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A superficial coating to improve oxidation and decarburization resistance of bearing steel at high temperature
Applied Surface Science 258 (2012) 4977–4982
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A superficial coating to improve oxidation and decarburization resistance of bearing steel at high temperature Xiaojing Wang a,b , Lianqi Wei a , Xun Zhou a,b , Xiaomeng Zhang a , Shufeng Ye a,∗ , Yunfa Chen a a b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, P. O. BOX 353, Beijing 100190, China Graduate University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 2 November 2011 Received in revised form 18 December 2011 Accepted 24 January 2012 Available online 2 February 2012 Keywords: Bearing steel High temperature oxidation Decarburization Coatings
a b s t r a c t The coating material consisted of aqueous slurry of dolomite, bauxite and silicon carbide mixture. Such a coating material when applied superficially on the steel surface not only enhances oxidation resistance but also helps in inhibiting the decarburization even up to 1250 ◦ C. Metalloscope, XRD and TG-DTA thermal analysis revealed that the formation of a newly densified coating comprised of spinels and the reducing atmosphere formed by the oxidation of SiC improved the resistance of oxidation and decarburization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years high carbon chromium bearing steel has been widely used because it has the advantages of comprehensive performance, convenient operation and low production cost [1,2]. However, Annealing and reheating treatments on bearing steels are generally carried out in furnaces having an oxidizing atmosphere. Scales are formed on the steel materials during their heating, resulting in a lowered product yield, accompanied with various problems, such as lowering in the commercial value of the products due to surface imperfection and lowering in the strength of the steel products due to decarburization [3,4]. Decarburization is a loss of carbon atoms from the surface of the work pieces, thereby producing a surface with lower carbon content than at some other distance beneath the surface. It implies changes of mechanical properties as, e.g., the changes of hardness and of fatigue resistance [5]. Particularly in the case of steel materials, such as bearing billets, which contain about 1.5% carbon, the influence on the steel quality by the lowering in strength due to the surface decarburization is very significant [6]. Decarburization and oxidation occur simultaneously. The reaction rates are high, and there is competition between the two phenomena: oxidation rapidly consumes the decarburized metal [3].
∗ Corresponding author. Tel.: +86 10 62588029; fax: +86 10 82544919. E-mail address: [email protected] (S. Ye). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.135
Therefore, it has a lot of difficulties to prevent steel from oxidation and decarburization simultaneously. The aim of this work is to fabricate a high temperature corrosion resistant coating. The microstructure and sintering behaviors of the coatings were investigated. The corrosion resistant mechanism also is discussed. 2. Experimental 2.1. Preparation of the coating The chemical composition (in weight percent) of the bearing steel used in the present study is C 1.00%, Cr 1.50%, Mn 0.30%, Si 0.25%, S 0.01%, P 0.01%, and Fe balance. Specimens with dimensions of 10 mm × 10 mm × 10 mm were prepared by rolling and intermediate annealing (at 500 ◦ C) operations. Subsequently, the specimens were mechanically polished by abrading on SiC papers in succession up to 800 grit followed by cleaning and rinsing in supersonic treatment and dried for coating protective layers. The coating materials, consisted of dolomite, bauxite and silicon carbide, were mixed in various proportions. The compositions of the dolomite and bauxite are listed in Tables 1 and 2. The mixture was diluted with water to secure the proper viscosity, which enabled deposition by painting or spraying. The addition of the binding agent (citric acid) was necessary for good adhesion to the surface of the steel after drying.
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X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
Table 1 Compositions of the dolomite.
Table 3 Compositions and the corrosion resistance of the coatings.
Component
MgO
CaO
SiO2
Fe2 O3
Al2 O3
Parts by weight
Content [wt.%]
39.82
58.72
0.64
0.46
0.36
Dolomite Silicon carbide Bauxite Citric acid distilled water Anti-oxidation effect, % Anti-decarburization effect, %
Table 2 Compositions of the bauxite. Component
Al2 O3
SiO2
TiO2
Fe2 O3
CaO
K2 O
Content [wt.%]
63.96
29.15
3.36
2.41
0.76
0.36
2.2. Experimental method Two types of bearing steel specimens, bare and coated, were weighted, heated for 0–120 min at temperatures of 950–1250 ◦ C, and then weighed again without scale. m = m1 − m2
(1)
m1 − m2 × 100% Steel loss = m1
(2)
Anti-oxidation effect ıoxi =
mbare − mcoated × 100% mbare
(3)
where m1 and m2 are the weights of the specimens before and after oxidation without scale, g. mbare and mcoated are the weight loss of the bare specimen and the coated one. Every specimen was cut into two parts by cutter bar and one of them was polished by abrading on SiC papers. The specimens were etched by 4 vol% admixture of nitric acid and absolute ethyl alcohol. Finally, they were dehydrated by absolute ethyl alcohol and used for metallographic observation to determine the decarburization layer. Anti-decarburization effect ıdec =
dbare − dcoated × 100% dbare
(4)
where dbare and dcoated are the decarburized depths of the bare and the coated specimens, m. 2.3. Characterization Oxidation-kinetics studies of bare and coated specimens were carried out by a thermobalance (RZ, Luoyang Precondar, China) which was equipped with a continuous weighing capacity of 500 g, sensitivity of 1 mg and data recorded every 60 s. The heating rate employed was 10 ◦ C/min up to a final temperature of 1250 ◦ C. Energy change with the temperature rising was detected by a TG-DTA thermal analysis system (TG-DTA, Beijing Scientific Instrument Factory, China). Microstructures of the decarburization were observed by an optical microscope (polarization microscope, DM LP/P, Leica, Germany). Data of X-ray diffractometer (XRD, X’Pert Pro, Philips, Netherlands) were collected to analyze the change of phases in the coating.
A
B
C
D
64 20 0 7 25 64.00
64 0 20 7 25 56.00
64 20 40 7 37 48.00
64 40 20 7 37 68.00
100.00
−14.03
3.36
100.00
100.00
is because the fine SiC powder was oxidized and changed gradually to protective cristobalite-SiO2 layer which acted as an excellent barrier to oxygen diffusion from atmosphere. The protective SiO2 was not formed from the SiO2 which was added initially but was newly formed through the oxidation process of the SiC. Graphite (C) was also formed due to the oxidation of SiC, keeping a reducing atmosphere even at quite high temperatures [7]. The presence of considerable graphite in this way indicated that the diffusion of C and O is impeded and the decarburization was effectively prevented. Compared B (without bauxite in the coating), the specimen A remarkably increased the decarburization resistance and also had a better anti-oxidation effect. This is based on the decomposition of bauxite and its reaction with Fe and dolomite. The products SiO2 , FeO·Al2 O3 , ␣-Al2 O3 and spinel were nonpermeable to oxygen [8,9]. However, by comparing specimens A and D, it has been found that overmuch bauxite would make the oxidation resistance drop down because the anti-oxidation effect of the silicon oxide produced by the decomposition of bauxite actually deteriorated at temperatures in excess of 1200 ◦ C. This is due to the formation of fayalite (2FeO·SiO2 ), which had a low melting point. Fayalite formed a eutectic with wustite above 1177 ◦ C that promoted the rapid grow of the oxide by liquid oxide attack [7,10]. The decarburization resistance did not decrease due to the increase of oxide scale which consumed the decarburized layer produced below 1177 ◦ C [3]. This can be explained by the competition between oxidation and decarburization. As shown in Fig. 1, the practical oxidation equals to the theoretical oxidation. The depth difference between the theoretical decarburization and the theoretical oxidation determines the practical decarburization. 3.2. The effects of heating temperature Coating A in Table 3 was applied in the test and the effects of heating temperature are shown in Table 4. From both the curves in Fig. 2, which shows the difference in the weight of the cleaned specimens before and after heating, it can be
3. Results and discussion 3.1. The effects of coating components The coating materials were mixed according to Table 3 and the effects of anti-oxidation and anti-decarburization at 1250 ◦ C for 60 min were evaluated. Dolomite was chosen as the main raw material because magnesium oxide has high melting point and can be regarded as the framework of the coating. As shown in Table 3, the oxidation and decarburization prevention were obviously improved when SiC is admixed into the coating by comparing the specimens A and C. This
E
64 20 20 7 29 68.00
Fig. 1. The competition between oxidation and decarburization.
X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
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Table 4 The oxidation and decarburization resistance of coating A heated for 0–120 min at temperatures of 950–1250 ◦ C. Temperature, time
ıoxi , %
ıdec ,%
Temperature, time
ıoxi , %
ıdec ,%
950 ◦ C, 60 min 1050 ◦ C, 60 min 1150 ◦ C, 60 min 1250 ◦ C, 60 min
44.20 60.43 61.93 64.44
6.45 6.85 7.05 100.00
1250 ◦ C, 0 min 1250 ◦ C, 30 min 1250 ◦ C, 90 min 1250 ◦ C, 120 min
67.19 67.17 60.43 63.60
65.14 100.00 100.00 100.00
observed that the steel loss increased with increasing temperature, for both types (bare and coated) of specimens. The steel loss of the coated specimen was smaller than that of the bare one during the heating process. The amount of SiO2 in bauxite was much higher than that in mullite; the excess SiO2 together with the impurities in bauxite formed an amorphous SiO2 and cristobalite to accompany the formation of mullite at temperature about 1000 ◦ C [11]. Bauxite is composed of diaspore (Al2 O3 ·H2 O) and kaolinite (Al2 O3 ·2SiO2 ·2H2 O). The reactions (5) and (6) occurred because alumina and metakaolinite (Al2 O3 ·2SiO2 ) were formed when diaspore and kaolinite were heated up to 500 ◦ C [12–14]. Heating at 980 ◦ C leaded to a direct formation of ␥-alumina and amorphous SiO2 or spinel (SiAl2 O4 ) according to reactions (7) and (8). Mullite phase first appeared at a temperature around 1100 ◦ C according to reactions (9) and (10), its amount increased with the increase of temperature. The amorphous SiO2 changed to cristobalite above 1200 ◦ C according to reaction (11). Al2 O3 ·H2 O (diaspore) → Al2 O3 (␥-alumina) + H2 O Al2 O3 ·2SiO2 ·2H2 O (kaolinite) → Al2 O3 ·2SiO2 (metakaolinite) + 2H2 O
(5) (6)
Al2 O3 ·2SiO2 (metakaolinite) → Al2 O3 (␥-alumina) + 2SiO2 (amorphous)
(7)
Al2 O3 ·2SiO2 (metakaolinite) → SiAl2 O4 (spinel) + SiO2 (amorphous)
(8)
SiAl2 O4 (spinel) + SiO2 (amorphous) → 1/3(3Al2 O3 ·2SiO2 ) (mullite) + 4/3SiO2 (amorphous)
(9)
Al2 O3 (␥-alumina) + 2SiO2 (amorphous) → 1/3(2Al2 O3 ·2SiO2 ) (mullite) + 4/3SiO2 (amorphous)
(10)
Fig. 2. Steel loss of GCr15 bearing steel specimens during heating for 60 min after scaling.
3Al2 O3 ·2SiO2 (mullite) + 4SiO2 (amorphous) → 3Al2 O3 ·2SiO2 (mullite) + 4SiO2 (cristobalite)
(11)
The amorphous SiO2 , cristobalite and mullite had certain effect in impeding the diffusion of oxygen. However, the coating was not compact enough that oxidation still existed [15]. At higher temperatures, the protection was much effective (Table 4), because on one hand, graphite (C) was formed due to the oxidation of SiC, keeping a reducing condition within the coating even at quite high temperatures [7]. On the other hand, new sintered phases were nonpermeable to oxygen at high temperature. Fig. 3 shows the new phases formed at 1150 ◦ C, including several spinels (MgCr2 O4 , (Mg, Fe) (Cr, Al)2 O4 , MgAl2 O4 and Fe (Cr, Al)2 O4 ) [8,9]. From Fig. 4 it is evident that the decarburization of the bare specimen increased below 1150 ◦ C. With increasing temperature above 1150 ◦ C, the growth rate of the decarburized layer decreased because the diffusions of C and O were retarded by the thick oxide scale of the bare specimen in a certain degree when heated at high temperature [16]. Below 1150 ◦ C, the anti-decarburization effect of the coating was less than 7.05%, because the decarburized depth of the bare specimen dbare was not large after the scale was cleaned. The bare specimen generated more serious oxidation on the surface (Fig. 2), consuming certain depth of decarburized layer. Although for the coated specimen, the coating protected it from decarburization in a certain degree due to the formation of mullite and glassy phase, the anti-decarburization effect ıdec was still small according to Eq. (4). At higher temperature, above 1150 ◦ C, the protection of decarburization was much effective, because the main reactions in the coating, which gave the protective properties, started at around 1050 ◦ C, as evident from the DTA curve. The scale and decarburized layer, formed at the previous stage, reacted with the coating materials to produce the newly densified film, which hindered the secondary decarburization. Therefore the coated specimen heated at 1250 ◦ C had no decarburization after scaling.
Fig. 3. XRD patterns of bare specimen (a) and coated specimen (b) after heating at 1150 ◦ C.
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X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
Fig. 4. Decarburization growth of bare and coated specimens during heating for 60 min.
Fig. 6. Isothermal oxidation kinetics of bare and coated specimens at 1250 ◦ C in air.
Fig. 5 shows the bare specimen experienced endothermic process at 780 ◦ C because of austenization. And at about 950 ◦ C, an evident exothermal process happened because of the oxidation. By contrast, the coated specimen experienced an endothermic process below 100 ◦ C because of the dehydration of coating. At 175 ◦ C
a stronger endothermic reaction started, assumed to be the melting process of citric acid. The exothermic reactions are observed from 450 ◦ C to 540 ◦ C due to the decarburization of the excessive citric acid and the combustion of the residual carbon [17,18]. However, metakaolinite were formed at around 500 ◦ C, therefore, there was an endothermic peak separating the exothermal peak of citric acid [12–14]. The reactions (12) and (13) are the two steps of thermal decomposition of dolomite. The reaction (12), happened at around 790 ◦ C, was not been clearly expressed in Fig. 5 because similar with the bare specimen, the coated specimen experienced austenization process at 780 ◦ C and its peak covered the peak of reaction (12). The endothermal peak can be seen at around 935 ◦ C due to reaction (13) and the peak of the decomposition of metakaolinite was overlapped with the reaction (13). The exothermal peak can be seen at around 1100 ◦ C and 1200 ◦ C because the formation of mullite phases according to reactions (9) and (10) and cristobalite according to reaction (11). CaMg(CO3 )2 → CaCO3 + MgO + CO2
(12)
CaCO3 → CaO + CO2
(13)
The reaction of coating materials with scale and decarburized layer started above 1000 ◦ C.
Fig. 5. DTA curve of specimens with and without coating: (a) complete and (b) partial enlarged.
Fig. 7. Steel loss of GCr15 bearing steel specimens at 1250 ◦ C in air.
X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
Fig. 8. Decarburization growth of bare and coated specimens at 1250 ◦ C in air.
3.3. The effects of holding time Coating A in Table 3 was applied in the test and the effects of holding time are shown in Table 4.
4981
The isothermal oxidation kinetic curves of bare and coated specimens at 1250 ◦ C in air for 6 h were plotted as a graph of weight gain versus time and are illustrated in Fig. 6. It can be seen the coating reduce oxidation rate of GCr15 bearing steel. Fig. 7 shows the bare specimens severely suffered oxidation, and quite a lot of steel was lost in the form of oxide scale. There was a marked reduction in loss of steel volume in the whole heating process when the specimens were coated. The effect of the coating was based on the formation of spinels, which exhibit low permeability to oxygen at temperature up to 1250 ◦ C. It can be observed in Fig. 8 that the decarburization of the bare specimen increased with the extension of holding time. On the contrary, the decarburization of the coated specimen inversely decreased with the increase of holding time. When the temperature just increased to 1250 ◦ C, the coated specimen had decarburization layer formed below 1250 ◦ C, because the reactions of the coating materials, which produced new densified coating, did not start at low temperature. Despite the glassy phase, cristobalite and mullite, formed by the decomposition of bauxite at low temperature, had certain effect in impeding the diffusion of oxygen, decarburization still existed when holding time was less than 30 min. When the holding time was prolonged at 1250 ◦ C, the coating materials, decarburization layer and the scale quickly took part in the formation of effective film and the decarburization, previously generated, disappeared quickly as a result. Then the film was so compact that it blocked the secondary diffusion of carbon and oxygen at high temperature. Fig. 9 shows that the decarburization depth of the bare specimen reached 839.51 m after heating for 2 h at 1250 ◦ C. In contrast, the coated specimen had no decarburization completely. The coating could diminish the decarburization on the steel surface for up to 100% (Table 4). 4. Conclusions 1. A novel coating was successfully fabricated by using dolomite, bauxite, silicon carbide and binding agent to prevent oxidation and decarburization during reheating process of bearing steels. 2. The performed tests confirmed that SiO2 in bauxite formed a glassy phase and cristobalite to accompany the formation of mullite at 1000 ◦ C. The products had certain effect in preventing oxidation and decarburization, but the coating was not compact enough that oxidation and decarburization still existed. 3. At higher temperature, the protection was much effective, because on one hand, graphite was formed due to the oxidation of SiC, keeping a reducing atmosphere. On the other hand, the scale and decarburized layer, formed at previous stage, reacted with the coating materials to produce the newly densified film above 1150 ◦ C. The film was mainly composed of spinels and it could hinder the secondary oxidation and decarburization. Acknowledgments The authors gratefully acknowledge the financial supports from Key Topics in Innovation Engineering Funded by the Chinese Academy of Sciences (No. KGCX2-YW-224), Key Projects in the National Science & Technology Pillar Program in the Eleventh Fiveyear Plan Period (No. 2006BAC02A14), and the National Natural Science Foundation of China (No. 50774073). References
Fig. 9. Microstructure of steel GCr15, after heating for 2 h at 1250 ◦ C in a muffle furnace: (a) bare and (b) coated.
[1] D.W. Hetzner, Laser glazed bearings, ASTM Spec. Tech. Publ. 1327 (1998) 471–495. [2] S. Ganguly, I. Chakrabarti, M.D. Maheshwari, T. Mukherjee, Ultra clean steel for anti-friction bearing applications, in: J.M. Beswick (Ed.), Bearing Steel Technology, 2002, 100 Barr harbor drive, West Conshohochen, pp. 47–70.
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[3] J. Baud, A. Ferrier, J. Manenc, J. Bénard, The oxidation and decarburizing of Fe–C alloys in air and the influence of relative humidity, Oxid. Met. 9 (1975) 69–97. [4] Y. Prawoto, N. Sato, I. Otani, M. Ikeda, Carbon restoration for decarburized layer in spring steel, J. Mater. Eng. Perform. 13 (2004) 627–636. [5] M. Hajduga, J. Kuˇcera, Decarburization of Fe–Cr–C steels during hightemperature oxidation, Oxid. Met. 29 (1988) 419–433. [6] M. Kitayama, H. Odashima, Method for preventing decarburization of steel materials, USA Patent no. 4,227,945 (1980). [7] H. Odashima, M. Kitayama, Oxidation-inhibition mechanism and performance of a new protective coating for slab reheating of 3-percent Si-steel, ISIJ Int. 30 (1990) 255–264. [8] M. Torkar, B. Glogovac, F. Kaucic, D. Finzgar, S. Triglav, Diminution of scaling by the application of a protective coating, J. Mater. Process. Technol. 58 (1996) 217–222. [9] L. Wei, P. Liu, S. Ye, Y. Xie, Y. Chen, Preparation and properties of anti-oxidation inoganic nano-coating for low carbon steel at an elevated temperature, J. Wuhan Univ. Technol. 21 (2006) 48–52. [10] L. Suárez, P. Rodríguez-Calvillo, Y. Houbaert, R. Colás, Oxidation of ultra low carbon and silicon bearing steels, Corros. Sci. 52 (2010) 2044–2049.
[11] O. Castelein, B. Soulestin, J.P. Bonnet, P. Blanchart, The influence of heating rate on the thermal behaviour and mullite formation from a kaolin raw material, Ceram. Int. 27 (2001) 517–522. [12] C.Y. Chen, G.S. Lan, W.H. Tuan, Microstructural evolution of mullite during the sintering of kaolin powder compacts, Ceram. Int. 26 (2000) 715–720. [13] C.J. McConville, W.E. Lee, J.H. Sharp, Microstrctural evolution infired kaolinite, Br. Ceram. Trans. 97 (1998) 162–168. [14] W.E. Lee, G.P. Souza, C.J. McConville, T. Tarvornpanich, Y. Iqbal, Mullite formation in clays and clay-derived vitreous ceramics, J. Eur. Ceram. Soc. 28 (2008) 465–471. [15] C.Y. Chen, G.S. Lan, W.H. Tuan, Preparation of mullite by the reaction sintering of kaolinite and alumina, J. Eur. Ceram. Soc. 20 (2000) 2519–2525. [16] N.P. Zhetvin, L.N. Podvoiskii, L.I. Krylova, Investigation of the kinetics of surface decarburization in ball bearing steel when heat treated, Met. Sci. Heat Treat. 2 (1960) 105–108. [17] M.M. Barbooti, D.A. Al-Sammerrai, Thermal decomposition of citric acid, Thermochim. Acta 98 (1986) 119–126. [18] J. Mastowska, Thermal decomposition and thermofractochromatographic studies of metal citrates, J. Therm. Anal. 29 (1984) 895–904.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A superficial coating to improve oxidation and decarburization resistance of bearing steel at high temperature Xiaojing Wang a,b , Lianqi Wei a , Xun Zhou a,b , Xiaomeng Zhang a , Shufeng Ye a,∗ , Yunfa Chen a a b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, P. O. BOX 353, Beijing 100190, China Graduate University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 2 November 2011 Received in revised form 18 December 2011 Accepted 24 January 2012 Available online 2 February 2012 Keywords: Bearing steel High temperature oxidation Decarburization Coatings
a b s t r a c t The coating material consisted of aqueous slurry of dolomite, bauxite and silicon carbide mixture. Such a coating material when applied superficially on the steel surface not only enhances oxidation resistance but also helps in inhibiting the decarburization even up to 1250 ◦ C. Metalloscope, XRD and TG-DTA thermal analysis revealed that the formation of a newly densified coating comprised of spinels and the reducing atmosphere formed by the oxidation of SiC improved the resistance of oxidation and decarburization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years high carbon chromium bearing steel has been widely used because it has the advantages of comprehensive performance, convenient operation and low production cost [1,2]. However, Annealing and reheating treatments on bearing steels are generally carried out in furnaces having an oxidizing atmosphere. Scales are formed on the steel materials during their heating, resulting in a lowered product yield, accompanied with various problems, such as lowering in the commercial value of the products due to surface imperfection and lowering in the strength of the steel products due to decarburization [3,4]. Decarburization is a loss of carbon atoms from the surface of the work pieces, thereby producing a surface with lower carbon content than at some other distance beneath the surface. It implies changes of mechanical properties as, e.g., the changes of hardness and of fatigue resistance [5]. Particularly in the case of steel materials, such as bearing billets, which contain about 1.5% carbon, the influence on the steel quality by the lowering in strength due to the surface decarburization is very significant [6]. Decarburization and oxidation occur simultaneously. The reaction rates are high, and there is competition between the two phenomena: oxidation rapidly consumes the decarburized metal [3].
∗ Corresponding author. Tel.: +86 10 62588029; fax: +86 10 82544919. E-mail address: [email protected] (S. Ye). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.135
Therefore, it has a lot of difficulties to prevent steel from oxidation and decarburization simultaneously. The aim of this work is to fabricate a high temperature corrosion resistant coating. The microstructure and sintering behaviors of the coatings were investigated. The corrosion resistant mechanism also is discussed. 2. Experimental 2.1. Preparation of the coating The chemical composition (in weight percent) of the bearing steel used in the present study is C 1.00%, Cr 1.50%, Mn 0.30%, Si 0.25%, S 0.01%, P 0.01%, and Fe balance. Specimens with dimensions of 10 mm × 10 mm × 10 mm were prepared by rolling and intermediate annealing (at 500 ◦ C) operations. Subsequently, the specimens were mechanically polished by abrading on SiC papers in succession up to 800 grit followed by cleaning and rinsing in supersonic treatment and dried for coating protective layers. The coating materials, consisted of dolomite, bauxite and silicon carbide, were mixed in various proportions. The compositions of the dolomite and bauxite are listed in Tables 1 and 2. The mixture was diluted with water to secure the proper viscosity, which enabled deposition by painting or spraying. The addition of the binding agent (citric acid) was necessary for good adhesion to the surface of the steel after drying.
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X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
Table 1 Compositions of the dolomite.
Table 3 Compositions and the corrosion resistance of the coatings.
Component
MgO
CaO
SiO2
Fe2 O3
Al2 O3
Parts by weight
Content [wt.%]
39.82
58.72
0.64
0.46
0.36
Dolomite Silicon carbide Bauxite Citric acid distilled water Anti-oxidation effect, % Anti-decarburization effect, %
Table 2 Compositions of the bauxite. Component
Al2 O3
SiO2
TiO2
Fe2 O3
CaO
K2 O
Content [wt.%]
63.96
29.15
3.36
2.41
0.76
0.36
2.2. Experimental method Two types of bearing steel specimens, bare and coated, were weighted, heated for 0–120 min at temperatures of 950–1250 ◦ C, and then weighed again without scale. m = m1 − m2
(1)
m1 − m2 × 100% Steel loss = m1
(2)
Anti-oxidation effect ıoxi =
mbare − mcoated × 100% mbare
(3)
where m1 and m2 are the weights of the specimens before and after oxidation without scale, g. mbare and mcoated are the weight loss of the bare specimen and the coated one. Every specimen was cut into two parts by cutter bar and one of them was polished by abrading on SiC papers. The specimens were etched by 4 vol% admixture of nitric acid and absolute ethyl alcohol. Finally, they were dehydrated by absolute ethyl alcohol and used for metallographic observation to determine the decarburization layer. Anti-decarburization effect ıdec =
dbare − dcoated × 100% dbare
(4)
where dbare and dcoated are the decarburized depths of the bare and the coated specimens, m. 2.3. Characterization Oxidation-kinetics studies of bare and coated specimens were carried out by a thermobalance (RZ, Luoyang Precondar, China) which was equipped with a continuous weighing capacity of 500 g, sensitivity of 1 mg and data recorded every 60 s. The heating rate employed was 10 ◦ C/min up to a final temperature of 1250 ◦ C. Energy change with the temperature rising was detected by a TG-DTA thermal analysis system (TG-DTA, Beijing Scientific Instrument Factory, China). Microstructures of the decarburization were observed by an optical microscope (polarization microscope, DM LP/P, Leica, Germany). Data of X-ray diffractometer (XRD, X’Pert Pro, Philips, Netherlands) were collected to analyze the change of phases in the coating.
A
B
C
D
64 20 0 7 25 64.00
64 0 20 7 25 56.00
64 20 40 7 37 48.00
64 40 20 7 37 68.00
100.00
−14.03
3.36
100.00
100.00
is because the fine SiC powder was oxidized and changed gradually to protective cristobalite-SiO2 layer which acted as an excellent barrier to oxygen diffusion from atmosphere. The protective SiO2 was not formed from the SiO2 which was added initially but was newly formed through the oxidation process of the SiC. Graphite (C) was also formed due to the oxidation of SiC, keeping a reducing atmosphere even at quite high temperatures [7]. The presence of considerable graphite in this way indicated that the diffusion of C and O is impeded and the decarburization was effectively prevented. Compared B (without bauxite in the coating), the specimen A remarkably increased the decarburization resistance and also had a better anti-oxidation effect. This is based on the decomposition of bauxite and its reaction with Fe and dolomite. The products SiO2 , FeO·Al2 O3 , ␣-Al2 O3 and spinel were nonpermeable to oxygen [8,9]. However, by comparing specimens A and D, it has been found that overmuch bauxite would make the oxidation resistance drop down because the anti-oxidation effect of the silicon oxide produced by the decomposition of bauxite actually deteriorated at temperatures in excess of 1200 ◦ C. This is due to the formation of fayalite (2FeO·SiO2 ), which had a low melting point. Fayalite formed a eutectic with wustite above 1177 ◦ C that promoted the rapid grow of the oxide by liquid oxide attack [7,10]. The decarburization resistance did not decrease due to the increase of oxide scale which consumed the decarburized layer produced below 1177 ◦ C [3]. This can be explained by the competition between oxidation and decarburization. As shown in Fig. 1, the practical oxidation equals to the theoretical oxidation. The depth difference between the theoretical decarburization and the theoretical oxidation determines the practical decarburization. 3.2. The effects of heating temperature Coating A in Table 3 was applied in the test and the effects of heating temperature are shown in Table 4. From both the curves in Fig. 2, which shows the difference in the weight of the cleaned specimens before and after heating, it can be
3. Results and discussion 3.1. The effects of coating components The coating materials were mixed according to Table 3 and the effects of anti-oxidation and anti-decarburization at 1250 ◦ C for 60 min were evaluated. Dolomite was chosen as the main raw material because magnesium oxide has high melting point and can be regarded as the framework of the coating. As shown in Table 3, the oxidation and decarburization prevention were obviously improved when SiC is admixed into the coating by comparing the specimens A and C. This
E
64 20 20 7 29 68.00
Fig. 1. The competition between oxidation and decarburization.
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Table 4 The oxidation and decarburization resistance of coating A heated for 0–120 min at temperatures of 950–1250 ◦ C. Temperature, time
ıoxi , %
ıdec ,%
Temperature, time
ıoxi , %
ıdec ,%
950 ◦ C, 60 min 1050 ◦ C, 60 min 1150 ◦ C, 60 min 1250 ◦ C, 60 min
44.20 60.43 61.93 64.44
6.45 6.85 7.05 100.00
1250 ◦ C, 0 min 1250 ◦ C, 30 min 1250 ◦ C, 90 min 1250 ◦ C, 120 min
67.19 67.17 60.43 63.60
65.14 100.00 100.00 100.00
observed that the steel loss increased with increasing temperature, for both types (bare and coated) of specimens. The steel loss of the coated specimen was smaller than that of the bare one during the heating process. The amount of SiO2 in bauxite was much higher than that in mullite; the excess SiO2 together with the impurities in bauxite formed an amorphous SiO2 and cristobalite to accompany the formation of mullite at temperature about 1000 ◦ C [11]. Bauxite is composed of diaspore (Al2 O3 ·H2 O) and kaolinite (Al2 O3 ·2SiO2 ·2H2 O). The reactions (5) and (6) occurred because alumina and metakaolinite (Al2 O3 ·2SiO2 ) were formed when diaspore and kaolinite were heated up to 500 ◦ C [12–14]. Heating at 980 ◦ C leaded to a direct formation of ␥-alumina and amorphous SiO2 or spinel (SiAl2 O4 ) according to reactions (7) and (8). Mullite phase first appeared at a temperature around 1100 ◦ C according to reactions (9) and (10), its amount increased with the increase of temperature. The amorphous SiO2 changed to cristobalite above 1200 ◦ C according to reaction (11). Al2 O3 ·H2 O (diaspore) → Al2 O3 (␥-alumina) + H2 O Al2 O3 ·2SiO2 ·2H2 O (kaolinite) → Al2 O3 ·2SiO2 (metakaolinite) + 2H2 O
(5) (6)
Al2 O3 ·2SiO2 (metakaolinite) → Al2 O3 (␥-alumina) + 2SiO2 (amorphous)
(7)
Al2 O3 ·2SiO2 (metakaolinite) → SiAl2 O4 (spinel) + SiO2 (amorphous)
(8)
SiAl2 O4 (spinel) + SiO2 (amorphous) → 1/3(3Al2 O3 ·2SiO2 ) (mullite) + 4/3SiO2 (amorphous)
(9)
Al2 O3 (␥-alumina) + 2SiO2 (amorphous) → 1/3(2Al2 O3 ·2SiO2 ) (mullite) + 4/3SiO2 (amorphous)
(10)
Fig. 2. Steel loss of GCr15 bearing steel specimens during heating for 60 min after scaling.
3Al2 O3 ·2SiO2 (mullite) + 4SiO2 (amorphous) → 3Al2 O3 ·2SiO2 (mullite) + 4SiO2 (cristobalite)
(11)
The amorphous SiO2 , cristobalite and mullite had certain effect in impeding the diffusion of oxygen. However, the coating was not compact enough that oxidation still existed [15]. At higher temperatures, the protection was much effective (Table 4), because on one hand, graphite (C) was formed due to the oxidation of SiC, keeping a reducing condition within the coating even at quite high temperatures [7]. On the other hand, new sintered phases were nonpermeable to oxygen at high temperature. Fig. 3 shows the new phases formed at 1150 ◦ C, including several spinels (MgCr2 O4 , (Mg, Fe) (Cr, Al)2 O4 , MgAl2 O4 and Fe (Cr, Al)2 O4 ) [8,9]. From Fig. 4 it is evident that the decarburization of the bare specimen increased below 1150 ◦ C. With increasing temperature above 1150 ◦ C, the growth rate of the decarburized layer decreased because the diffusions of C and O were retarded by the thick oxide scale of the bare specimen in a certain degree when heated at high temperature [16]. Below 1150 ◦ C, the anti-decarburization effect of the coating was less than 7.05%, because the decarburized depth of the bare specimen dbare was not large after the scale was cleaned. The bare specimen generated more serious oxidation on the surface (Fig. 2), consuming certain depth of decarburized layer. Although for the coated specimen, the coating protected it from decarburization in a certain degree due to the formation of mullite and glassy phase, the anti-decarburization effect ıdec was still small according to Eq. (4). At higher temperature, above 1150 ◦ C, the protection of decarburization was much effective, because the main reactions in the coating, which gave the protective properties, started at around 1050 ◦ C, as evident from the DTA curve. The scale and decarburized layer, formed at the previous stage, reacted with the coating materials to produce the newly densified film, which hindered the secondary decarburization. Therefore the coated specimen heated at 1250 ◦ C had no decarburization after scaling.
Fig. 3. XRD patterns of bare specimen (a) and coated specimen (b) after heating at 1150 ◦ C.
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Fig. 4. Decarburization growth of bare and coated specimens during heating for 60 min.
Fig. 6. Isothermal oxidation kinetics of bare and coated specimens at 1250 ◦ C in air.
Fig. 5 shows the bare specimen experienced endothermic process at 780 ◦ C because of austenization. And at about 950 ◦ C, an evident exothermal process happened because of the oxidation. By contrast, the coated specimen experienced an endothermic process below 100 ◦ C because of the dehydration of coating. At 175 ◦ C
a stronger endothermic reaction started, assumed to be the melting process of citric acid. The exothermic reactions are observed from 450 ◦ C to 540 ◦ C due to the decarburization of the excessive citric acid and the combustion of the residual carbon [17,18]. However, metakaolinite were formed at around 500 ◦ C, therefore, there was an endothermic peak separating the exothermal peak of citric acid [12–14]. The reactions (12) and (13) are the two steps of thermal decomposition of dolomite. The reaction (12), happened at around 790 ◦ C, was not been clearly expressed in Fig. 5 because similar with the bare specimen, the coated specimen experienced austenization process at 780 ◦ C and its peak covered the peak of reaction (12). The endothermal peak can be seen at around 935 ◦ C due to reaction (13) and the peak of the decomposition of metakaolinite was overlapped with the reaction (13). The exothermal peak can be seen at around 1100 ◦ C and 1200 ◦ C because the formation of mullite phases according to reactions (9) and (10) and cristobalite according to reaction (11). CaMg(CO3 )2 → CaCO3 + MgO + CO2
(12)
CaCO3 → CaO + CO2
(13)
The reaction of coating materials with scale and decarburized layer started above 1000 ◦ C.
Fig. 5. DTA curve of specimens with and without coating: (a) complete and (b) partial enlarged.
Fig. 7. Steel loss of GCr15 bearing steel specimens at 1250 ◦ C in air.
X. Wang et al. / Applied Surface Science 258 (2012) 4977–4982
Fig. 8. Decarburization growth of bare and coated specimens at 1250 ◦ C in air.
3.3. The effects of holding time Coating A in Table 3 was applied in the test and the effects of holding time are shown in Table 4.
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The isothermal oxidation kinetic curves of bare and coated specimens at 1250 ◦ C in air for 6 h were plotted as a graph of weight gain versus time and are illustrated in Fig. 6. It can be seen the coating reduce oxidation rate of GCr15 bearing steel. Fig. 7 shows the bare specimens severely suffered oxidation, and quite a lot of steel was lost in the form of oxide scale. There was a marked reduction in loss of steel volume in the whole heating process when the specimens were coated. The effect of the coating was based on the formation of spinels, which exhibit low permeability to oxygen at temperature up to 1250 ◦ C. It can be observed in Fig. 8 that the decarburization of the bare specimen increased with the extension of holding time. On the contrary, the decarburization of the coated specimen inversely decreased with the increase of holding time. When the temperature just increased to 1250 ◦ C, the coated specimen had decarburization layer formed below 1250 ◦ C, because the reactions of the coating materials, which produced new densified coating, did not start at low temperature. Despite the glassy phase, cristobalite and mullite, formed by the decomposition of bauxite at low temperature, had certain effect in impeding the diffusion of oxygen, decarburization still existed when holding time was less than 30 min. When the holding time was prolonged at 1250 ◦ C, the coating materials, decarburization layer and the scale quickly took part in the formation of effective film and the decarburization, previously generated, disappeared quickly as a result. Then the film was so compact that it blocked the secondary diffusion of carbon and oxygen at high temperature. Fig. 9 shows that the decarburization depth of the bare specimen reached 839.51 m after heating for 2 h at 1250 ◦ C. In contrast, the coated specimen had no decarburization completely. The coating could diminish the decarburization on the steel surface for up to 100% (Table 4). 4. Conclusions 1. A novel coating was successfully fabricated by using dolomite, bauxite, silicon carbide and binding agent to prevent oxidation and decarburization during reheating process of bearing steels. 2. The performed tests confirmed that SiO2 in bauxite formed a glassy phase and cristobalite to accompany the formation of mullite at 1000 ◦ C. The products had certain effect in preventing oxidation and decarburization, but the coating was not compact enough that oxidation and decarburization still existed. 3. At higher temperature, the protection was much effective, because on one hand, graphite was formed due to the oxidation of SiC, keeping a reducing atmosphere. On the other hand, the scale and decarburized layer, formed at previous stage, reacted with the coating materials to produce the newly densified film above 1150 ◦ C. The film was mainly composed of spinels and it could hinder the secondary oxidation and decarburization. Acknowledgments The authors gratefully acknowledge the financial supports from Key Topics in Innovation Engineering Funded by the Chinese Academy of Sciences (No. KGCX2-YW-224), Key Projects in the National Science & Technology Pillar Program in the Eleventh Fiveyear Plan Period (No. 2006BAC02A14), and the National Natural Science Foundation of China (No. 50774073). References
Fig. 9. Microstructure of steel GCr15, after heating for 2 h at 1250 ◦ C in a muffle furnace: (a) bare and (b) coated.
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