Interface reactions between steel 42CrMo4 and mullite

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Interface reactions between steel 42CrMo4 and mullite Tilo Zienert ∗ , Olga Fabrichnaya 1 TU Bergakademie Freiberg, Institute of Material Science, Gustav-Zeuner Str. 5, 09599 Freiberg, Germany Received 29 July 2014; received in revised form 24 October 2014; accepted 29 October 2014

Abstract Reactions between steel 42CrMo4 and mullite were investigated using differential thermal analysis (DTA) up to 1650 ◦ C and examined afterwards by X-ray diffraction (XRD), and scanning electron microscopy (SEM/EDX, SEM/EBSD). Several reactions were found and compared with thermodynamic calculations of pure steel, iron, and the mullite-steel interface at different partial pressures of oxygen. Two mullite phases with different composition were found after all experiments: composition of one mullite phase was in agreement with reported Al:Si ratio from literature and contained very low amount of metal impurities, whereas the second mullite had lower Al:Si ratio and relatively high amounts of dissolved manganese, magnesium, and iron. © 2014 Elsevier Ltd. All rights reserved. Keywords: Interface; Chemical properties; Steel; Mullite; Refractory

1. Introduction Interfacial reactions between metals and ceramics are of wide interest and were studied since several decades. Interactions between iron melts and oxide ceramics are important in steel metallurgy. A new idea is to improve the properties of steel products with special ceramic filters in the steel casting process.1 One method is the using of special coatings on the ceramic filter which can react with impurity particles. This coatings must be stable against liquid steel under casting conditions to not increase the oxide content in the melt. In a parallel work by Dudczig et al.2 several chemical reactions between a mullite coating material and a commercial steel 42CrMo4 were reported. In particular, it was found that molten steel reacted with mullite in presence of carbon at 1650 ◦ C. The mullite coating decomposed to an alumina-rich phase and a phase enriched by Mn with a substantially decreased Al:Si ratio in comparison with initial mullite material. The aim of this work is to investigate reactions between steel and mullite

∗

Corresponding author. Tel.: +49 03731 39 3156; fax: +49 03731 39 2604. E-mail addresses: [email protected] (T. Zienert), [email protected] (O. Fabrichnaya). 1 Tel.: +49 03731 39 3156; fax: +49 03731 39 2604.

in more detail using differential thermal analysis (DTA) and to characterise the samples with X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX and SEM/EBSD) after the DTA experiments. The interpretation of the DTA results will be based on the SEM/EDX, SEM/EBSD and XRD results. Results of thermodynamic calculations will be compared with the obtained experimental data. 2. Materials and methods Experiments were done using commercial powders of steel 42CrMo4 (Saarstahl AG, Germany) with the chemical composition as shown in Table 1 and mullite ‘Symolox M72 K0C’ (Nabaltec AG, Germany). The mullite powder is a mixture of mullite with an atomic ratio of Al:Si equal to 3:1 and alumina. The composition of the M72 powder is presented in Table 2. The starting materials were weighted and mixed to get homogenised powder samples with the desired compositions of 10 mass% (SM-I) and 20 mass% (SM-II) of the mullite powder mixture. The DTA experiments were performed using a SETARAM SetSys Evolution DTA 1750 device. Alumina crucibles were used to investigate steel and steel + mullite mixtures. The mullite powder M72 was investigated using a platinum crucible. All DTA experiments were performed under flowing argon atmosphere with heating rates of 20 K/min and cooling rates of 30 K/min.

http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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Table 1 Composition of the steel 42CrMo4.

Table 3 Phase names, diverging captions in figures and corresponding models used in thermodynamic calculations.

Element

Mass%

Element

Mass%

C Mn S Ni Al Co

0.409–0.468 0.256–0.774 0.031–0.035 0.187–0.209 0.021–0.08 0.0099–0.012

Si P Cr Mo Cu Ti

0.114–0.251 0.012–0.015 0.977–1.03 0.192–0.195 0.289–0.302 0.0008–0.0021

The highest temperature was always 1650 ◦ C if not otherwise mentioned. The relatively high heating and cooling rates were chosen to minimise possible reactions between the sample and the crucible at high temperatures. After the DTA experiments, the samples were grinded and polished and interface reactions were examined by SEM (Leo1530 GEMINI) with back scattered electron contrast and EBSD. Energy dispersive X-ray spectroscopy (EDX Bruker AXS Mikroanalysis GmbH) was employed to obtain the chemical compositions of found phases. The XRD patterns of powdered specimen were recorded using the Freiberger Präzisionsmechanik diffractometer URD63 (CuKα radiation; FPM Freiberg, Germany). Qualitative and quantitative analysis were done by Rietveld analysis using MAUD.3 The ThermoCalc software4 was used for thermodynamic calculations.

Phase

Name in figures

SGTE database Liquid FCC A1

fcc

BCC A2

bcc

Cementite Graphite MC SHP M3C2 M7C3 M23C6

MoC

Oxide database GTT Liquid Slag

Fe-fcc, Fe-bcc Corundum β-Cristobalite Olivine Halite

Al2 O3 ss. Fe2 O3 ss. SiO2 Fayalite FeO ss.

Mullite Spinel

Hercynite

3. Results

Model

(C, Cr, Fe, Mn, Mo, Ni, Si)1 (Cr, Fe, Mn, Mo, Ni, Si)1 (C, Va)1 (Cr, Fe, Mn, Mo, Ni, Si)1 (C, Va)3 (Cr, Fe, Mn, Mo, Ni)3 (C)1 (C)1 (Mo)1 (C)1 (Cr, Mo)3 (C)2 (Cr, Fe, Mn, Mo, Ni)7 (C)3 (Cr, Fe, Mn, Ni)20 (Cr, Fe, Mn, Mo, Ni)3 (C)6 (Al2 O3 , Si0.5 Al1.5 O3.25 , Fe2 , Fe2 O3 , Fe2 O2 , Fe0.67 Al1.33 O2.67 , Si2 O4 , Fe1.33 Si0.67 O2.67 , FeSiO3 , Fe0.36 Si0.91 Al0.73 O3.27 )1 (Fe)1 (Al+3 , Fe+3 )2 (O−2 , Va)3 (Si)1 (O)2 (Fe+2 )1 (Fe+2 )1 (Si+4 )1 (O−2 )4 (Al+3 , Fe+2 , Fe+3 , Va)1 (O−2 )1 (Al+3 )1 (Al+3 )1 (Al+3 , Fe+3 , Si+4 )1 (O−2 , Va)5 (Al+3 , Fe+2 , Fe+3 )1 (Al+3 , Fe+2 , Fe+3 , Va)2 (Fe+2 , Va)2 (O−2 )4

Magnetite

3.1. Thermodynamic calculations The temperatures of phase transformations in 42CrMo4 steel were calculated using the steel database compiled by SGTE.5 The compound energy formalism, in the form of the sublattice model, was used to desribe solid phases and the substitutional model was used to describe liquid phase. The phase names and models used to describe phases found in calculations are listed in Table 3. The steel composition used in calculation was in mass percent: 0.409 C, 0.114 Si, 0.256 Mn, 0.977 Cr, 0.187 Ni, 0.192 Mo and 97.865 Fe. The calculated molar phase fraction vs. temperature diagram is shown in Fig. 1. The bcc phase transformed into fcc on heating between 732 and 776 ◦ C. The beginning of melting was calculated as 1443 ◦ C which is completed at 1499 ◦ C. The fcc transformed to bcc phase during melting at 1490 ◦ C. At temperatures below the bcc → fcc transformation at 732 ◦ C different carbides and graphite were in equilibrium with the bcc phase. The calculated temperatures of bcc → fcc

transformation and the melting for the 42CrMo4 steel composition were lower than in pure Fe (911 and 1538 ◦ C) whereas the high-temperature transformation of fcc to bcc was shifted to higher temperature in comparison to pure Fe (1394 ◦ C). Thermodynamic calculations of the ceramic–metal interface under different partial pressures of oxygen were done using the

Table 2 Composition of mullite M72 K0C. Component

Mass%

Component

Mass%

Al2 O3 SiO2 Fe2 O3 TiO2

71–73 25–27 <0.4 <0.3

CaO MgO Na2 O K2 O

<0.1 <0.1 <0.3 <0.7 Fig. 1. Calculated phase fraction diagram of the used steel 42CrMo4.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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3

Fig. 2. Interface between Fe and Symolox M72 KOC with log(pO2 ) = −16.

GTT database for oxides.6 The compound energy formalism, in the form of sublattice model, was used to describe solid solution phases. The liquid phase was described by the associate model.7 The interface reactions were calculated for a mixture of 50 mol% iron and 50 mol% of mullite M72 which results in following composition (in mol%): 15.3846 Al, 3.8462 Si, 50.0 Fe and 30.9544 O. A partial pressure of oxygen equal to log(pO2 ) = −16 was assumed at casting conditions of liquid steel with other conditions being temperature equal to 1650 ◦ C and total pressure of 101325 Pa. Thermodynamic calculations of molar phase fraction vs. temperature diagrams were done for this and higher oxygen partial pressures, and in the temperature range between 300 and 1700 ◦ C. The phase fraction diagram with log(pO2 ) = −16 assumed at casting condition is shown in Fig. 2. Mullite was stable in contact with solid iron, but not in contact with molten iron. The metallic liquid dissolved more oxygen than solid iron. At lower temperatures, the amount of mullite and alumina was changing because of the temperature dependence of the homogeneity range of mullite. With increasing oxygen partial pressure, the Fe+2 containing phases, such as hercynite (FeAl2 O4 ) and fayalite (Fe2 SiO4 ), became stable as it is shown in Fig. 3 in the calculated phase fraction diagram for log(pO2 ) = −9. At temperatures higher than 1458 ◦ C two liquid phases were in equilibrium: metallic and oxide liquid (slag). Crystallisation started with precipitation of hercynite at temperatures below 1492 ◦ C. At temperatures slightly above the end of crystallisation at 1250 ◦ C, FeO formed in addition to hercynite. Crystallisation of the slag phase ended with fayalite phase formation at 1200 ◦ C. Magnetite was the second spinel phase forming at temperatures below 695 ◦ C and mullite became stable only at temperatures below 551 ◦ C. Hercynite decomposed at 473 ◦ C to alumina and an additional amount of magnetite. With increasing the oxygen partial pressure to 10−5 , only oxide phases were stable as it is shown in Fig. 4. Magnetite crystallised from liquid at 1626 ◦ C, mullite started forming at 1320 ◦ C, and silica crystallised in addition from the last melt at 1303 ◦ C. Therefore, crystallisation ended with three solid phases being stable: magnetite, mullite and silica. The amount of silica

Fig. 3. Interface between Fe and Symolox M72 KOC with log(pO2 ) = −9.

decreased with temperature decrease until it was dissolved completely in mullite at 1100 ◦ C. At 803 ◦ C magnetite transformed into hematite (Fe2 O3 ). According to equilibrium calculations the mixture of mullite and iron was not stable in the temperatures from 473 to 1492 ◦ C, and mullite was not stable at temperature above 1492 ◦ C. It can be concluded that if the oxygen partial pressure during the casting procedure is locally higher than log(pO2 )=-9 the mullite phase will not be stable and a slag phase will be formed which may crystallise forming hercynite and fayalite phases. 3.2. DTA In order to understand the reactions between mullite and steel, the powders of mullite, steel, and their mixtures containing 10 and 20 mass% of mullite were investigated using DTA. The obtained heating and cooling curve of the pure steel sample is shown in Fig. 5. Three endothermic reactions can be observed on heating which were interpretated using thermodynamic calculations: The first heat effect, observed on heating, at 740 ◦ C was in a very good agreement with the calculated

Fig. 4. Interface between Fe and Symolox M72 KOC with log(pO2 ) = −5.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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0

heating cooling

151

-5

80

0 °C

1460°C 0 °C 137

-10

60

-15

0

-25 -30

2 °C

-40

139

-40

C 800

1000

1200

1400

42CrMo4 SM-I SM-II

-45

14 76 °

-60 -80 600

-20

-35

67 6°

-20

Heat flow [μV]

1 14 460 98 °C °C

74 0°

C

20

C

Heat flow [μV]

40

-50 1350

1400

1450

1600

1500

1550

1600

Temperature [ °C]

Temperature [°C]

steel–mullite samples may be explained by inhomogeneity of the samples. To check the reproducibility of DTA experiments for SM-I and SM-II samples, the experiments were repeated under the same conditions using larger sample mass to distinguish better the observed heat effects. The heating curves of these experiments (SM-I-2 and SM-II-2) were the same as described for the first series of experiments, but different peaks were measured even for the same composition on cooling (see Fig. 8). Therefore it is important to indicate groups of reactions which were the same for all four experiments. A first group of reactions (group 1) was observed between 1239 and 1225 ◦ C on cooling and at least two peaks can be distinguished in each experiment. 100 42CrMo4 SM-I SM-I I

1476°C

°C

80

13

17

60 40 20 1285°C

0

-60 -80 1150

1200

1250

1300

1350

°C 13 92

-40

13 13 19 ° 37 C °C

C

-20 12 25 °

(bcc → fcc) transformation. The determined onset points of the last two reactions were 1460 (fcc → bcc) and 1498 ◦ C (bcc → L) respectively which were also in a good agreement with the thermodynamic calculations. According to the calculation results described above, the steel melting started at 1443 ◦ C, then fcc transformed into bcc phase at 1490 ◦ C, and finally melting was completed at 1499 ◦ C. On the cooling curve, the same transformations can be seen with some undercooling effect. The DTA experiment was also performed for mullite powder at the same conditions, but no heat effects were observed in the investigated temperature range up to 1650 ◦ C. The heating curves of the samples SM-I and SM-II are shown in comparison to the heating curve of the pure steel sample in Fig. 6. The melting behaviour of both steel–mullite mixtures were very similar to each other, but different from steel sample in several ways. Three reactions were observed in steel–mullite mixtures as well as in the pure steel samples, but the reaction temperatures were shifted to lower and higher values. The temperature of bcc → fcc transition was 740 ◦ C in all three experiments. The onset point of second heat effect was determined as 1370 ◦ C in steel–mullite mixture which was lower than in pure steel sample. The onset point of the third transformation was with a temperature of 1510 ◦ C slightly above that in pure steel. The uniform behaviour of the two steel–mullite mixtures was changed on cooling. The cooling curve of the pure steel sample, presented in Fig. 5, indicated crystallisation of bcc, bcc → fcc, and fcc → bcc transitions took place at 1476, 1392 and 676 ◦ C respectively with undercooling of 20, 68 and 66 K respectively. The cooling curves at high temperatures of SM-I and SM-II are shown in comparison to the one of the pure steel sample in Fig. 7. Different heat effects were observed for the two steel–mullite mixtures which were substantially different from crystallisation of pure steel sample. The difference in crystallisation of

Fig. 6. DTA heating curves of the samples SM-I and SM-II in comparison to the pure steel sample.

Heat flow [µV]

Fig. 5. DTA heating and cooling curve of the pure steel sample.

1400

1450

1500

Temperature [°C]

Fig. 7. DTA cooling curves of the samples SM-I and SM-II in comparison to the pure steel sample.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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1360°C

-40

st

SM-I-3, 1nd heat. SM-I-3, 2 heat.

151 3 °C

°C 17 15

st

°C 58 13

Heat flow [μV], 1 heat.

1352°C 125 9

°C

1340°C

Heat flow [µV]

C 1°

-30

-20

-40

-50 -60

1200

1250

1300

1350

1400

Temperature [°C]

-60

A second group of reactions (group 2), with onset points between 897 and 881 ◦ C, was also observed in all cooling curves of steel–mullite mixtures. The respective cooling curves can be seen in Fig. 9. Last, a third group of heat effects, with onset points between 770 and 763 ◦ C, was found. The temperatures of the last group were ≈100 K higher than the one observed in the cooling curve of pure steel. The last observed heat effect can be related with the fcc → bcc transformation in steel expected to occur at 776 ◦ C from the results of thermodynamic calculations. The examination of microstructure and X-ray diffraction patterns of the samples after DTA was necessary to explain the nature of the group 1 and the group 2 heat effects which is reported in Section 3.3 ‘Phase identification after DTA’. At the present stage, we can conclude that group 2 effects were related -30 Group 3 770-763°C

-35

Group 2 897-881°C

-40 -45 -50 -55 -60 SM-I SM-I-2 SM-II SM-II-2 700

750

800

850

700 800 900 1000 1100 1200 1300 1400 1500 1600 Temperature [ °C]

Fig. 8. DTA cooling curves of the two runs of samples SM-I and SM-II.

Heat flow [µV]

93

-20

-70

-100 1150

-70 650

C

-50

9°

Group 1 1239-1225°C

0

74

50

-10

C 0°

100

-65

137 0 °C

74 0

0

20

10

nd Heat flow [μV], 2 heat.

200

150

5

900

950

1000

Temperature [ °C]

Fig. 9. DTA cooling curves of the two runs of samples SM-I and SM-II below 1000 ◦ C.

Fig. 10. The two DTA heating curves of the sample SM-I-3.

with the amount of mullite in the mixture and the effects were more pronounced for mixtures SM-II containing more mullite. A third series of experiments (SM-I-3 and SM-II-3) were performed to understand the nature of the observed heat effects. The samples were heated to 1650 ◦ C, cooled down to 600 ◦ C, and were heated up again to 1650 ◦ C and cooled to room temperature. The used heating and cooling rates were the same as described above. The two heating curves of SM-I-3 are shown in Fig. 10. The original bcc → fcc transformation in 42CrMo4 steel was found in both heating curves, but the heat effect on the second heating was strongly decreased. The determined onset points on the first and second heating run were 740 and 749 ◦ C respectively. In addition to the described peaks on heating (see above), one new effect was observed on the second heating curve, with an onset point of 931 ◦ C. It can be noted that the temperature of this effect was slightly above the bcc → fcc transformation in pure Fe. The value of this effect was substantially larger than the effect at 749 ◦ C. Changes in reactions were also observed above 1350 ◦ C. Two heat effects at 1370 and 1513 ◦ C were normally observed on heating. On second heating the onset point of the first reaction was shifted to the slightly lower temperature of 1358 ◦ C. The onset point of the second heat effect was slightly shifted to the higher temperature of 1517 ◦ C in comparison with first heating (1513 ◦ C). The same characteristics were found for the composition of SM-II on second heating in the temperature range between 600 and 1650 ◦ C. The next series of DTA experiments were performed by heating up to 1450 ◦ C to avoid melting of steel. The idea of this series of experiments was to check if the reaction between mullite and steel occurred before melting and to avoid the influence of nonreproducible steel crystallisation. The samples were again heated up two times, but only up to 1350 ◦ C (SM-I-4) and 1400 ◦ C (SM-(I,II)-5). After reaching the highest temperature (HT), the samples were hold on this temperature for one hour and were then cooled to 600 ◦ C before the second heating to 1450 ◦ C and

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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SM-I-5, 1nd heat. SM-I-5, 2 heat.

0

-5

745°C

Heat flow [μV]

90 6°

C

5

1365°C

Fig. 13. Typical microstructure of SM-I after reaching the melting point of steel within DTA. Phases: (1) steel, (2) Al2 O3 , (3) ‘normal’ mullite and (4) Mn-rich mullite.

-10

800

900

1000

1100

1200

1300

1400

Temperature [ °C]

Fig. 11. DTA heating curves of sample SM-I-5.

cooling to room temperature without an isothermal step. The different HT-temperatures were chosen to investigate the kind of reaction at around 1360 ◦ C (first HT below and the second above the temperature of reaction). With that series of experiments it was clarified that the shifting of the bcc → fcc in steel transformation was the result of a diffusion process between solid phases which did not need the presence of liquid steel. The DTA heating and cooling curves of samples SM-I after heating to 1400 and 1450 ◦ C are shown in Figs. 11 and 12 respectively. It can be seen that the obtained results were reproducible. The heat effects on the first heating and the second heating were the same as for SM-I-3 first and second heating. The second effect at 906 ◦ C appeared on the second heating. It can be seen that on the second 0

-10

st

SM-I-51 nd cool. SM-I-5 2 cool.

7°

C

-30

88

4-

88

2°

-40

-50

1380-1360°C

C

77

Heat flow [µV]

-20

-60

-70 600

700

800

900

1000 1100 1200 1300 1400

Temperature [ °C]

Fig. 12. DTA cooling curves of sample SM-I-5.

heating in experiment SM-I-5 the heat effect at around 1365 ◦ C consisted from two peaks which were also observed on the first and second cooling at temperatures ≈1380–1360 ◦ C. The results obtained for the second sample SM-II were very similar to SM-I and the measured onset points for both samples coincide within uncertainty limits. It can be concluded that cooling of samples heated up to 1450 ◦ C occurred in more equilibrium way and there were no substantial undercooling effects. The temperature of the heat effect of transformations at ≈1360 ◦ C were observed at the same temperature range on cooling. 3.3. Phase identification after DTA The microstructure of the samples SM-I and SM-II after DTA were very similar. A typical microstructure is shown in Fig. 13. The chemical compositions of the different phases were measured with EDX. Four phases were always observed in SEM micrographs: steel and three oxide phases. One of them is alumina. Main components of the other two phases were aluminium, silicon and oxygen. However the Al:Si ratio were different in these phases and one of them was enriched more then the other one by Mn, Mg and Fe. Only three phases were distinguished with XRD investigation: (1) steel or Fe, (2) alumina and (3) mullite. No reflexes from other phases were found. Therefore it can be assumed that the last two oxide phases found in the micrographs were mullite with different compositions and very similar lattice parameters. This assumption was confirmed by EBSD investigations which make it possible to index the measured EBSD patterns of the specific grains: The determined structure of both oxide phases was mullite. A SEM micrograph of the microstructure of SM-II after heating two times up to 1650 ◦ C is shown in Fig. 14. The same phases were observed as in the SM-I samples after DTA, but the contrast between them in the micrograph is less clear in comparison to the SM-I samples. The mean compositions of both mullite phases measured by EDX are presented in Table 4. It was found that one mullite phase had a very similar chemical composition to that of the starting material with only very small fractions of impurity elements, and the Al:Si ratio equal to 3:1 was similar to that reported

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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Fig. 14. Typical microstructure of SM-II after reaching the melting point of steel within DTA. Phases: (1) steel, (2) Al2 O3 , (3) ‘normal’ mullite, (4) Mn-rich mullite, Black: Pores.

in literature8 . The second mullite was different: The ratio of Al:Si equal 0.5:1 was substantially different from literature data and a high amount of manganese (15 at%) was dissolved in the structure. It should be mentioned that the amount of magnesium was found locally up to 2 at% in the Mn-rich mullite which was the same as the dissolved amount of iron. A higher amount of other elements like K, Na and S was also found in comparison to the ‘normal’ mullite. In the ‘normal’ mullite, only traces of impurity elements (less than 3 at% in sum) were found. It can be concluded from the XRD and EDX measurements that manganese, magnesium and iron were dissolved in mullite due to reaction with steel. A phase separation in mullite occurred and newly mullite phase enriched by Mn having unusual Al:Si ratio was formed. The interface between SM-I and the alumina crucible after DTA can be seen in Fig. 15. The upper pores of the crucible were filled with the Mn-rich mullite phase and the sample was stuck on the crucible surface. It is unlikely that this was a process of solid state diffusion because of the long diffusion length (more than 100 ␮m) and short heating times. Therefore it was assumed that the Mn-rich mullite was melted in the experiment and that the reactions found in DTA around 1360 ◦ C on heating and cooling were related to the melting and crystallisation of that mullite phase (see Fig. 11 and 12).

7

Fig. 15. Interface between the alumina crucible and the sample. Crucible pores (1) were partially filled with Mn-rich mullite (2).

An example of a SEM micrograph of steel–mullite samples, investigated by DTA up to temperatures of 1450 ◦ C, is shown in Fig. 16 for sample SM-I-5 (isothermal treatment at 1400 ◦ C, HT = 1450 ◦ C) which also indicated presence of two mullite phases. The chemical compositions of the two mullite phases measured by EDX were the same as reported above. X-ray diffraction measurements were done on the following samples after DTA: pure mullite M72, SM-I-2, SM-II-2, and SM-II-3. The sample SM-I-2 consisted mainly of alumina and steel with only a small amount of mullite. Therefore the results of this measurement were not used to study the mullite phases in detail. The structural model of mullite by Ban and Okada9 was used in the perform of the Rietveld refinement. It was not possible to fit the obtained diffraction pattern by using two different mullite phases and no broden mullite peaks were observed in the measured XRD pattern. Therefore the two mullite phases must had very similar lattice parameters. In Table 5 the obtained lattice parameters and the volume of the elemental cells for mullite and iron are shown. Only the lattice parameter a of the mullite phase after DTA was slightly smaller than the one of the mullite from

Table 4 Chemical compositions of the two mullite phases determined by EDX without oxygen. ‘Normal mullite’ Element Al Si Fe Mn Na K Cr Ti

‘Mn-rich mullite’ at% 73.5 23.6 0.8 0.6 0.4 0.4 0.4 0.3

Element

at%

Si Al Mn Na K Fe Mg S Ti

50.4 25.8 14.9 2.1 2.0 1.9 1.8 0.8 0.3

Fig. 16. Microstructure of sample SM-I-5 after isothermal treatment and highest temperature of 1450 ◦ C with phases (1) steel, (2) Al2 O3 , (3) ‘normal’ mullite and (4) Mn-rich mullite.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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Table 5 Measured lattice parameters and the calculated volumes of the elemental cells of selected phases from the samples SM-II-2, SM-II-3 and M72. Sample

Phase

˚ Lattice parameter (A) a

Volume b

c

˚ 3) (A

SM-I-2

α-Fe

2.8693(1)

SM-II-2

Mullite α-Fe

7.5627(2) 2.86864(6)

7.6860(2)

2.88562(7)

167.732 23.606

SM-II-3

Mullite α-Fe

7.5690(1) 2.86956(6)

7.6896(1)

2.88692(6)

168.026 23.621

M72

Mullite

7.57435(9)

7.68956(8)

2.88709(3)

168.153

starting material which resulted in a slightly decreased elemental cell. The other parameters were measured to be the same within experimental uncertainties. The measured lattice parameter of the steel is only slightly higher than the one which is reported for ˚ V = 23.554 A ˚ 3 ).10 pure iron at room temperature (a = 2.8665 A, 4. Discussion Based on the DTA experiments, it can be concluded that reaction between steel and mullite occurred in solid state. The partial oxygen pressure in the protective gas is around 10−5 -10−6 atm. Therefore manganese, iron and magnesium in form of oxides were dissolved in mullite, and the steel appeared to be depleted in manganese and other alloying elements. Presence of mullite phase enriched by manganese and normal mullite after DTA was confirmed by SEM/EDX and SEM/EBSD investigations. However, since manganese diffused into mullite from steel it can be concluded that steel should become depleted by several alloying elements mainly by manganese. It can be assumed that phase separation occurred not only in mullite, but also in steel. This assumption is consistent with DTA results and the reasons could be shortly described as following. After interaction of steel with mullite during heating, the effects related with transformations fcc → bcc (≈780 ◦ C) on cooling were substantially weaker in all investigated steel–mullite mixtures. Another observation that reverse bcc → fcc (≈750 ◦ C) on the second heating was substantially weaker than in pure steel sample and on the first heating. On cooling and second heating of steel–mullite samples additional strong effect appeared at 910 ◦ C close to the bcc → fcc transformation temperature in pure iron (see Figs. 9 and 10). Also, it can be seen that melting of steel in the steel–mullite samples was shifted to higher temperature closer to the melting of pure iron too (see Fig. 6). With the investigation of samples after DTA by SEM/EDX, SEM/EBSD and XRD it was possible to attribute observed heat effects on heating and cooling curves to specific phase transformations which appeared due to reactions between steel and mullite. The first heat effect on heating curves of steel–mullite mixture was already explained by bcc → fcc transformation in steel 42CrMo4. Heat effects observed on heating at ≈1360 ◦ C can be related with two transformations: melting of Mn-enriched mullite phase and fcc → bcc transformation in the depleted steel phase. It can be clearly seen for samples heated up to 1450 ◦ C

23.623

that on the second heating the effect at ≈1360 ◦ C consists from two peaks at 1363 and 1390 ◦ C (see Fig. 11). This effect was also observed on the cooling curves as double peak with onset points at 1380 and 1360 ◦ C both on the first and second cooling (see Fig. 12). It should be mentioned that the effect at ≈1360 ◦ C was substantially larger for the samples SM-II containing more mullite phase (see Fig. 8). The effect at 1380–1390 ◦ C was close to fcc → bcc transformation in pure iron. Therefore, it can be concluded that the diffusion of manganese into the mullite phase must have been occurred at temperature below 1360 ◦ C. This process cannot be seen as distinct heat effect, but only as a change of slope of the DTA baseline. At a temperature of 1360 ◦ C, the Mn-rich mullite melted and the transformation of fcc to bcc in the depleted steel occurred at 1390 ◦ C (close to transformation temperature in pure Fe). The samples heated up to temperatures below the melting of steel demonstrated reproducible behaviour not only on heating, but also on cooling occurred in an equilibrated way. In the samples heated up to 1650 ◦ C the steel was completely melted. As mentioned above the behaviour of steel–mullite was completely reproducible on heating. The crystallisation of steel on cooling in steel–mullite samples heated up to 1650 ◦ C was complicated and occurred as one or more peaks observed at different temperatures in the range 1440–1270 ◦ C. Crystallisation behaviour was not reproducible in this temperature range. There were large undercooling effects and crystallisation of steel occurred in non-equilibrium way (see Figs. 7 and 8). However the heat effect at 1225 ◦ C consisting of several peaks was reproduced in all measurements and may be attributed to transformations bcc → fcc and crystallisation of Mn-enriched mullite. It should be mentioned that at temperatures below 1200 ◦ C the behaviour of all steel–mullite samples on cooling can be considered as the same within uncertainty of measurement. Already during the first cooling the transformation of fcc to bcc was separated into two effects: the larger one at ≈883 ◦ C and smaller one at 767 ◦ C. The first one can be related with transformation in the depleted steel, while the second one with transformation in steel of unchanged composition. On the second heating a very small effect was observed at 745–749 ◦ C close to the transformation temperature in pure steel sample and close to the first effect on first heating of steel–mullite mixtures. The second effect observed on the second heating at 906–931 ◦ C was substantially larger than the first one (see Figs. 10 and 11).

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033

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It should be noted that the temperature of the second effect was close to transformation in pure Fe. Therefore this effect was attributed to bcc → fcc transformation in depleted steel. The results obtained in this work can be compared with literature data. Solubility of iron in mullite was already investigated at air in several works.11–13 It was found that Fe+3 substituted Al+3 in mullite crystal structure. Similar interaction was observed between Cr and mullite.11,14 Wahlster et al.15 investigated interaction between manganese-enriched steel and mullite at 1600 ◦ C and found presence of MnAl2 O4 spinel (galaxite) in the reaction layer together with mullite. Formation of slag was reported for the SiO2 -enriched compositions.15 The investigation of present work was performed under protective gas atmosphere with partial pressure of oxygen ≈10−5 –10−6 atm. In this condition Fe+3 , Fe+2 and Mn+2 should be stable. It was mentioned by Schneider13 that Fe+3 substituted Al+3 in mullite structure with increase of lattice parameters due to larger ionic radius of Fe+3 . It was also mentioned that in the reducing atmosphere solubility of iron in mullite decreased because Fe+2 was too large to be incorporated into mullite structure. Therefore Fe+3 substitution of Al+3 is consistent with,13 while different mechanism should be suggested for incorporation of divalent ions especially in case of manganese which was found to reach 15 at% (without counting the amount of oxygen). To explain noticeable solubility of Mn+2 in mullite structure heterovalent substitution of Al+3 by Mn+2 and Si+4 can be suggested. This is also consistent with decrease of Al:Si ratio in Mn-enriched mullite. The solubility of Fe2 O3 in mullite phase at air condition was taken into account in GTT oxide database.6 However calculations did not show decomposition of mullite into two phases. It is not clear if the phase separation of mullite is an equilibrium process. Further phase equilibrium studies are necessary in the systems iron–mullite and manganese–mullite under protective gas atmosphere to conclude about the mechanism of interaction between steel and mullite. The phase separation in steel most probably occurred due to the fact that the reaction between mullite and steel was not completed and some steel grains did not change its composition. However, the amount of unreacted steel was substantially reduced and the transformation in it was seen as small effect on the second heating within DTA. Concerning the miscibility gap in the mullite phase it needs to be clarified if it was equilibrium process or if the presence of the two mullite phases was due to the fact that the reaction with steel was not completed and some of the mullite grains did not change their compositions. 5. Conclusion With the experimental investigations of steel 42CrMo4 and mullite powder mixtures by DTA and further characterisation of samples by XRD and electron microscopy the formation of a manganese-rich mullite coexisting with unchanged mullite was established. Experiments clearly demonstrated that the reaction between steel and mullite occurred in the solid state. SEM investigations of microstructures indicated the melting of Mn-enriched mullite phase which can be attributed to the

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observed heat effects around 1360 ◦ C on heating and cooling curves for samples heated up to 1450 ◦ C. However it was not possible to distinguish two mullite phases using XRD and no detailed information of the mullite structure could be obtained from the Rietveld refinement. This needs to be investigated in further work. Other important question for future investigations is the mechanism of divalent metal solubility in mullite structure. Investigations of reaction between pure manganese and mullite and pure iron and mullite would be important to provide information for the thermodynamic database development. Thermodynamic calculations indicated no reaction between Fe and mullite in the solid state in the range of oxygen partial pressure between 10−16 and 10−9 . At oxygen partial pressure equal to 10−9 calculations indicated formation of hercynite and fayalite. However experimental investigations performed in the present work did not indicate presence of these phases. Calculations at partial pressure of oxygen equal to 10−5 , which correspond to conditions of DTA investigations under protective gas, the stability of mullite and magnetite was indicated. It should be noted that solubility of Fe2 O3 in mullite was accounted in thermodynamic model of mullite, while solubility of divalent metals ions such as Mn+2 was not included in the modelling. Therefore further equilibrium studies and incorporation of obtained results into thermodynamic description of phases are necessary. It should be stressed that the results obtained in the present work are similar to ones of Dudczig et al.2 who found that the mullite coatings on alumina filters were not stable in contact with molten steel. The conditions applied in the present work were different – interaction of steel and mullite were studied under protective gas atmosphere and no carbon was used in the base materials. However, in both works Mn-enriched alumosilicate phase with substantially lower Al:Si ratio than in normal mullite was found. In the present work it was established that this phase had mullite structure and reaction between steel and mullite occurred in solid state. Therefore the results of present work confirmed that mullite coating on alumina filters can not be recommended for steel melt filtration. Acknowledgement This study was financially supported by the German Research Foundation (DFG) in frame of the subproject A03 within the Collaborative Research Centre SFB 920. References 1. Emmel M, Aneziris CG. Functionalization of carbon-bonded alumina filters through the application of active oxide coatings for steel melt filtration. J Mater Res 2013;28:2234–42. 2. Dudczig S, Aneziris CG, Dopita M, Rafaja D. Application of oxide coatings for improved steel filtration with the aid of a metal casting simulator. Adv Eng Mater 2013;15(12):1177–87. 3. Lutterotti L, Bortolotti M, Ischia G, Lonardelli I, Wenk HR. Rietveld texture analysis from diffraction images. Z für Krist Suppl 2007;26:125–30. 4. Andersson JO, Helander T, Höglund L, Shi P, Sundman B. Thermocalc & dictra, computational tools for materials science. Calphad 2002;26(2):273–312.

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5. SGTE. Thermodynamic properties of inorganic materials, vol. 19, Subvolume C, Part 1, binary systems and ternary systems from C–Cr–Fe to Cr–Fe–W of Landolt-Börnstein new series. Heidelberg: Springer Berlin; 2011. 6. http://www.gtt-technologies.de/; 2012. 7. Lukas H, Fries S, Sundman B. Computational thermodynamics – the Calphad method. Cambridge UK: Cambridge University Press; 2007. 8. Klug FJ, Prochazka S, Doremus RH. Alumina-silica phase diagram in the mullite region. J Am Ceram Soc 1987;70(10):750–9. 9. Ban T, Okada K. Structure refinement of mullite by the Rietveld method and a new method for estimation of chemical composition. J Am Ceram Soc 1992;75(1):227–30.

10. von Batchelder F, Raeuchle R. Re-examination of the symmetries of iron and nickel by the powder method. Acta Crystallogr 1954;7(5):464. 11. Murthy MK, Hummel F. X-ray study of the solid solution of TiO2 , Fe2 O3 , and Cr2 O3 in mullite (3Al2 O3 ·2SiO2 ). J Am Ceram Soc 1960;43(5):267–73. 12. Schneider H, Rager H. Iron incorporation in mullite. Ceram Int 1986;12(3):117–25. 13. Schneider H. Temperature-dependent iron solubility in mullite. J Am Ceram Soc 1987;70:C43–5. 14. Rager H, Schneider H, Graetsch H. Chromium incorporation in mullite. Am Miner 1990;75:392–7. 15. Wahlster M, Mass H, Abratis H, Choudhury A. Reaktionen zwischen Feuerfesten Stoffen des Systems SiO2 –Al2 O3 und manganhaltigen Eisenschmelzen. Arch für das Eisenhüttenwes 1970;41(1):37–42.

Please cite this article in press as: Zienert T, Fabrichnaya O. Interface reactions between steel 42CrMo4 and mullite. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.033