Effect of nitrogen addition on microstructure and erosion-corrosion behavior of stainless steel in acidic slurry

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1 Effect of nitrogen addition on microstructure and erosion-corrosion behavior of stainless steel in acidic slurry *Li Ping 1-3, Cai Qi zhou 2, Wei Bo kang 2 and Li Feng jun 3 (1. School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo , Henan, China; 2. School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan , China; 3. China Yituo Group Co., Ltd., Luoyang , Henan, China ) Abstract: Effect of nitrogen content on the microstructure and erosion-corrosion (E-C) behavior of stainless steel (containing about 25%Cr, 5% 6%Ni, and 1% 2%Mo) was investigated by using optical microscope(om), potentiodynamic polarization curve and immersion method, a self-made rotating disk apparatus and scanning electron microscopy (SEM) and so on. The results show that with the increase of nitrogen content from 0 (no adding) to wt.%, the Cr eq /Ni eq ratio of the tested materials decreases from 4.14 to 1.23 and the ferrite volume percentage of the steel tested linearly decreases approximately from 90% to 10%. The effect of nitrogen content on polarization behavior of the studied steel was not signifi cant. All the passive current densities ( I p ) and pure corrosion rates (V c ) of the different N content specimens in the acidic solution (slurry containing 0.1 M H 2 SO %Cl %F -, the simplifi ed zinc hydrometallurgy slurry) are too low and nearly negligible. Whereas the pure erosion rates (V e ) of the four scenarios of the steels tested (N content, wt.%: N1, no adding; N2, 0.143%; N3, 0.289%; N4, 0485%, respectively) are much higher than their V c values. The V e values of the tested steels are in the order of N4 > N1 > N3 > N2, at the same time, the total weight loss rates (V t ) follow the order of N4 > N1 >N3 > N2, too. N2 steel consisting of approximately equal volume fractions of ferrite (α) and austenite (c) exhibits the most excellent E-C resistance and erosion resistance, while the stainless steels possessing the big difference between α phase and c phase, such as N1 steel nearly possessing single ferrite or N4 steel nearly possessing single austenite, show poorer E-C resistance and erosion resistance. Key words: nitrogen; microstructure; erosion-corrosion (E-C); duplex stainless steels CLC number: TG Document code: A Article ID: (2009) Duplex stainless steels (DSSs) are characterized by having a two-phase microstructure consisting of approximately equal volume fractions of ferrite (a) and austenite (c), generally speaking, the fraction (by volume) of the phase at least is usually desired above 30% [1-3]. DSSs have a combination of excellent stress corrosion resistance and high strength similar to that of ferritic steels and high toughness similar to that of austenitic steel [1-3]. These excellent properties have made DSSs widely applied in industries, such as oil and gas refineries, offshore platforms, chemical plates, pulp and paper industries, nuclear reactors and process systems [2,4-6]. Nitrogen as a kind of austenite stabilizing elements, its Ni equivalents (Ni eq ) is 30 [7] and pitting corrosion resistance *Li ping Male, born in 1968, postdoc., associate professor. Research interests: casting process and new materials, wear-resistant and corrosion-resistant materials and their application leeping68@163.com Received: ; Accepted: equivalent (PREW) is 16 times of Cr [8], and therefore nitrogen addition, usually only a small amount like 0.1 wt.% 0.3 wt.% [9-12], can enable to substitute nickel that is expensive and scarce on the earth. Many researches have emphasized on the effect of nitrogen on microstructure and mechanical property of stainless steel [9-12], however, few researches have drawn attention to the effect of nitrogen on erosion-corrosion (E-C) behavior of stainless steel [13,14]. The purpose of this study is to investigate effect of nitrogen alloying addition on the microstructure and E-C behavior of stainless steel (containing about 25%Cr, 5% 6%Ni, and 1% 2%Mo) in order to develop a cost effective E-C resistant material of the impeller of slurry pump used in traditional zinc hydrometallurgy industry. 1 Experimental procedure The studied materials were firstly melted in a 15 kg vacuum furnace and cast into 180 mm 150 mm 60/25 mm trapezoid ingots. For adding nitrogen, Fe-59.4Cr-5.5N master alloy was 197

2 CHINA FOUNDRY used. The chemical compositions of the ingots are shown in Table 1. The ingots were machined into 20 mm 20 mm 15 mm of cubic samples for microstructure analysis and into U10 mm 50 mm of cylindrical bars for both E-C and corrosion Vol.6 No.3 inspections, and into U11.29 mm 15 mm of cylindrical coupons for electrochemical test, respectively. The samples were solution treated at 1,050 for 1.5 h and water quenched, then tempered at 550 for 1.5 h and air cooled. Table1: Chemical composition of the studied duplex stainless steel samples (wt.%) Alloys C Si Mn P S Cr Mo Ni Nb Cu V N Fe N no adding Bal. N Bal. N Bal. N Bal. Before the tests, the E-C samples were wet ground sequentially with grit emery paper, then degreased with acetone, washed with distilled water and dried, weighed prior to and after each erosion test run by an analytical balance scale with an accuracy of 0.1 mg. After the tests, the eroded surfaces were cleaned and covered with enamel, to avoid oxidation and other types of contamination. Immediately after the removal of the enamel, the eroded surfaces were observed under a scanning electron microscopy (SEM) in order to analyze the eroded mechanisms. Pure corrosion rates were measured using an immersion method by placing the samples in the tested solution. Immersion tests were conducted in the same solution at room temperature for 720 h. Electrochemical polarization curves were obtained by using CS200 type electrochemical cell comprising a saturated calomel electrode as reference and a platinum counter electrode as an auxiliary electrode, from -0.6 V to 1.0 V (SCE) at a sweep rate of 1 mv/s. All electrochemical potentials referred in this work are indicated relatively to the saturated calomel electrode (SCE). All specimens were mechanically polished with SiC paper up to 1200 grit, washed, cleaned ultrasonically, dried in air for use. The exposed area of the specimens was of approximately 1 cm 2. The pure erosion and E-C experiments were carried out using a self-made rotating disk apparatus as shown in reference [15]. The medium of pure erosion test consisted of 10wt.% silica sand (average granularity 415 µm) and distilled water. E-C test was conducted in the slurry medium of 0.1M H 2 SO %Cl %F - +10wt.% silica sand and distilled water. All tests were controlled at the temperature of 70 ± 3. The slurry flow velocity of the test was 16 m/s. The test duration was 10 h. The erosion or E-C rate of a sample was evaluated by measuring its mass loss per unit area and per unit time. 2 Results and discussion 2.1 Effect of nitrogen content on the microstructure Figure 1 is the optical microstructures of the DSSs samples containing 0 (no adding nitrogen), 0.143wt.% N, 0.289wt.% N and 0.485wt.% N under 1,050 solution treated and followed by water quenching and 550 tempering followed by air cooling, respectively. Nitrogen is a kind of strong austenite stabilizing elements, Long and DeLong [7] suggested the effect of the elements on Cr equivalents (Cr eq ) and Ni equivalents (Ni eq ) with equations: Cr eq = wt.% Cr + wt.% Mo wt.% Si wt.% Nb Ni eq = wt.% Ni wt.% Mn + 30 (wt.% N + wt.% C) Table 2 is the calculated results of the tested steels containing different N contents according to the above equations. Table 2 shows that with the increase of nitrogen content it causes the significant increase of Ni equivalents of the tested steels, while almost the same Cr equivalents (29%) of the four tested steels remained, as a result, a remarkable decrease of Cr eq /Ni eq ratio occurred. Figure 2 illustrates the analyzed method of phases change of the tested stainless steels used in Schaeffler drawing modified by Schneider in terms of Table 2 and the result is shown in Fig.3. From Fig.3 we could clearly see that with the increase of nitrogen content from 0 (no adding) to wt.% the Cr eq / Ni eq ratio of the tested materials decreases from 4.14 to 1.23 and the ferrite volume percentage linearly decreases approximately from 90% to 10%. The result is in good accordance with the optical micrograms of the studied specimens from Fig.1. Table 2: Cr and Ni equivalents of the tested steels Alloy N content (wt.%) Cr eq (wt.%) Ni eq (wt.%) Cr eq / Ni eq N N N N Polarization curves Potentiodynamic polarization curves of the specimens with different N contents in the tested solution were shown in Fig.4. From Fig.4 we could find that the corrosion potential (Ecorr) and the passive current density (I p ) decreased, passive region widened with the increase of nitrogen content, whereas, all of I p values are very small, so in general, the effect of N content on the polarization behavior of the studied steel was insignificant. All the I p values of the different N content specimens in the tested solution are too low and nearly negligible. 198

3 (a) No adding nitrogen (b) wt.% N (c) 0.289% wt.% N (d) 0.485% wt.% N (Austenite in black, ferrite in white) Fig.1: The optical micrograms of the studied specimens Fig.2: Analyzed method of phase change of the tested DSSs used in Schaeffler diagram 1 no adding nitrogen; wt.% N; wt.% N, wt.% N Fig.4: Polarization curves of DSSs containing different N contents in the tested medium, 60 mv/min, SCE. Fig.3: Change of Cr eq / Ni eq and ferrite volume fraction with nitrogen content 2.3 The effect of N content on E-C rate Figure 5 presents the results of the pure erosion weight loss rate (V e ) and the E-C total weight loss rate (V t ) of the tested steels in the tested slurry at flow velocity of 16 m/s, together with the pure corrosion loss rate (V c ). All the V c values of the tested steels are very low and the change of the V c values with N contents is almost negligible. This shows that the tested materials have excellent corrosion resistance in the tested slurry and that excellent corrosion resistance of the tested specimens results from higher concentration of corrosion resistance elements such as Cr, Ni, Mo rather than the N content. The V e values of the four kinds of N content steels tested are much higher than the V c values. The order of V e value of the tested steels follows N4 > N1 > N3 > N2, at the same time, the order of the V t values is just N4 > N1 > N3 >N2, too. Apparently, we can clearly see that the V t values are not equal to the sum of the V e value and the V c value. The large interaction (V s ) between corrosion and erosion resulted 199

4 CHINA FOUNDRY Fig. 5: The tested result of four kinds of N content steels in a significant increase of the total material loss, even if the V c values are very small [14,15]. N2 steel tested consisting of approximately equal volume fractions of ferrite (a) and austenite (c) exhibits the most excellent E-C resistance and erosion resistance, while either N1 steel nearly possessing single ferrite or N4 steel nearly possessing single austenite shows poor E-C resistance and erosion resistance. From the above results, we could conclude that good correlativity of the total weight loss rate (V t ) of the tested materials with their pure erosion mass loss rate (V e ), that is, E-C resistance of the tested steels strongly depends on their pure erosion resistance. Because stainless steels depend on a passive film for corrosion resistance, the effect of wear on the corrosion is dramatic [10-12]. Wear can produce great shifts in the corrosion Vol.6 No.3 potential of 400 mv, and increase anodic current densities by more than orders of magnitude. From the surface morphologies of SEM (Fig.6) after E-C test, we can clearly find the damage feature of brittle fracture and flake-off within N1 steel. Severe directional erosion ravines and press pits could be clearly observed on the worn surface of N4 steel. Simultaneously, the slighter erosion damage has been revealed on the worn surface of N2 and N3 steel, especially N2 steel. The above considerable difference in E-C rate and surface damage of four specimens can be attributed to the difference of their microstructures resulted from different N contents. It is reported that the austenitic phase has a low stacking fault energy (SFE) and develops a planar dislocation structure during deformation, while the ferritic phase exhibits a relatively high SFE and cross slip can easily occur during deformation [16]. N1 specimen containing nearly single-phase ferrite microstructure exhibited serious brittle flake-off resulting from smaller deformation strengthening ability of ferrite phase during strong E-C. For N4 specimen containing single-phase austenite microstructure, evident ploughed marks appeared at the worn surface under rapid erosion of solid particles in the slurry due to low hardness of N4 specimen, at the same time the martensitic transformation induced by surface deformation occurred during E-C made the surface hard and brittle. Therefore, the surface was easy to be fractured or spalled, as shown in Fig.6(d), and then fresh austenite surface exposed again. Due to this continuing plough and brittle spallation action, serious surface damage and great E-C material removal occurred within N4 specimen. The reason that the slighter surface and lower E-C rate occurred at N2 and N3 specimens can be explained by the (a) N1 steel, no adding (b) N2 steel, 0.143%N (c) N3 steel, 0.289%N (d) N4 steel, 0.485%N Fig.6: E-C tracks of DSSs containing different N contents in the tested slurry, flow velocity of 16 m/s. 200

5 different abilities of surface deformation strengthening of austenite phase (c) and ferrite (a) in DSSs. High-density dislocations in the c phase of DSS made the c phase at surface and subsurface harder than that in the a phase of DSS. The a phase and the c phase were beneficial to each other to keep the surface tough. At the same time the steels are also strengthened by the nitride formation due to the higher nitrogen addition. Furthermore, keeping a proper ratio of the c phase in DSSs is very important for corrosive wear [15], This might explain why N2 steel consisting of approximately equal volume fractions of c phase and a phase exhibited most excellent E-C resistance. 3 Conclusions (1) As a kind of strong austenite stabilizing element, nitrogen, even a small amount of addition, can substitute costly and scarce nickel. With the increase of nitrogen content in the studied range of nitrogen content, the ferrite volume percentage of the tested steel linearly decreases. (2) The effects of nitrogen contents on polarization behaviors and V c values of the four steels tested are not significant in the tested slurry, whereas large influence on the V e and V t values of the tested steels, and V e are much higher than their V c values. Though the V c values are very small, they result in the evident increase of their V t values. (3) N2 steel and N3 steel show stronger erosion and E-C resistance, especially N2 steel consisting of approximately equal volume fractions of ferrite (a) and austenite (c) exhibits the most excellent E-C resistance and erosion resistance in the tested slurry, while the stainless steel possessing the big difference of a phase and c phase, such as N1 steel nearly possessing single ferrite or N4 steel nearly possessing single austenite, shows poorer E-C resistance and erosion resistance. Therefore, the fraction (by volume) of ferrite phase in DSSs is desirable to control in the range from 30% to 50%. References [1] Solomon H D and Devine J T M. Duplex Austenitic-Ferritic Stainless Steels. In: Proceedings of Conference on Duplex Stainless Steels, R.A. Lula (Ed.), ASM, Metals Park, OH, 1982: [2] Datta P, Upadhyaya G S. Sintered duplex stainless steels from premixes of 316L and 434L powders. Materials Chemistry and Physics, 2001, 67 : [3] Merello R, Botana FJ, Botella J, et al. Infl uence of chemical composition on the pitting corrosion resistance of non-standard low-ni high-mn N duplex stainless steels. Corrosion Science, 2003, 45: [4] Charles J. Composition and properties of duplex stainless steels. Welding in the World,1995, 36: [5] Ravindranath K, Mailhotra S N. The influence of aging on the intergranular corrosion of 22 chromium-5nickel duplex stainless steel. Corrosion Science,1995,37 (1): [6] Moll M A. The world of duplex steels supply and demand of a heavy duty group of steels. In: Proceedings of the 5th International Conference on Duplex Stainless Steels, October Zutphen (Netherlands): Stainless Steel World, KCl Publishing BV, 1997: [7] Long C J, DeLong W T. The ferrite content of austenitic stainless steel weld metal. Welding J., 1973: [8] Okamoto H. The Effect of Tungsten and Molybdenum on the Performance of Super Duplex Stainless Steels. In: Proceedings of Application of Stainless Steel 92, Stockholm, Sweden, Jernkontoret, 1992, [9] Park Y H, Lee Z H. The effect of nitrogen and heat treatment on the microstructure and tensile properties of 25Cr 7Ni 1.5Mo 3W xn duplex stainless steel castings. Mater. Sci. and Eng. A, 2001, 297: [10] Park Y H, Lee Z H. Nitrogen alloying in Duplex Stainless Steel and its effect on microstructure. In: Proceedings of the 4th Asian Foundry Congress, 32nd Annual Convention of Australian Foundry Institute, Broadbeach, Qld, Australia, Oct. 1996: [11] Bi H Y, Jiang. X X, Li S Z. The corrosive wear behavior of Cr Mn N series casting stainless steel. Wear,1999, : [12] Simmons J W. Overview: high-nitrogen alloying of stainless steels. Mater. Sci. Eng. A,1996, 207: [13] Lu Xin chun, Shi Ke, Li Shizhuo, Jiang Xiao xia. Effects of surface deformation on corrosive wear of stainless steel in sulfuric acid solution. Wear, 1999, : [14] Zheng Yugui, Yao Zhiming, Wei Xiangyun and Ke Wei. The synergistic effect between erosion and corrosion in acidic slurry medium. Wear, 1995, : [15] Li Ping, Cai Qi zhou, Wei Bokang, et al. Foundry, 2006, 55(2): (in Chinese) [16] Moverare Johan J, Odeen Magnus. Deformation behaviour of a prestrained duplex stainless steel. Mater. Sci. Eng. A, 2002, 337: The study was supported by the postdoctoral foundation for Yituo Group Co., Ltd., P. R. China (No.TKJ ) and the doctoral foundation for Henan Polytechnic University, Henan (No. B ). 201