Evolution of Microstructure, Tensile Mechanical and Corrosion Properties of a Novel Designed TRIP-Aided Economical 19Cr Duplex Stainless Steel After Aging Treatment

Abstract

In this experiment, a novel designed Mn-N-bearing, nearly Ni-free, TRIP-aided economical 19Cr (Fe-18.9Cr-10.1Mn-0.3Ni-0.26N-0.03C) duplex stainless steel (DSS) was prepared, and it exhibited a good combination of strength and toughness after suitable solution treatment, showing good application potential. The deformation mechanisms of ferrite and austenite are different during tensile deformation at room temperature: the ferrite phase was deformed by a dislocation slip mechanism and formed a cell structure due to its higher stacking fault energy; the lower stacking fault energy of austenite resulted in a strain-induced martensite phase transformation mechanism. With an increase in aging time from 1 h to 7 h at 750 °C in air, the σ phase precipitates in the ferrite triple grain boundary junction, which leads to an increase in ultimate tensile strength, acts as an obstacle to the dislocation motion and decreases the ductility, deteriorating the pitting corrosion resistance in 3.5 wt.% NaCl solution at the same time. The σ phase precipitation behavior does not alter the deformation mechanism of the phases of the solution-treated TRIP-aided economical DSS.

1. Introduction

Duplex stainless steels (DSSs) are composed of an austenite phase (γ phase) and a ferrite phase (α phase), with the austenite phase typically accounting for approximately 50% to 60% of the microstructure [1,2]. This dual-phase composition endows DSSs with a blend of properties [3,4], combining the strong corrosion resistance [5,6], good toughness and excellent weldability [7] of austenitic stainless steel with the good strength and chloride (Cl⁻) stress corrosion resistance of ferritic stainless steels [8,9,10]. These comprehensive properties make DSSs highly versatile materials that are widely utilized in demanding environments [11,12,13] such as the petrochemical, paper transport and aviation power industries. However, the differing deformation mechanisms of the ferrite and austenite phases present challenges [14,15]. In conventional high-alloyed DSSs, this disparity can lead to reduced ductility and increase the risk of cracking [16] during the material forming process. In recent years, the development of economical DSSs has gained momentum, driven by the significant price fluctuations of raw materials like nickel and molybdenum, as well as the growing environmental pressures associated with steel production. The goal is to create advanced steel structural materials with superior mechanical properties but with a reduced reliance on costly nickel and molybdenum [17,18].

A key advancement in this area is the Mn-N-bearing alloying design philosophy, firstly proposed by Herrera for economical DSS product development [19]. This approach offers a twofold advantage: Mn-N alloying can replace the expensive nickel element, thereby reducing the cost, and the austenite phase is metastable, gradually transforming into a martensite phase during plastic deformation. This transformation-induced martensite contributes significantly to the excellent mechanical properties of Mn-N-bearing economical DSS, particularly enhancing ductility during plastic deformation [20,21]. Recent years have witnessed the design and investigation of a new series of TRIP plastic economical DSSs across different systems. For instance, Jiaxin Pan et al. [22] designed a nearly nickel-free vanadium-containing lean DSS with good mechanical properties and demonstrated that the annealing treatment temperature substantially influences the microstructure and mechanical properties of the DSS. Similarly, JeomYong Choi et al. [23] developed a class of molybdenum-free TRIP-aided lean DSSs. The absence of molybdenum in these steels suppresses the precipitation of the harmful σ phase. In TRIP-aided DSSs, the deformation temperature plays a crucial role in controlling the stability of the austenite phase, then influencing the kinetics of the deformation-induced martensite transformation and the final mechanical properties [24].

Economical TRIP-aided DSSs stand out with their relatively low cost and excellent mechanical properties, showing great potential for application in engineering construction [25,26]. Hot-forming methods are extensively used in the production of DSS products; examples of these methods include hot rolling [27], hot forging [28], welding [29], heat treatment and so on. The type, amount and characteristics of precipitations could affect the production and application of economical TRIP-aided DSSs [30,31].

Heat treatment critically influences duplex stainless steel properties by altering the material’s microstructure, phase balance, mechanical performance and corrosion resistance. For example, 2205 DSS [32] shows increased austenite formation at higher solution temperatures, optimizing phase balance. However, improper thermal processing risks generating detrimental phases (σ, χ, secondary austenite), which degrade toughness and corrosion resistance. Controlled heat treatment effectively dissolves these phases, enhancing material quality. Mechanically, heat-treated 2205 DSS exhibits superior hardness at 1050 °C, while 2209 DSS [33] achieves enhanced impact toughness after a 1 h treatment at the same temperature. However, high-temperature treatments reduce pitting corrosion resistance [34] due to alloy element redistribution and coarse ferrite grains increasing stress corrosion cracking susceptibility. Optimal heat treatment balances austenite–ferrite ratios and eliminates harmful phases but requires precise temperature/time control to prevent property degradation [35]. Therefore, it is necessary to study the influence of heat treatment on duplex stainless steel.

This study focuses on the design and preparation of a novel, low-cost, manganese- and nitrogen-enhanced Fe-Cr-Mn-N series of economical 19Cr DSS sheet materials, leveraging their phase transformation plasticizing effects. This research investigates the precipitation behavior during aging treatment at 750 °C in air and its effect on the microstructure, mechanical tensile properties and pitting corrosion behavior in NaCl solutions. The findings aim to provide scientific and theoretical guidance for the production and application of economical TRIP-aided 19Cr DSSs.

2. Experimental Details

In this experiment, a 50 kW ZG-25 vacuum medium frequency induction furnace was selected to cast novel economical 19Cr DSS experimental samples. The rated weight of the ingots for each furnace was about 10.00 kg. The main raw materials for smelting the designed DSS were pure Fe bar, pure Ni bar, Mn, FeCr and FeCrN. The chemical composition of the cast ingot is shown in

Table 1. Chemical composition of experimental duplex stainless steel (wt.%).

Steel	Cr	Ni	Si	Mn	C	N	Fe
Cr19	18.9	0.3	0.5	10.1	0.030	0.261	Bal.

. After checking the qualified chemical composition, the cylindrical castings with dimensions of ϕ 80 mm were hot-forged into bars of 25 mm × 40 mm × L mm (L represents the length of the hot-forged bar). The hot-forging temperature should not be lower than 950 °C to preventing forging cracks. In this experiment, all the subsequent samples were cut from the hot-forged bars. To investigate the evolution of the microstructure, tensile mechanical properties and corrosion properties of the DSS during aging treatment at 750 °C in air, sheets with dimensions of 5 mm × 25 mm × 60 mm were cut from the hot-forged bar along the length of the hot-forged bar by a wire-electrode cutting machine. (Shenwei CNC Machine Tool (Jiangsu) Co., Ltd., Suzhou, China). Initially, the cut hot-forged DSS sheet specimens were subjected to full solution treatment at 1100 °C for a duration of 30 min and then quenched in water. After that, these solution-treated samples were subjected to multi-pass cold rolling to achieve a sheet thickness of 1 mm. Next, the cold-rolled plates underwent annealing recrystallization at 1100 °C for 3 min and were subsequently quenched in water. The samples treated with annealing recrystallization were then subjected to isothermal aging at 750 °C for 1 h, 3 h, 5 h and 7 h, respectively, and finally quenched in water. The aging-treated sheets were applied to investigate the microstructure, mechanical properties and corrosion properties.

The microstructure characteristics of these aged DSS samples were observed using a KEYENCE VH-S1 optical metallographic microscope. (Keyence (China) Co., Ltd., Shanghai, China). The austenite phase appeared bright and the ferrite phase appeared gray after electrochemical etching in a 10 wt.% KOH solution for 45 s. The relationship between phase and temperature in 00Cr19Mn10Ni0.3N0.25 DSS was calculated using Thermo-Calc thermal simulation software (Thermal-Calc 2015b) based on the TCFE7 database. The model diagram of temperature and phase facilitates the analysis and investigation of precipitates in the experimental process.

Mechanical properties investigation experiments were performed in accordance with the Chinese standard GB/T228-2002. Tensile tests were carried out at room temperature with a strain rate of 1.67 × 10⁻³ s⁻¹ using specimens with a length of 25 mm and a diameter of 5 mm. The TEM microstructure of the samples was observed using a JEM-2010F field emission transmission electron microscope (Japan Electronics Co., Ltd., Tokyo, Japan) under liquid nitrogen cooling conditions. The cooling temperature should be below −30 °C. The electrolyte used in the double-jet electrolytic thinning device (MTP-1A) consists of 90% absolute ethanol (C₂H₅OH) and 10% perchloric acid (HClO₄) solution, with a thinning voltage of 35 V.

To study the pitting corrosion behavior, the polarization curves of the test materials were measured using a CHI-900D electrochemical workstation. The samples’ corrosion behavior in a 3.5 wt.% NaCl solution was investigated using potentiodynamic polarization tests with a scanning rate of 0.5 mV/s at room temperature. The active surface of the electrode was diminished to 1.0 cm² with epoxy resin, so as to bypass edge effects. The aging-treated samples were used as the working material. Ag-AgCl electrode was used as the reference electrode and platinum electrode as the counter electrode. The specimen was left in the pitting solution for 30 min to reach its free corrosion potential.

3. Results and Discussion

3.1. Effect of 750 °C Aging Treatment on Microstructure Evolution of This Novel Designed DSS

In order to predict the potential effects of heat treatment solutions on the microstructure and properties of the novel designed DSS, Thermo-Calc calculation software was employed in this experiment. The software calculated the phases and temperature relationship based on the TEFE7 database, and result was presented in Figure 1. As indicated in Figure 1, when the heat treatment temperature surpasses approximately 840 °C, the equilibrium microstructure of the novel designed 19Cr DSS consists of ferrite (α) and austenite (γ) phases. As the temperature increases from about 840 °C to about 1340 °C, the volume fraction of austenite phase decreases. Once the temperature exceeds approximately 1340 °C, the initial melting region begins to form within the material. However, when the heat treatment temperature is below 850 °C, precipitates such as σ, Cr₂N and M₂₃C₆ may form in the matrix. These precipitates can significantly affect the microstructure, mechanical properties and corrosion resistance. Meanwhile, the type, quantity and morphology of the specific precipitates are also influenced by heat treatment conditions. Although Thermo-calc software and its associated database serve as thermodynamic calculation tools, they cannot address precipitation dynamics issues. Therefore, aging treatment test experiments are essential for investigating the characteristic of precipitates and their impact on mechanical and pitting corrosion behavior.

Figure 2 displays the metallographic optical microstructures of samples aged at 750 °C for different times. The typical ferrite/austenite dual-phase structure dominated the microstructure when the DSS was aged at 750 °C for 1 h to 7 h. The dark gray ferrite phases formed bands in the austenite matrix along the rolling direction, and no obvious precipitate phases were observed between ferrite and austenite in the metallographic microstructure. This contradicts the thermodynamic calculation results from the Thermo-Calc result in Figure 1. This discrepancy may arise because the precipitates are too small to be detected by optical microscopy. Consequently, in subsequent experiments, a field-emission transmission electron microscope (TEM) was utilized to investigate the more detailed microstructural characteristics of the aged DSS.

3.2. Effect of 750 °C Aging Heat Treatment on Tensile Mechanical Properties at Room Temperature

As shown in Figure 3, the engineering stress–strain curves of the solution-treated and corresponding aging-treated samples were obtained via tensile testing with a strain rate of 1.67 × 10⁻³ s⁻¹ at room temperature. The cold-rolled DSS sheet solution-treated at 1100 °C for 3 min exhibited a good combination of strength and ductility. Its tensile strength was about 820 MPa and the elongation to fracture was close to 70%. The solution-treated DSS sheet demonstrates considerable engineering application potential due to its desirable balance of strength and ductility. In contrast, when the solution-treated cold-rolled DSS sheets underwent aging treatment at 750 °C, the tensile mechanical properties were significantly altered compared to the solution-treated DSS sheet. During room-temperature tensile testing deformation, the tensile strength increased while the elongation to fracture decreased rapidly with the increase in aging time after aging treatment at 750 °C. Moreover, the aging-treated sheet exhibited greater work-hardening capacity compared to the solution-treated samples.

After aging treatment at 750 °C for 1 h, the tensile strength was nearly close to about 900 MPa, with the elongation to fracture at about 56%. As the aging time at 750 °C was further extended, the tensile strength of the DSS sheet continued to rise. When the aging treatment time reached 7 h, the tensile strength approached 1000 MPa, while the fracture elongation was less than 50%. This trend of increasing tensile strength alongside decreasing elongation in the sheet aged at 750 °C is most likely attributed to precipitate formation and its hindering effect on dislocation motion. However, based on the aforementioned results, the optical microscope is incapable of revealing the microstructure evolution mechanism. To gain a deeper understanding of the effect of the 750 °C aging heat treatment on the microstructure precipitation behavior of this novel DSS and the microstructure evolution mechanism during room-temperature tensile deformation, a TEM-2010F field-emission transmission electron microscope was employed. The specimen near the tensile fracture of the sample aged at 750 °C for 7 h after tensile testing at room temperature was selected for investigation, with the corresponding results presented in Figure 4, Figure 5 and Figure 6.

Figure 4 illustrates the TEM micrograph of the deformed ferrite phase near the tensile fracture of the sample aging-treated at 750 °C for 7 h. Dislocations filled the ferrite grains, exhibiting noticeable pile-up at the ferritic grain boundaries, which acted as obstacles to dislocation motion. At this magnification, no obvious precipitates were observed within the ferrite grains and at their grain boundaries. However, a higher-magnification TEM micrograph revealed special microstructural characteristics, as shown in Figure 5. The TEM bright-field image (Figure 5a) shows that dislocation slip occurred in the ferrite phase, forming a large number of dislocation cells. The phenomenon of dislocation pile-up at ferrite grain boundaries was particularly evident, especially at triple grain boundary junctions. The corresponding TEM dark-field image (Figure 5b) reveals irregular spheroidal precipitates generated at the ferrite triple grain boundary, with a length size of approximately 143 nm. According to the TEM selected-area electron diffraction results, this precipitate was identified as the σ phase, a typical precipitate in duplex stainless steels that forms at ferritic grain boundaries. The presence of such precipitates can effectively impede dislocation movement during the material’s plastic deformation, and the extensive entanglement of dislocations can induce stress concentration, thereby enhancing the material’s tensile strength. With a prolonged aging time, the growth of the precipitates significantly intensifies their hindering effect on dislocations during tensile deformation, leading to a gradual increase in the material’s tensile strength over time.

The microstructure characteristics of the deformed austenite phase near the tensile fracture were markedly different from those of the aforementioned deformed ferrite phase. The corresponding TEM micrographs of the deformed austenite phase are shown in Figure 6. In this deformation region, distinct lath-like microstructures were observed. According to the TEM dark-field image (Figure 6a), these lath-like structures originated from the grain boundaries and extended into the interior of the austenite grains. The lath structure corresponds to the α’-martensite, which has a cubic structure. The orientation relationship between the lath structure and the austenitic matrix structure accords with $(110)\alpha ’//(\stackrel{\_}{1}1\stackrel{\_}{1})\gamma \&[001]\alpha ’//[011]\gamma$ based on the selected area electron diffraction result.

Based on the TEM micrographs of the deformation region near the tensile fracture presented in Figure 4, Figure 5 and Figure 6, the deformation mechanism of the DSS aged at 750 °C during room-temperature tensile deformation is now clear. The ferrite phase deformed via the dislocation slip, forming a cellular microstructure. In contrast, the deformation mechanism of the austenite phase was dominated by strain-induced martensite transformation. This research conclusion aligns with our prior study on the plastic deformation mechanism at room temperature of the 19Cr DSS solution-treated at 1100 °C [36]. In previous studies, the phase transformation of the austenite phase during the tensile process of solution-treated samples had a deformation-induced plastic effect, transforming into the martensitic phase. This allowed the material to exhibit a favorable combination of strength and elongation at room temperature. Notably, the deformation mechanisms of ferrite and austenite in the 19Cr DSS at room temperature remain unchanged even after aging treatment at 750 °C. However, σ phase precipitates form during aging treatment at 750 °C in air. As a well-known intermetallic compound in DSSs, the σ phase tends to form at ferritic grain boundaries. Its presence not only impedes dislocation motion but also alters the local stress distribution within the material. With prolonged aging time, the growth of σ phase particles intensifies their hindering effect on dislocations, leading to a progressive increase in tensile strength. However, this strengthening mechanism comes at the cost of reduced ductility, as the increased stress concentration around the precipitates can promote crack initiation and propagation, thereby lowering the fracture elongation. This conclusion is in agreement with the tensile mechanical property results presented in Figure 3.

In comparison to the dislocation-dominated deformation in the ferrite phase, the strain-induced martensite transformation in the austenite phase represents a distinctive deformation mechanism for this 19Cr DSS. Unlike conventional austenitic stainless steels where deformation twinning may prevail, the martensite transformation here provides an additional strain hardening mechanism. The formation of α’-martensite laths, extending from the grain boundaries into the austenite grains, not only contributes to the overall strength of the material but also influences its ductility. This dual-phase deformation mechanism, combining the dislocation slip in ferrite and martensite transformation in austenite, endows the DSS with its unique balance of mechanical properties. Furthermore, the specific orientation relationship between the α’-martensite and the austenitic matrix, as revealed by the selected-area electron diffraction, highlights the epitaxial nature of this transformation, which is crucial for understanding the compatibility of strains between the two phases during deformation.

3.3. Effect of Aging Treatment at 750 °C on Pitting Corrosion Properties

To investigate the effects of aging treatment at 750 °C on the pitting corrosion properties of this DSS, potential dynamic curves were measured at a scanning rate of 0.5 mV/s in a 3.5 wt.% NaCl solution at room temperature. Figure 7 presents the polarization curves of solution-treated and aging-treated DSSs. The results indicate that the self-corrosion potentials of solution-treated DSS and DSS aging-treated at 750 °C are roughly similar. However, the solution-treated DSS at 1100 °C exhibits the widest stable passivation region, with a pitting corrosion potential of approximately 260 mV, demonstrating the best pitting corrosion resistance compared to aging-treated DSS. In contrast, aging treatment at 750 °C significantly degrades the pitting corrosion resistance of solution-treated DSS. When solution-treated DSS is subjected to aging treatment at 750 °C for 1 h, the stable passivation region narrows, and the pitting potential decreases markedly to about 215 mV. As the aging time at 750 °C increases further, the pitting corrosion resistance of DSS continues to decline. When samples are aged at 750 °C for 7 h, the pitting corrosion potential drops to approximately 75 mV. TEM observations reveal that σ phase precipitates at ferrite grain boundaries during aging treatment at 750 °C, and the amount of σ phase increases with prolonged aging time. The presence of σ precipitates exacerbates the pitting corrosion resistance of DSS in NaCl solutions.

Figure 7 vividly illustrates the impact of different treatments on the polarization behavior of DSS. For solution-treated DSS at 1100 °C, its wide stable passivation region indicates that the passive film on its surface is highly stable and capable of effectively preventing localized corrosion. The high pitting corrosion potential suggests that the initiation of pitting corrosion requires a higher potential, meaning the material has strong resistance to pitting corrosion under chloride ion attack. In contrast, aging treatment at 750 °C negatively affects the passivation performance of DSS. As aging time increases, the narrowing of the stable passivation region and the decrease in pitting potential reflect the gradual degradation of the passive film’s stability and the material’s reduced ability to resist pitting corrosion. This may be attributed to the fact that during aging treatment, carbides, nitrides, and intermetallic compounds such as σ phase precipitate at grain boundaries and phase boundaries. These precipitates disrupt the continuity of the passive film, create galvanic corrosion cells and increase the likelihood of localized corrosion. Additionally, the precipitation of these phases may alter the microstructure and composition of the material, reducing its corrosion resistance.

Optical micrographs of the samples after polarization tests, following electrolytic etching in a 10 wt.% KOH solution, are shown in Figure 8. Observations reveal that all pitting corrosion sites in the experimental alloys initiate within the ferrite phase. As aging time increases, the size of the pitting pits enlarges, and the resistance to pitting corrosion in chloride solutions diminishes. Pitting [37,38] pits preferentially form at the grain boundaries between ferrite and austenite. Research indicates that during aging treatment at 750 °C, σ phase precipitates at ferrite grain boundaries, and the quantity of σ phase increases with prolonged aging time. The susceptibility of ferrite to pitting corrosion is higher than that of austenite. Pitting corrosion typically begins at the boundaries between ferrite and austenite. During aging treatment, σ phase precipitation at ferrite grain boundaries further weakens the local structure and composition of the ferrite phase, creating more active sites for corrosion. Chloride ions in the solution readily adsorb onto these sites, leading to the breakdown of the passive film and the initiation and propagation of pitting corrosion. Moreover, the mismatch in corrosion rates between ferrite and austenite can induce micro-galvanic corrosion, accelerating the pitting corrosion process.

4. Conclusions

The present study systematically investigated the microstructure evolution, mechanical properties and pitting corrosion behavior of a novel Mn-N alloyed Ni-free TRIP-aided 19Cr DSS during isothermal aging at 750 °C for different time. The main findings can be summarized as follows:

• Solution-treated specimens exhibited an exceptional combination of strength and ductility, with an ultimate tensile strength of approximately 820 MPa and a fracture elongation approaching 70%. This superior mechanical performance originates from the coordinated deformation mechanisms: dislocation slip-dominated deformation in ferrite, which forms dislocation cells, and strain-induced martensitic transformation in austenite.
• Aging treatment at 750 °C induced a time-dependent evolution of mechanical properties. The tensile strength increased by 16.3%, from 820 MPa to 954 MPa, after 7 h of aging, while the elongation reduced by 38.6%, from 70% to 43%. This hardening effect is attributed to σ-phase precipitation at the ferrite triple junctions, which impedes dislocation motion effectively.
• Pitting corrosion resistance showed progressive deterioration with increasing aging duration. Evidence of this includes a 52% decrease in pitting potential after 7 h of aging. The increased susceptibility correlates with σ-phase acting as preferential sites for corrosion initiation.
• Microstructural analysis revealed that the phase-specific deformation mechanisms remained unchanged post-aging. No secondary phase formation occurred except for σ precipitates in the ferrite boundaries. The stability of austenite was maintained during deformation, as confirmed by the retained TRIP effect.