High Temperature Oxidation and Wear Resistance of Y-modified Hot Dipping Aluminized Coating on SCH12 Steel

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1 Rare Metal Materials and Engineering Volume 41, Suppl. 1, February 212 Cite this article as: Rare Metal Materials and Engineering, 212, 41(S1): High Temperature Oxidation and Wear Resistance of Y-modified Hot Dipping Aluminized Coating on SCH12 Steel Fan X S, Yang Z G, Zhang C State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 184, China Abstract: Microstructures, high-temperature oxidation and wear resistance of hot dipping Al-Si-Y coating on SCH12 heat resistant cast steel were investigated in this study. The aluminized coating was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The results showed that the coating was composed of the Al-rich outer layer and the intermetallics inner layer. In the Al-rich layer, some Y-rich precipitates and Fe-Al-Si-Cr precipitates could be observed. The intermetallics layer presented three layers induced by the increase of Fe, Cr, Ni content and the corresponding decrease of Al, Si content. The oxidation tests were conducted in still air at 85 for up to 1 h. After oxidation, a top oxide scale composed of mainly α-al 2 O 3, Al 5 Y 3 O 12 was formed on the steel surface. The intermetallics beneath the oxide scale consisted of mainly FeAl and small amount of Fe 2 Al 5 and Cr 3 Si phase. The mass gain of the coated and uncoated SCH1steel is.45 mg/cm 2 and.57 mg/cm 2, respectively. The wear resistance was investigated using a high-temperature pin-on-disc tribometer at 65. The wear rate for the coated and uncoated steel is.45 μm 3 /μm N and 3.1 μm 3 /μm N, respectively. The high temperature wear tests and oxidation tests results demonstrated that the yttrium-modified aluminized specimen had significantly improved high-temperature wear resistance and equivalent oxidation resistance compared with the original SCH12 specimen. Key words: hot dipping aluminizing; oxidation resistance; high-temperature wear resistance Austenitic heat resistant steels have been commonly used in mechanical components working under high temperature conditions, such as aerospace, steam turbine applications and so on, because they render excellent oxidation resistance, good processing properties and mechanical properties at high temperature. However, the applications of the austenitic heat resistant steels are limited because of the poor wear resistance induced by the low hardness. Therefore, surface treatment is indispensable for the austenitic heat resistant steels components working under wear conditions at high temperature. Hot dipping aluminizing process is an effective technology of producing a heat and oxidation resistant coating on steels [1-5]. The aluminized coating is composed of alumina layer and the Fe-Al intermetallics layer when being used at high temperature in atmosphere. The dense and continuous alumina layer can protect the steel substrate from further oxidation. Moreover, the microhardness of the alumina layer and the Fe-Al intermetallics layer is much higher than that of austenitic heat resistant steels [3]. It is probably to improve the wear resistance of the austenitic heat resistant steels by the hot dipping aluminizing process. Unfortunately, previous studies on hot dipping aluminizing process always focus on the oxidation behavior but rarely on the wear resistance of the aluminized layers. In addition, it is a known fact that alloying elements can significant influence on the oxidation resistance and the microstructures of the aluminized layer in the hot dipping aluminizing process [4]. It was reported that yttrium can significantly improve the aluminized surface and its oxide film, resulting in enhanced resistance to wear at both elevated and room temperatures [5]. Silicon can reduce the thickness of intermetallics layer and improve the toughness and heat resistance [6]. Moreover, Cr-Si ceramics reinforced precipitates may be formed in the aluminized layer because there is sufficient Cr in austenitic heat resistant steels, which was used to improve the high temperature wear resistance of substrate in the previous studies [7]. Therefore, yttrium and silicon were selected as alloying element in the present study. 1 Experiment 1.1 Specimens preparation Commercial SCH12 heat resistant cast steel was used in this study. The specimens for wear test were sized to 3 mm in diameter and 1 mm in thickness and the other specimens were sized to 17 mm 7 mm 4 mm. All the specimens were Received date: May 25, 211 Corresponding author: Yang Z G, Ph. D., Professor, Department of Materials Science and Engineering, Tsinghua University, Beijing 184, P. R. China, Tel: , zgyang@tsinghua.edu.cn

2 Fan X S et al. / Rare Metal Materials and Engineering, 212, 41(S1): ground up to 12 mesh emery paper then polished After being degreased in 2 wt% NaOH solution at 8 for 1 min, rinsed with deionized water, and then descaled in 5 wt% HCl solution at 8 for 3 min, rinsed with deionized water again, the specimens were dipped into a passivation solution with the composition of.37 wt% NaNO wt% CrO wt% K 2 MnO 4 at room temperature for 5 min. The hot dipping aluminizing process were performed at 75 for 5 min in the melting Al-8 wt% Si-5 wt% Y alloy melted in a alumina crucible in a well-type electrical resistance furnace. 1.2 Microstructures and hardness characterization The microstructures and the element distributions of the coatings and substrate were analyzed using FEI Quanta 2 FEG scanning electron microscopy (SEM) with an energy dispersive X-ray facility (EDX). The XRD profiles of the specimens were recorded using D/max255HB+/PC high power (18 kw) polycrystalline X-ray diffractometer with Cu Kα radiation. The scan was performed in the angular range of 2º-9 with a step size of.2 and a scanning ratio of 6 /min. The microhardness was measured by a Vikers microhardometer under an indentation load of 25 g for 1 s. The microhardness values for each specimen were measured for more than six times and averaged. 1.3 High-temperature oxidation tests The aluminized specimens for high-temperature oxidation tests were pretreated at 1 for 4 h. The high-temperature oxidation tests were carried out in a box furnaces at 85 for up to 1 h in still air. The mass gain of the specimens was measured by a BS21s electronic balance with a weighting accuracy of.1 mg. The oxidation kinetics curves were employed to evaluate the high temperature oxidation resistance of specimens. 1.4 High-temperature wear tests A ball-on-disk SRV tribometer with an oscillating ball was used to test unlubricated wear resistances at 65. The friction coefficient limit for the Optimol tribometer is 1.5, which means that the tribometer will stop if the friction coefficient is higher than 1.5 and last for 3 s. Before the wear test, the aluminized samples were pretreated at 85 for 4 h. The wear tests were carried out against a Si3N4 ball under normal load of 5 N. The oscillating amplitude is 1 mm and the frequency is 5 Hz, 3 oscillating was carried out. The wear tests were done three times for each specimen. The volume loss evaluated by the method presented in Ref.[8]. The wear volume loss W was calculated as DL 2 1 arcsin H W sin 2arcsin H 4 D 2 D Where D is the diameter of the Si 3 N 4 ball, L is the length of the wear scar and H is the width of the wear scar (observed by a reading optical microscope). The ware rate was calculated by Wear rate=volume loss/(load total oscillating distance). 2 Results and Discussion 2.1 Microstructure of the specimens Aluminized specimens Fig.1a presents the cross-section morphology of the SCH12 steel dipping in the melting Al-8 wt% Si-5 wt% Y alloy at 75 for 5 min. The aluminized coating shows three distinct regions: (a) the top Al-rich layer with some white and gray precipitates; (b) the intermetallics layer and (c) the SCH12 substrate. From the high magnification cross-section photographs presented in Fig.1b, the intermetallics layer can be further divided into three layers: (1) the outer intermetallics layer with some large crystals; (2) the intermediate intermetallics layer and (3) the inner intermetallics layer adjacent to the substrate. There is a distinct, flat interface between the intermetallics layer and the substrate. It s probably because that the presence of certain level of Si in the molten aluminum can inhibit the diffusion of Al into the substrate effectively and change the intermetallics layer/substrate interface morphology [2]. However, the intermetallics layer/al-rich layer interface is irregular and a large amount of large crystals can be observed. The element distribution along the line shown in Fig.1b is presented in Fig.2. It can be seen that the content of inward diffusion element, Al and Si, decrease with the increased distance from Al-rich layer in the intermetallics layer. However, the content of outward diffusion element Al, Cr and Ni decrease with the increased distance from substrate. In addition, scarcely Ni can be detected in the outer intermetallics layer. The delamination of the intermetallics layer is probably induced by the variety of the content of different elements. The element distribution along the line shown in Fig.1b is presented in Fig.2. It can be seen that the content of inward diffusion element, Al and Si, decrease with the increased distance from Al-rich layer in the intermetallics layer. However, the content of outward diffusion element Al, Cr and Ni decrease with the increased distance from substrate. In addition, scarcely Ni can be detected in the outer intermetallics layer, which is an interesting results but the mechanism is unclear. The delamination of the intermetallics layer is probably induced by the variety of the content of different elements. Fig.1 Cross-section morphology of the aluminized SCH12 specimen

3 Fan X S et al. / Rare Metal Materials and Engineering, 212, 41(S1): Content/at% Content/at% Fig.2 Element distribution along the line shown in Fig.1b The cross-sections of the specimen were also analyzed by means of EDX point analyses. The Al-rich layer is mainly compose of Al ( 98 wt%) with a minor amount of Si. The composition of the Al-rich layer is different from the aluminum bath probably due to the Si and Y precipitate in the solidification process. The EDX results of the white and gray precipitates in the Al-rich layer were presented in Table 1. It can be seen that the white precipitates contains mainly Al, Si, Y and small amount of Fe and Ni. The gray precipitates contain mainly Al, Si, Fe and small amount of Cr, which is similar with the outer intermetallics layer and the large crystals. The Fe, Cr and Ni elements in the precipitates may be from the SCH12 substrate due to that the Fe, Cr and Ni in the SCH12 substrate can dissolve into molten Al. It was pointed out that the precipitates in the Al-rich layer obtained by dipping in pure Al were found to be crystals of FeAl 3 [9]. In the present work, because of the addition of Si in the aluminum bath and the containing of Cr in the substrate, the gray precipitates and the outer intermetallics layer was determined to be (Fe,Cr) 2 SiAl 7 phase, and the intermediate intermetallics layer and the inner intermetallics layer was Si-containing (Fe,Cr)Al 3 phase and Si-containing (Fe,Cr)Al 2 phase, respectively [1] Aluminized specimens after oxidation Fig.3 presents the cross-section microstructures beneath the oxide scale of the aluminized specimen after oxidation at 85 for 2 h. The Al-rich layer disappeared by the inter-diffusion of Al and Fe, Cr, Ni. The heat treatment of the aluminized layer brings about the oxidation of the top aluminum layer. And also results in the increase in thickness of the intermetallics layer. The intermetallics layer was composed of Al Fe Si Cr Ni Fig.3 Cross-section microstructure beneath oxide scale on aluminized SCH12 specimen after oxidation at 85 for 2 h Table 1 EDX results of the white precipitates, gray precipitates and the outer intermetallics layer (at%) Element Al Si Cr Fe Ni Y White precipitates Gray precipitates Outer intermetallics mainly FeAl phase and small amount of Fe 2 Al 5 and Cr 3 Si phase based on the EDX result. Fig.4a presents the surface morphology of the aluminized SCH12 specimen after oxidation for 8 h. The EDX results showed that the gray regions are Al 2 O 3 and the bright regions are the mixed oxides of Y and Al. Fig.4b presents the XRD Intensity/a.u. α-al 2 O 3 β-feal Al 5 Y 3 O 12 Cr 3 Si θ/(º) Fig.4 Surface morphology (a) and XRD pattern (b) of the aluminized SCH12 specimens after oxidation at 85 for 8 h

4 Fan X S et al. / Rare Metal Materials and Engineering, 212, 41(S1): pattern of the aluminized SCH12 specimen after oxidation for 8 h. It can be seen that the oxide scale were composed of mainly α-al 2 O 3 and Al 5 Y 3 O 12. Besides the oxides phase, strong peaks of FeAl and peaks of Cr 3 Si were observed, which is in accordance with the EDX results of the specimen after oxidation for 2 h. 2.2 Evaluation of high temperature properties Wear properties Fig.5 presents the results of wear test on the SCH12 samples at 65. It can be seen that the wear rate of the uncoated specimen is higher than that of the aluminized specimen by a factor of six. The wear resistance for the aluminized specimen is obviously improved compared with the uncoated specimens. The improvement of the wear resistance is probably induced by the high hardness of the oxide scale and the intermetallics layer formed during the heat treatment process. The micro-hardness values of the oxide scale, the intermetallics layer and the SCH12 substrate are presented in Fig.6. It can be seen that the micro-hardness value of the aluminized specimen is far higher than that of the SCH12 substrate, which is beneficial to improve the wear resistance. In addition, it has been indicated that Cr 3 Si phase was formed in the intermetallics layer in the further oxidation at high temperature. Cr 3 Si is one of transition metal silicide with excellent elevated-temperature creep strength, high-temperature hardness and high-temperature wear resistance [7, 11]. It was reported that some composite coating with Cr 3 Si as reinforced phase had excellent wear resistance under high-temperature and room-temperature sliding wear conditions [12, 13]. In this study, a large amount of Cr 3 Si distributed in the intermetallics layer as showed in Fig.3, which may be favorable to the wear resistance and oxidation resistance at high temperature for long time Oxidation resistance Fig.7 presents the mass gain of the uncoated and the aluminized SCH12 after oxidation at 85 from 2 to 1 h. After oxidation for 1 h, the mass gain of the uncoated and aluminized SCH12 specimens is.57 mg/cm 2 and.45 mg/cm 2, respectively. It can be seen that the oxidation resistance of the aluminized SCH12 was improved. Previous studies showed that the most effective coatings are those which are the densest and most continuous. It had been indicated that α-al 2 O 3 was formed on the aluminized SCH12 specimen by the XRD results. The α-al 2 O 3 is dense and protectively, which can inhibit the diffusion of oxygen to the substrate. In addition, the Y oxides were the most beneficial, reducing the growth rate of the Al 2 O 3 scale, improving the oxide adhesion and causing a change in the direction of growth of the scale [14]. Combination the wear tests results and the oxidation tests results, it can be demonstrated that the yttrium-modified aluminized specimen has significantly improved high-temperature wear resistance and equivalent oxidation resistance compared with the original SCH12 specimen. The aluminized SCH12 was possible to use in tribological applications at high temperature. 3 (µm N) -1 Wear Rate/µm Fig.5 Wear test results on uncoated and aluminized specimens Microhardness, HV/ 1MPa Fig.6 Microhardness of the SCH12 steel, α-al 2 O 3 [3] and FeAl Mass Gain/mg cm Fig.7 Oxidation kinetics curves of the uncoated and aluminized SCH12 specimens 3 Conclusions 3.1 SCH Aluminized 115 SCH12 FeAl α-al 2 O 3 Uncoated SCH12 Aluminized SCH Oxidation Time/h 1) After aluminizing in Al-Si-Y melting, the SCH12 heat resistant cast steel was coated by the Al-rich outer layer and the intermetallics inner layer. Y-rich precipitates and Fe-Al-Si-Cr precipitates were formed in the Al-rich outer layer. The intermetallics layer contained the outer intermetallics layer, intermediate intermetallics layer and inner intermetallics layer, which were composed of (Fe,Cr) 2 SiAl 7 phase, Si-containing (Fe,Cr)Al 3 phase and Si-containing (Fe, Cr)Al 2

5 Fan X S et al. / Rare Metal Materials and Engineering, 212, 41(S1): phase, respectively. The delamination of the intermetallics layer is probably induced by the variety of the content of different elements. 2) After oxidation, an oxide scale was formed on the surface of the specimen. The Al-rich layer disappeared and thickness of the intermetallics layer was increased by the inter-diffusion. The compound layer was composed of mainly FeAl phase and small amount of Fe 2 Al 5 and Cr 3 Si phase. The oxide scale was mainly composed of mainly α-al 2 O 3 and Al 5 Y 3 O 12 after oxidation at 85 for 8 h. 3) Under the wear test conditions in the present study, the wear rate of the aluminized and the uncoated specimens were 3.1 μm 3 /μm N and.45 μm 3 /μm N, respectively. The high-temperature wear resistance of the aluminized SCH12 heat resistant cast steel was improved significantly. 4) The mass gain of the aluminized and uncoated SCH12 steel after oxidation at 85 for 1 h is.45 mg/cm 2 and.57 mg/cm 2, respectively. The oxidation resistance of the aluminized specimen at 85 is a little better than that of the uncoated specimen. 5) The wear test and oxidation test results demonstrated that yttrium-modified aluminized specimen has significantly improved high-temperature wear resistance and equivalent oxidation resistance compared with the original SCH12 specimen. The aluminized SCH12 was probably to use in tribological applications at high temperature. References 1 Sasaki T, Yakou T. Surface and Coating Technology[J], 26, 21: Awan G H, Hasan Faiz ul. Material Science Engineering A[J], 28, 472: Deqing W. Applied Surface Science[J], 28, 254: Glasbrenner H, Nold E, Voss Z. Journal of Nuclear Material[J], 1997, 249: 39 5 Ahmadi H, Li D Y. Wear[J], 23, 255: Cao Y, Sun K N, Zhou R D. Electroplating & Pollution Control[J], 2, 2: 26 7 Duan G, Wang H M. Scripta Materialia[J], 22, 46: 17 8 Wang Z B, Lu J, Lu K. Surface and Coating Technology[J], 26, 21: Han S L, Li H L, Wang S M et al. International Journal of Hydrogen Energy[J], 21, 35: Liu B J. Hot Dipping Aluminizing Process of Steels[M]. Beijing: Metallurgical Industry Press, Duan G, Wang H M. Tribology[J], 22, 4: Duan G, Zhao H Y, Wang H M. Rare Metal Materials and Engineering[J], 23, 32: Duan G, Wang H M. Rare Metal Materials and Engineering[J], 23, 32: Hussey R J, Graham M J. Oxidation of Metals[J], 1996, 45: 349