EFFECT OF Ni 3 Al ADDITION AND HEATING MODE ON THE ELECTROCHEMICAL RESPONSE ON AUSTENITIC AND FERRITIC STAINLESS STEELS

Transcription

1 DOI /s Powder Metallurgy and Metal Ceramics, Vol. 55, Nos. 5-6, September, 2016 (Russian Original Vol. 55, Nos. 5-6, May-June, 2016) EFFECT OF Ni 3 Al ADDITION AND HEATING MODE ON THE ELECTROCHEMICAL RESPONSE ON AUSTENITIC AND FERRITIC STAINLESS STEELS A. Raja Annamalai, 1,2,4 A. Upadhyaya, 2 and D. Agrawal 3 UDC ; ; The effect of intermetallic Ni 3 Al addition and heating mode on the electrochemical behavior of sintered austenitic (316L) and ferritic (434L) stainless steels is studied. The green compacts were sintered by conventional and microwave methods in solid state at 1350 C in H 2 for a period of 60 min. The sintered samples were then subjected to several characterization techniques for evaluating their density and hardness, and finally their electrochemical response was evaluated through potentiodynamic polarization scan (1 mv/sec) in 0.1 N H 2 SO 4. Upon addition of nickel aluminide, there was an increase in the densification of the composites, which was found more pronounced in case of 434L stainless steel with nickel-aluminide dispersoid composites. A positive response was also observed on the corrosion resistance of the composites as compared with the bare stainless steel compacts. Keywords: stainless steel, sintering, intermetallic, density, hardness. INTRODUCTION Powder metallurgy (P/M) processing of stainless steels offers advantage of net-shaping, high material utilization, and ability to tailor microstructures and modify compositions using dispersoid additions. The parts of complex geometry can be produced to close tolerances in an economical manner through P/M route. Despite these advantages, the P/M stainless steels suffers relatively poor mechanical and corrosion properties attributed to their inherent porosity when compared to their wrought and cast counterparts and which have been reported by various researchers in different environments [1, 2]. To enhance the mechanical and tribological properties of the P/M stainless steels, hard dispersoids such as Al 2 O 3 and SiC have been reinforced. Powder metallurgy processing allows the microstructural modification with respect to the reinforcement size, shape, and placement [3]. Several researchers have reported the beneficial mechanical and wear behavior of these MMCs (Metal Matrix Composites) [4 7]. However, the major drawback of these composites is their poor corrosion behavior compared to straight stainless steels. The poor interaction between the matrix and reinforcements promotes the onset of the corrosion attack. Shankar et al. [8] reported that Y 2 O 3 addition marginally increases densification in 1 Department of Manufacturing Engineering, School of Mechanical Engineering, VIT University, Vellore, Tamil Nadu , India. 2 Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, UP, , India. 3 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA. 4 To whom correspondence should be addressed; raja.anna@gmail.com. Published in Poroshkovaya Metallurgiya, Vol. 55, Nos. 5 6 (509), pp , Original article submitted January 12, /16/ Springer Science+Business Media New York

2 316L compacts in solid state and supersolidus sintering condition. This finding was attributed to the interaction of Cr 2 O 3 with the Y 2 O 3 dipersoids. Subsequent analysis revealed no significant degradation of the corrosion resistance in 316L Y 2 O 3 composites as compared to straight 316L compacts. Jain et al. [7] have also observed similar interaction for YAG additions to 316L and 434L stainless steels. An improve ment in the wear and corrosion resistance has been observed as reinforcing with intermetallics produced in situ through reactive sintering [9]. As compared to the oxide and carbide reinforcement the aluminide based intermetallics have been shown to have better interaction with the stainless steel matrix. It is envisaged that such an interaction results in enhanced corrosion resistance [10]. Furthermore, the interaction can aid in smoother compositional variation, which can assist in reducing localized (e.g., intergranular) corrosion. The present work investigates the effect of intermetallic (Ni 3 Al) addition and heating mode on the electrochemical behavior of sintered austenitic (316L) and ferritic (434L) stainless steels. The compacts were sintered in solid-state (1350 C) conditions. The sintered samples were characterized for their corrosion resistance by potentiodynamic polarization scan in 0.1 N H 2 SO 4. EXPERIMENTAL PROCEDURES The stainless steels (316L and 434L) and nickel-aluminide (Ni 3 Al) powders were used for the present study. The powders were supplied by Ametek Speciality Metal Products (USA) and Xform Inc. (USA). The chemical composition of the stainless steel powders is summarized in Table 1. The aluminide powders were found to have a purity more than 99.7%; their characteristics are summarized in Table 2. The composites investigated in the present work are 316L and 434L stainless steels with different proportions (4, 8, and 12 vol.%) of aluminides (Ni 3 Al). The powders were mixed in the required proportions in a turbula mixer (model: T2C, supplier: Bachofen, Basel, Switzerland) for 20 min. Cylindrical pellets (16 mm diameter and 6 mm average thickness) were compacted from the mixed compositions at 600 MPa using a 50 ton hydraulic press (model: CTM-50; supplier: FIE, Ichalkaranji, India) with a floating die, using zinc stearate as a die wall lubricant. The green densities of the compacts were around 80% of theoreti-cal density. The green compacts were sintered in a MoSi 2 heated horizontal tubular sintering furnace (model: OKAY 70T-7; supplier: Bysakh, Kolkata, India) at a heating rate of 5 C/min. The as-pressed compacts were consolidated isothermally at 1350 C, which correspond to solid-state sintering. Sintering was carried out in hydrogen atmosphere (dew point: 35 C). TABLE 1. Composition of the As-Received Powders (in wt.%) Used in the Present Investigation Powder C Si Mn Ni Cr Mo S P Fe 316L Bal. 434L Bal. TABLE 2. Characteristics of the As-Received Powders with Spherical Shape of the Particles Property Powder 316L 434L Ni 3 Al Cumulative powder size, m: D D D Apparent density, g/cm Flow rate, sec/50 g Theoretical density, g/cm

3 The sintered densities were determined from dimensional and weight measurements. Microwave sintering of the green compacts was carried out using a multimode cavity 2.45 GHz, 6 kw commercial microwave furnace (model: RC/20SE; supplier: Amana Radarange). The sintered compacts were prepared for metallography and chemically etched in Marvels reagent. The microstructural observations were carried out through an optical microscope (model: Lunn Major; supplier: Leitz, Germany) as well as the scanning electron microscope (model: JSM-840A; supplier: JEOL, Japan). For electrochemical evaluation, the cylindrical samples were polished to mirror finish and ultrasonically cleaned in acetone. The potentiodynamic scans were conducted using a flat cell, supplied by Accutrol Inc., USA, at room temperature. A standard three-electrode (reference, counter, and working) technique was used for the measurement. The reference electrode used for the present experiment was a calomel electrode (SCE) saturated with saturated KCl and the sintered alloy was chosen as the working electrode. A platinum mesh acted as a counter electrode. The area of the sintered sample covered to the solution was 1 cm 2. The electrochemical experiments were performed using Gamry Instruments, Inc., potentiostat, (model: PC4) on a CMS100 Framework. Prior to the polarization test, each sample was stabilized for about 3600 sec in the solution to get a stable open circuit potential (OCP). The potentiodynamic polarization tests were conducted at-least three times at a constant scan rate of 1 mv/sec in freely aerated 0.1 N H 2 SO 4. The potentiodynamic scan of a sample was given in graphical form (voltage versus current density). The critical parameters like corrosion potential (E corr ), corrosion current (I corr ), and corrosion rate were evaluated from the polarization curves. The passivation current density (I p ), critical current density (I crit ), primary passivation potential (E pp ), and the trans-passive potential were also determined from the potentiodynamic polarization scan. RESULTS AND DISCUSSIONS The variation of sintered density with Ni 3 Al addition for 316L and 434L stainless steels consolidated using conventional and microwave furnace is pre-sented in Table 3. The effect of Ni 3 Al addition on densification improved the density of stainless steel compacts. Microwave sintering of stainless steel grades 316L and 434L results in higher sintered density. In comparison to the density of conventional processed compacts, the enhancement in densification during microwave sintering is more pronounced for 434L Ni 3 Al composites. During conventional sintering, the dispersoids, which are in general refractory in nature, remain segregated at the metal metal interparticle regions and inhibit compact densification [11]. These observations suggest that Ni 3 Al particles in the composite independently couple with microwaves. The interaction of microwaves with this reinforcement is perhaps more effective than even stainless steel powders. This results in formation of localized hot spots within the compacts during sintering. This not only activates densification but also concomitantly results in microstructural coarsening. In summary, it is possible to TABLE 3. Effect of Ni 3 Al Addition and Heating Mode on the Sintered Density of Austenitic (316L) and Ferritic (434L) Stainless Steel Compacts Sintered at 1350 C Ni 3 Al content, vol.% % Theoretical density 316L 434L Conventional Microwave Conventional Microwave Average grain size, µm % Theoretical density Average grain size, µm % Theoretical density Average grain size, µm % Theoretical density Average grain size, µm (*) 83.0 (*) 77.2 (*) (*) Grain size could not be determined. 290

4 Fig. 1. Optical micrographs of 316L Ni3Al composites with varying amounts of nickel aluminide addition (0, 4, 8, 12 vol.%) and sintered in conventional and microwave furnaces engineer local thermal instability and induce anisothermal effects in microwave-sintered ferrous systems by appropriately tailoring the composition. It is envisaged that more in-depth investigation of this effect can result in engineering a novel microstructure through alloying iron with reinforcements having different coupling behavior with microwaves. Peelameduet et al. [12] and Roy and co-workers [13] have demonstrated that in multiphase systems, the phases heat up at different rates. This was shown to result in local anisothermal heating effect. Through model experiments in systems, such as Y2O3 Fe3O4, BaCO3 Fe3O4, and NiO Al2O3, Peelamedu et al. [12] demonstrated that the hotter species diffuse rapidly into the relatively colder ones and correlated it with the very rapid diffusion rates observed in microwave processed compacts. To test this hypothesis and to utilize its effect on the enhancement of densification and corrosion resistance in ferrous alloys, stainless steel was selected as a candidate system. 291

5 Fig. 2. Optical micrographs of 434L Ni3Al composites with varying amounts of nickel aluminide addition (0, 4, 8, 12 vol.%) and sintered in conventional and microwave furnace Figures 1 and 2 show the microstructures of 316L Ni3Al and 434L Ni3Al composites consolidated in conventional as well as microwave furnace. In the both grades, aluminide dispersoids are uniformly distributed. Both austenitic and ferritic stainless steel with dispersed Ni3Al show relatively coarse grains in microwave-sintered condition (Figs. 1b 1d and 2b 2d). The extent of microstructural coarsening seems to enhance with increasing aluminide content. In case of conventional sintering, aluminide addition seems to have an opposite influence on the microstructural coarsening and seems to act as grain growth inhibitors. For conventionally sintered 316L Ni3Al and 434L Ni3Al composites, increasing dispersoid addition results in smaller grain size. Figure 3 shows the effect of Ni3Al addition and sintering mode on the bulk hardness of 316L and 434L compacts. The addition of aluminide increases the hardness of stainless compacts. In general, the hardness increases with increasing Ni3Al addition. For each composition, as compared to conventional sintering, microwave sintering results in either equivalent or moderately higher hardness. 292

6 a b Fig. 3. Effect of heating mode and varying Ni 3 Al content addition on the bulk Vickers hardness of 316L and 434L stainless steels Figures 4a and 4b present the potentiodynamic polarization curves for 316L Ni 3 Al composites sintered in conventional and microwave sintering furnaces, respectively. The corresponding polarization curves for Ni 3 Al composites are shown in Fig. 5. The trends in anodic polarization for conventionally sintered 316L Ni 3 Al composites reveal a passive behavior in 0.1 N H 2 SO 4 up to an applied potential of 1000 mv. Beyond that, the increase in the current density increases progressive breakdown of the passive layer on the surface. A similar trend is exhibited by microwave sintered 316L Ni 3 Al composites as well (Fig. 4b). A comparison of Figures 4a and 4b reveals that current density in the anodic region for microwave sintered compacts is lower than its conventionally sintered counterpart. This is reflected in the relatively lower I corr and corrosion rates in microwave sintered 316L Ni 3 Al compacts (Table 4). Unlike the 316L Ni 3 Al compo-sites, the 434 Ni 3 Al compacts processed through conventional as well microwave sintering routes exhibit active passive transition (Figs. 5a and 5b). However, unlike the austenitic steels, microwave sintering does not result in a marked improvement in the corrosion rate of 434L Ni 3 Al composites (Table 4). Velasco et al. [10] have reported that intermetallic additions, such as Cr 2 Al and TiAl, improve the corrosion response of austenitic stainless steels. In sintered multiphase systems, such as composites, the presence of features having different electrochemical potentials results in the formation of microcells. Sites of high energy (e.g., grain boundaries) tend to be anodic, whereas reinforcements tend to be of relatively lower energy and hence are cathodic [12]. Thus, both topological as well as compositional features on the exposed surface influence the corrosion results. a b Fig. 4. Comparison of the potentiodynamic polarization curves in 0.1N H 2 SO 4 of 316L Ni 3 Al composites with varying aluminide addition: the compacts were consolidated using conventional (a) and microwave (b) furnace 293

7 TABLE 4. Effect of Heating Mode and Aluminide Content on the Passivity Parameters of 316L and 434L Compacts Obtained from the Potentiodynamic Polarization Investigation Composition Sintering mode I corr, A/cm 2 E corr, mv Corrosion rate, mpy 316L compacts 316L Conventional Microwave L 4Ni 3 Al Conventional Microwave L 8Ni 3 Al Conventional Microwave L 12Ni 3 Al Conventional Microwave L compacts 434L Conventional Microwave L 4Ni 3 Al Conventional Microwave L 8Ni 3 Al Conventional Microwave L 12Ni 3 Al Conventional Microwave The corrosion resistance of any material strongly depends on its microstructural features. From the standpoint of corrosion, the addition of the chemically noninert dispersoids to stainless steels has two opposing influences. While Ni 3 Al addition in optimal level increases the densification, the microstructure coarsening too is restricted. Hence, the structure is more refined. This would contribute to corrosion through formation of local galvanic couples betweenstainless steel and aluminides. This can be termed as microstructural effect. On the other hand, the activation of densification will reduce the pores that are active sites and thereby result in enhancement in corrosion resistance. In case of microwave sintering, the anisothermal heating effect results in coarser microstructures in stainless steel Ni 3 Al composites, which further contributes to corrosion resistance. This benign role of Ni 3 Al on the corrosion response is further confirmed for ferritic stainless steel. However, the high corrosion rates made the comparison between individual compositions rather difficult. In summary, the trend in the corrosion resistance of sintered stainless steels and their composites can be summarized as. 294 a Fig. 5. Potentiodynamic polarization curves of conventionally (a) and microwave (b) sintered 434L Ni 3 Al composites with varying aluminide addition b

8 The effect of heating mode and Ni 3 Al addition on the corrosion resistance (increasing trend) of austenitic and ferritic stainless steel can be summarized as follows. Conventional sintering: 316L 12Ni 3 Al 316L 8Ni 3 Al straight 316L 316L 4Ni 3 Al, straight 434L 434L 8Ni 3 Al 434L 4Ni 3 Al 434L 12Ni 3 Al; Microwave sintering: 316L 8Ni 3 Al straight 316L 316L 4Ni 3 Al 316L 12Ni 3 Al, 434L 4Ni 3 Al 434L 8Ni 3 Al straight 434L 434L 12Ni 3 Al (the compositions that resulted in significant reduction in the corrosion rate have been underlined). In case of microwave sintering, the positive effect of aluminide addition on the corrosion response can be attributed to the enhanced microstructural coarsening (Figs. 1 and 2). The sintered microstructures of aluminidereinforced austenitic and ferritic stainless steels suggest that Ni 3 Al retains its identity and appears as distinct phases. The Ni 3 Al boundaries appear as a dark rim. The phase formation in the interfacial region was critically evaluated through EDS analysis by Upadhyaya and Balaji [14, 15]. The interface between the aluminide particles and stainless steel that appear in the sintered structure was confirmed to predominantly containing Al 2 O 3. Similar observation has been reported by Abenojar and co-workers [16] in 316L 3 vol.% Cr 2 Al stainless steels sintered at 1230 C. Extensive interaction between the intermetallic reinforcements (TiAl, Cr 2 Al, and TiCr 2 ) with the stainless steel matrix has been reported by Abenojar and co-workers [16]. The reduction in corrosion rate with Al 2 O 3 addition to the stainless steel was reported by Mukherjee and Upadhyaya [17]. They attributed this to the interaction between Cr 2 O 3 and Al 2 O 3 that results in solid solution formation [18]. Thus, it can be concluded that the improvement in corrosion properties of nickel-aluminide reinforced stainless steels is due to the interaction of Cr 2 O 3 with the Al 2 O 3 that forms in situ at the Ni 3 Al 316L/434L interface during sintering. CONCLUSIONS The microwave sintering enhances the densification in both austenitic 316L and ferritic 434L stainless steels aluminide composites. This improvement can be attributed to the faster heating rate in microwave, which enhances the sintering kinetics. A change in grain morphology and respective grain size and pore closure is observed in case of microwave sintered compacts. The addition of aluminides to both 316L and 434L stainless steels increases the bulk hardness. This effect is attributed to the higher hardness of the dispersoid particles. In the case of microwave sintered composites, I crit decreases for Ni 3 Al addition. This could be due to the effect of Ni present in the Ni 3 Al, which is known to decrease the I crit in the stainless steel systems. The slight increase in E corr for the microwave sintered compacts indicates its noble behavior as compared to the conventional sintering compacts. REFERENCES 1. J. Shankar, A. Upadhyaya, and R. Balasubramaniam, Electrochemical Behavior of Sintered Oxide Dispersion Strengthened Stainless Steels, Corros. Sci., 46, No. 2, (2004). 2. E. Otero, A. Pardo, E. Saenz, et al., Corrosion behaviour of AISI 304L and 316L stainless steels prepared by powder metallurgy in the presence of sulphuric and phosphoric acid, Corros. Sci., 40, No. 8, (1998). 3. R. M. German, Powder Metallurgy Science, Metal Powder Industries Federation, New Jersey, USA (1994). 4. R. M. German, Supersolidus liquid-phase sintering of prealloyed powders, Metall. Mater. Trans. A., 28, (1997). 5. F. Velasco, N. Anton, J. M. Torralba, and J. M. Ruiz-Prieto, Mechanical and Corrosion Behavior of Powder-Metallurgy Stainless-Steel Based Metal-Matrix Composites, Mater. Sci. Technol., 13, Issue 10, (1997). 295

9 6. S. N. Patankar and M. J. Tan, Role of reinforcement in sintering of SiC/316L stainless steel composite, Powder Metall., 43, No. 4, 350 (2000). 7. J. Jain, A. M. Kar, and A. Upadhyaya, Effect of YAG addition on sintering of P/M 316L and 434L stainless steels, Mater. Lett., 58, (2004). 8. J. Shankar, A. Upadhyaya, and R. Balasubramaniam, Supersolidus liquid phase sintering of austenitic stainless steel yttria composites, in: Advances in Powder Metallurgy Particulate Materials 2002, Vol. 13, Metal Powder Industries Federation, Princeton, New Jersey, USA (2202), pp P. Jackson, C. C. Degnan, and J. V. Wood, Reactive sintering of 316L stainless steel by the formation of a nickel aluminide liquid phase, in: Proc. EUROMAT Materials Development and Processing, Vol. 8, Wiley VCH, London (1999), pp F. Velasco, W. M. Lima, N. Anton, et al., Effect of intermetallic particles on wear behaviour of stainless steel matrix composites, Tribol. Int., 36, 547 (2003). 11. Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM Handbook, Vol. 1, ASM International (1990), pp R. D. Peelamedu, R. Roy, and D. Agrawal, Anisothermal reaction synthesis of garnets, ferrites, and spinels in microwave field, Mater. Res. Bull., 36, (2001). 13. R. Roy, Y. Fang, J. Cheng, and D. K. Agrawal, Decrystallizing solid crystalline titania, without melting, using microwave magnetic fields, J. Amer. Ceram. Soc., 88, No. 6, (2005). 14. S. Balaji, G. Joshi, and A. Upadhyaya, Corrosion behavior of sintered aluminide reinforced ferritic stainless steels, Scripta Mater., 56, (2007). 15. S. Balaji and A. Upadhyaya, Electrochemical behavior of sintered YAG dispersed 316L stainless steel composites, Mater. Chem. Phys., 101, Nos. 2 3, (2007). 16. J. Abenojar, F. Velasco, J. M. Torralba, et al., Reinforcing 316L stainless steel with intermetallic and carbide particles, Mater. Sci. Eng. A., 335, 1 5 (2002). 17. S. K. Mukherjee and G. S. Upadhyaya, Sintering of 434L ferritic stainless steel containing Al 2 O 3 particles, Int. J. Powder Metall. Powder Technol., 19, (1983). 18. M. C. Baran and B. A. Shaw, Corrosion modes in P/M ferritic stainless steels, Int. J. Powder Metall., 36, No. 4, (2000). 296