ISSN 1318-0010
KZLTET 32(3-493)165(1998)
DECARBURIZATION MECHANISM OF Fe-Si-Al ALLOY WITH ANTIMONY ADDITION
MEHANIZEM RAZOGLJIČENJA ZLITINE Fe-Si-Al, LEGIRANE Z
ANTIMONOM
DARJA STEINER PETROVIČ1, M. JENKO1, V. GONTAREV2, H. J. GRABKE3
1Institute of Metals and Technology, Ljubljana, Slovenia 2University of Ljubljana, NTF-OMM, Ljubljana, Slovenia 3Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Prejem rokopisa - received: 1998-12-06; sprejem za objavo - accepted for publication: 1998-12-14
The mechanism of decarburization process of Fe-Si-Al alloy was investigated on samples of cold rolled sheets; some of them alloyed with 0.048 wt.% Sb. The decarburization was performed in a H2-H2O gas mixture at 840°C. It was influenced by the oxidation of the surface. Antimony influence on decarburization mechanism of Fe-Si-Al alloy depends on carbon content in the steel.
Key words: decarburization, silicon steels, surface oxidation, antimony
Raziskali smo proces razogljičenja vzorcev Fe-Si-Al zlitin v obliki hladno valjane pločevine, ki smo jo legirali z 0.048 mas.% Sb. Razogljičenje je potekalo v atmosferi vlažnega vodika pri temperaturi 840°C. Na kinetiko razogljičenja vpliva tudi oksidacija površine jekla. Vpliv antimona na mehanizem razogljičenja zlitine Fe-Si-Al je bil povezan s koncentracijo ogljika v jeklu.
Ključne besede: razogljičenje, silicijeva jekla, oksidacija, oksidna plast, antimon
1 INTRODUCTION
The decarburization of Fe-Si-Al alloy - silicon steel is performed by annealing in a gas mixture of hydrogen and water vapour H2-H2O in the temperature range from 700 to 900°C with a controlled partial pressure ratio of water vapour and hydrogen p(H2O)/p(H2). The decarburi-zation process of steel in a gas mixture H2-H2O can be described by1:
1.	Diffusion of carbon to the steel surface
2.	Transport of water vapour to the steel surface and equilibration at the phase boundary steel-gas mixture
3.	Dissociation of water vapour molecules into a hydrogen and oxygen and adsorption on the steel surface
4.	Oxidation of carbon
5.	Oxidation of iron and alloying elements
The major chemical reactions of decarburization of steel are:
[C]Fe + O2(g) — CO2(g)
[C]Fe + CO2(g) — 2 CO(g)
[C]Fe + 2 H2(g) — CH4(g)
[C]Fe + H2O(g) — CO(g) + H2(g) [C]Fe + 2 H2O(g) — CO2(g) + 2 H2(g)
(1) (2)
(3)
(4)
(5)
The decarburization in a gas mixture of H2-H2O from 700 to 900°C proceeds predominantly according to reaction [C]Fe + H2O(g) — CO(g) + H2®, the reaction according to [C]Fe + 2 H2(g) — CH4(g) can be neglected at p(H2O)/ p(H2) greater than 0.012.
While the carbon is oxidised to gases CO and CO2, the steel surface is continually oxidised to an oxide layer,
KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 
which is influenced by alloying elements, affecting the scaling process of iron. When the alloying elements are less noble than iron then the possibility of internal oxidation arises, an external oxide layer may also be formed under circumstances which are normally protective to iron3. The following reactions of oxidation occur at the surface of the steel:
2 Few + O2(g) — 2 FeO,
3 Fe(s) + 2 O2(g)
2 Few + 3/2 O2(g) :
(s)
Fe3O4(s) Fe2O3(s)
Fe(s) + CO2(g)
Si(s) + O2(g)
: FeO(S) + CO(g)
SiO
2(s)
2 Al(S) + 3/2 O2(g) — ALAW Mn(S) + 1/2 O2(g) — MnO(S)
2 EXPERIMENTAL
(6)
(7)
(8) (9)
(10) (11) (12)
The samples were laboratory manufactured Fe-Si-Al alloys, hot and cold rolled strips with a final thickness of 1.5 mm and composition shown in Table 1. For thermo-gravimetric measurements samples were cut to a final dimension of 20 x 10 x 1,5 [mm3], ground and polished with 1 mm grade diamond paste. All samples were cleaned in an ultrasound acetone bath.
The samples were submitted to decarburization annealing at 840°C in a gas mixture of hydrogen and water vapour with ratio p(H2O)/p(H2) of 0.03 to 0.3. After de-carburization the specimens were investigated by optical microscopy, SEM, and AES.
3-4	165
D. STEINER PETROVIČ ET AL.: DECARBURIZATION MECHANISM..
Table 1: Chemical composition of Fe-Si-Al alloys (wt.%) Tabela 1: Kemi~na sestava zlitin Fe-Si-Al (mas.%)
Alloy No.	%C	%Si	%Al	%Mn	%P %S	%Sb
2	0.260	1.74	1.19	0.23	0.0015 0.0023	-
3	0.009	1.40	0.70	0.25	0.0015 0.0020	0.048
4	0.010	1.55	0.70	0.25	0.0015 0.0024	-
6	0.280	1.94	0.80	0.17	0.0015 0.0023	0.048
Figure 1: Ferrite microstructure of alloy No.2 after decarburization (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, M 500x, Nital) Slika 1: Feritna mikrostruktura zlitine No.2 po žarjenju za razoglji~enje (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, P 500x, Nital)
3 RESULTS AND DISCUSSION
Metallografic examination of annealed samples of Fe-Si-Al alloys showed that the decarburization process of Fe-Si-Al alloys at 840°C was efficient with exception of the alloy No.6 containing 0.28 wt.% C and 0.048 wt.% Sb. The microstructure of all decarburized steels after heat treatment consisted of polyhedral ferrite grains (Figures 1,3,4). The grain growth of recrystallized
Figure 3: Ferrite microstructure of alloy No.3 (with 0.048 wt.% Sb) after decarburization (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, M 500x, Nital)
Slika 3: Feritna mikrostruktura zlitine No.3 (z 0.048 mas.% Sb) po žarjenju za razoglji~enje (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, P 500x, Nital)
grains depends on carbon content in the steel No.4,5. The grain growth rate was greater in decarburized steels and a coarse average grain size was obtained after decar-burization. Decarburization in alloy No.6 took place only in a shallow layer under the surface (Figure 2), where a ferritic microstructure with internal oxidation along the grain boundaries was formed. The microstructure of the non-decarburized part of the sample No.6 with 0.28 wt.%C and 0.048 wt.% Sb remained ferrite-pearlite.
The samples of investigated steels were found to oxidise initially in a decarburizing atmosphere. The layers formed on the surface consisted of iron, aluminium and manganese oxides. The thickness of the oxide layer increased with increasing the time of annealing and the oxygen partial pressure in the gas mixture (p(H2O)/ petto). The surface oxidation during the decarburization was function of the temperature and oxygen potential and it was predominantly influenced by the behaviour of alloying elements Al and Mn.
Antimony was found to promote pearlite formation6, therefore, the phase equilibrium in ferritic steel with an-
Figure 2: Ferrite-pearlite microstructure of alloy No.6 (with 0.048 wt.% Sb) after decarburization (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, M 500x, Nital)
Slika 2: Feritno-perlitna mikrostruktura zlitine No.6 (z 0.048 mas.% Sb) po žarjenju za razoglji~enje (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, P 500x, Nital)
Figure 4: Ferrite microstructure of alloy No.4 after decarburization (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, M 500x, Nital) Slika 4: Feritna mikrostruktura zlitine No.4 po žarjenju za razoglji~enje (p(H2O)/p(H2)=0.03, t=15 min, T=840°C, P 500x, Nital)
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D. STEINER PETROVIČ ET AL.: DECARBURIZATION MECHANISM..
Figure 5: Mechanism of decarburization of Fe-Si-Al alloy with 0.28 wt.% C and 0.048 wt.% Sb
Slika 5: Mehanizem razoglji~enja zlitine Fe-Si-Al z 0.28 mas.% C in 0.048 mas.% Sb
timony addition is established by more precipitated and less carbon in solid solution7. The smaller rate of decar-burization of steel with antimony addition supports this hypothesis. Since the oxidation of carbon in Fe-Si-Al alloy with 0.28 wt.% C and 0.048 wt.% Sb was rather slow, the oxide layer formed on the surface of alloy No.6 was very compact. The surface and the internal oxidation during the decarburization of antimony doped Fe-Si-Al alloy has already been discussed in reference8. The continuous formation of gas molecules of CO and CO2 during the decarburization of alloy No.2 resulted in a blis-
Figure 6: Kinetics of oxidation of Fe-Si-Al alloy with 0.009 wt.% C and 0.048 wt.% Sb during heat treatment at 840°C Slika 6: Kinetika oksidacije zlitine Fe-Si-Al z 0.009 mas.% C in 0.048 wt.% Sb med žarjenjem pri 840°C
495	KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6
Figure 7: Mechanism of decarburization of Fe-Si-Al alloy with 0.009 wt.% C and 0.048 wt.% Sb
Slika 7: Mehanizem razoglji~enja zlitine Fe-Si-Al z 0.009 mas.% C in 0.048 mas.% Sb
ters and cracks in the oxide scale8. The transport of carbon oxides can occur only through cracks and pores because the solubility and diffusivity of carbon in oxides and oxide scales are extremely small9.
On the thermogravimetric curve of Fe-Si-Al alloy No.3 containing 0.009 wt.% C and 0.048 wt.% Sb the mass loss due to decarburization at p(H2O)/p(H2) = 0.03 was more pronounced than the mass gain due to oxidation (Figure 6).
The less oxidised surface of Fe-Si-Al alloy No.3 after decarburization is likely to be the result of antimony surface segregation. Due to a low carbon content in the steel, the surface segregation of antimony10-17 hindered the dissociation reaction of water vapour molecules and the oxidation of the steel surface as well18 (Figure 7).
4 SUMMARY
The decarburization mechanism of Fe-Si-Al alloys in a H2-H2O gas mixture at 840°C depends on the diffusion of carbon to the steel surface and chemical reactions on the surface.
During the decarburization of steel sheets in a flow apparatus mass gain was observed which implies on the oxidation of the steel surface. The oxidation of carbon on the steel surface was a function of temperature, oxygen potential of the atmosphere and was influenced by the selective oxidation of alloying elements Si, Al and Mn which have a higher affinity toward oxygen than iron. The thickness of the oxide layers increased with increasing time of decarburization annealing. The formation of oxide layers was a function of temperature and oxygen potential of the atmosphere as well. Formation of gaseous molecules due to high decarburization ability of antimony free alloy with 0.26 wt.% C resulted in porous and cracked oxide layer.

D. STEINER PETROVIČ ET AL.: DECARBURIZATION MECHANISM..
The influence of antimony on the decarburization mechanism of Fe-Si-Al alloys is associated with carbon content in the steel.
Antimony addition in Fe-Si-Al alloy decreases the solubility of carbon, promotes precipitation of carbides and delays decarburization process. The decarburization process of Fe-Si-Al alloy containing 0.28 wt.% C and 0.048 wt.% Sb was hindered by a protective oxide layer and the steel could be decarburized only beneath the surface. The oxide layer was more compact in comparison to antimony free alloys. Internal oxidation along grain boundaries affected the course of oxidation in the Fe-Si-Al alloys containing 0.048 wt.% Sb.
On the other hand, the decarburization process of Fe-Si-Al alloy containing 0.009 wt.% C and 0.048 wt.% Sb at p(H2O)/p(H2) = 0.03 was more efficient due to a surface segregation of antimony. Antimony segregating to the steel surface hindered the dissociation reaction of water vapour molecules and oxidation of the steel surface.
ACKNOWLEDGEMENT
The investigation was supported by Ministry of Science and Technology, Slovenia through the project J2-7228-206 and Max-Planck-Gesellschaft, Germany.
5 REFERENCES
1 L. L. Shreir, R. A. Jarman, G. T. Burstein, Corrosion, Vol.1, Metal/Environment Reactions, Third Edition, Butterworth-Heinemann, Oxford, 1995
2H. J. Grabke, G. Tauber, Arch. Eisenhüttenwes., 46 (1975) 3, 215-222
3N. Birks, in Decarburization, The Iron and Steel Institute, London, 1970, 1-11
4D. Steiner, M. Jenko, F. Vodopivec, Kovine, zlitine, tehnologije, 28 (1994) 1-2, 125-129
5	D. Steiner Petrovi~, M. Jenko, F. Vodopivec, Journal de Physique IV, 5 (1995) C7, 255-258
6	E. N. Pan, C. Y. Chen, American Foundry Men Society Transactions, 70 (1996) 845-858
7	F. Vodopivec, M. Jenko, D. Steiner Petrovi~, B. Breskvar, F. Marin{ek, Steel Research, 68 (1997) 80-86
8	D. Steiner Petrovi~, M. Jenko, F. Vodopivec, H. J. Grabke, Kovine, zlitine, tehnologije, 31 (1997) 3-4, 209-211
9	P. Kofstad, A. Rahmel, R. A. Rapp, D. L. Douglass, Oxidation of Metals, 32 (1989) 1/2, 125-166
10H. J. Grabke, ISIJ Int., 29 (1989) 529-533
11 C. L. Briant, A. M. Ritter, Acta Metall., 32 (1984) 2031-2042
12G. Lyudkovsky, P. K. Rastogi, M. Bala, Journal of Metals, 1 (1986) 18-25
13M. Rüsenberg, H. Viefhaus, Surf. Sci., 172 (1986) 615-619
14 M. Jenko, F. Vodopivec, B. Pra~ek, M. Godec, D. Steiner, J. Mag. Mat., 133 (1994) 229-232
15M. Jenko, F. Vodopivec, H. Viefhaus, M. Milun, T. Valla, M. Godec, D. Steiner Petrovi~, Fizika, A4 (1995) 3, 91-98
16	M. Jenko, F. Vodopivec, H. J. Grabke, H. Viefhaus, M. Godec, D. Steiner Petrovi~, Journal de Physique IV, 5 (1995) C7, 225-231
17	R. Mast, PhD Thesis, University of Dortmund, Germany, 1996
18	D. Steiner Petrovi~, MD Thesis, University of Ljubljana, Slovenia, 1998
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