Effect Of Sintering Temperature, Heating Mode And Graphite Addition On The Corrosion Response Of Austenitic And Ferritic Stainless Steels

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1 Trans. Indian Inst. Met. Vol. 61, Nos. 2-3, April-June 2008, pp TP 2216 Effect Of Sintering Temperature, Heating Mode And Graphite Addition On The Corrosion Response Of Austenitic And Ferritic Stainless Steels C. Padmavathi, G. Joshi, A. Upadhyaya and D. Agrawal * Department of Materials & Metallurgical Engineering, Indian Institute of Technology, Kanpur, India * Materials Research Institute, The Pennsylvania State University, University Park, PA, USA cpadma@iitk.ac.in (Received 10 December 2007 ; in revised form 15 March 2008) ABSTRACT In recent years, sintered stainless steel with carbon addition has been proposed for potential use as exhaust flanges in automotive applications. The net-shaping requirements for such applications necessitate the use of powder metallurgical (P/M) processing route. However, due to the presence of porosity, most sintered steels have poor corrosion resistance. The present study compares the effect of sintering temperature, heating mode and graphite addition (up to 1.5%) on the densification and electrochemical response of both ferritic (434L) and austenitic (316L) stainless steels. The compacts were sintered in both conventional (radiatively heated) as well as microwave furnace. As compared to conventional sintering, samples consolidated in microwaves have higher densification (particularly at 1200 C) and exhibit better corrosion properties. 1. INTRODUCTION As compared to conventional casting techniques, powder metallurgical (P/M) processing is a near-net shape technique that offers greater material utilization (>95%); more microstructural homogeneity and ability to tailor novel compositions 1,2. P/M stainless steels are usually solid-state sintered. As compared to their wrought counterparts, the solid-state sintered stainless steels have usually poor mechanical properties and corrosion resistance due to inherent porosity 3. Consequently, the sintered compacts require costly post-sintering operations, such as hot pressing, repress-sintering, or forging which increases the cost. The mechanical properties of sintered stainless steels be improved through consolidation at higher sintering temperatures; novel sintering techniques; and use of alloying additives that aid densification kinetics through liquid phase sintering. Many of these sintering aids however often lead to deterioration in the corrosion resistance 4. This work investigates the influence of graphite as a sintering aid and the electrochemical response of the graphite-added ferritic (434L) and austenitic (316L) stainless steels as compared to their wrought counterparts. Recently, Panda et al. 5-7 have shown that stainless steels powder compacts couple with microwaves and can be rapidly heated and sintered. Microwave sintering results not only in significant reduction in the processing time (~70%) but also restricts microstructural coarsening, which in turn, leads to greater compact densification. More recently, Padmavathi et al. 8 have reported that stainless steel composites sintered a relatively higher temperatures in microwave furnace have somewhat better electrochemical response. This paper investigates the corrosion behavior of austenitic and ferritic grade stainless steel - with and without graphite addition consolidated through conventional as well as microwave sintering The electrochemical behavior has been correlated with the densification response and microstructural attributes. 2. EXPERIMENTAL For the present investigation, austenitic and ferritic grades were selected. The nominal compositions of the powders used in the present investigation are presented in Table 1. For both grades, the powders were prepared by gas atomization and are in prealloyed form. The characteristics of the as-received powders is detailed in Table 2. To both the grades, graphite powder was mixed in different proportions (0.5, 1.0 and 1.5 wt %) in a Turbula mixer (model: T2C, supplier: Bachoffen, Basel, Switzerland) for 20 min. The powders were then uniaxially pressed to cylindrical pellets (16 mm dia, and 6 mm height) 600 MPa in a hydraulic press (model: CTM-50, supplier: FIE, Ichalkaranji, India). To minimize the friction along the walls of die zinc stearate was used as a die wall lubricant. The green densities of pure and graphite added 316L and 434L stainless steel samples were found to be ± 0.04 and ± 0.05, respectively.. Conventional sintering of green compacts were carried out in a MoSi 2 heated horizontal tubular sintering furnace (model: OKAY 70T-7, supplier: Bysakh, Kolkata, India). The compacts were heated at a constant heating rate of 5 C/min and isothermally held for 1h at 1200 C and 1300 C. Sintering was Table 1 Composition of the austenitic and ferritic grade stainless steels used in the present study. Grade * 316L 434L Composition, wt.% Fe-16.5Cr-12.9Ni-2.48Mo-0.93Si-0.21Mn C-0.008S-0.01P Fe-17Cr-1Mo-0.71Si-0.2Mn-0.023C-0.02S-0.02P * Supplier: Ametek Specialty Metals Products, USA

2 240 Padmavathi et al. : Effect of sintering temperature, heating mode and graphite addition Table 2 Characteristics of the powders in as-received conditions used in the present study Property 316L Powder 434L Processing technique Gas atomization Gas atomization Powder shape Rounded Spherical Cumulative powder size, μm D D D Apparent density, g/cm Flow rate, s/50g Theoretical density, g/cm Fig. 1 : Effect of graphite addition and heating mode (MWS vs CON) on the density of 316L compacts sintered at (a) 1200 C and (b) 1300 C. carried out in hydrogen (dew point: -35 C). Microwave sintering of green compacts was carried out using a 6 kw, 2.45 GHz multi-mode cavity microwave furnace. Further detail of the microwave sintering setup used for the present study is provided elsewhere 6,7. The sintered density was determined by dimensional measurements. The microstructural studies were carried out using optical microscope. The sintered compacts were etched for about 10 s by Marble s reagent (50 ml HCl + 25 ml of saturated aqueous CuSO 4 solution). The electrochemical behavior of all microwave and conventionally sintered samples was studied in 0.1N H 2 SO 4 solution using a DC potentiodynamic polarization equipment (model: PC4, supplier: Gamry Instruments Inc.). Before corrosion testing, samples were polished with 600 SiC papers for obtaining smooth surface and then stabilized for 3600s in 0.1N H 2 SO 4 for obtaining stable open circuit potential (OCP). Potentiodynamic polarization testing was carried out in a flat corrosion cell (supplier: Accutrol Inc., USA) using a standard three-electrode configuration with KCl as the counter electrode. The polarization testing was carried out from 800 mv to 1800 mv (w.r.t. SCE) at a scan rate of 1 mvs -1 to construct the Tafel plots (logarithmic variation as a function of voltage). The corrosion potential (E corr ) and corrosion current (I corr ) were determined from the intersection of these two linear plots 9. The corrosion rate can be determined using 1st-Stern method and is expressed as follows: Corrosion Rate= CωI corr / ρ (1) where C is conversion factor, ω is the equivalent weight (g); ρ is the density of the material (mgm 3 ) and I corr the corrosion current (ma/m 2 ). The Tafel slopes were estimated from corresponding anodic and cathodic curves. 3. RESULTS AND DISCUSSION 3.1 Densification response Figures 1a and 1b shows effect of heating mode and graphite addition on the density of 316L compacts sintered at 1200 C Fig. 2 : Optical micrographs of (a) pure 316L and (b) 316L- 1.5% graphite sintered in conventional and microwave furnace at 1300 C. and 1300 C, respectively. Irrespective of the heating mode, at both the temperatures, graphite addition results in density enhancement. As compared to 1200 C, compacts sintered at higher temperature results in greater densification. This can be attributed to the faster diffusion kinetics at 1300 C. Graphite addition, lowers the solidus temperature of stainless steel and hence results in activating the sintered density. As compared to conventional sintering (CON), microwave sintered (MWS) compacts have densification behaviour at 1200 C. In contrast, at higher temperature (1300 C) conventional furnace sintering is found to be more effective. Figure 2 compares the microstructures of straight 316L and 316L-1.5C sintered at 1300 C in conventional and microwave furnace. It is to be noted that for both compositions, microwave sintering results in a more refined microstructure. Figure 3 shows the effect of heating mode and sintering temperature on the densification behaviour for both straight 434L as well as graphite-added 434L samples. The behaviour is similar to 316L compacts. For both sintering temperatures,

3 Padmavathi et al. : Effect of sintering temperature, heating mode and graphite addition 241 Fig. 3 : Effect of graphite addition and heating mode on the sintered density of 434L compacts consolidated at (a) 1200 C and (b) 1300 C. graphite addition leads to enhanced densification. As compared to austenitic stainless steels, in ferritic steels microwave sintering leads to significant increase in the density. This can be attributed to the better H-field interaction with the ferritic stainless particulate compacts 5. In case of ferritic stainless steels microwave sintering and graphite addition both seem to be having a synergistic effect upon densification at both 1200 C as well as 1300 C. The only exception to this is the sintering response of 434L-1.5% graphite at 1300 C. A comparison of the microstructures of 434L and 434L-1.5C compacts (Fig. 4) reveals that conventionally sintered compacts have a coarser microstructure than their microwave sintered counterparts. 3.2 Corrosion behaviour Figures 5a and 5b show the potentiodynamic polarization curves for microwave and conventionally sintered 316L and 316L-graphite compacts samples at 1200 and 1300 C, respectively. Table 3 summarizes the corresponding corrosion parameters. From Fig. 5 and Table 3, it is well evident that as compared to conventional sintering, the E corr values are become nobler with decreasing anodic current density for all microwave sintered samples. For conventionally sintered Fig. 4 : Optical micrographs of (a) pure 434L and (b) 434L- 1.5% graphite sintered in conventional and microwave furnace at 1300 C. compacts at 1200 C and 1300 C it is clearly evident that graphite addition results in increased corrosion resistance. This correlates well with the densification results shown in Fig. 1. As compared to conventional sintering, superior corrosion resistance was observed for microwave sintered samples. This correlates well with the densification enhancement in microwave sintering owing to microstructural refinement during microwave sintering (Fig. 2). Overall, for austenitic stainless steels, combination of high sintering temperature and graphite addition favours an improvement in the corrosion resistance. For both the sintering temperatures, all compacts sintered in microwaves exhibit active-passive transition, which is similar to the behaviour expected for wrought stainless steel 10. From Table 3, it is to be noted that at 1300 C, the conventionally sintered 316L with 1% C resulted in least corrosion. For microwave sintering, pure Table 3 Effect of sintering temperature and graphite addition on electrochemical behavior of 316L stainless steel sintered at 1200 and 1300 C in conventional and microwave furnace. Sample E corr (mv) I corr (μacm -2 ) Corrosion rate (mmpy) Temperature ( C) L pure (CON) L-0.5%C (CON) L-1.0%C (CON) L-1.5%C (CON) L pure (MWS) L-0.5%C (MWS) L-1.0%C (MWS) L-1.5%C (MWS)

4 242 Padmavathi et al. : Effect of sintering temperature, heating mode and graphite addition Table 4 Effect of sintering temperature and graphite addition on electrochemical behavior of 434L stainless steel sintered at 1200 and 1300 C in conventional and microwave furnace. Sample E corr (mv) I corr (macm -2 ) Corrosion rate (mmpy) L pure (CON) L-0.5%C (CON) L-1.0%C (CON) L-1.5%C (CON) L pure (MWS) L-0.5%C (MWS) L-1.0%C (MWS) L-1.5%C (MWS) Fig. 5 : Potentiodynamic polarization curves for 316L and 316Lgraphite compacts sintered conventionally and in microwave furnace at (a) 1200 C and (b) 1300 C. Fig. 6 : Potentiodynamic polarization curves for conventional and microwave sintered 434L and 434L-graphite compacts sintered at (a) 1200 and (b) 1300 C. 316L and 316L-1.5C compacts (sintered at 1300 C) exhibit superior corrosion resistance which is equivalent to the level expected for wrought stainless steels 11. Figures 6a and 6b show the potentiodynamic curves for pure 434L and graphite-added samples sintered in conventional and microwave furnace at 1200 and 1300 C, respectively. Table 4 summarizes the corrosion parameters for all the sintered compacts. It is worth noting that as compared to samples sintered conventionally, those sintered in microwave furnace at 1200 C have marginal differences in corrosion potential and current densities. At 1200 C, pure 434L compacts has the least corrosion rate. Similarly at 1300 C, lower anodic current densities and nobler corrosion potential was observed in 434L-1.5C consolidated through microwave as well as conventional sintering. This correlates well with the densification behavior. While all microwave sintered samples resulted in passive behavior, it is rather surprising to note that this did not lead to significant improvement in their corrosion resistance. In fact, from the corrosion results (Table 4), it can be inferred that for all compositions the corrosion rates are comparable to those of conventional sintered compacts. This indicates that the nature of passive film formation on the surface in microwave sintered compacts is probably not that stable. It would be interesting to conduct systematic evaluation of the nature of the passive film formation in microwave sintered stainless steels through Electrochemical Impedance Spectroscopy (EIS) techniques.. 4. CONCLUSIONS The present research work focuses on developing austenitic (316L) and ferritic (434L) stainless steel grades with improved corrosion properties through variation in the sintering temperature (1200 C and 1300 C); heating mode (conventional vs microwave); and graphite addition (up to 1.5%). For both the grades, higher sintering temperature and graphite addition lead to enhanced compact density. For ferritic stainless steel, microwave sintering is effective at both temperatures (1200 C and 1300 C). In case of straight 316L and 316L-graphite compacts consolidated at 1200 C, microwave sintering is more effective than conventional sintering. Microwave sintered compacts have a more refined structure as compared their conventionally sintered counterparts. Optimal amount of graphite addition in conjunction with microwave sintering demonstrates an improvement in the corrosion resistance

5 Padmavathi et al. : Effect of sintering temperature, heating mode and graphite addition 243 which is correlated to the microstructural and densification response. ACKNOWLEDGMENTS The authors acknowledge the assistance provided by Mr. Gaurav Mishra and Mr. Joe Stearns in the experimentation. REFERENCES 1. German R M, Powder Metallurgy Science, Metal Powder Industries Federation, Princeton, NJ, USA (1994). 2. Kaysser W A and Petzow G, Present State of Liquid Phase Sintering, Powder Metallurgy, 28 (1985) German R M, Powder Metallurgy of Iron and Steel, John Wiley, NY, USA (1998). 4. Tandon R and German R M, Sintered and Mechanical Properties of Boron Doped Austenitic Stainless Steel, International Journal of Powder Metallurgy, 34 (1998) Panda S S, Upadhyaya A and Agrawal D, Effect of heating mode and temperature on sintering of YAG dispersed 434L ferritic stainless steel, J. Mater. Sci., 42 (2007) Panda S S, Upadhyaya A, Agrawal D, Effect of Conventional and Microwave Sintering on the Properties of Yttria Alumina Garnet-Dispersed Austenitic Stainless Steel, Metallurgical and Materials Transactions A, 37A (2006) Panda S S, Upadhyaya A and Agrawal D, Effect of heating mode and temperature on sintering of YAG dispersed 434L ferritic stainless steel, J. Mater. Sci, 42 (2007) Padmavathi C, Upadhyaya A and Agrawal D, Corrosion Behavior of Microwave Sintered Austenitic Stainless Steel Composites, Scripta Materialia, 57 (2007) Fontana M G, Corrosion Ion Engineering, McGraw-Hill, New York, USA, (1986). 10. Sedriks A J, Corrosion of Stainless Steels, John Wiley & Sons, Inc, New York, NY, USA, (1996). 11. Klar E and Samal P, Powder Metallurgy Stainless Steel, ASM International, Materials Park, OH, USA, (2007).