Aged Duplex Stainless Steel Research Articles

Effect of thermal ageing on the deformation behaviour of ferrite and austenite in a duplex stainless steel: Micropillar characterisation and dislocation density modelling

The deformation behaviour of the austenitic and ferritic phases in an aged duplex stainless steel is investigated experimentally and numerically. The effect of the ferritic phase’s spinoidal decomposition during thermal ageing on the mechanical behaviour of the alloy is studied through micropillar compression tests on the individual single crystal phases before and after ageing, and APT and TEM investigations of the decomposed ferrite and corresponding dislocation structures. Dislocation density-based crystal plasticity models are formulated for the austenitic and ferritic phases, and calibrated from the measured micropillars responses.A considerably higher and more homogeneously distributed initial dislocation density is observed in the austenite, resulting in a more homogeneous deformation of the austenitic micropillars than the ferritic ones. The spinoidal decomposition of the ferrite leads to a considerable increase of its initial flow stress. The strengthening effect of thermal ageing and the resulting localised deformation response of the aged ferritic micropillars are correctly described by the crystal plasticity results, which required a higher initial obstacle (dislocations + inter-phases of the newly formed α′ domains) density to dislocation motion. Moreover, a strong size effect was observed on the behaviour of the as-received and aged austenitic and as-received ferritic micropillars, but not in the aged ferritic ones. Such contrasting behaviour originates from the fact that, for the size sensitive cases, the dislocation mean free path is controlled by the mean micropillar diameter, whereas in the aged ferrite micropillars, it is by the ∼ 5 nm wide Cr-rich domains within the spinoidal decomposed ferrite.

New insights into the hardening and pitting corrosion mechanisms of thermally aged duplex stainless steel at 475 °C: A comparative study between 2205 and 2101 steels

• Ferrite hardening is caused by α'-α phase separation. • In LDSS 2101, Cr nitrides impact pitting corrosion more than α'-evolution. • G-phase contributes to a decrease in pitting corrosion in long-term aging. • Spinodal decomposition is the main cause of the drop-in pitting corrosion of DSS 2205. In this study, the relationship between spinodal decomposition and the formation of Ni-rich clusters and G-phase in the ferrite on hardening and pitting corrosion of two thermally aged duplex stainless steels (DSSs) at 475 °C was investigated. Results indicate that, for 2205 DSS, pitting corrosion behavior is influenced by the presence and size of G-phase precipitates for longer aging times, but this contribution is masked by the advanced stage of spinodal decomposition in the ferritic structure. On the other hand, for 2101 DSS, the formation of Cr-richer nitrides impairs pitting corrosion resistance more than spinodal decomposition.

Correlation of ferrite-phase reconfiguration and mechanical properties in thermally aged duplex stainless steel

The thermal aging behavior and subsequent recovery treatment of duplex stainless steel are investigated by mechanical properties testing and microstructure characterization. After thermal aging at 475 °C for 500 h, the ferrite phase undergoes severe spinodal decomposition and precipitation, which results in a significant increase in yield strength, and a substantial reduction in ductility. Combining the nano-indentation, geometric phase analysis, and tensile testing, it can be confirmed that spinodal decomposition is the main contributor to the hardening of the ferrite phase. The hardening of the ferrite phase is induced by the semi-coherent interface between α´-phase and α-phase and higher shear modulus of Cr-enriched domains. The increased yield stress calculated by the simplified equation is in good agreement with the experimental value. When the aged samples were treated by high-temperature annealing and pulsed electric field, the hardening is significantly eliminated, and the ductility is correspondingly restored. Further, the essence of the difference in required temperature to restore deteriorated performance is the change of system free energy caused by temperature and pulsed electric field. • The ferrite phase undergoes severe decomposition after aging at 475 °C for 500 h. • Ferrite hardening is mainly caused by spinodal decomposition. • Pulsed electric field and annealing can restore the deteriorated performance. • Electric free energy decreases the required temperature for restoring performance.

The Effect of Microstructural Changes on Mechanical and Electrochemical Corrosion Properties of Duplex Stainless Steel Aged for Short Periods.

This work reports the effects of Microstructural changes due to the secondary phases, in particular sigma (σ), on the mechanical properties and electrochemical behavior of thermally aged duplex stainless steel (DSS). Structural, morphological, mechanical, and electrochemical characterizations were performed. Sigma phase content increased with increasing aging treatment time. It had a net-like shape, as observed by electron backscatter diffractometry (EBSD). Its presence directly damaged mechanical properties. The corrosion assessment included electrochemical impedance spectroscopy (EIS) in 1 M NaCl solution at temperatures of 25, 40, and 65 °C. EIS results demonstrate that an increase in the σ phase content decreased the corrosion resistance (21.1–0.8, 3.5–0.3, and 3.1–0.2 kΩ cm2 at 25, 40, and 60 °C, respectively).

Improvement of Fatigue Resistance and Fracture Toughness of Thermally Aged Duplex Stainless Steel by Laser Shock Peening

Duplex stainless steels (DSSs) offer high strength compared to common austenitic stainless steels; however, when this steel type is exposed to high temperature, the precipitation of sigma phase due to its fast formation kinetics may affect significantly the mechanical properties of the alloy. In this research, the feasibility of using laser shock peening (LSP) to reduce the negative effects of sigma phase precipitation on the mechanical properties of samples with thermal aging is studied. The LSP treatment was performed using a Nd:YAG pulsed laser operating at 10 Hz. The LSP setup was the waterjet arrangement without sample coating. The tensile, yield strength, fatigue crack growth, fracture toughness and hardness of samples aged at 750 °C for different holding times were determined. In order to assess the effect of LSP on fatigue crack growth and fracture behavior, another set of thermally aged compact tension specimens with pre-crack was LSP-treated on both faces and then fatigue loading was applied. Experimental results demonstrate that, while thermal aging accelerates fatigue crack growth of DSS samples, LSP significantly reduces the effect of thermal aging, by reducing the fatigue crack growth rate and increasing the fracture toughness of DSS aged samples.

Electrochemical Changes with Thermal Aging of Duplex Stainless Steels

Duplex stainless steels are extensively used in pressurized water reactors (PWR) in the form of welds and castings (pumps, valves, etc.). Long-term thermal aging of duplex stainless steels leads to embrittlement due to α-αʹ phase separation in the ferrite grains and this phase separation results in degradation of the corrosion resistance of these alloys. This study investigated the effect of thermal aging time on the electrochemical and corrosion properties of as-received and thermally aged duplex stainless steels in a range of PWR water chemistries. The duplex stainless steels such as 2101, 2003, 2205 and welded 2101 and 2209, were subjected to electrochemical characterization in the as-received condition or after thermal aging for 100, 1000 and 10,000 hours. The electrochemical characterization constituted of open-circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS), linear polarization spectroscopy (LPR) and cyclic polarization (CP) in near neutral aqueous solutions, containing 2.2-5 ppm Li (LiOH) and 800-2000 ppm B (H3BO3). In addition, to investigate the effect of chlorides on the pitting resistance of these alloys, 1M concentration of chlorides (NaCl) were added to the base solution. The α-αʹ phase separation has been characterized by atom probe tomography, nanoindentation and impact toughness testing. The results indicated degradation in impact toughness and corrosion resistance with increasing aging time and phase separation. Some base cases were further characterized for microstructures and rust analysis using optical microscopy and scanning electron microscopy. The results of this work will be used to benchmark corrosion simulation models.

Pitting Corrosion of Thermally Aged Duplex Stainless Steels at Different Temperature for Long Time

The pitting corrosion of three duplex steels (DSS), SAF2205, SAF2507, Z3CN20.09M thermally aged at 350 - 500 ℃ for 5500 h was investigated by the means of electrochemical techniques. The pitting potential (Ep) of thermally aged Z3CN20.09M specimens decreased with increasing of ageing temperature, however, the Ep of SAF2205 and SAF2507 thermally aged specimens decreased first and reached the lowest value at 450 ℃ and then increased slightly. The electrochemical impedance spectroscopy results are coincident with the potentiodynamic polarisation testing results. The variation in the charge transfer resistance of specimens coincided with the Ep values. There is little influence of low temperature (below 350 ℃) on resistance to pitting of SAF2205 and SAF2507 DSS. But, the pitting resistance of them degrades drastically ageing at higher temperature above 450 ℃. The deterioration in pitting resistance of thermally aged DSS specimens mainly attributes to the precipitation of Cr-rich α′ phases which affected by temperature markedly. At the range of 350 -500 ℃, the temperature the higher, the precipitation of Cr-rich α′ phases the more. The formation of G phases and the healing process during the long thermal ageing at 500 ℃ is the other reasons for the change in pitting resistance.

Quantification of hardening contribution of G-Phase precipitation and spinodal decomposition in aged duplex stainless steel: APT analysis and micro-hardness measurements

Ageing of cast duplex stainless steels (DSS) is attributed to the decomposition of the ferrite: spinodal decomposition and precipitation of G-phase particles. This leads to an increase in hardness and a decrease in Charpy toughness. According to the literature, spinodal decomposition is accepted to play a major role on the hardening even if the role of G-phase precipitation on mechanical properties is still not clear. This work links microstructural characterization performed using atom probe tomography to micro-hardness of the ferrite for a wide variety of duplex steels (from cast steels with and without Mo to lean steels) aged under different conditions. An attempt to quantify the contribution of both spinodal decomposition and G-phase precipitation is made by applying linear and square superposition principle of Ardell, Orowan and a modified BKS models. The models used are shown to give an excellent estimation of the experimental values of the hardness increase of the ferrite of the cast and lean steels for a wide range of composition and temperature. This work shows that, conversely to what is said in the literature, spinodal decomposition is not systematically the main contributor to hardening.

A brittle fracture mechanism in thermally aged duplex stainless steels revealed by in situ high-energy X-ray diffraction

A direct relationship between the microstructural evolution and macroscopic fracture behaviors of a thermally aged (at 475 °C for 400 h) duplex stainless steel (DSS) has been established with experimental results from atom probe tomography (APT), nanoindentation and in situ synchrotron-based high-energy X-ray diffraction (HE-XRD). The APT experiments demonstrate that ferrite in DSS spinodally decomposes into Cr-enriched and Cr-depleted domains during thermal aging, which leads to a severe hardening effect in ferrite. The lattice strain development during deformation acquired with the in situ HE-XRD measurements confirms the cleavage fracture of α{110} and α{200} ferrite grains aligned perpendicular to loading direction. The thermally aged DSS can be readily fractured by connecting the cleavage cracks in ferrite. The spinodal-decomposition-induced hardening in ferrite and the premature failure of ferrite control the final brittle fracture in the aged DSS.

Nano-Deformation Behavior of a Thermally Aged Duplex Stainless Steel Investigated by Nanoindentation, FIB and TEM

Nanoindentation and electron backscattering diffraction were conducted on a thermally aged duplex stainless steel to investigate the effect of crystal orientation on the plastic deformation behavior during indentation testing. Both nanohardness H and indentation modulus E are correlated with the orientation factor averaged over three normal directions of the contact surface. After thermal aging, the orientation dependence of the hardness and indentation modulus in ferrite significantly changed. For both the ferrite and austenite phases, the maximum and minimum values of the hardness and indentation modulus are given by the near- $$\left\langle {111} \right\rangle$$ and near- $$\left\langle {001} \right\rangle$$ -oriented grains, respectively. The TEM analysis results indicate that the area of plastic deformation in the ferrite grain decreases after thermal aging. The interactions between precipitates and dislocations are considered to be responsible for the degradation of plastic deformation ability in ferrite. The anisotropy of hardness is related to the crystallographic nature and shearing mechanisms.

Hardening Contribution of G-Phase Nanoparticle Precipitation and Spinodal Decomposition in Aged Duplex Stainless Steel Studied by APT Analysis and Micro-Hardness of Ferrite

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Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD

The effect of thermal aging on phase transformation and impact toughness of an as-cast duplex stainless steel was investigated at room temperature. After long-term thermal aging, the impact toughness decreases significantly and the cracks initiate and propagate more easily. The plastic deformation ability of the ferrite phase decreases after thermal aging, which leads to the degradation of impact toughness. High stress concentration occurs on the grain boundaries of the austenite phase in the aged materials. Meanwhile, high-stress concentration areas are also observed in the austenite phase near the grain boundaries. After long-term thermal aging, pinned dislocations in ferrite and along phase boundaries lead to the high stress concentration. Micro-cracks preferentially initiate in the ferrite phase and propagate via separation of phase boundaries. The blocking influences of spinodal decomposition precipitates and G-phase precipitates are stronger than the effect of grain boundaries and phase boundaries on the dislocation movement.

Influence of the Heterogeneous Nucleation Sites on the Kinetics of Intermetallic Phase Formation in Aged Duplex Stainless Steel

The aim of this work is to study the influence of the heterogeneous nucleation site quantity, observed in different ferrite and austenite grain size samples, on the phase transformations that result in intermetallic phases in a UNS S31803 duplex stainless steel (DSS). Solution treatment was conducted for 1, 24, 96, or 192 hours at 1373 K (1100 °C) to obtain different ferrite and austenite grain sizes. After solution treatment, isothermal aging treatments for 5, 8, 10, 20, 30, or 60 minutes at 1123 K (850 °C) were performed to verify the influence of different amounts of heterogeneous nucleation sites in the kinetics of intermetallic phase formation. The sample solution treated for 1 hour, with the highest surface area between matrix phases, was the one that presented, after 60 minutes at 1123 K (850 °C), the smaller volume fraction of ferrite (indicative of greater intermetallic phase formation), higher volume of sigma (that was present in coral-like and compact morphologies), and chi phase. It was not possible to identify which was the first nucleated phase, sigma or chi. It was also observed that the phase formation kinetics is higher for the sample solution treated for 1 hour. It was evidenced that, from a certain moment on, the chi phase begins to be consumed due to the sigma phase formation, and the austenite/ferrite interface presents higher S V for all solution treatment times. It was also observed that intermetallic phases form preferably in austenite-ferrite interfaces, although the higher occupation rate occurs at triple junction ferrite-ferrite-ferrite. It was verified that there was no saturation of nucleation sites in any interface type nor triple junction, and the equilibrium after 1 hour of aging at 1123 K (850 °C) was not achieved. It was then concluded that sigma phase formation is possibly controlled by diffusional processes, without saturation of nucleation sites.

Influence of Ni, Mo and Mn Content on the G-Phase Precipitation and Spinodal Decomposition of Aged Duplex Stainless Steels.

Influence of Ni, Mo and Mn Content on the G-Phase Precipitation and Spinodal Decomposition of Aged Duplex Stainless Steels.

Pitting corrosion of thermally aged cast duplex stainless steel for primary coolant pipes of nuclear power plants

ABSTRACTThe pitting corrosion of thermally aged Z3CN20.09M cast duplex stainless steel (CDSS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy. Both pitting potential and charge transfer resistance values of thermally aged specimens became less positive with aging time, while passive current density increased, indicating that thermal aging causes degradation in pitting corrosion resistance. The pitting potential of thermally aged specimens after annealed is higher than that of thermally aged specimens revealing that the pitting corrosion resistance of annealed specimens has been improved. The α’ phase precipitation during thermal aging in ferrite brings the Cr-enriched and Cr-depleted zone microstructures which mainly induced the degradation of pitting corrosion resistance. The microsegregation of Cr element or Cr enriched and Cr-depleted zones will be alleviated or even removed due to the Cr-rich α’ phases dissolution during the process of annealing. So, the pitting corrosion resistance of annealed specimens is improved.

Characterization of Plastic Deformation Behavior of a Thermally Aged Duplex Stainless Steel

In situ tensile tests at room temperature have been conducted on a duplex stainless steel (DSS) thermally aged at 400 °C for 10,000 h to investigate both the plastic deformation mechanisms and the effect of long-term thermal aging on crack initiation. After thermal aging, the ultimate tensile strength of the DSS increases and the plasticity significantly decreases. The fracture morphology changes from ductile fracture with shallow dimples to a mixture of cleavages in ferrite and tearing in austenite. Electron backscattered diffraction (EBSD) technique has been used to determine the crystallographic orientations of austenite and ferrite grains on three areas deformed differently. The EBSD analysis results indicate that high strain occurs near grain boundaries and phase boundaries. The localized strain incompatibility is considered to be responsible for high stress concentration and crack initiation. The long-term thermal aging affect on the crack initiation and the cleavage cracks are found to be initiated in aged ferrite grains.

M23C6 carbides and Cr2N nitrides in aged duplex stainless steel: A SEM, TEM and FIB tomography investigation

The precipitation evolution during ageing of a 2101 lean duplex stainless steel was investigated, revealing that the precipitate type and morphology depends on the nature of the grain boundary. Triangular M23C6 carbides precipitate only at γ/δ interfaces and rod-like Cr2N nitrides precipitate at both γ/δ and δ/δ interfaces. After 15min of ageing, the M23C6 size no longer evolves, whereas that of the Cr2N continues to evolve. For Cr2N, the morphology is maintained at γ/δ interfaces, whereas percolation occurs to form a continuous layer at δ/δ interfaces. By combining 2D and 3D characterisation at the nanoscale using transmission electron microscopy (TEM) and focused ion beam (FIB) tomography, a complete description of the precipitation evolution was obtained, including the composition, crystallographic structure, orientation relationship with the matrix phases, location, morphology, size and volume fraction.

Deformation behavior of thermal aged duplex stainless steels studied by nanoindentation, EBSD and TEM

The cast duplex stainless steels were investigated in order to understand the plastic deformation behaviors and fracture mechanisms after thermal aging at 400°C for up to 10,000 h. Tensile strength has an obvious improvement, but the yield strength has a slightly increase after long-term thermal aging. Microcracks initiate in the ferrite and extend to phase boundaries, leading to the fracture of ferrite before the tensile failure of specimen. Nanoindentation tests indicate that hardness in ferrite continuously increases with aging time, but not strongly affected by deformation degree. Electron backscattered diffraction observations reveales that high-stress concentration on the phase boundaries causes the phase boundary separating. The dislocations pile-up at spinodal decomposition precipitates in ferrite phases and along G-phase boundaries, leading to the reduction of mechanical property of the ferrite.

Corrosion behaviour of sites containing (Cr, Fe)2N particles in thermally aged duplex stainless steel studied using capillary techniques, atomic force microscopy and potentiostatic pulse testing method

The microstructure and corrosion behaviour of sites containing (Cr, Fe)2N in UNS S32202 duplex stainless steel after heat treatment at 900°C were studied in 2M NaCl using surface observation techniques at the micro- and nanoscale, the electrochemical microcell technique at the microscale and the potentiostatic pulse testing method. No zone depleted in chromium was detected in the matrix around (Cr, Fe)2N. Although there is no depleted zone, pitting corrosion was observed at particles. It is shown that particles first dissolve. A crevice is formed and the bare substrate is exposed to the aggressive solution. Large pits propagate.

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20 000 h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.