Aluminizing and Oxidation Treatment of 1Cr18ni9 Stainless Steel
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http://www.paper.edu.cn 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 0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.11.076 转载 中国科技论文在线 http://www.paper.edu.cn 256 D. Wang, Z. Shi / Applied Surface Science 227 (2004) 255–260 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 tempera- tures 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 Æ1 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, mid- dle 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, alu- minum 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.