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Article

Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750

by
Dohyung Kim
1,
Wonsub Chung
2,* and
Byung-Hyun Shin
3,*
1
The Institute of Materials Technology, Pusan National University, Busan 46241, Republic of Korea
2
Eco-Friendly Smart Ship Parts Technology Innovation Center, Busan 46241, Republic of Korea
3
Innovative Graduate Education Program for Global High-Tech Materials and Parts, Pusan National University, Busan 46241, Republic of Korea
*
Authors to whom correspondence should be addressed.
Metals 2023, 13(5), 957; https://doi.org/10.3390/met13050957
Submission received: 24 April 2023 / Revised: 9 May 2023 / Accepted: 12 May 2023 / Published: 15 May 2023
(This article belongs to the Special Issue Corrosion Detection and Protection of Steel Pipelines)
Browse Figures
Versions Notes

Abstract

:
Super duplex stainless steel (SDSS) is used for manufacturing large valves and pipes in offshore plants because of its excellent strength and corrosion resistance. Large valves and pipes are manufactured by forging after casting, and the outside and inside microstructures are different owing to the difference in the cooling rate caused by the thermal conductivity. This microstructural variation causes cracks during solution annealing, which breaks the materials. To study the corrosion resistance of the SDSS forged material, the influence of the microstructure according to the difference between the inside and outside cooling rates of the cast SDSS was evaluated. To analyze the effects of the secondary phase fraction before solution annealing on the solution and corrosion resistance, the corrosion resistance with and without solution annealing was measured using the potentiodynamic polarization test and critical temperature test after the precipitation of the secondary phase. In the potentiodynamic polarization test, the secondary phase decreased the activation polarization and increased the corrosion rate. The critical pitting temperature exhibited the effect of the secondary phase.

1. Introduction

Stainless steel is an alloy widely used in industrial fields because it has excellent corrosion resistance and mechanical properties. Stainless steel is classified into four main phases: austenite (high corrosion resistance), ferrite (high-temperature strength), martensite (high strength), and duplex (high corrosion resistance and strength).
Duplex stainless steel (DSS) refers to an alloy composed of a nearly 1:1 volume fraction of austenite and ferrite phases (Dual phase). It has high corrosion resistance to local corrosion and stress corrosion cracking and excellent mechanical properties (ultimate tensile strength: 780 MPa; elongation: 30%). When the volume fraction of DSS is 1:1, the chemical composition of austenite and ferrite is the best condition to prevent pitting corrosion. DSS, like austenite stainless steel, is evaluated for corrosion resistance based on the pitting resistance equivalent number (PREN: wt.% Cr + 3.3 wt.% Mo + 16 wt.% N). PREN is a numerical value of resistance to pitting corrosion in a chloride-containing environment; the higher the number, the higher the resistance to pitting corrosion. DSS is classified into four grades (lean, standard, super, and hyper) according to the PREN. If the PREN exceeds 32, it is considered to have a pitting corrosion resistance against seawater, and super duplex stainless steel (SDSS) has a PREN of 40 or more. These properties make it suitable for use as a structural material to improve safety and productivity in offshore plants, seawater desalination, and semiconductor industries.
Valves and pipes used in offshore plants require high strength and pitting corrosion resistance because they are used in seawater (high corrosion environment by chloride ion) [1,2,3]. Among various materials, super duplex stainless steel (SDSS) is suitable for application in offshore plants because of its high corrosion resistance and strength [4,5,6]. SDSS has very high pitting corrosion resistance with a PREN of 40 to 50. However, the application of SDSS as a material part was limited due to the burden of the issue of rising raw material prices and difficult formability. As safety and maintenance costs increase, the application of materials with excellent properties such as SDSS is becoming more important. When SDSS is heat treated, the change in the fraction of austenite and ferrite is important because it affects corrosion resistance. The volume fraction issue is an important problem in SDSS due to the difference in cooling rate according to the size.
Further, SDSS is a dual phase stainless steel composed of austenite and ferrite, which causes various issues regarding heat treatment, such as high-temperature cracking due to phase transformation (the change in the volume fraction of austenite and ferrite) during high-temperature forging to use the valve and pipe of offshore plants [7,8,9]. From the high-temperature crack analysis, the secondary phase (Sigma, Chi, CrN, and Cr23C6) is confirmed to develop within 20 mm of the interior because of the low cooling rate. The SDSS is broken by cracks caused by the difference in cooling rate between the inside and outside.
The large casting of SDSS occurs with a difference in the volume fraction of austenite, ferrite, and secondary phase owing to the difference in the cooling rate between the inside and outside, causing various issues [4,5]. When the secondary phase is precipitated inside, hot temperature cracking occurs, decreasing the corrosion resistance. The solution annealing of SDSS is conducted to optimize corrosion resistance by re-dissolving the secondary phase and stabilizing the fraction of the austenite phase and the ferrite phase. In order to obtain excellent corrosion resistance of SDSS, the ratio of the austenite phase and the ferrite phase should be close to 1:1, and appropriate heat treatment temperature conditions are required to control the ratio of the two phases. Although significant research efforts have been devoted to suppressing the precipitation of secondary phases, no studies have been conducted on the effects of the secondary phase on solution annealing at 1100 °C [10,11,12].
Extensive research on the manufacturing of SDSS for welding and heat treatment has also been conducted. Videira studied increasing the corrosion resistance of SDSS through hybrid GTAW-Laser welding and nitrogen [13]. Nilson and Shin studied the heat treatment effect on the volume fraction of phased and electrochemical properties [4,5,14,15]. Liu studied the effect of cooling rate on the initial structure of SDSS after solution annealing [16]. The studies investigated the effect of the equilibrium volume fraction on the electrochemical properties but did not examine the effect of the secondary phase on the solution annealing process. Kose conducted a study on the microstructure and strength of steel after laser welding for advanced pipe manufacture [6]. Kim studied the corrosion behavior of commercial STS materials used as highly corrosion-resistant materials [17]. Although many studies on SDSS have been conducted, studies on the effect of secondary phase precipitation on solution annealing have not been conducted.
In this study, in order to apply SDSS as various material parts, we tried to understand and solve the problems caused by the precipitation of secondary phases that may occur due to manufacturing and processing. Therefore, in this study, the change in solution annealing according to the volume fraction of the secondary phase was measured electrochemically to research the effect of phase fraction on the phase in SDSS. After heat treatment at 900–1050 °C to precipitate the secondary phase, solution annealing was performed at 1100 °C. The microstructure under the heat treatment conditions was analyzed using field emission scanning electron microscopy (FE-SEM), and the volume fraction of each phase was measured at a magnification of ×200. Electron probe micro analysis (EPMA) and Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis were used to confirm the precipitation of the secondary phase. The corrosion properties were measured to check the secondary phase with heat treatment temperature using potentiodynamic polarization and critical pitting temperature (CPT) tests.

2. Materials and Methods

2.1. Materials and Heat Treatment

In this study, a super duplex stainless steel, specifically SDSS2507, was used, commonly used for manufacturing valves and pipes in offshore plants. Its components are listed in Table 1. After casting and cooling, the microstructure and chemical composition after heat treatment were examined at four temperatures 900–1050 °C (tests were conducted for four temperature; 50 °C increments) in increments of 50 °C to confirm the effect of the secondary phase on the high-temperature cracking and corrosion resistance of SDSS2507 [4,5]. The cooling method was water quenching, and the cooling rate was 70 °C/s. The cooling rate is calculated as the energy loss per unit time.

2.2. Microstructure and Volume Fraction

For the experiment, to analyze the microstructure and phase fraction, the specimens were polished using diamond paste. After etching for 1 min in 5 wt.%KOH electrolyte solution, it was confirmed using FE-SEM (Hitachi, Ltd., Tokyo, Japan) [14]. The secondary phase was confirmed using X-ray diffraction (XRD, Rigaku, Co., Ltd., Tokyo, Japan), and the chemical composition was determined via an electron probe micro-analyzer (EPMA, JEOL, Ltd., Tokyo, Japan) and energy dispersive x-ray spectroscopy (EDS, Hitachi, Ltd., Tokyo, Japan) after polishing. The chemical composition of nitrogen was calculated according to the maximum solution of ferrite; it is impossible to analyze the concentration quantitatively using EDS (maximum solution of ferrite is 0.05 wt.%) [4,5,6]. After the secondary phase (Sigma, Chi et al.) was precipitated, solution annealing was performed at 1100 °C, and the microstructure was confirmed using FE-SEM at a resolution of ×200. The heat treatment conditions are shown in Figure 1. The volume fractions of austenite, ferrite, and secondary phase were evaluated to ASTM E 1245-3 by etched FE-SEM (×200) image analysis. The reported volume fraction analysis results are averaged from at least five measurements.

2.3. Corrosion Properties

The corrosion properties with the volume fraction of secondary phase with or without solution annealing need to check the effect of the volume fraction of secondary phase on solution annealing. The corrosion properties were analyzed using potentiodynamic polarization and CPT tests as well as utilizing a potentiometer (Versa STAT, AMETEK, Inc., PA, USA) and a three-electrode cell. The three-electrode cell comprised a working electrode (WE, specimens, 10 × 10 mm2), reference electrode (RE, saturated calomel electrode (SCE of KCl (Potassium chloride, PIM430))), and counter electrode (CE, platinum mesh, 20 × 20 mm2). In order to eliminate the effect of dissolved oxygen on the corrosion behavior, nitrogen gas was purged to remove dissolved oxygen before the test.
Potentiodynamic polarization tests measure the change in the current with potential, which can be used to analyze the corrosion behavior. In this study, the material was tested in an electrolyte of 3.5 wt.% NaCl, and the potential was measured from −0.6 to 1.2 V (ASTM G 5–14). The scan rate was set to 0.17 mV/s [4,5]. The potentiodynamic polarization test was conducted after immersion for 10 min to measure the open circuit potential and stabilize the potential.
The CPT test is used to classify duplex stainless steel grades and identify the temperature at which the passivation layer breaks down in the electrolyte (ASTM G 150–99). The electrolyte used for the CPT measurement was 5.85 wt.% NaCl (1 mol), and the starting temperature of the test was 1–2 °C. The increasing rate of temperature was 1 °C/min, and the CPT is defined as the temperature at which the current density exceeds 100 μA/cm2 and is maintained for more than 1 min.

2.4. Corrosion Morphology

Because the corrosion morphology of SDSS occurs in a weak phase (low PREN), it is necessary to confirm the effect of the secondary phase with or without solution heat treatment. The morphologies of the corroded samples were confirmed after the CPT tests with heat treatment condition. The pitting morphology was confirmed at ×1000 magnification using FE-SEM to check the pitting site [15].

3. Results and Discussion

3.1. Microstructure

Although SDSS is composed of austenite and ferrite, the secondary phase is known to precipitate owing to the segregation of Cr and Mo at 975 °C [10,11,12]. To confirm the secondary phase, heat treatment was performed from 900 to 1050 °C to check the inside microstructure. The microstructure images according to each heat treatment temperature are shown in Figure 2 [4]. It was confirmed that the microstructure of SDSS changed according to the heat treatment temperature. The austenite phase was formed in the form of islands, the ferrite phase was a matrix structure, and the secondary phase was confirmed at the phase boundary. The presence of the secondary phase is confirmed via XRD, as shown in Figure 3. Most of the secondary phase was identified at 900 °C and formed from 900–1000 °C. The secondary phase precipitated at the phase boundary between austenite and ferrite, and EPMA and EDS were used to confirm the chemical composition. The chemical composition of the secondary phase presented in Figure 4 and Table 2 indicates that the secondary phase has high Cr and Mo (Sigma) chemical composition and low Cr and Mo chemical composition (Chi) [18]. The secondary phase appears as a white and black dot because of the difference in the surface roughness (no etching); the volume fraction of each phase (austenite, ferrite, and secondary phase) is shown in Figure 5. The volume fraction of austenite was maintained; however, the volume fraction of ferrite changed based on the volume fraction of the secondary phase due to the high Cr of ferrite. The secondary phase on the grain boundary of ferrite and austenite was identified until 1020 °C and could not be identified beyond 1030 °C [14].
Solution annealing at 1100 °C is performed to optimize the corrosion resistance of stainless steel; for SDSS, this process is generally performed at 1100 °C [19]. The microstructure and volume fraction after solution annealing are shown in Figure 6 and Figure 7, respectively. After solution annealing, the microstructures of the four specimens appear similar; however, differences are observed in the volume fraction. Austenite represents a case of the solution annealing of a specimen with a high volume fraction of the secondary phase [15]. The secondary phase affected the solution annealing process; this confirmed that the solution of the alloys influenced the volume fraction of austenite and ferrite.
Considering the volume fraction and chemical composition, the secondary phase was formed at the boundary of austenite and transformed into ferrite because of the segregation Cr and Mo into ferrite (high chemical composition of Cr and Mo). During solution annealing, the secondary phase transforms into ferrite, as shown in Figure 8. Because high amounts of Cr and Mo in the secondary phase do not dissolve in austenite but readily dissolve in ferrite, the transformation results in ferrite. This difference in volume fraction on austenite and ferrite causes the difference in chemical composition on each phase [4,5,20,21,22,23,24,25].

3.2. Electrochemical Properties

The potentiodynamic polarization test is used to elucidate the corrosion behavior of a material by measuring the current density with respect to potential [4,5]. From the potentiodynamic polarization test results shown in Figure 9 and Table 3, the presence of the secondary phase decreases the potential (Ecorr) and increases the current density (Icorr) in the activation polarization. The results of the potentiodynamic polarization test confirm that the solution annealing increases the corrosion resistance. Solution annealing optimizes the corrosion resistance because it equalizes the pitting resistance (PRE, wt.% Cr + 3.3 wt.% Mo + 16 wt.% N) of austenite and ferrite [4].
The CPT is used to evaluate the corrosion resistance of duplex stainless steel by comparing the temperature at which the passivation layer is destroyed [5,20,21]. The corrosion resistance was compared considering the volume fraction of the secondary phase, and the results are shown in Figure 10. The secondary phase is a factor that decreases CPT and influences solution annealing [15]. The CPT test shows a difference in the corrosion resistance with PRE because PRE is calculated using the chemical composition, which is affected by the elements present in each phase [4]. This relationship is related to the volume fraction, and the fractions of austenite and ferrite that are not equalized by the secondary phase, which affect the chemical composition and corrosion resistance.
Figure 10a shows the effect with the volume fraction of secondary phase. The increased volume fraction of the secondary phase decreased CPT because the secondary phase is a pitting site due to the Cr defected zone Chi. Further, solution annealing shows the increased CPT but that was affected by the secondary phase. Figure 10b shows the effect of the secondary phase on solution annealing. CPT shows that the secondary phase decreased the effect of solution annealing.
The pitting morphology after CPT test was confirmed at the location of the electrochemical characterization analysis, as shown in Figure 11 and Figure 12 [22]. The secondary phase is the corrosion site that causes corrosion in the form of a secondary phase. The pitting after 1050 °C worked on austenite (PREN = 41.1) due to its lower PREN compared with ferrite (PREN = 43.6). The chemical composition at 1050 °C is shown in Table 4. When the volume fraction of austenite to ferrite is equal (1050–1100 °C), corrosion occurs at the grain boundary between the austenite and ferrite.
The secondary phase is the corrosion site owing to the decreased corrosion resistance (Chi, 22 wt.% Cr and 2.2 wt.% Mo) before and after solution annealing, resulting in the segregation of the alloy [24,25]. To prevent the decrease in the corrosion resistance, solution annealing is performed; however, when a secondary phase is precipitated, the volume fraction cannot be equalized even with solution annealing, and the corrosion resistance cannot be optimized.

4. Discussion

To prevent high-temperature cracking in SDSS, the precipitation of secondary phases must be limited. However, in large castings, it is difficult to control the secondary phase precipitation at high temperature. The secondary phases were precipitated at the grain boundary of austenite via the segregation of Cr and Mo from 900 °C to 1020 °C. The solution annealing at 1100 °C is a heat treatment to optimize the pitting corrosion resistance [4,5,6]. However, SDSS, in which the secondary phase is precipitated, does not have an optimized corrosion resistance [10,11,12]. The secondary phase is transformed into ferrite due to high Cr and Mo, and austenite should be transformed into ferrite during solution annealing [19]. However, as a sufficient driving force to realize this transformation does not exist, it remains as austenite. When the volume fraction of austenite was high, the pitting corrosion resistance was decreased because of the low pitting corrosion resistance (low PREN) of austenite.
The secondary phase of SDSS was decreased to the potential and increased the corrosion rate in the activation polarization of the potentiodynamic polarization curve [4]. After solution annealing at 1100 °C, the secondary phase of SDSS was dissolved to ferrite; however, the corrosion resistance was not optimized owing to the difference in the volume fraction of austenite and ferrite because of variations in the pitting resistance by chemical composition.
The solution annealing improves the corrosion resistance but does not provide a sufficient driving force for dissolving the secondary phase and increasing the volume fraction of ferrite [4,19]. Therefore, when the secondary phase is precipitated, the volume fraction of ferrite must be increased to prevent the decrease in the corrosion resistance; when precipitating 30% of the secondary phase, the heat treatment at a high temperature of up to 1200 °C results in equivalent fractions of austenite and ferrite [5,26,27]. The studies of Y. Kong and M. Li show grain growth to higher heat input energy of stainless steels [26,27]. When the cooling rate is equal, the grain growth is easy because of enough energy and time. When the secondary phase is precipitated, the solution annealing treatment should be conducted at a temperature higher than the conventional solution annealing temperature (1100 °C), and the corrosion resistance can be improved.

5. Conclusions

By simulating the microstructure of SDSS2507, the volume fraction of secondary phase was precipitated by controlling these three types. Solution annealing was performed to solidify the secondary phase, and the following conclusions were drawn after analyzing the electrochemical properties using potentiodynamic polarization curve and CPT test analysis.
(1)
The secondary phase precipitated at the grain boundary of austenite and grew into ferrite because of the high chemical composition of Cr and Mo. After solution annealing at 1100 °C, the secondary phase dissolved as ferrite. Solution annealing optimized the corrosion resistance by rendering the fraction of austenite and ferrite as 1:1; however, solution annealing after the precipitation of the secondary phase did not result in a volume fraction of 1:1.
(2)
The secondary phase is formed by the segregation of Cr and Mo, which decreased the pitting corrosion resistance, as confirmed by the potentiodynamic polarization curve and CPT test analysis. The solution annealing after the precipitation of secondary phase fully dissolved the secondary phase but did not result in the growth of ferrite or optimization of the pitting corrosion resistance. To improve the pitting corrosion resistance by the dissolution of the secondary phase and growth of ferrite, solution annealing after precipitation of the secondary phase should be performed at a higher temperature than the conventional temperature (1100 °C).
(3)
SDSS2507 is manufactured by high-temperature forging because it is used in valves and pipes in offshore plants. However, owing to the difference in the cooling rate between the inside and outside, a secondary phase is formed inside and hot cracks occur. To stabilize the microstructure and improve the corrosion resistance, the solution annealing of SDSS2507 at a temperature higher than that of the solution heat treatment can prevent high-temperature cracking because the high temperature over the solution annealing temperature at the same cooling rate can prevent the precipitation of the secondary phase.

Author Contributions

Conceptualization, D.K.; methodology, D.K.; validation, D.K., W.C. and B.-H.S.; formal analysis, D.K., W.C. and B.-H.S.; investigation, D.K.; resources, W.C. and B.-H.S.; data curation, D.K.; writing—original draft preparation, D.K.; writing—review and editing, D.K., W.C. and B.-H.S.; visualization, D.K.; supervision, D.K., W.C. and B.-H.S.; project administration, D.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) [grant number 2020R1A5A8018822]; the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean Government (MOTIE) (P20002019, Human Resource Development Program for Industrial Innovation); and the BK21 FOUR program [grant number 4120200513801] funded by the Ministry of Education (MOE, Korea) and the National Research Foundation of Korea (NRF).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematics diagrams of heat-treatment conditions for examining the precipitation of the secondary phase and the effect of solution annealing of super duplex stainless steels UNS S 32750.
Figure 1. Schematics diagrams of heat-treatment conditions for examining the precipitation of the secondary phase and the effect of solution annealing of super duplex stainless steels UNS S 32750.
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Figure 2. SEM images after heat treatment for the precipitation of the secondary phase of SDSS UNS S32750: (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1050 °C.
Figure 2. SEM images after heat treatment for the precipitation of the secondary phase of SDSS UNS S32750: (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1050 °C.
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Figure 3. XRD patterns with and without the secondary phase (900 and 1050 °C) of SDSS UNS S32750.
Figure 3. XRD patterns with and without the secondary phase (900 and 1050 °C) of SDSS UNS S32750.
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Figure 4. EPMA images after the precipitation of the secondary phase after heat treatment at 900 °C of SDSS UNS S32750.
Figure 4. EPMA images after the precipitation of the secondary phase after heat treatment at 900 °C of SDSS UNS S32750.
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Figure 5. Volume fraction of each phase with respect to the heat-treatment temperature of SDSS UNS S32750.
Figure 5. Volume fraction of each phase with respect to the heat-treatment temperature of SDSS UNS S32750.
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Figure 6. SEM images after solution annealing at 1100 °C with the volume fraction of the secondary phase of SDSS UNS S32750: (a) 900–1100 °C, (b) 950–1100 °C, (c) 1000–1100 °C, and (d) 1050–1100 °C.
Figure 6. SEM images after solution annealing at 1100 °C with the volume fraction of the secondary phase of SDSS UNS S32750: (a) 900–1100 °C, (b) 950–1100 °C, (c) 1000–1100 °C, and (d) 1050–1100 °C.
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Figure 7. Volume fraction after solution annealing at 1100 °C with volume fraction of secondary phase of SDSS UNS S32750.
Figure 7. Volume fraction after solution annealing at 1100 °C with volume fraction of secondary phase of SDSS UNS S32750.
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Figure 8. Volume fraction with or without solution annealing at 1100 °C of SDSS UNS S32750.
Figure 8. Volume fraction with or without solution annealing at 1100 °C of SDSS UNS S32750.
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Figure 9. Potentiodynamic polarization curve: potential vs. current density curve in the electrolyte comprising 3.5 wt.% NaCl of SDSS UNS S32750 (a) before solution annealing and (b) after solution annealing.
Figure 9. Potentiodynamic polarization curve: potential vs. current density curve in the electrolyte comprising 3.5 wt.% NaCl of SDSS UNS S32750 (a) before solution annealing and (b) after solution annealing.
Metals 13 00957 g009aMetals 13 00957 g009b
Metals 13 00957 g010 550
Figure 10. CPT curve: time (s) vs. current density (A/cm2) with the heat-treatment condition of SDSS UNS S32750 (a) before and (b) after solution annealing.
Figure 10. CPT curve: time (s) vs. current density (A/cm2) with the heat-treatment condition of SDSS UNS S32750 (a) before and (b) after solution annealing.
Metals 13 00957 g010
Metals 13 00957 g011 550
Figure 11. Corrosion morphology with the heat-treatment temperature for the precipitation of the secondary phase of SDSS UNS S32750: (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1050 °C.
Figure 11. Corrosion morphology with the heat-treatment temperature for the precipitation of the secondary phase of SDSS UNS S32750: (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1050 °C.
Metals 13 00957 g011
Metals 13 00957 g012 550
Figure 12. Corrosion morphology after the solution annealing of SDSS UNS S3275: (a) 900–1100 °C, (b) 950–1100 °C, (c) 1000–1100 °C, and (d) 1050–1100 °C.
Figure 12. Corrosion morphology after the solution annealing of SDSS UNS S3275: (a) 900–1100 °C, (b) 950–1100 °C, (c) 1000–1100 °C, and (d) 1050–1100 °C.
Metals 13 00957 g012
Table
Table 1. Chemical composition of the casted SDSS UNS S32750.
Table 1. Chemical composition of the casted SDSS UNS S32750.
CNMnNiCrMoFePRE
SDSS25070.010.30.796.825.03.8Bal42
Table
Table 2. Chemical composition of the secondary phase (Sigma and Chi) after heat treatment at 900 °C of SDSS UNS S32750.
Table 2. Chemical composition of the secondary phase (Sigma and Chi) after heat treatment at 900 °C of SDSS UNS S32750.
Chemical Composition (wt.%)SigmaChi
Cr30.1 ± 2.122.1 ± 1.5
Mo8.8 ± 0.62.2 ± 0.1
Ni4.6 ± 0.49.5 ± 0.3
Mn1.1 ± 0.11.0 ± 0.1
FeBalBal
Table
Table 3. Potentiodynamic polarization test results (Figure 9) with heat treatment conditions of super duplex stainless steel UNS S32750.
Table 3. Potentiodynamic polarization test results (Figure 9) with heat treatment conditions of super duplex stainless steel UNS S32750.
(a)(b)
Ecorr (mV)Icorr (A/cm2)EPit (mV)Ecorr (mV)Icorr (A/cm2)EPit (mV)
900 °C−150 ± 10 9 × 10−7 990 ± 10−100 ± 51 × 10−71020 ± 10
950 °C−150 ± 10 9 × 10−71000 ± 10−40 ± 51 × 10−71020 ± 10
1000 °C−140 ± 10 8 × 10−71010 ± 10−40 ± 51 × 10−71020 ± 10
1050 °C−80 ± 102 × 10−71030 ± 10−40 ± 51 × 10−71030 ± 10
Table
Table 4. Chemical composition of austenite and ferrite after heat treatment at 1050°C of super duplex stainless steel UNS S32750.
Table 4. Chemical composition of austenite and ferrite after heat treatment at 1050°C of super duplex stainless steel UNS S32750.
PhaseAustenite (%)Ferrite (%)
Chemical CompositionCrMoNFeCrMoNFe
23.23.10.48Bal26.35.00.05Bal
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Kim, D.; Chung, W.; Shin, B.-H. Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750. Metals 2023, 13, 957. https://doi.org/10.3390/met13050957

AMA Style

Kim D, Chung W, Shin B-H. Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750. Metals. 2023; 13(5):957. https://doi.org/10.3390/met13050957

Chicago/Turabian Style

Kim, Dohyung, Wonsub Chung, and Byung-Hyun Shin. 2023. "Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750" Metals 13, no. 5: 957. https://doi.org/10.3390/met13050957

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