Austenite Matrix Research Articles

Static investigation of martensitic nucleation criticality in an atomistic model material

ABSTRACT This contribution investigates the nucleation of martensitic transformations in the bulk of austenitic single crystals. The transformation is associated with a decrease in the potential energy that is the driving force for the transformation. The coordinated atom displacements, which generate the martensite, produce large elastic strains in the material, which are associated with an elastic energy that opposes the transformation. A thermodynamic nucleation criterion for this type of transformations is derived. The criterion is evaluated by the method of static lattice relaxation for homogeneous nucleation in an atomistic 2D model material. The potential energy of martensitic clusters of different sizes and shapes, embedded in a stabilised austenitic matrix are calculated. The results exhibit non-monotonous progressions of the potential energy with the cluster size. This indicates that the elastic strain energy scales non-linearly with the size to allow the formation of local energy maxima at critical cluster sizes, interpreted as nucleation barriers. These barriers separate domains of stability from domains of instability in the configurations space, similar to Kelvin's classical theory of spherical fluid phase nucleation. The critical nuclei sizes are in the order of 100–150 unit cells, and their shapes are plate-like with long dimension parallel to the shear direction. The critical size of twinned clusters decreases with the twinning multiplicity.

The Effect of Vanadium on Modified Z-Phase Characteristics in Austenitic Steels

In austenitic steels, the tetragonal Z-phase (NbCrN) has frequently been credited with beneficial strengthening effects during dislocation creep. In the modified Z-phase, niobium is partially substituted by vanadium. The basic objective of this contribution is a detailed characterization of the modified Z-phase in vanadium bearing austenitic AISI 316LN+Nb+V and HR3C steels. Experimental activities were focused on crystallography, thermodynamic and dimensional stability, kinetics of precipitation (TTP diagram) and solvus temperature of the modified Z-phase in the steels examined. Thermodynamic modelling was used for prediction of stable minor phases and solvus temperature of the modified Z-phase. Kinetics of precipitation of the (Nb,V)CrN phase in the AISI 316LN+Nb+V steel was experimentally investigated in the temperature interval of 550–1250 °C. The kinetics of precipitation of the modified Z-phase in austenitic matrix was fast. Results of diffraction studies on particles of the modified Z-phase confirmed the existence of the tetragonal unit cell already after short-term annealing. The solvus temperature of the modified Z-phase in austenitic steels was determined to be lower than that for the NbCrN phase. The decrease in the solvus temperature is dependent on the vanadium content in austenitic steels. Both thermodynamic calculations and experimental results proved that the thermodynamical stability of the modified Z-phase in austenite was high. More data are needed for evaluation of long-term dimensional stability of the (Nb,V)CrN phase in austenitic steels at temperatures for their engineering applications.

Effects of Mn content on austenite stability and mechanical properties of low Ni alumina-forming austenitic heat-resistant steel: a first-principles study

Low Ni alumina-forming austenitic (AFA) heat-resistant steel is an advanced high-temperature stainless steel with reduced cost, good machinability, high-temperature creep strength, and high-temperature corrosion resistance. Using the First-principles approach, this study examined the effect of Mn content on austenite stability and mechanical properties at the atomic level. Adding Mn to low Ni-AFA steel increases the unit cell volume with an accompanying increase in the absolute value of formation energy; the austenite formed more easily. The austenitic matrix binding energy decreases and remains negative, indicating austenite stability. As the Mn content increases from 3.2 to 12.8 wt%, the system's bulk modulus (B) rises significantly, and the shear modulus (G) falls. In addition, the system's strength and hardness decrease, and the Poisson ratio of the austenite matrix increases with improved elasticity; the system has excellent plasticity with an increase in the B/G. For the Fe22–Cr5–Ni3–Al2 system, with the increase of Mn content, the electron density distribution between the atoms is relatively uniform, and the electrons around the Mn atoms are slightly sparse, which will slightly reduce the structural stability of the matrix. The experiment demonstrated the matrix maintains the austenitic structure when adding 3.2–12.8 wt% Mn elements to low Ni-AFA steel. At an Mn content of 8 wt%, the overall mechanical properties of the high-Mn AFA steel are optimal, with a tensile strength of 581.64 MPa, a hardness of 186.17 HV, and an elongation of 39%.

Microstructural, crystallographic, and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

• Magnetic field-assisted directional solidification (MFADS) can control structure. • Macro-/micro-segregation and orientation can be adjusted by MFADS. • Microsegregation-induced microstructure influences mechanical properties. • Stress-induced crystallographic evolution is traced by in-situ EBSD. Abstract In this study, the effect of transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field, causing the emergence of martensite clusters in the austenite matrix. Moreover, the magnetic field alleviates the microsegregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of the above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation. The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the microsegregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor. The stress-induced superelasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Abnormal grain growth under solution-aging treatments enables convergence mechanical properties of a Fe-Ni-Cr based austenitic alloy

In this study, systematic heat treatment routes including solution (980 °C to 1130 °C) and aging (675 °C to 775 °C) processes were used to tailor the microstructural evolution and mechanical performance of a Fe-Ni-Cr based austenitic alloy, strengthened by a γ′ phase (Ni3(Ti, Al)). Grain growth was observed with increasing solution treatment temperature from nearly 90 μm (980 °C) to nearly 200 μm (1130 °C). Grain refinement during solution heat processes was found to ascribe to the solute drag effect and the pinning effect of nickel-titanium-enriched segregates and precipitates. During aging, the precipitation behavior of γ′ is found to be almost independent of solution treatment temperatures. Interestingly, abnormal grain growth during aging was observed. The motion of grain boundaries was ascribed to the formation of nickel-titanium-enriched γ′ in the austenitic matrix, thereby contributing to the dissolution of the previously enriched segregates and precipitates from grain boundaries. The studied alloy shows a wide range of mechanical properties, from tensile strength of 1131 MPa with ductility at 36%, to tensile strength of 880 MPa with ductility at 50%. The current study demonstrates that grain refinement of the alloy at solution treatment may not benefit the final mechanical properties.

Anomalous fatigue crack propagation behavior in near-threshold region of L-PBF prepared austenitic stainless steel

Laser powder bed fusion (L-PBF) process produces a specific non-equilibrium microstructure with properties significantly different from those of conventionally processed materials. In the present study, the near-threshold fatigue crack propagation in austenitic stainless steel 304L processed by L-PBF was investigated. Three series of specimens with different orientation of initial notch with respect to build direction were manufactured in order to evaluate the effect of specimen orientation on the near-threshold fatigue crack propagation behavior. The results showed absence of the orientation dependence of the fatigue crack propagation behavior. In addition, abnormally low threshold stress intensity factor values were recorded, which were attributed to the absence of crack closure even at low load ratio (R = 0.1). In order to explain observed behavior, the specimens of selected orientation were heat treated to relieve build-in residual stresses (HT1) and to create recrystallized microstructure (HT2) comparable to conventionally processed (wrought) stainless steels. It was found that the characteristic microstructure produced by L-PBF is the main reason for the absence of crack closure and the low threshold values at low load ratios. As-built microstructure containing sub-micron dislocation cell-substructure is prone to cyclic instability. In the crack tip region, cyclic plasticity results in strain-induced phase transformation and continuous thin martensitic layer is formed in the crack vicinity. The induced martensite phase is softer compared to the austenite matrix strengthened by cell-substructure. Together with cyclic instability of the matrix, the macroscopic cyclic softening occurs as the result within the crack tip region. Under such conditions, the formation of the plasticity-induced and roughness-induced crack closure is significantly reduced and macroscopic resistance to the fatigue crack propagation is low.

Weld Repair Technique for Petroleum Product Pipeline in Extreme Conditions of Prolonged Thermal Cycling

The paper presents the results of experimental research on the development of a repair technique for a pipeline transporting hydrogen mixed with petroleum vapors, which has been in operation for 25 years at Mozyr Oil Refinery, the largest oil enterprise of the Republic of Belarus. The failure of this pipeline was caused by a crack formed in the weld root and its propagation along the fusion line of the heterogeneous weld joint. The developed technique made it possible to prevent further crack propagation without interrupting the refinery fractionation column operation. Welding of the main weld should be performed on a previously deposited damping layer. The damping layer must be deposited by using welding materials, the welding method and the technique that lead to creating a nonoriented dendritic structure. For this purpose, a welding material that has an energy of crack initiation and propagation greater than the energy of crack propagation in the base metal must be used. The composition of the weld must include chemical elements that inhibit the intensity of diffusion processes and increase the strength of the interatomic forces of the austenitic matrix at the pipeline operating temperature.

Microstructural characteristics, mechanical properties and corrosion performance of super austenitic stainless steel 904L produced by wire arc additive manufacturing

This study involves the fabrication of a super austenitic stainless steel 904 L thin wall using wire arc additive manufacturing (WAAM) technology. The microstructure, mechanical properties, and localized corrosion performance of WAAM-processed 904 L were investigated. The average surface roughness varied between 22.43 and 28.51 µm due to the presence of delineations in the wall. Microstructural analysis revealed the presence of equiaxed, columnar, and elongated dendrites along the building direction (BD) of the as-deposited wall. Secondary phases were not noticed in the as-deposited wall due to repeated thermal cycles. Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the uniform dissolution of elements such as Ni, Cr, and Mo within the austenitic matrix. The hardness mapping in WAAM 904 L indicated the variation at the top, middle, and bottom regions and was 220 ± 11 HV, 235 ± 9 HV, and 260 ± 8 HV, respectively. The tensile test outcomes of WAAM 904 L specimens at 0°, 45°, and 90° orientation showed lower anisotropy and observed maximum tensile properties in the horizontal direction (0 °), i.e., YS= 312 ± 12 MPa, UTS= 658 ± 24 MPa & %E = 36 ± 1.5%. The mode of fracture was ductile in nature, with dimples and microvoids. The Potentiodynamic polarization (PDP) tests in 3.5% NaCl solution highlighted the formation of stable passive film on the surface of the as-produced 904 L specimens, and superior resistance to pitting corrosion was noticed. Electrochemical impedance measurements revealed the stability of WAAM specimens and showed a similar trend to PDP outcomes. The degree of sensitization (DOS) of the WAAM 904 L samples was lower, and the corrosion rate ranged between 0.07 and 0.70 mils per year (mpy).

Effect of Mn Content on the Toughness and Plasticity of Hot-Rolled High-Carbon Medium Manganese Steel

The tensile and impact deformation behavior of three different Mn content test steels, xMn-1.0C-0.25V-1.5Cr-0.3Mo (5, 8 and 13 wt%), were investigated using mechanical properties testing, SEM-EBSD and TEM. The elongation and -20 °C impact energy of the three types of Mn content test steels increased as the Mn content increased. The room temperature tensile elongation was 9%, 23% and 81%, and the -20 °C impact energy was 9 J, 99 J and 241 J, respectively. The fracture morphologies of 5 Mn and 8 Mn were found to be cleavage fractures with secondary cracks and micro-voids. The 13 Mn fracture morphology was a plastic fracture with many coarse dimples. Transverse cracks perpendicular to the tensile direction occurred on the surface of the gauge area of 5 Mn and 8 Mn tensile specimens, reducing plasticity dramatically. This was mainly related to the martensitic transformation produced by stress. We characterized the martensite near the tensile fracture and speculated the main mode of crack propagation. Furthermore, a little amount of sharp-shaped BCC phase was found in the 5 Mn, which was determined to be a hard phase relative to the austenite matrix by nanoindentation test. These steels have stacking fault energies ranging from ~15 to ~29 mJ/m2 with increasing Mn content 13 Mn has high stacking fault energy (SFE) and austenite stability. Twin-induced plasticity (TWIP) was the deformation mechanism.

Microstructure, mechanical properties and deformation behavior of Cr-containing triplex low-density steels with different C content

In this work, Fe–20Mn–9Al–3Cr-xC (wt.%, x = 0.8–1.4) steels were designed to investigate the microstructure, mechanical properties and strain hardening behavior of Cr-containing Triplex low-density steel with different C content. Due to the presence of 3 wt% Cr, all the steel strips with different C content consisted of ferrite and austenite matrix with κ-carbide precipitation. With increasing the C content from 0.8 wt% to 1.4 wt%, the ferrite content decreased from 36 vol% to 4 vol%. Simultaneously, the precipitation and growth of κ-carbides in austenite was accelerated significantly. Quantitative analysis of the yield strength indicated that the strength contribution by κ-carbides increased from 36.3 MPa in 0.8C steel strip to 409.4 MPa in 1.4C steel strip, and became the primary strengthening factor when the C content exceeded 1.0 wt%. Fortunately, due to Cr addition, no intergranular κ-carbide was observed even in the 1.4C steel strip, and the steel strip demonstrated a good elongation of more than 33%. During plastic deformation, the slip band-dominated dislocation substructure made the 0.8C steel strip show a three-stage strain hardening, whereas a four-stage strain hardening occurred in the 1.0C, 1.2C and 1.4C steel strips due to the successive formation of slip band and microband. In addition, it was found that the plastic deformation in ferrite was prior to that in austenite, and the formation of ferrite improved the strain hardening capacity of steel strips at the early deformation stage. The findings provided valuable insights into the design of high-performance Triplex low-density steels by composition optimization.

High Ductile Medium Mn Lightweight Alloy: The Role of Intensive Quenching and Deep Cryogenic Treatment

Medium Mn lightweight steels with a relatively higher Mn content of 9–12 wt% have been actively developed recently to meet the demands of crashworthiness and lightweight vehicles. In this study, a combined intensive quenching (IQ) and deep cryogenic treatment (DCT) was first proposed to achieve the microstructural homogeneity as well as the final strength–ductility synergy of medium Mn lightweight steels with Mn segregation bands, together with a comparison with the conventional intercritical annealing. The proposed IQ and DCT process induced the formation of finer large fractioned plate-like martensite in the austenite matrix and thereby contributed to finer and uniform austenite grains after subsequent intercritical annealing. The martensitic transformation rate (dVγ/dε) and transformation kinetics (k value) were used to evaluate the mechanical stability of retained austenite, showing that the D700&750 sample exhibited a similar dVγ/dε value and extremely low k value when compared to the conventional IA650–850 samples, implying that the former had the higher mechanical stability of austenite. The higher mechanical stability of austenite enabled the TRIP effect to occur in a larger strain range, leading to continuous strain hardening behavior. Thus, the highest yield strength (728 MPa) and the largest total elongation of 61.6% were achieved in the D700&750 sample, where the ductility was more than three times higher than that of the conventional IA samples. The grain size and morphologies of retained austenite were believed to be the main factors influencing the strain-hardening behavior of this type of ultrafine lamellar and equiaxed ferrite and austenite duplex structure.

Behavior of a superaustenitic stainless steel weld cladding deposited by the gas metal arc welding process

The performance of a superaustenitic stainless steel weld metal containing 6%Mo deposited by the GMAW process was studied. Multipass welding by the GMAW process with three layers was performed on a carbon steel plate with average heat input of 0.8 kJ/mm. After welding, microstructure characterization by scanning electron microscopy and electron backscattering diffraction, and corrosion performance by cyclic polarization and potentiostatic tests to determine the pitting potential and critical pitting temperature were performed on samples extracted at 3 mm from the fusion line, the minimum thickness required. The weld metal containing 6%Mo showed a microstructure consisting of an austenitic matrix with a volume fraction of Chi and Laves phases inferior to 1% and a PREN value higher than 40 at the dendrite core. As a consequence, similar pitting potential and critical pitting temperature compared to original alloys containing 6%Mo were obtained. Thus, superaustenitic stainless steel weld metals containing 6%Mo have the potential to be suggested as an alternative for weld claddings in the oil and gas industry.

Effects of dispersoid preforming via multistep sintering of oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy

• A newly designed ODS manufacturing process, multi-step sintering process, consisting of a dispersoid preforming heat treatment followed by sintering to achieve the dispersoid refinement in HEA matrix was suggested. • An atypical form of Y 2 O 3 , FCC-Y 2 O 3 , formed adjacent to the dislocation inside the matrix grain during the dispersoid preforming heat treatment of multi-step sintering process with a cube-on-cube orientation relationship with the matrix. • The FCC-Y 2 O 3 formed in the multi-step sintered bulk acts as an effective obstacle to grain boundary and dislocation movement resulting in a microstructure refinement and the increment of mechanical strength. • In the bulk prepared by the conventional ODS manufacturing process, coarsened dispersoids consisting of a complex structured oxide with BCC-Y 2 O 3 as the core were predominantly formed along the grain boundary. • Cr-C-O region with a crystal structure of Cr 23 C 6 formed in the HEA matrix serves as an oxygen-supplying reservoir for dispersoids, promoting coarsening of the dispersoids. Dispersoid formation and microstructural evolution in an oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy (HEA) using a newly designed multistep sintering process are investigated. The proposed multistep sintering consists of a dispersoid preforming heat treatment of as-milled 0.1wt% Y 2 O 3 -CoCrFeMnNi high-entropy alloy powders at 800°C, followed by sintering at 800–1000°C under uniaxial pressure. In the conventional single-step sintered bulk, the coarsened BCC Y 2 O 3 dispersoids mainly form with an incoherent interface with the HEA matrix. In contrast, finer FCC Y 2 O 3 dispersoids, an atypical form of Y 2 O 3 , are formed in the matrix region after multistep sintering. Nucleation of FCC Y 2 O 3 dispersoids is initiated on the favorable facet, the {111} plane of the austenitic matrix, with the formation of a semi-coherent interface with the matrix during the dispersoid preforming heat treatment and it maintains its refined size even after sintering. It is found that dispersoid preforming prior to sintering appears promising to control the finer dispersoid formation and refined grain structure.

Studies on the effects of cryogenic cooling on microstructure and mechanical properties of plasma arc welded SS 316

The stainless steel of grade 316 has significant use in nuclear engineering, aerospace industry and submarines and has become a material of choice due to its diversified properties. While welding this material, the heat input supplied weakens the fusion zone by grain growth, widening the heat-affected zone (HAZ), reducing yield and tensile strengths, and introducing distortion. Therefore, to address these issues, this study investigated the effect of cryogenic cooling during the welding process of thin sheets of SS 316 to improve the microstructure, mechanical properties, and reducing the distortion of the welded material. The keyhole mode plasma arc welding process was used to weld in a single pass without using filler with three different cooling methods. The microstructures, microhardness, and tensile properties of cryogenically and conventionally cooled weld samples were investigated at room temperature. The microstructural behavior of samples was characterized by metallurgical microscopy and scanning electron microscopy. The SEM Analysis reveals γ austenite and δ ferrite phases in conventionally welded test samples. M23C6 is formed in small amounts from δ ferrite, surrounding the δ ferrite on grain boundaries. In cryogenic cooled samples, delta ferrite is detected on grain boundaries of the austenitic matrix. Additionally, traces of (Cr, Fe)2 N are also revealed in specific cryogenic cooled samples due to liquid nitrogen impingement. In gel cooled samples, M23C6 is shown due to comparatively prolonged duration of cooling, and the results reveal that the liquid nitrogen and thermo gel improved average grain size up to 83.53% and 66.84%, respectively, as compared to an average grain size of conventional plasma weld. The reduction in HAZ is observed to be about 43.38% and 7.92% for liquid nitrogen cooled and thermo-gel samples, respectively, compared to conventional weld. Moreover, the tensile and yield strength of liquid nitrogen-cooled weldments increased up to 22.28% and 28.96%, respectively, while for gel-cooled welded sample, a 10.50% improvement in tensile strength and 3.10% in yield strength was observed. Furthermore, a reduction of 75% in distortion is achieved for welded samples with liquid nitrogen cooling.

Alkaline corrosion resistance of slag‐free self‐shielded titanium‐added metal‐cored welding overlay

AbstractThe corrosion resistance of slag‐free self‐shielded metal‐cord welding overlay with different ferrotitanium additions in alkaline solution (pH=14) was investigated. The results show that the self‐corrosion potential Ecorr is the highest and the self‐corrosion current density Icorr is the minimum when the addition of ferrotitanium is 6 wt. %. titanium carbide corrodes before other carbides in alkaline solution due to the absence of iron. A ferrotitanium addition of 6 wt. % can provide the best corrosion resistance to the fabricated welding overlays, as an increased ferrotitanium addition will result in a decrease in chromium carbide and an increase in titanium carbide, of which titanium carbide is the most prone to corrosion, followed by chromium carbide, and finally the austenitic matrix.

Changes in Abrasion Resistance of Cast Cr-Ni Steel as a Result of the Formation of Niobium Carbides in Alloy Matrix

Cast austenitic chromium-nickel steel is commonly used for the manufacture of machine parts and components, which are exposed to the attack of corrosive media and abrasive wear during operation. The most commonly used grades include GX2CrNi18-9 and X10CrNi18-8 as well as GX2CrNiMo17-12-2 and X6CrNiMoNb17-12-2. To improve the abrasion resistance of cast chromium-nickel steel, primary niobium carbides were produced in the metallurgical process by increasing the carbon content and adding Fe-Nb. The microstructure of the obtained test castings consisted of an austenitic matrix and primary niobium carbides evenly distributed in this matrix. The measured hardness of the samples after heat treatment ranged from 215 to 240 HV and was higher by about 60 units than the hardness of the reference cast GX10CrNi18-9 steel, which had a hardness of about 180 HV. Compared to the reference cast steel, the abrasive wear resistance of the tested cast chromium-nickel steel (measured in Miller test) with contents of 4.4 and 5.4 wt% Nb increased only slightly, i.e., by 5% for the lower niobium content and 11% for the higher niobium content. Compared to ordinary cast GX10CrNi18-9 steel, the addition of 9.2 wt% Nb reduced the abrasive wear by almost 2.5 times.

Microstructural Evolution and Mechanical Performance of Two Joints of Medium-Mn Stainless Steel with Low- and High-Alloyed Steels

In this work, 2 mm thick medium-Mn austenitic stainless steel (MMn-SS) plates were joined with austenitic NiCr stainless steel (NiCr-SS) and low-carbon steel (LCS) using the gas tungsten arc welding technique. A precise adjustment of the welding process parameters was conducted to achieve high-quality dissimilar joints of MMn-SS with NiCr-SS and LCS. The microstructural evolution was studied using laser scanning confocal and electron microscopes. Secondary electron imaging and electron backscatter diffraction (EBSD) techniques were intensively employed to analyze the fine features of the weld structures. The mechanical properties of the joints were evaluated by uniaxial tensile tests and micro-indentation hardness (HIT). The microstructure of the fusion zone (FZ) in the MMn-SS joints exhibited an austenitic matrix with a small fraction of δ-ferrite, ~6%. The tensile strength (TS) of the MMn-SS/NiCr-SS joint is significantly higher than that of the MMn-SS/LCS joint. For instance, the TSs of MMn-SS joints with NiCr-SS and LCS are 610 and 340 MPa, respectively. The tensile properties of MMn-SS/LCS joints are similar to those of BM LCS, since the deformation behavior and shape of the tensile flow curve for that joint are comparable with the flow curve of LCS. The HIT measurements show that the MMn-SS/NiCr-SS joint is significantly stronger than the MMn-SS/LCS joint since the HIT values are 2.18 and 1.85 GPa, respectively.

Investigation of microstructural and corrosion behavior of 316LSi structure developed by wire arc additive manufacturing

SS316LSi austenitic stainless steel thin wall was deposited using wire arc additive manufacturing technology, which is known for its high deposition rate and effective material utilization. The optical micrograph revealed the formation of vermicular δ-ferrite, dispersed in the austenite matrix in the deposited wall. The X-ray diffraction patterns confirmed the formation of γ-austenite, δ-ferrite, and chromium carbide (Cr23C6) phases. The SEM and energy dispersive spectroscopy study exhibited the formation of intergranular precipitates. In the magnetic study, ferrite number was found from 3 to 7 in a different portion of the wall which revealed the formation of the ferromagnetic δ-ferrite phase. The Cr and Ni equivalent ratio, 1.66, ferrite number, and presence of vermicular δ-ferrite in the microstructure showed that ferrite austenite can be the mode of solidification in the alloy. The preferred grain growth direction was determined by electron backscattered diffraction analysis to be < 001 > orientation along the build-up direction. In the potentiodynamic polarization and double loop electrochemical potentiodynamic reactivation study, the corrosion rate and intergranular corrosion were correlated to the amount of δ-ferrite and Cr23C6 respectively. The degree of sensitization confirmed that the amount of Cr23C6 formed is least in the lowermost and highest in the central portion of the deposited wall.

Aspects of Bending High-Borated Austenitic Stainless Steel Sheets for Interim Storage of Spent Nuclear Fuel

The presented work is aimed at the evaluation of the cold bending capacity of high-borated austenitic stainless steel sheets. Due to their excellent neutron-absorbing capability, borated stainless steels belong to the group of tailor-made structural materials widely used for vertical storage baskets for holding spent nuclear fuel assemblies in cooling pools at dry or wet storage facilities. The basket consists of individual fuel assembly cells. Each polygonal cell is usually welded from several steel strips. It would be advantageous to use bent steel semi-product to avoid welded seams as much as possible. Welded seams are difficult to make, and moreover, they are susceptible to corrosion. However, high-borated stainless steels, because of their boron content, show limited hot and cold workability. Thus, their cold bending capacity would be the primary issue. Their austenitic matrix with embedded hard and brittle boride particles is prone to the evolution and fast propagation of dimple transgranular fracture. This work is focused on the bending aspects of borated steel sheets with respect to the most commonly used hexagonal cell geometry. Experimental results provide practical recommendations for the rack design. The damage criterion has also been proposed using FEM simulations in DEFORM®.

Effect of Fluoride Content of Mouthwashes on the Metallic Ion Release in Different Orthodontics Archwires.

Metal ion release studies were carried out on three of the most commonly used orthodontic wires in the clinic: austenitic stainless steel, Ti-Mo, and superelastic NiTi, using three mouthwashes with different fluoride concentrations: 130, 200, and 380 ppm. Immersions were carried out in these mouthwashes at 37 °C for 1, 4, 7, and 14 days, and the ions released were determined by inductively coupled plasma-mass spectrometry (ICP-MS). All wires were observed by scanning electron microscopy (SEM). The results showed a moderate ion release in the stainless steel wires, with nickel and chromium values of 500 and 1000 ppb in the worst conditions for the wires: concentrations of 380 ppm fluoride and 14 days of immersion. However, in the Ti-Mo and NiTi alloys, an abrupt change in release was observed when the samples were immersed in 380 ppm fluoride concentrations. Titanium releases in Ti-Mo wires reached 200,000 ppb, creating numerous pits on the surface. Under the same conditions, the release of Ni and Ti ions from the superelastic wires also exceeded 220,000 ppb and 180,000 ppb, respectively. This release of ions causes variations in the chemical composition of the wires, causing the appearance of martensite plates in the austenitic matrix after 4 days of immersion. This fact causes it to lose its superelastic properties at a temperature of 37 °C. In the case of immersion in 380 ppm mouthwashes for more than 7 days, rich-nickel precipitates can be seen. These embrittle the wire and lose all tooth-correcting properties. It should be noted that the release of Ni ions can cause hypersensitivity in patients, particularly women. The results indicate that the use of mouthwashes with a high content of fluoride should not be recommended with orthodontic archwires.