Effect of Annealing Temperature on the Novel Lean Duplex Stainless Steel

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The effect of annealing temperature (1000–1150 °C) on the microstructure evolution, mechanical properties, and pitting corrosion behavior of a newly developed novel lean duplex stainless steel with 20.53Cr-3.45Mn-2.08Ni-0.17N-0.31Mo was studied by means of optical metallographic microscopy (OMM), scanning electron microscopy (SEM), magnetic force microscopy (MFM), scanning Kelvin probe force microscopy (SKPFM), energy dispersive X-ray spectroscopy (EDS), uniaxial tensile tests (UTT), and potentiostatic critical pitting temperature (CPT).
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   Materials   2014 ,  7  , 6604-6619; doi:10.3390/ma7096604   materials   ISSN 1996-1944 www.mdpi.com/journal/materials  Article Effect of Annealing Temperature on the Mechanical and Corrosion Behavior of a Newly Developed Novel Lean Duplex Stainless Steel Yanjun Guo 1 , Jincheng Hu 2 , Jin Li 1, *, Laizhu Jiang 2 , Tianwei Liu 3  and Yanping Wu 4   1  Department of Materials Science, Fudan University, No. 220, Handan Road, Shanghai 200433, China; E-Mail: 13110300027@fudan.edu.cn 2  Research and Development Center, Baosteel Co., Ltd., No. 885, Fujin Road, Shanghai 201900, China; E-Mails: hujincheng@baosteel.com (J.H.); lzjiang@baosteel.com (L.J.) 3  Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box 718-35, Mianyang 621907, Sichuan Province, China; E-Mail: liutianwei@caep.cn 4  China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan Province, China; E-Mail: wuyanping@caep.cn *  Author to whom correspondence should be addressed; E-Mail: jinli@fudan.edu.cn; Tel./Fax: +86-21-6564-3648.   Received: 4 May 2014; in revised form: 27 August 2014 / Accepted: 29 August 2014 /  Published: 12 September 2014 Abstract:  The effect of annealing temperature (1000–1150 °C) on the microstructure evolution, mechanical properties, and pitting corrosion behavior of a newly developed novel lean duplex stainless steel with 20.53Cr-3.45Mn-2.08Ni-0.17N-0.31Mo was studied  by means of optical metallographic microscopy (OMM), scanning electron microscopy (SEM), magnetic force microscopy (MFM), scanning Kelvin probe force microscopy (SKPFM), energy dispersive X-ray spectroscopy (EDS), uniaxial tensile tests (UTT), and potentiostatic critical pitting temperature (CPT). The results showed that tensile and yield strength, as well as the pitting corrosion resistance, could be degraded with annealing temperature increasing from 1000 up to 1150 °C. Meanwhile, the elongation at break reached the maximum of 52.7% after annealing at 1050 °C due to the effect of martensite transformation induced plasticity (TRIP). The localized pitting attack preferentially occurred at ferrite phase, indicating that the ferrite phase had inferior pitting corrosion resistance as compared to the austenite phase. With increasing annealing temperature, the pitting resistance equivalent number (PREN) of ferrite phase dropped, while that of the austenite phase rose. Additionally, it was found that ferrite possessed a lower Volta potential OPEN ACCESS   Materials   2014 , 7    6605 than austenite phase. Moreover, the Volta potential difference between ferrite and austenite increased with the annealing temperature, which was well consistent with the difference of PREN. Keywords:  lean duplex stainless steel; annealing temperature; mechanical properties; TRIP effect; pitting corrosion behavior; PREN; Volta potential 1. Introduction Duplex stainless steels (DSSs), strongly relying on a balanced two-phase microstructure of ferrite ( α ) and austenite ( γ ), have attractive mechanical and corrosion properties and are thus widely used in the chemical, petrochemical, nuclear, marine and paper industries [1–4]. In past years, the development of DSSs has followed two routes. On one side, great efforts have been made to improve the corrosion resistance of DSSs by increasing the content of chromium (Cr), molybdenum (Mo) and nitrogen (N), e.g., the DSS UNS S32750 with high mass fractions of 25%–27% Cr, 3%–4.5% Mo and 0.25%–0.28%  N, which has been developed to meet with the requirement of good resistance and high strength in severe service environment [5–7]. On the other side, in order to conserve resources, “lean” route DSSs, e.g., UNS S32101 type with less nickel (Ni) but higher yield strength and better pitting resistance than standard austenitic grades, have been developed to meet the demand for grades with a lower cost [8–12]. However, the elongation of commercial UNS S32101 was around 30%, which limited the application of lean DSSs in many fields [8,13,14]. Recently, some developed lean DSSs with martensitic transformation have been designed to bring transformation induced plasticity (TRIP) effect and improve the ductility of lean DSSs. Herrera has designed a novel Mn based ductile lean duplex stainless TRIP steel (Fe-19.9Cr-0.42Ni-0.16N-4.79Mn-0.11C-0.46Cu-0.35Si, wt%), which had 1 GPa ultimate tensile strength and an elongation to fracture of above 60% resulted from a sequential martensite transformation of γ   →   ε   →   α ’ [14]. Choi has reported the effects of nitrogen addition on the strain-induced martensitic transformation of Fe-20Cr-5Mn-0.2Ni duplex stainless steel and found that the elongation was up to 60% in some lean DSSs containing 0.3% N [15]. In our work, a new lean duplex stainless steel with a composition of 20.53Cr-3.45Mn-2.08Ni-0.17N-0.31Mo has been developed. With a low Ni content, this resource-saving DSS can be less affected by price fluctuation of precious metal and reduce the risk of both production and uses. Additionally, the elongation has been improved to around 50% thanks to martensitic TRIP effect, which is obviously higher than that of 30% in S32101. Thus, this newly developed DSS holds a great  potential to be the promising replacement of UNS S32101 in the advantage of improved properties. In order to expand the application of this newly developed DSS used in this work, it is of great importance to obtain the suitable mechanical properties and good corrosion resistance. Solution heat treatment shows great impact to the mechanical and corrosion properties of DSS. The aim of the  present work is to find the optimum solution heating temperature for the studied specimen based on investigating the relationship between the solution heating temperature and the corresponding mechanical properties and pitting corrosion resistance. The effect of annealing temperature on the   Materials   2014 , 7    6606 microstructure, mechanical properties and corrosion behavior of this novel lean duplex stainless steel has been characterized by microscope, mechanical test and electrochemical technique. 2. Experimental Section 2.1. Materials The studied material in the paper was a novel developed lean duplex stainless steel with the chemical composition shown in Table 1. It was melted in a 50 kg vacuum furnace and then cast as a single square ingot. After removing the oxide skin, the ingot was forged into square slabs at the temperature ranging from 900 to 1200 °C with a thickness of 40 mm. The slabs were reheated at 1200 °C for 2 h and hot-rolled, using a laboratory hot-rolling mill, into 4 mm thick plates and then cold-rolled into 1.5 mm. The specimens were machined into blocks of dimension of 10 mm × 10 mm. The phase diagram of the newly developed DSS used in present work was calculated by using the Thermo-calc software, as shown in Figure 1. It could be seen that mainly ferrite and austenite were formed in the temperature range between 1000 and 1200 °C. Moreover, the specimen aged between 500 and 980 °C was subjected to precipitation of secondary phases, such as Cr  2  N, σ  and secondary austenite ( γ 2 ), which would seriously deteriorate the corrosion resistance of the duplex stainless steel [16,17]. Therefore, in the present work, the solution heating temperature below 1000 °C was not chosen so as to avoid the formation of these detrimental secondary phases. Thus, the specimens were subjected to different annealing treatment at 1000, 1020, 1050, 1080, 1110, and 1150 °C for 30 min, respectively, and quenched in water to avoid intermetallic precipitates. Table 1.  Chemical composition (wt%) of the duplex stainless steels (DSS). UNS No. C Si Mn P S Cr Ni Mo Cu N S32750 ≤ 0.03 ≤ 0.8 ≤ 1.2 ≤ 0.035 ≤ 0.0224.0/26.06.0/8.0 3.0/5.0 ≤ 0.5 0.24/0.32S32101 <0.04 <1.0 4.0/6.0 <0.04 <0.0321.0/22.01.35/1.700.10/0.80 0.10/0.80 0.20/0.25 New DSS   0.03 0.32 3.45 0.01 0.00420.53 2.08 0.31 0.34 0.17 Figure 1.  The phase diagram calculated by using the Thermo-Calc software for the newly developed DSS used in the present work.   Materials   2014 , 7    6607 2.2. Uniaxial Tensile Test The uniaxial tensile tests were conducted at room temperature with specimens having a gage thickness of 1.5 mm and width of 20 mm annealed at different temperatures using an Instron 5985 (Instron company, Pittsburgh, PA, USA) testing machine according to BS EN ISO 6892-1:2009 [18]. The test was carried out at an extension rate of 3 mm/min before reaching yield point and 20 mm/min after reaching yield point. The test was conducted with three parallel specimens for every annealing temperature and the average data were used for determination of the tensile strength, yield strength and elongation at break values in this paper. In order to observe the microstructure of annealed specimens, which were pre-stretched to 40%, the specimens were etched in 2% HF solution for 30 s, to make the ferrite, austenite, and martensite be distinguished in the optical images. 2.3. Electrochemical Measurement Electrochemical corrosion behavior of the specimens were performed with a CHI 660D potentiostat (Shanghai Chenhua Instrument Co., Ltd, Shanghai, China). All the measurements were carried out with a three-electrode cell where a Pt foil acted as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The specimens acting as working electrodes were sealed in epoxy resin. Prior to each experiment, the working electrode was wet mechanically ground, subsequently polished with a 1.5 μ m diamond paste, ultrasonically cleaned in ethanol and distilled water, and then dried in air. To avoid the crevice corrosion, interfaces between specimen and resin were sealed with silica gel sealant, and dried in air. The exposed electrode surface area was 100 mm 2 . The test solution was 500 mL of l mol/L NaCl. According to ASTM G 150-99 (2010) [19], critical  pitting temperature (CPT) determination of this novel lean duplex stainless steel was performed via  potentiostatic measurements by applying a constant potential of 250 mV SCE and increasing the solution temperature at a rate of 1 °C/min from 2 °C. The CPT was defined as the temperature at which the current sharply increased to 100 µA. After testing, specimens were electrochemically etched for 8 s in 30 wt% KOH solution at applied voltage of 2 V, to make the ferrite and austenite be distinguished in the optical images, and then examined for pits [20]. The CPT measurements of the same specimen for every annealing temperature were repeated at least six times, and the typical data were chosen as the CPT values in this work. 2.4. Optical Metallographic Microscopy and SEM-EDS Analysis Microstructural examination of specimens after the electrochemical measurements was conducted using optical metallographic microscopy and SEM (SCE Phillips XL30 FEG, Amsterdam, The  Netherlands). The volume fraction of ferrite and austenite was evaluated by quantitative metallography  based on the software attached to the microscope. Energy dispersive X-ray spectroscopy (EDS) was  performed on the individual phases to obtain their elemental composition. Because of the insensitivity of EDS techniques to N, an approximate calculation was conducted to determine the N fraction in both  phases. The level of N in ferrite phase was assumed to be the saturation value (around 0.05%) and that of austenite was calculated based on the content of N in the whole alloy and the phase volume fraction [21].
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