Aluminizing and Oxidation Treatment of 1Cr18ni9 Stainless Steel

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Applied Surface Science 227 (2004) 255–260

Aluminizing and oxidation treatment of 1Cr18Ni9 stainless steel Deqing Wang*, Ziyuan Shi Department of Materials Science and Engineering, Dalian Railway Institute, 794 Huanghe Road, Dalian 116028, Liaoning, PR China Received 8 July 2003; received in revised form 30 November 2003; accepted 30 November 2003

Abstract

The process of hot dipping pure aluminum on a stainless steel (1Cr18Ni9) followed by oxidation was studied to form a surface oxide layer. The thickness of the top aluminum on the steel substrate increases with increasing aluminizing time, while the thickness of the aluminum layer in the steel decreases as the increase in dipping temperature. Lower temperature and longer time favor a thicker layer of the aluminum on the substrate. The thickness of the intermetallic layer in the steel substrate increases with dipping temperature and time. However, the higher aluminizing temperature does not appear to have a signiﬁcant effect on the thickness of the intermetallic layer. The oxidation treatment of the aluminized steel at 800 8C results the formation of a top oxide layer on the steel surface, composed of a-alumina, Al4Cr and Al17Cr9. The aluminizing and oxidation treatment of the stainless steel creates about 120 mm thickness of top oxide layer which has an extremely sound adherency to the steel substrate and a greatly improved properties of thermal shock withstanding, high temperature oxidation resistance and anti-liquid aluminum corrosion. # 2003 Elsevier B.V. All rights reserved.

PACS: 81.65.K; 71.20.L; 81.65.M

Keywords: Aluminizing; Diffusion; Intermetallic compounds; Corrosion resistance

1. Introduction thermal erosion at temperatures between 450 and 980 8C [5,6]. By oxidation of the aluminum layer Aluminizing in aluminum melt [1,2] has long been on the surface of steel substrate, the steel substrate successfully used to form a thin layer of aluminum on will be protected by a layer of aluminum oxide that has the surface of steel substrate for improving the service high melting point, great hardness, thermodynamic property of steels, especially in corrosion resistance stability and poor wetting with aluminum melt [7]. applications [3,4]. In the process, when wetting the The wetting angle between alumina and aluminum surface of steel substrate, Al diffuses into steel to form melt below 900 8C is 1388, which is the largest among intermetallics. Due to the high microhardness and high common metal oxides [8]. aluminum content, the surface layer of the interme- The current work was one of a series study of tallics has extremely good resistance to wear and surface modiﬁcation of austenitic stainless steels to improve the properties such as resistance to high * Corresponding author. Tel.: þ86-411-368-3348; temperature oxidation and anti-aluminum corrosion fax: þ86-411-460-6139. when they were used in liquid aluminum processing. E-mail address: [email protected] (D. Wang). This paper reports the effects of temperature and time

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aluminizing during aluminizing and oxidation on the 2.4. Thermal and corrosion tests microstructure of a 1Cr18Ni9 stainless steel. More- over, the properties of microhardness, anti-aluminum The samples for thermal and corrosion tests were melt corrosion, high temperature oxidation resistance aluminized at 750 8C for 10 min and oxidized at 800 8C and anti-flash heating and quenching are also evalu- for 6 h. The high temperature oxidation property of the ated. sample was evaluated by the weight ratio, Wt/Wo, where Wo is the original weight of the sample, and Wt presents the weight of the sample after oxidation in air at 800 8C 2. Experimental for certain time. To measure the adherency of the oxide formed on the steel substrate, a thermal cycle test were 2.1. Materials carried out by repeatedly putting the sample in a resistance furnace at 800 8C for 10 min and then A 1Cr18Ni9 stainless steel (Fe–0.08C–17.4Si– quenching the specimen in water at room temperature. 9.5Ni in mass%) was used with the dimensions of The corrosion test was conducted in pure aluminum 20 mm 20 mm 2 mm. Commercial grade pure bath at 750 8C by immerging the original steel substrate aluminum with a purity of 99.7% was used as the and the sample after aluminizing and oxidation treat- molten aluminum bath. ments, and the corrosion property was assessed by the sample condition at different time. 2.2. Aluminizing 2.5. Microhardness measurement The aluminum ingots were heated to the temperatures between 700 and 750 8C in a graphite crucible The microhardness of the specimens was measured using a resistance furnace. The temperature of the using a Vickers microhardometer (FM700). The hard- molten aluminum bath was controlled to be within ness tests were performed under an indentation load of 1 8C. The steel samples were first degreased in a 25 g for 10 s. Analysis points were spaced so as to 100 g/l sodium hydrate solution at 50 8C for 5 min, eliminate the effect of neighboring indentations. The rinsed with water, and then descaled in aqua regia, microhardness was evaluated by taking five indenta- raised with water again. After being surface-pretreated tions on each specimen, and only the three middle in a molten salt mixture at 700 8C for 2 min, the steel values were averaged. specimens were immersed in the molten aluminum bath at each dipping temperature for different time 2.6. X-ray diffraction and energy dispersive X-ray before being cooled in air. analysis

2.3. Oxidation X-ray diffractometry (XRD) analysis in 2y range from 20 to 1208 using Cu Ka radiation was conducted The sample aluminized at 750 8C for 10 min was to determine phase structures of the samples at different placed in a resistance furnace where it was heated in conditions. Scanning electron microscopy (SEM) with air to a temperature of 650 8C over a 1 h heat-up an energy dispersive X-ray facility (EDX) was per- period, and then maintained at 650 8C for 1 h. This formed to analyze the element distributions of the heating permitted the formation of a certain thickness coatings. of oxide layer on the aluminized sample surfaces sufﬁcient per se to prevent aluminum from dripping, 3. Experimental result and discussion and thus to maintain the smoothness and uniformity of the surface layer. At this point, the furnace tempera- 3.1. Thickness of the aluminum and intermetallic ture was increased to 800 8C over a 1 h period. There- layers after, the specimen was cooled inside the furnace to room temperature after holding for a predetermined A typical cross-sectional morphology of the steel period of time. aluminized at 710 8C for 20 min is shown in Fig. 1 中国科技论文在线 http://www.paper.edu.cn

D. Wang, Z. Shi / Applied Surface Science 227 (2004) 255–260 257

240

o 180 730 C m µ 710 oC 120

Thickness, 60

0 0 102030 Time, min Fig. 1. Microstructure of the steel aluminized at 710 8C for 20 min. Fig. 3. Effect of dipping temperature and time on the thickness of intermetallic layer. where three layers are presented, top aluminum, middle intermetallics and bottom steel substrate. Unlike perature and longer time favor the acquirement of the tongue shaped morphology in aluminized carbon thicker aluminum layer on the steel. steel [9–11], the aluminum diffusion front of this steel As shown in Fig. 3, the thickness of the interme- is ﬂat. tallic layer in the steel substrate increases with dipping Fig. 2 shows the thickness variations of the pure temperature and time. However, It is worth noting that aluminum layer on the steel substrate with aluminiz- the aluminizing temperature does not appear to have a ing temperatures and time. At the dipping tempera- signiﬁcant effect on the thickness of the intermetallic tures, the thickness of the pure aluminum layer on the layer according to Fick’s law of diffusion. steel is increased with the increase in dipping time. When dipping time is kept constant, the thickness of 3.2. Oxidation treatment the pure aluminum layer on the steel substrate is reduced as the aluminizing temperature increases. The heat treatment of the aluminized stainless steel Whereas, Fig. 2 also illustrates that at each given time at 800 8C brings about the oxidation of the top alu- from short to long, the thickness difference of the minum coating. By XRD (Fig. 4), the phase evolution aluminum layers between the two temperatures on the surface of the specimens at different conditions becomes bigger, which indicates that the lower tem- is revealed. The original steel is composed of a and g phases. After aluminizing at 750 8C for 10 min, aluminum has reacted with iron and other alloying ele- 400 ments to form mainly Al5Fe2 and Al13Cr2, together

o with a little amount of Al3Ni2. The presentation of 350 710 C Al5Fe2 phase, instead of Al3Fe on the steel surface, 300

m according Al–Fe phase diagram [12] is due to the µ 730 oC preferential formation for its low atom concentration 250 along the C-axis [13]. Accordingly, the Al13Cr2 differs 200 a little from the stable Al7Cr in equilibrium Al–Cr Thickness, phase diagram [14]. Further oxidation treatment of the 150 aluminized sample at 800 8C for 6 h results in the

100 formation of a-alumina and the total vanish of the 0102030 Al13Cr2 peaks. Instead, Al17Cr9 and Al4Cr are formed. Time, min The reduction of aluminum content in the Al–Cr Fig. 2. Effect of dipping temperature and time on the thickness of intermetallics may come from the diffusion into the pure aluminum layer. steel substrate and the partial oxidation of aluminum 中国科技论文在线 http://www.paper.edu.cn

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Fig. 4. X-ray diffraction patterns of the steel at different conditions.

in high aluminum content Al–Cr intermetallics. More- 1500 over, the presentation of the Al17Cr9 and Al4Cr phases in the oxide layer may imply that they have very good 1200 oxidation resistance at high temperature. 900 The oxidation treatment also results in the increase

in thickness of both the oxide and the intermetallic 600 layers to about 120 mm in 6 h, and little thickness

gains are obtained since, as shown in Fig. 5. The 300 Microhardness, HV microhardness measurement (Fig. 6) reports that the aluminum layer has the hardness of about HV1120 0 with a little lower value closed to surface due to its 0 40 80 120 160 200 µ porous structure as shown in Fig. 7, and the hardness Distance, m of the steel substrate is about HV250. The decreasing Fig. 6. Microhardness measurement in the cross-section of the hardness of the intermetallic layer between the oxide sample after aluminizing at 750 8C for 10 min and oxidizing at 800 8C for 12 h.

170

140 Oxide m µ Intermetallics 110

Thickness, 80

50 03691215 Time, Hour

Fig. 5. Thickness of the oxide and intermetallic layers vs. time at Fig. 7. Indentations of the microhardness measurement on the 800 8C for the steel aluminized at 750 8C for 10 min. aluminized and oxidized sample. 中国科技论文在线 http://www.paper.edu.cn

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diffuse through the dense layer of the surface oxide for coming into contact with metal elements below the oxide layer. However, the original steel suffers from a weight loss over 0.3% in 4 h, which can be explained by the excess of surface element volatilization over the oxidation. After 4 h, the weight of the original steel declines very slowly. As other studies have shown that Al could effec- tively improve high temperature oxidation property of steels [15,16]. When Al content was varied from 8 to 16 at.% in carbon steels, the preferitial oxidation of element occurred from Fe to Al from metrix at 800 8C [17,18]. A much greater oxidation resistance of an Fe– Al alloy was achieved by a further increase in Al Fig. 8. Line scanning proﬁles of EDX element distribution on the content to 40 at.% [16]. In general, the oxidation cross-section of the aluminized and oxidized sample. resistance of Fe–Al alloys at high temperature is directly related to Al content which determines the 0.9 extent of surface coverage by aluminum oxide formed. Besides, high Al-content alloys were free from dec- 0.6 arburization, whereas low Al-content alloys suffered 0.3 Oxidized from decarburization [19]. In this study, the aluminiz- 0.0 ing process and the oxidation treatment produce the high Al-content surface which is fully covered by a -0.3 dense top layer of aluminum oxide. The oxide layer

Weight gain, % Original -0.6 acts as a barrier for oxygen coming into contact with steel substrate and provides an aditional protection for -0.9 the steel from high temperature oxidation. Hence, the 0 3 6 9 12 15 18 high temperature oxidation property of the stainless Time, Hour steel has been greatly improved. Fig. 9. Weight change of the samples oxidized at 800 8C. 3.3.2. Corrosion in aluminum melt The surface of the aluminized and oxidized sample and the steel substrate should be accounted for the does not wet with the liquid aluminum when it is changes of the intermetallic phases which are identi- immersed into the melt at 750 8C. It undergoes good ﬁed by the number 1, 2 and 3 in Fig. 7. By EDX line wetting in 50 h, pitting in 70 h and partial etching in scanning on the cross-section of the oxidized speci- 100 h. The penetration of the specimen appears in men, three peaks for aluminum distribution reveal that 148 h. However, the sample of the original steel in the intermetallic region, the phases presented in 1, 2 suffers the partial etching within 5 h and penetration and 3 zones should be the different aluminum-contain- failure occurs in 48 h. Covered by the 120 mm thick ing intermetallics, as shown in Fig. 8. oxide layer, the anti-liquid aluminum corrosion property of the stainless steel has been considerably 3.3. Property evaluation improved. The possible roles of the oxide layer played in the enhanced aluminum melt corrosion property of 3.3.1. High temperature oxidation the stainless steel are that it decreases wetting with As shown in Fig. 9, the aluminized and oxidized liquid aluminum, blocks direct contact between alu- steel obtains a very little weight gain, about 0.05% minum melt and the steel substrate, and accordingly, after the oxidation for 4 h, and its weight keeps nearly presents impedance to substrate dissolution into alu- constant. The reason is that it is difﬁcult for oxygen to minum melt. 中国科技论文在线 http://www.paper.edu.cn

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3.3.3. Adherency of the oxide on the steel substrate 4. The aluminizing and oxidation treatment of the The aluminized and oxidized sample remains a stainless steel creates about 120 mm thickness of perfect oxide surface for 22 thermal cycles of the top oxide layer which has an extremely sound rapid heating and cooling process before it experi- adherency to the steel substrate and a greatly ences oxide scaling off the steel substrate, and six improved properties of thermal shock withstand- more thermal cycles causes 50% exposure of the ing, high temperature oxidation resistance and substrate surface, whereas the hot spray coating of anti-liquid aluminum corrosion. aluminum oxide on surfaces of the same steel suffers peeling off after only one thermal cycle. Unlike traditional spray coating, the greatly improved adher- References ency of the oxide layer on the stainless steel substrate can be attributed to the oxide self-formation process in [1] G.A. Mollere, US Patent 2315725 (1948). which an anchor-like bonding presents between the [2] T. Sendzimir, US. Patent 2110893 (1938). oxide layer and the steel substrate. In addition, the [3] G. Willam, Wood Metal Handbook, ninth ed., vol. 5, Surface gradual distribution of the intermetallic components Cleaning, Finishing and Coating, ASM, OH 1982 p. 333. [4] T.C. Simpson, Corrosion 49 (7) (1993) 550. integrates the oxide layer with the substrate and makes [5] D. Liang, et al., Scripta. Metall. Meter. 34 (10) (1997) 1513. a relatively good match in expansions and contractions [6] Ni Zhijian, Ren Zhongyuan, Huang Diguang, J. Northwestern during thermal shock between the top layers. Inst. Arch. Eng. 3 (1997) 42 [in Chinese]. [7] R. Asthana, Metall. Mater. Trans. 26A (1995) 1307. [8] Liu Yaohui, He Zhenming, Yu Sirong, Dong Guitian, Li Qingchun, J. Mater. Sci. Lett. 11 (1992) 896. 4. Conclusions [9] Wang Deqing, Shi Ziyuan, Zou Longjiang, Appl. Surf. Sci. 241 (1–4) (2003) 304. 1. The thickness of the top aluminum on the steel [10] Shigeaki Kobayashi, Takao Yakou, Mater. Sci. Eng. A 338 substrate increases with increasing aluminizing (2002) 44. time, while the thickness of the aluminum layer in [11] K. Bouche, F. Barbier, A. Coulet, Mater. Sci. Eng. A 249 (1998) 167. the steel decreases with the increase in dipping [12] B. Massalski, Binary Phase Diagram ASM Int. 1 (1990) 148. temperature. Lower temperature and longer time [13] L.N. Larikov, V.M. Falchenko, D.F. Polishebuk, V.R. Ryabov, favor a thicker layer of the aluminum on the A.V. Lonovskays, in: G.V. Samsonov (Ed.), Protective substrate. Coatings on Metals, vol. 3, Consultant Bureau, New York, 2. The thickness of the intermetallic layer in the steel 1971 p. 56. [14] B. Massalski, Binary Phase Diagram ASM Int. 1 (1990) 139. substrate increases with dipping temperature and [15] Y.J. Li, J. Wang, X. Holly, Mater. Sci. Technol. 19 (2003) time. However, the higher aluminizing tempera- 657. ture does not appear to have a signiﬁcant effect on [16] M.A. Montealegre, J.L. Gonzalez-Carrasco, M.A. Munoz- the thickness of the intermetallic layer. Morris, Intermetallics 9 (2001) 487. 3. The oxidation treatment of the aluminized steel at [17] V. Shankar Rao, R.G. Baligidad, V.S. Raja, Intermetallics 10 (2002) 73. 800 8C results the formation of a top oxide layer [18] V. Shankar Rao, Intermetallics 11 (2003) 713. on the steel surface, composed of a-alumina, [19] V.S. Rao, V.S. Raja, High Temp. Mater. Processes 21 (2002) Al4Cr and Al17Cr9. 143.