 1,00% Cu (Figure 12 b).

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a)

o

0 100 200 300 400 500

, s

-4 -3 -2 -1 0 1

d

t/

d

,

C

/s

900 1000 1100 1200 1300 1400

t,

C

t = f( ) dt/d = f`( ) A B DEF H K L

A 94 1183 -0,12

B 116 1171 -0,93

D 177 1141 –

E 188 1141 0,06

F 201 1141 –

H 286 1107 -1,09

K 358 1039 -0,68

L 392 1010 -1,01

b)

microstructure: nodular graphite, ausferrite, ledeburitic carbides Fig. 12 (a, b). TDA curves (a) and the microstructure (b) of nodu-lar cast iron with carbides containing: 3,32% C, 2,48% Si, 0,24%

Mn, 1,50% Mo, 0,50% Cr, 1,00% Cu (Ec = 4,09%)

From Fig. 12 a results, that it is hypoeutectic cast iron (Ec = 4,09%). Eutectic transformation temperature is similar to the temperature of this transformation taking place in cast iron with chromium and molybdenum and without copper. Cast iron micro-structure consist of: nodular graphite, ausferrite and ledeburitic carbides. Ausferrite is highly advisable phase in wear resistant materials, because of its possibility to pressure hardening [6]. Carbides presence should cause high wear and adhesive resis-tance.

The average hardness of casting made of ausferritic cast iron with carbides was amount to 340HB.

4. Conclusions

Results have indicated the following:

metal matrix microstructure consisting large amount of ferrite and pearlite and small amount of upper and lower bainite is obtained in cast iron containing 1,45% Mo and 0,25% Cr, increase of chromium content to 0,50% decreases ferrite amount and increases carbides amount in cast iron with about 1,50% Mo,

1,00% chromium addition makes impossible obtain bainite as-cast in as-cast iron with 1,50% Mo,

a presence of small amount of lower bainite and pearlite and decrease of ferrite amount is caused by 0,50% Cu addition in cast iron with molybdenum,

austenite stability is increased because of 1,50% Cu addition, synergic addition of 1,50% Mo, 1,00% Cu and 0,50% Cr make possible to obtain an ausferrite in metal matrix microstructure of nodular cast iron with carbides.

Scientific project financed from means of budget on science in years 2009 ÷ 2012 as research project N508 411437.

References

[1]S. Pietrowski, G. Gumienny, Carbides in Nodular Cast Iron with Cr and Mo, Archives of Foundry Engineering, Vol. 7, Issue 3 (2007) 223-230.

[2] S. Pietrowski, G. Gumienny, Crystallization of nodular cast iron with carbides, Archives of Foundry Engineering, Vol. 8, Issue 4 (2008) 236-240.

[3] G. Gumienny, Bainitic nodular cast iron with carbides ob-taining with use of Inmold method, Archives of Foundry En-gineering, Vol. 9, Issue 3 (2009) 243-248.

[4] G. Gumienny, Bainitic-martensitic nodular cast iron with carbides, Archives of Foundry Engineering, Vol. 10, Issue 2 (2010) 63-68.

[5] The Sorelmetal Book of Ductile Iron, Metals Minerals, Warsaw, 2006.

[6] E. Guzik, Ausferritic cast iron and its kinds – structure and selected properties, Production system optimization tenden-cies in foundries, Katowice-Gliwice 2010,105-110.