Super Austenitic Stainless Steel Research Articles

Mechanical behavior of dissimilar AL6XN-IN718 welded joint obtained by cold metal transfer

Cold metal transfer (CMT) welding process was used to join the AL6XN super-austenitic stainless steel to a nickel-based super alloy IN718. Microhardness, tensile and low cycle fatigue tests were carried out to determine the mechanical properties of the welded joint. The heat used to perform the weld decreases the microhardness in the heat affected zone (HAZ) of the IN718 material. A decrement of about 50% with respect to the base material (∼410 HV0.2) was observed, which is attributed to the microstructural transformation produced by the weld thermal cycles, inducing the partial dissolution of the γ'′ phase in the nickel matrix. In contrast, the HAZ on the AL6XN side, the microhardness tends to be similar with respect to the base material (∼203 HV0.2). In terms of the fracture energy, determined from the stress-strain plot, the welded joint decreases about 60–65% with respect to the base materials. This tendency was also observed in the case of the Charpy impact test behavior. From the low cycle fatigue test, it was observed that AL6XN material and AL6XN-IN718, performed better than the IN718 alloy, i.e. more tolerance to the strain cyclic, increasing the number of cycles to failure. A 3-D transient thermal conduction finite element model was developed to correlate the thermal history with the microstructural transformation on the HAZ and mechanical properties. The weld thermal cycles are in good agreement with the experimental measurements obtained by K-type thermocouples.

Effect of Ce on the Segregation and Secondary-Phase Precipitation During the Solidification of S31254 Super-Austenitic Stainless Steel

Effect of Ce on the Segregation and Secondary-Phase Precipitation During the Solidification of S31254 Super-Austenitic Stainless Steel

Comparing Electrochemical Passivation and Surface Film Chemistry of 654SMO Stainless Steel and C276 Alloy in Simulated Flue Gas Desulfurization Condensates.

This research examines the behavior of electrochemical passivation and the chemistry of surface films on 654SMO super austenitic stainless steel and C276 nickel-based alloy in simulated condensates from flue gas desulfurization in power plant chimneys. The findings indicate that the resistance to polarization of the protective film on both materials initially rises and then falls with either time spent in the solution or the potential of anodic polarization. Comparatively, 654SMO exhibits greater polarization resistance than C276, indicating its potential suitability as a chimney lining material. Mott-Schottky analysis demonstrates that the density of donors in the passive film formed on 654SMO exceeds that on C276, potentially due to the abundance of Fe oxide in the passive film, which exhibits the characteristics of an n-type semiconductor. The primary components of the passive films on both materials are Fe oxides and Cr oxides. The formation of a thin passive film on C276 in the simulated condensates is a result of the low Gibbs free energy of nickel oxide and low Cr content. The slower diffusion coefficient of point defects leads to the development of a thicker and more compact passive film on the surface of 654SMO.

Inhibition of the second phase precipitation and improvement of intergranular corrosion resistance by boron segregation at the grain boundary of S31254 superaustenitic stainless steel

The inhibition of Mo segregation and phase precipitation is vital for improving the hot workability and corrosion resistance of superaustentic stainless steels (SASS). The boron non-equilibrium segregation of S31254 SASS was implemented through solid solution, air cooling, and diffusion at low-temperature treatment (SADT). The precipitation process and intergranular corrosion (IGC) of S31254 SASS with various boron distributions were researched at a sensitive temperature. The second phases were observed and identified by SEM and TEM. IGC susceptibility was evaluated by double-loop potentiodynamic reactivation (DL-EPR) measurements. The SADT treatment promoted more segregation of B at the grain boundary, leading to lower amounts of grain boundary precipitation before aging for 6 h. The decrease of σ phases in B-regulated samples enhances the IGC resistance, compared with the samples without B addition specimens.

Mechanism of improving solidification structure of super austenitic stainless steel by feeding strip into molten steel

Feeding steel strip into molten steel is an effective technique for refining the solidification structure of steel, especially for high-alloy steel. This study experimentally and numerically investigated the effects of feeding strip on the solidification structure of super austenitic stainless steel S32654. The results revealed that feeding strip noticeably increased the proportion of equiaxed grains, refined the dendritic structure, and improved Mo segregation and the precipitation of σ phase. The relevant mechanism was clarified as follows. Firstly, feeding strip promoted the formation and survival of a large number of floating dendrites by cooling the molten steel. As the temperature decreased, these floating dendrites sharply increased their quantity by repeated splitting and then acted as nucleation particles. Numerous equiaxed grains grew around these floating dendrites, significantly refining the central dendritic structure. Meanwhile, the doubling of the equiaxed grains has two effects on Mo segregation. First, it prevented the growth of columnar grains and the movement of Mo segregation interface towards the center, thus reducing the Mo macrosegregation. On the other hand, it dispersed and refined the residual liquid phase (Mo segregation regions), thereby reducing the Mo microsegregation. Additionally, the reduced Mo segregation lowered the σ phase precipitation temperature, allowing it to form in solid austenite with a shortened growth duration. The lighter Mo segregation and slower Mo diffusion in solid austenite led to insufficient Mo supply for the σ phase growth. Moreover, dendritic refinement limited the growth space of the σ phase. Finally, less and finer σ phase particles precipitated in the S32654 steel fed with strip. The improvement of feeding strip on the solidification structure will reduce the temperature and time required for high-temperature homogenization, which is beneficial for the subsequent hot working.

Effect of Nitrogen on the Corrosion Resistance of 6Mo Super Austenitic Stainless Steel

6Mo super austenitic stainless steel (SASS) with nitrogen contents of 0.2 and 0.4 (wt.%) was melted, and solution treatments at 1100, 1180, and 1250 °C for 30 min were performed. The effects of nitrogen on the microstructure and pitting resistance of the two steels that signed as 0.2N and 0.4N samples were investigated. At a heat-treatment temperature of 1180 °C, the alloy demonstrates the highest corrosion resistance, attributed to the combined effects of grain size and precipitates. The structure of the passivation film changes with increasing nitrogen content, with the Cr/Fe ratio is significantly higher in the 0.4N sample compared to the 0.2N sample. Moreover, the increase in nitrogen content results in thicker Cr and Mo oxide layers and higher levels of NH3 and NH4+, thereby improving the corrosion resistance of the stainless steel.

Effect of Mo on intergranular corrosion behavior in super-austenitic stainless steel

The effect of the Mo element on the intergranular corrosion behavior was investigated on super-austenitic stainless steels. The σ-phase structure and compositional changes of stainless steels with different Mo contents during aging at 800–900°C were investigated by TEM, combined with electrochemical tests, and two different intergranular corrosion mechanisms were revealed. The low-Mo 904 L stainless steel induces severe intergranular electrochemical corrosion due to selective anodic dissolution at grain boundaries caused by decreased passivation ability and σ-phase composition changes. The σ-phase precipitation of 254SMo and AL-6XN with high-Mo leads to Cr and Mo depletion reduces passivation film continuity, and causes intergranular corrosion.

Machinability investigation of 254 SMO super austenitic stainless steel in end milling under different cutting and lubri-cooling conditions

Machinability investigation of 254 SMO super austenitic stainless steel in end milling under different cutting and lubri-cooling conditions

Thermal deformation behavior and microstructure analysis of the as-cast super austenitic stainless steel Sanicro35

Thermal compression experiments on the super austenitic stainless steel Sanicro35 were carried out using a Gleeble 3800 thermal simulation laboratory machine to investigate its thermal deformation behavior at different deformation temperatures (900 °C–1150 °C) and strain rates (0.001–10 s−1). The microstructure of the large deformation zone of the specimen was investigated using electron backscatter diffraction (EBSD). The results show that the thermal compression rheological stress of the super austenitic stainless steel Sanicro35 decreases with increasing temperature and decreasing strain rate. Dynamic recrystallization (DRX) is the main softening mechanism for this material. The morphology characteristics, recrystallization fraction, dislocation density and twin grain boundary distribution of the microstructure were analyzed by EBSD. With the increase of deformation temperature, the higher grain boundary mobility contributed to the growing of DRX grains. As the strain rate increases, the larger deformation storage energy provides sufficient activation energy for DRX grain nucleation, and the nucleation of DRX grains becomes denser. The twin boundaries are mainly distributed within the DRX grains. The smaller the grain size of DRX, the denser the nucleation of twin boundaries, and the generation of twins can promote the development of DRX. The softening mechanism under most deformation conditions is discontinuous dynamic recrystallization (DDRX). However, at 10 s−1, the high strain rate causes microbands to be generated within the deformed grains, and the microband boundaries evolve toward the high-angle grain boundaries (HAGBs) with increasing temperature, which promotes the occurrence of Continuous dynamic recrystallization (CDRX).

Effect of Mo on the σ-phase precipitation behavior of super-austenitic stainless steel

The aging precipitation behavior of super-austenitic stainless steels 904L (4.32% molybdenum (Mo)), 254SMo (6.06% molybdenum) and AL-6XN (6.29% molybdenum) with different molybdenum contents at 800–900°C was investigated by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The effect of molybdenum content on the σ-phase precipitation behavior of commonly used super-austenitic stainless steel was investigated. The results show that the super-austenitic stainless steels with different molybdenum contents formed different numbers and shapes of σ phases in the isothermal aging process, and the number of σ phases of 254SMo and AL-6XN is much larger than that of 904L, and the shape of the σ phases mainly shows coarse acicularity and elongation, while the σ phases of 904L show fine granularity.

Effect of post-weld heat treatment on microstructure, crystallography, and mechanical properties of laser beam welded AISI 904L super austenitic stainless steel

This research examines the crystallography, microstructure, and mechanical properties of AISI 904L super austenitic stainless steel (SASS) joint by laser beam welding (LBW). The results indicated that the B3 and B4 specimen’s weld metal (WM) microstructure (the minimum value at the welding heat input of 0.117 kJ/mm and the maximum value at the welding heat input of 0.200 kJ/mm) comprised almost all austenite (dendritic + interdendritic) phases. Some Cr23C6 and Cr2N precipitates appeared at the grain boundaries and in the interdendritic region due to post-weld heat treatment (PWHT). Besides, it was obtained that the dendritic structure increased due to the rise in heat input. While most of the grains in WM displayed orientation in [111] and [101], it was detected that they displayed orientation in [001] and [101] with PWHT. While the high-angle grain boundary (HAGB) ratio of the B4 specimen was found to be high, the low-angle grain boundary (LAGB) ratio of the B4H specimen was found to be also high. In addition, the KAM was reduced with the effect of PWHT. Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (NW) orientation between grains with FCC/BCC structure was observed in WMs of B4 and B4H specimens. The highest mechanical properties were mostly obtained in the specimens joined by the lowest welding heat input. Fracture surfaces of tensile and impact samples of laser welded joints showed mixed mode fractures, i.e. ductile and partially cleavage fractures.

Characterisation of TCP precipitation sequences of superaustenitic stainless steel and correlation with electrochemical and mechanical properties

This study investigates the influence of precipitation sequences formed at critical high-temperature range for various ageing time of super austenitic stainless steel 654SMO against the evolution of its corrosive resistance performance through the microstructural characterization, hardness and electrochemical corrosion testing. The specific type of stainless-steel alloy is one of the most corrosion resistant along with exceptional mechanical properties, tailor-made for pressurized and erosive systems handling chlorinated sea water, plate heat exchangers, and fuel gas cleaning applications and an excellent case study of in-depth phase transformations-electrochemical properties characterization phenomena. The microstructure evolution is characterized by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), hardness testing and potentiodynamic polarization curves. The corresponding precipitation characteristics and sequences of secondary phases are discussed and correlated with electrochemical properties. The results demonstrate σ phase with various morphologies formed during isothermal aging and the formation of three more secondary topologically close pack (TCP) phases, such as chi phase (χ), laves phase (A2B), and Cr2N against corrosive resistance and mechanical performance, which are of primary importance for understanding electrochemical phenomena of different phases and useful for optimizing such alloys for various applications.

Combined role of molybdenum and nitrogen in Limiting corrosion and pitting of super austenitic stainless steel

The molybdenum and nitrogen content of super austenitic stainless steel in Cl− solution is shown to influence pitting resistance using immersion, electrochemical testing, and simulation. Variations in Mo and N content affect the defect density, resistance, and densification of the passive film, thereby reducing the number of pitting. A higher local pH associated with the pitting pits and an increase in NH3(NH4+) are the results of increased N content, which also slows the rate of pitting expansion. The combined effects of fewer actively reactive spots within the passive film retarded pitting, and decreased corrosion rates due to NH3(NH4+) mitigation of local acidity which serves to reduce the corrosion rate. The work function is improved to a greater extent when Mo and N are co-doped compared with individual Mo and N doping, and the adsorption energy is significantly increased when Mo and N are co-doped, indicating a synergistic role for Mo and N in the prevention of corrosion by Cl−.

Controlling plasma-based surface modifications of an austenitic alloy by thermochemical and athermal diffusions

Under nitrogen plasma immersion ion implantation, the energy density per pulse EPulse/A was found to be proportional to the amount of the N-expanded austenite phase (γN) in a superaustenitic stainless steel, as previously seen for an austenitic-ferritic alloy. Since γN occurs in a range of stoichiometries, the parameter was also contrasted with the ion fluence Γ, which affects the retained dose. Γ varies inversely with EPulse/A and influences oppositely the properties of the modified surfaces. The temperature was the same among the treatments (320 °C) to rule out thermal diffusion as an extra variable. The increase of EPulse/A (for applied voltages 6.2–10.4 kV) produces thicker layers (1.4–2.2 µm), as an increase in lattice defects and stresses enhances N-diffusion, while high Γ values favor the N-saturation at the top surface for the opposite reason. The lowest Epulse/A (6.2–9.5 kV) result in brittle cases with a possible thin nitrides layer on top, whereas γN prevails in the ductile layer produced by the highest one (10.4 kV). The N-concentration governs the strength against plastic deformation: hardness varies up to 32% from the highest (6.2 kV) to the lowest Γ treatment (10.4 kV). In summary, the correlation of Epulse/A with Γ is indispensable for controlling properties of the modified surfaces for performance purposes.

Superior resistance to cavitation erosion and corrosion of a novel SiC-reinforced superaustenitic stainless steel coating fabricated by high-velocity air fuel spraying

In this work, we developed a SiC-reinforced superaustenitic stainless steel (SS) coating aimed at enhancing resistance against cavitation erosion (CE) and corrosion. Relevant results indicated that the coating exhibited a diminished mean erosion depth compared to that of 304 SS in 3.5 wt% NaCl solution. Furthermore, a pronounced inhibitory effect on the synergistic interplay between CE and corrosion was demonstrated. Electrochemical assessments underscored the superior passivation and repassivation properties of the coating during CE, contrasting markedly with the compromised passivation capacity of 304 SS, which assumed an active dissolution state characterized by a substantially elevated corrosion current density. The exceptional resistance to CE and corrosion of the coating can be attributed to its low porosity, the elevated levels of corrosion-resistant elements, and the nanoscale grain size. Consequently, this coating emerges as a highly promising candidate for the protective coating on 304 SS flow-handling components.

Deformation behavior of super-austenitic stainless steel sandwich panels subjected to air blast and underwater explosion

Super-austenitic stainless steel is a class of high alloy steels with enhanced toughness, outstanding strength, and corrosion resistance. In this article, the structural integrity of super-austenitic stainless steel square sandwich panels subjected to surface and underwater explosions is investigated through numerical simulations, and the results are validated with experiments. Abaqus Explicit Solver is used to predict the face deformation and core crushing failure modes of the sandwich panels. Simulations are performed for three levels of impulse load by varying the explosive quantity. The fluid-structure interaction is established by CONWEP and UNDEX algorithms for air blasts and underwater explosions respectively. The dynamic deformation behavior of the sandwich panel is modeled using the Johnson-Cook (JC) plasticity model incorporating strain rate and temperature effects. Good agreement is witnessed between the numerical predictions and experimental observations. An increment in deformation of the face sheet, cell wall buckling, and core compaction is observed with the increase in the impulse. From a series of air and underwater explosion simulations, the advantage of using sandwich structures for blast mitigation is demonstrated. In addition, the significance of numerical simulations to limit experiments and predict the deformation of the sandwich panels is presented.

Revealing cation and metal gradients in and underneath passive films of the stainless steel 1.4652 in acidic and alkaline electrolytes with angular resolved dual energy X‐ray photo‐electron spectroscopy

AbstractPassive films on the superaustenitic stainless steel 1.4652 were studied using angular resolved hard X‐ray photo‐electron spectroscopy (HAXPES), which provides an increased information depth compared to conventional X‐ray photo‐electron spectroscopy (XPS). Elemental depth profiles in the oxide as well as gradients in the underlying metal were established using a dedicated algorithm. The utility was exemplified for three different conditions of film formation: (i) as polished and as polarized to the high passive end in (ii) 1 M H2SO4 and (iii) 1 M NaOH. The use of a superaustenitic material ascertained that metal signals from the main contributors to film formation: Fe, Cr, Ni, and Mo remained above the detection limit throughout the data set. With the dual‐source technique (XPS, HAXPES), it was possible to investigate metal gradients underneath the passive film. For polarizations in an acidic environment, a metallic nickel enrichment of the order of 5 Å was found. For a corresponding polarization in an alkaline environment, the metal region closest to the oxide/metal interface was enriched in iron, followed by concentration gradients down to about 50 Å into the metal.

Studies on the Cooperative Influence of Cr and Mo on the Pitting Corrosion Resistance of Super Austenitic Stainless Steels

The effects of varying Cr and Mo concentrations on the pitting corrosion resistance of super austenitic stainless steels in Cl− solutions were investigated using a combination of immersion experiments, electrochemical measurements, X-ray photoelectron spectroscopy, and first-principles computational simulations. The surface characteristics, impedance, and defect concentration of the passive film were changed, and this eventually resulted in a decrease in the number of pitting pits. Due to a decrease in active sites within the passive film, a delayed beginning of pitting, and the combined effect of MoO42− inhibitors, it was discovered that an increasing Mo concentration slows the rate of pitting extension, resulting in reduced maximum pitting area and depth. Additionally, Mo increased the adsorption energy of nearby atoms, whereas Cr raised the adsorption energy of itself. Interestingly, compared with individual doping, co-doping of Cr and Mo increased work function and adsorption energy, indicating a synergistic impact in enhancing resistance to Cl− corrosion.

Precipitate composition regulation and corrosion resistance improvement of boron-containing S31254 steels through a pre-aging treatment

Boron (B) at the grain boundary (GB) influences the segregation of Cr and Mo in S31254 super austenitic stainless steels, which inhibits the formation of precipitates. In this work, to make full use of B at the GB, a pre-aging treatment schedule at 300 ℃ for 50 and 150 min was proposed. After pre-aging treatment and high-temperature aging at 950 ℃, B-containing S31254 was used to investigate the effect of heat treatment on the precipitates. Results verified that pre-aging treatment for 50 and 150 min could promote the segregation of B at GB, and the nucleation of precipitates was inhibited during high-temperature aging. Consequently, the B-containing and Mo-rich precipitates were formed at 950 ℃. Especially, pre-aging treatment for 50 min showed the optimum effect on regulating precipitate compositions. B-containing and Mo-rich phase reduced the Cr-depleted region near the precipitates, which could remarkably improve corrosion resistance. Pre-aging treatment and high-temperature aging can regulate precipitate compositions, which can in turn improve corrosion resistance.

Strain-induced precipitation behaviors of 7Mo super-austenitic stainless steel

Strain-induced precipitation (SIP) behaviors of 7Mo super-austenitic stainless steel (SASS) were studied by strain relaxation test at 850 °C. Our results reveal that sigma (σ) phases are the predominant SIP of 7Mo SASS, with the chemical formula of (FeNi) (CrMo). SIP first precipitates in granular shape at HAGBs, such as deformed grain boundaries (SIP-GBs), interfaces between deformation twin layers and matrix (SIP-TW), and HAGBs inside deformed grains (SIP-HAGBs). This is attributed to the ability of HAGBs for providing high deformation stored energy, nucleation sites, and efficient element diffusion channels. When the nucleation sites at HAGBs become saturated with increasing holding time, needle-like SIPs (SIP-NL) are precipitated along closely packed planes of γ-Fe matrix. The specific orientation relationships between SIP-NL and matrix, namely (−111)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ or (-1-1-1)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ, and the needle-like shape are contribute to minimizing interfacial energy and elastic strain energy of SIP, respectively, thereby promoting SIP-NL. The absence of SIP on dislocations can be attributed to the large critical nucleus size of σ phase (about 4.87 nm), which poses challenges for SIP nucleation through dislocation distortion energy. Mo is the controlling element for the nucleation and growth of SIP. SIP can promote cDRX through particle-induced nucleation (PSN) mechanism during the deformation process, while residual high-density dislocations around SIP can also promote cSRX during the holding process. During holding process, the boundaries of recrystallized grains can also serve as nucleation sites for SIP, thereby promoting SIP, and finally form a multi-layer structure of “σ phase/Recrystallization/σ phase”.