Aluminizing and oxidation treatment of 1Cr18Ni9 stainless steel

Transcription

1 Applied Surface Science 227 (2004) 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 , 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, Al 4 Cr and Al 17 Cr 9. 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: K; L; M Keywords: Aluminizing; Diffusion; Intermetallic compounds; Corrosion resistance 1. Introduction Aluminizing in aluminum melt [1,2] has long been successfully used to form a thin layer of aluminum on the surface of steel substrate for improving the service property of steels, especially in corrosion resistance applications [3,4]. In the process, when wetting the surface of steel substrate, Al diffuses into steel to form intermetallics. Due to the high microhardness and high aluminum content, the surface layer of the intermetallics has extremely good resistance to wear and * Corresponding author. Tel.: þ ; fax: þ address: wdeqing@online.ln.cn (D. Wang). thermal erosion at temperatures between 450 and 980 8C [5,6]. By oxidation of the aluminum layer on the surface of steel substrate, the steel substrate will be protected by a layer of aluminum oxide that has high melting point, great hardness, thermodynamic stability and poor wetting with aluminum melt [7]. The wetting angle between alumina and aluminum melt below 900 8C is 1388, which is the largest among common metal oxides [8]. The current work was one of a series study of surface modification of austenitic stainless steels to improve the properties such as resistance to high temperature oxidation and anti-aluminum corrosion when they were used in liquid aluminum processing. This paper reports the effects of temperature and time /$ see front matter # 2003 Elsevier B.V. All rights reserved. doi: /j.apsusc 转载

2 256 D. Wang, Z. Shi / Applied Surface Science 227 (2004) aluminizing during aluminizing and oxidation on the microstructure of a 1Cr18Ni9 stainless steel. Moreover, the properties of microhardness, anti-aluminum melt corrosion, high temperature oxidation resistance and anti-flash heating and quenching are also evaluated. 2. Experimental 2.1. Materials A 1Cr18Ni9 stainless steel (Fe 0.08C 17.4Si 9.5Ni in mass%) was used with the dimensions of 20mm 20 mm 2 mm. Commercial grade pure aluminum with a purity of 99.7% was used as the molten aluminum bath Aluminizing The aluminum ingots were heated to the temperatures between 700 and 750 8C in a graphite crucible using a resistance furnace. The temperature of the molten aluminum bath was controlled to be within 1 8C. The steel samples were first degreased in a 100 g/l sodium hydrate solution at 50 8C for 5 min, rinsed with water, and then descaled in aqua regia, raised with water again. After being surface-pretreated in a molten salt mixture at 700 8C for 2 min, the steel specimens were immersed in the molten aluminum bath at each dipping temperature for different time before being cooled in air Oxidation The sample aluminized at 750 8C for 10 min was placed in a resistance furnace where it was heated in air to a temperature of 650 8C over a 1 h heat-up period, and then maintained at 650 8C for 1 h. This heating permitted the formation of a certain thickness of oxide layer on the aluminized sample surfaces sufficient per se to prevent aluminum from dripping, and thus to maintain the smoothness and uniformity of the surface layer. At this point, the furnace temperature was increased to 800 8C over a 1 h period. Thereafter, the specimen was cooled inside the furnace to room temperature after holding for a predetermined period of time Thermal and corrosion tests The samples for thermal and corrosion tests were aluminized at 750 8C for 10 min and oxidized at 800 8C for 6 h. The high temperature oxidation property of the sample was evaluated by the weight ratio, W t /W o, where W o is the original weight of the sample, and W t presents the weight of the sample after oxidation in air at 800 8C for certain time. To measure the adherency of the oxide formed on the steel substrate, a thermal cycle test were carried out by repeatedly putting the sample in a resistance furnace at 800 8C for 10 min and then quenching the specimen in water at room temperature. The corrosion test was conducted in pure aluminum bath at 750 8C by immerging the original steel substrate and the sample after aluminizing and oxidation treatments, and the corrosion property was assessed by the sample condition at different time Microhardness measurement The microhardness of the specimens was measured using a Vickers microhardometer (FM700). The hardness tests were performed under an indentation load of 25 g for 10 s. Analysis points were spaced so as to eliminate the effect of neighboring indentations. The microhardness was evaluated by taking five indentations on each specimen, and only the three middle values were averaged X-ray diffraction and energy dispersive X-ray analysis X-ray diffractometry (XRD) analysis in 2y range from 20 to 1208 using Cu Ka radiation was conducted to determine phase structures of the samples at different conditions. Scanning electron microscopy (SEM) with an energy dispersive X-ray facility (EDX) was performed to analyze the element distributions of the coatings. 3. Experimental result and discussion 3.1. Thickness of the aluminum and intermetallic layers A typical cross-sectional morphology of the steel aluminized at 710 8C for 20 min is shown in Fig. 1

3 D. Wang, Z. Shi / Applied Surface Science 227 (2004) Thickness, µm o C 710 o C Fig. 1. Microstructure of the steel aluminized at 710 8C for 20 min. Thickness, µm Time, min 710 o C 730 o C Fig. 2. Effect of dipping temperature and time on the thickness of pure aluminum layer where three layers are presented, top aluminum, middle intermetallics and bottom steel substrate. Unlike the tongue shaped morphology in aluminized carbon steel [9 11], the aluminum diffusion front of this steel is flat. Fig. 2 shows the thickness variations of the pure aluminum layer on the steel substrate with aluminizing temperatures and time. At the dipping temperatures, the thickness of the pure aluminum layer on the steel is increased with the increase in dipping time. When dipping time is kept constant, the thickness of the pure aluminum layer on the steel substrate is reduced as the aluminizing temperature increases. Whereas, Fig. 2 also illustrates that at each given time from short to long, the thickness difference of the aluminum layers between the two temperatures becomes bigger, which indicates that the lower temperature and longer time favor the acquirement of thicker aluminum layer on the steel. As shown in Fig. 3, the thickness of the intermetallic layer in the steel substrate increases with dipping temperature and time. However, It is worth noting that the aluminizing temperature does not appear to have a significant effect on the thickness of the intermetallic layer according to Fick s law of diffusion Oxidation treatment Time, min Fig. 3. Effect of dipping temperature and time on the thickness of intermetallic layer. The heat treatment of the aluminized stainless steel at 800 8C brings about the oxidation of the top aluminum coating. By XRD (Fig. 4), the phase evolution on the surface of the specimens at different conditions 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 elements to form mainly Al 5 Fe 2 and Al 13 Cr 2, together with a little amount of Al 3 Ni 2. The presentation of Al 5 Fe 2 phase, instead of Al 3 Fe on the steel surface, according Al Fe phase diagram [12] is due to the preferential formation for its low atom concentration along the C-axis [13]. Accordingly, the Al 13 Cr 2 differs a little from the stable Al 7 Cr in equilibrium Al Cr phase diagram [14]. Further oxidation treatment of the aluminized sample at 800 8C for 6 h results in the formation of a-alumina and the total vanish of the Al 13 Cr 2 peaks. Instead, Al 17 Cr 9 and Al 4 Cr are formed. The reduction of aluminum content in the Al Cr intermetallics may come from the diffusion into the steel substrate and the partial oxidation of aluminum

4 258 D. Wang, Z. Shi / Applied Surface Science 227 (2004) Fig. 4. X-ray diffraction patterns of the steel at different conditions. in high aluminum content Al Cr intermetallics. Moreover, the presentation of the Al 17 Cr 9 and Al 4 Cr phases in the oxide layer may imply that they have very good oxidation resistance at high temperature. The oxidation treatment also results in the increase in thickness of both the oxide and the intermetallic layers to about 120 mm in 6 h, and little thickness gains are obtained since, as shown in Fig. 5. The microhardness measurement (Fig. 6) reports that the aluminum layer has the hardness of about HV1120 with a little lower value closed to surface due to its porous structure as shown in Fig. 7, and the hardness of the steel substrate is about HV250. The decreasing hardness of the intermetallic layer between the oxide Microhardness, HV Distance, µm Fig. 6. Microhardness measurement in the cross-section of the sample after aluminizing at 750 8C for 10 min and oxidizing at 800 8C for 12 h. 170 Thickness, µm Oxide Intermetallics Time, Hour Fig. 5. Thickness of the oxide and intermetallic layers vs. time at 800 8C for the steel aluminized at 750 8C for 10 min. Fig. 7. Indentations of the microhardness measurement on the aluminized and oxidized sample.

5 D. Wang, Z. Shi / Applied Surface Science 227 (2004) Fig. 8. Line scanning profiles of EDX element distribution on the cross-section of the aluminized and oxidized sample. Weight gain, % and the steel substrate should be accounted for the changes of the intermetallic phases which are identified by the number 1, 2 and 3 in Fig. 7. By EDX line scanning on the cross-section of the oxidized specimen, three peaks for aluminum distribution reveal that in the intermetallic region, the phases presented in 1, 2 and 3 zones should be the different aluminum-containing intermetallics, as shown in Fig Property evaluation Time, Hour Oxidized Original Fig. 9. Weight change of the samples oxidized at 800 8C High temperature oxidation As shown in Fig. 9, the aluminized and oxidized steel obtains a very little weight gain, about 0.05% after the oxidation for 4 h, and its weight keeps nearly constant. The reason is that it is difficult for oxygen to 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 effectively 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 content to 40 at.% [16]. In general, the oxidation resistance of Fe Al alloys at high temperature is directly related to Al content which determines the extent of surface coverage by aluminum oxide formed. Besides, high Al-content alloys were free from decarburization, whereas low Al-content alloys suffered from decarburization [19]. In this study, the aluminizing process and the oxidation treatment produce the high Al-content surface which is fully covered by a dense top layer of aluminum oxide. The oxide layer acts as a barrier for oxygen coming into contact with steel substrate and provides an aditional protection for the steel from high temperature oxidation. Hence, the high temperature oxidation property of the stainless steel has been greatly improved Corrosion in aluminum melt The surface of the aluminized and oxidized sample does not wet with the liquid aluminum when it is immersed into the melt at 750 8C. It undergoes good wetting in 50 h, pitting in 70 h and partial etching in 100 h. The penetration of the specimen appears in 148 h. However, the sample of the original steel suffers the partial etching within 5 h and penetration failure occurs in 48 h. Covered by the 120 mm thick oxide layer, the anti-liquid aluminum corrosion property of the stainless steel has been considerably improved. The possible roles of the oxide layer played in the enhanced aluminum melt corrosion property of the stainless steel are that it decreases wetting with liquid aluminum, blocks direct contact between aluminum melt and the steel substrate, and accordingly, presents impedance to substrate dissolution into aluminum melt.

6 260 D. Wang, Z. Shi / Applied Surface Science 227 (2004) Adherency of the oxide on the steel substrate The aluminized and oxidized sample remains a perfect oxide surface for 22 thermal cycles of the rapid heating and cooling process before it experiences oxide scaling off the steel substrate, and six more thermal cycles causes 50% exposure of the substrate surface, whereas the hot spray coating of aluminum oxide on surfaces of the same steel suffers peeling off after only one thermal cycle. Unlike traditional spray coating, the greatly improved adherency of the oxide layer on the stainless steel substrate can be attributed to the oxide self-formation process in which an anchor-like bonding presents between the oxide layer and the steel substrate. In addition, the gradual distribution of the intermetallic components integrates the oxide layer with the substrate and makes a relatively good match in expansions and contractions during thermal shock between the top layers. 4. Conclusions 1. 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 with the increase in dipping temperature. Lower temperature and longer time favor a thicker layer of the aluminum on the substrate. 2. 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. 3. 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, Al 4 Cr and Al 17 Cr 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. References [1] G.A. Mollere, US Patent (1948). [2] T. Sendzimir, US. Patent (1938). [3] G. Willam, Wood Metal Handbook, ninth ed., vol. 5, Surface Cleaning, Finishing and Coating, ASM, OH 1982 p [4] T.C. Simpson, Corrosion 49 (7) (1993) 550. [5] D. Liang, et al., Scripta. Metall. Meter. 34 (10) (1997) [6] Ni Zhijian, Ren Zhongyuan, Huang Diguang, J. Northwestern Inst. Arch. Eng. 3 (1997) 42 [in Chinese]. [7] R. Asthana, Metall. Mater. Trans. 26A (1995) [8] Liu Yaohui, He Zhenming, Yu Sirong, Dong Guitian, Li Qingchun, J. Mater. Sci. Lett. 11 (1992) 896. [9] Wang Deqing, Shi Ziyuan, Zou Longjiang, Appl. Surf. Sci. 241 (1 4) (2003) 304. [10] Shigeaki Kobayashi, Takao Yakou, Mater. Sci. Eng. A 338 (2002) 44. [11] K. Bouche, F. Barbier, A. Coulet, Mater. Sci. Eng. A 249 (1998) 167. [12] B. Massalski, Binary Phase Diagram ASM Int. 1 (1990) 148. [13] L.N. Larikov, V.M. Falchenko, D.F. Polishebuk, V.R. Ryabov, A.V. Lonovskays, in: G.V. Samsonov (Ed.), Protective Coatings on Metals, vol. 3, Consultant Bureau, New York, 1971 p. 56. [14] B. Massalski, Binary Phase Diagram ASM Int. 1 (1990) 139. [15] Y.J. Li, J. Wang, X. Holly, Mater. Sci. Technol. 19 (2003) 657. [16] M.A. Montealegre, J.L. Gonzalez-Carrasco, M.A. Munoz- Morris, Intermetallics 9 (2001) 487. [17] V. Shankar Rao, R.G. Baligidad, V.S. Raja, Intermetallics 10 (2002) 73. [18] V. Shankar Rao, Intermetallics 11 (2003) 713. [19] V.S. Rao, V.S. Raja, High Temp. Mater. Processes 21 (2002) 143.