Cr Ferritic Steel Research Articles

Effect of Aggressive Anions on the Stress Corrosion Cracking of Stainless Steel in Hot Pressurized Alkaline Water

Abstract U-bend stress corrosion cracking (SCC) testing of low Cr ferritic (Type 409) and austenitic (Type 304L) stainless steel was conducted in a hot pressured alkaline water to study the effect of aggressive anions (Cl− and HS−) on the relative susceptibility. SCC was only observed in Type 304L when immersed in the solution that contained both aggressive anions. Critical factors were identified based on cross-section examination of the U-bends after exposure using complementary electron microscopy techniques. These factors include: (i) preferential oxidation of deformation bands (arising from cold working), (ii) Ni-S compound formation at the oxide/metal interface and (iii) S and Cl incorporation into the inward growing Cr-rich oxide. These critical factors were considered within an overall slip dissolution-type mechanism to account for the SCC observed.

The Influence of the Hot-Rolling Temperature on the Microstructure and Mechanical Properties of Ti-Nb Microalloyed 21%Cr Ferritic Stainless Steel

Microalloying and heat treatment are essential processing techniques for ferritic stainless steel (FSS). Three different compositions of 21%Cr FSS with 0.28Ti, 0.21Ti + 0.05Nb, and 1.05Ti + 0.17Nb were prepared. The interaction effects of the Nb and Ti contents and hot-rolling annealing on the microstructure, mechanical properties, and precipitate phases of FSS were studied. The microstructure, crystal structure, and precipitation phase of steel at 930, 980, and 1030 °C with Ti-Nb microalloying were investigated using an optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM). The room-temperature tensile properties, surface roughness, and hardness were tested separately. This study found that the composite addition of Ti and Nb had a dual effect of fine-grain strengthening and precipitation strengthening. The 1.05Ti + 0.17Nb steel specimen had a moderate grain size and the best uniformity after hot rolling at 980 °C. The tensile strength and elongation were 454 MPa and 34.2%, which achieved an optimal balance between strength and plasticity.

Identification of Precipitate Phases in an 11 Pct Cr Ferritic and Martensitic Steel after Thermomechanical Treatment

Identification of Precipitate Phases in an 11 Pct Cr Ferritic and Martensitic Steel after Thermomechanical Treatment

Effect of Carburizing Atmosphere on High-Temperature Oxidation Resistance of 17％ Cr Ferritic Stainless Steel Doped with Cu and Ni

Effect of Carburizing Atmosphere on High-Temperature Oxidation Resistance of 17％ Cr Ferritic Stainless Steel Doped with Cu and Ni

Development of creep resistant high yttria 18Cr ferritic ODS steel through hot powder forging route

The present work reports the creep behaviour of a high yttria Fe-18Cr-2W-0.57Ti-1Y2O3 (all compositions in wt.%) ferritic oxide dispersion strengthened (ODS) steel produced by powder forging. The creep tests were carried out in a temperature range of 873 K to1023K and a stress range of 150 MPa to 425 MPa. Creep data is analysed using the power law creep equation. A large value of stress exponents (n ∼ 10 at 873 K, ∼ 18 at 923 K and 8.5 at 973 K) and a high activation energy (Q ∼581 kJ/mol) are obtained. These high values are attributed to threshold stress (σth) generated due to attractive interaction between dislocation and oxide particles. Threshold stress for different creep conditions was determined. True stress exponent values of 4 to 5 were obtained after accounting for the threshold stress. Electron back scattered diffraction (EBSD) revealed an equiaxed strain free grain structure in the powder forged ODS steel. Transmission electron microscopy (TEM) of creep tested samples indicates that microstructure remains stable after creep with no significant coarsening of nano size oxide particles. Dislocation pinning by the oxide particles was observed in creep tested samples. The observed values of true stress exponent suggest dislocation climb over the nano oxide particles to be the creep mechanism operating in the ODS steel. Samples sections close to creep fracture surfaces were investigated by EBSD. Void formation at triple junctions followed by crack propagation normal to the loading direction was observed. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. Conventional ODS steel compositions contain up to 0.35% yttria. A significant improvement in creep life as well as an increase in activation energy for creep was observed because of high (1%) yttria content used in the ODS steel studied.

Three-step learning strategy for designing 15Cr ferritic steels with enhanced strength and plasticity at elevated temperature

15Cr ferrite steels are urgently required in advanced Ultra-supercritical power plants but meet design challenges in balancing excellent strength and plasticity at high temperatures. We developed a three-step learning strategy based on mutually driven machine learning and purposeful experiments to complete this multi-objective task. Compared with traditional adaptive learning and local-interpolation learning, this step-by-step modular manner provides good transparency and interpretability of the information flow, which is ensured by identifying essential factors from an exquisitely prepared composition-microstructure dataset, and learning valuable knowledge about the composition-property relationship. The requirement of only two groups of experiments indicates the low cost and high efficiency of the strategy. Performing the strategy, we found that Ti is another key element affecting the Laves phase besides Mo and W, and their effects on ultimate tensile strength (UTS) and elongation were also uncovered. Importantly, several low-cost steels free of Co were successfully designed, and the best steel exhibited 156%, 31%, and 62% higher UTS and elongation at 650 °C than the typical 9Cr, 15Cr, and 20Cr steels, respectively. Based on the advantages and success of the strategy in terms of alloy improvement, we believe the strategy suits other multi-objective design tasks in more materials systems.

Microstructural, textural, and residual stress evolution alongside the magnetic properties through isothermal static recrystallization of cold-drawn Fe–Cr–Si–S–C ferritic high-alloy stainless steel and by JMAK modelling

The evolution of microstructures, textures, and residual stresses scale of cold-drawn (CD) 18%Cr ferritic stainless steels (FSS) round bars under isothermal static recrystallization annealing are investigated. As for determination of the new grains' formation, the size, growth, and orientation, as well as the nucleation of grains, alongside revealing the recrystallization fractions are studied by a mixed experimental and theoretical work. The recrystallization kinetics are calculated according to the Johnson-Mehl-Avrami-Kolmogorov theory. Electron backscatter diffraction (EBSD) analyses are applied for the actual specimens of an industrial wire drawing process with 20 and 35% reduction rates. The heat treatment has been implemented at three different set ratio temperatures of 0.65, 0.68, and 0.71, defining as a ratio between the temperatures of annealing and melting point (TRAM). According to the results, the nucleation is occurred at faster incubation time with higher temperature and reduction rate. The grain average size (AGS) increases gradually whereas the temperature increases. This is while the 0.71TRAM-scale annealed samples illustrate finer and more equiaxed recrystallized grains. Following recovery, recrystallized grains, and the grain growth, the overall varied AGS of 27–86 μm in the core region are achieved. Moreover, the JMAK model agrees well with the obtained experimental data. All in all, this 18Cr-FSS material is taken into account for the optimal industrial annealing production as well as the targeted material selection of the market applications. It is observed that the higher recrystallization leads the higher magnetic permeability, lower coercivity, and lower residual stresses compared to the unheated specimens.

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.

Role of the alloy grain boundaries in the high-temperature oxidation and Cr volatilization of 22 wt% Cr ferritic stainless steel for SOFC applications

The effects of alloy grain size of high-Cr ferritic stainless steel were investigated using a sheet (t = 2 mm) and plate (t = 22 mm) of Cr ferritic stainless steel. The average grain size of each sample was measured as ∼ 25 µm and ∼ 500 µm, respectively. The large-grained plate sample exhibited a 41% oxidation rate reduction, and a slight decrease in Cr volatilization was observed in the small-grained sheet sample. A numerical examination revealed that inhomogeneous GB-related oxidation is a major cause of the different oxidation kinetics affected by metal grain size.

Creep-ratcheting effect and lifetime prediction of advanced 9-12% Cr ferritic steel at 600 ℃

The creep-ratcheting behavior of advanced 9–12 % Cr ferritic steel at 600 °C under peak holding time of 0 min, 2 min, 5 min, 10 min, 20 min and 30 min were investigated and a creep-ratcheting constitutive model was developed. Results present that the anelastic strain during the whole creep-fatigue life for various holding time approximately keep constant. However, it decreases rapidly during the last several cycles. Moreover, the primary creep regeneration (PCR) behavior during repeated loading and unloading process was observed. It keeps almost the same with increasing number of cycles and changes slightly for different peak holding time under the same stress amplitude and stress ratio. Additionally, a modified plasticity-creep superposition model was established considering the combination effects of creep damage, ratcheting evolution, anelastic recovery and PCR. The comparison between tested and predicted data shows that the proposed model has high accuracy for the prediction of both creep-ratcheting strain and creep-fatigue lifetime. It is of interest that the proposed model can also well simulate the evolution of stress–strain during the whole lifetime, especially for the reproduction of anelastic creep recovery and PCR along the load history.

Effect of solute Ni on thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars

The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 μm were fabricated on the surface of Fe–18%Cr–Ni alloy sheets containing different contents of Ni (0–2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 μm, the Fe–18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 μm. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe–18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions.

Correlation of deformation with damage progression behavior around a notch tip under creep and fatigue conditions for W-added 9Cr steel including weld joint

Research concerning heat-resistant steels for the application in fossil-fired power plants has progressed remarkably during the past 60 years. This has resulted in improvements in the electrical efficiency of fossil-fired power plants. Currently, there are plans and programs to develop ultra-supercritical plants designed to operate at steam temperature and pressure conditions of 600/650 °C and 32 MPa. The W-added 9%Cr ferritic heat-resistant steel, that is, ASME grade P92, has been developed as a boiler material for this ultra-supercritical plant. Boiler materials, whose performance is critical for ultra-supercritical plant, are required to possess high creep resistant properties. In addition, these materials are exposed to fatigue induced by thermal stresses, that is, they are operated under creep-fatigue interacting conditions. In this study, mechanical tests under the condition of high temperature creep-fatigue interaction were conducted for P92 steel under stress-controlled and various load frequency conditions using the in-situ observational creep-fatigue testing machine to observe the damage formation behavior around a notch tip composed of voids in mesoscale. On the basis of these results, the effects of damage formation behavior on crack growth life were clarified. Furthermore, for the case of creep deformation, the numerical analyses of vacancy diffusion and concentration around a notch tip were conducted using our proposed numerical method of local stress-induced vacancy diffusion behavior, which is a nanoscale phenomenon to relate these behaviors to the damage formation behavior in mesoscale (μm scale).

Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels

Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels

Role of Alloy Grain Boundaries in the High-Temperature Oxidation and Cr Volatilization of 22 Wt.% Cr Ferritic Stainless Steel for Sofc Applications

Role of Alloy Grain Boundaries in the High-Temperature Oxidation and Cr Volatilization of 22 Wt.% Cr Ferritic Stainless Steel for Sofc Applications

Tribocorrosion in Ferritic Stainless Steels: an Improved Methodological Approach

This study aimed to improve a potentiodynamic-based methodological approach for characterising the tribocorrosion of ferritic stainless steel. Synergistic effects in tribocorrosion systems have been widely investigated and debated under potentiostatic conditions. Because potentiostatic tests can ignore essential phenomena that are typically noticeable in potentiodynamic tests (potential scanning), such as variations in the friction coefficient that are motivated by potential scanning, e.g., during active, passive, and transpassive domains, a potentiodynamic-technique-based methodology was enhanced to analyse tribocorrosion in ferritic stainless steels. The proposed method facilitates the analysis of all essential parameters for tribocorrosion phenomena based on a single figure directly associating the average friction coefficient curves from sliding tests and tribocorrosion tests. It consists of performing sliding tribological tests to evaluate mechanical wear accurately, potentiodynamic corrosion tests to determine corrosion resistance in the absence of mechanical wear, and tribocorrosion tests to associate mechanical wear and corrosion degradation caused by chemical/electrochemical effects. Validation of the methodology consisted of its application to an 11% Cr ferritic stainless steel. The results demonstrate that tribocorrosion intensifies the material loss, but only by a minimal amount. The results demonstrate that the average passivation current density in the tribocorrosive tests was significantly higher. Apparently, the corrosion products generated during the tribocorrosion tests acted as solid lubricants inducing a lower friction coefficient in the tribocorrosive tests. Finally, tribocorrosion intensifies the material loss, but only by a minimal amount.

Fabrication routes for advanced first wall design alternatives

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 °C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets.

Time-lapse observation of pitting corrosion in ferritic stainless steel under bipolar electrochemistry control

A bipolar electrochemistry approach has been applied to characterise pit nucleation and growth kinetics in 13% Cr ferritic stainless steel. In-situ time-lapse imaging, supported by laser confocal microscopy measurements, were applied to obtain pit nucleation frequencies and associated pit growth characteristics. The development of champion pits is observed, and the effect of pit shape and aspect ratio on pit nucleation and growth are discussed. Pits preferentially nucleated and grew with depth to width aspect ratios of 0.4–0.6. The observed pit growth kinetics are independent of pit volume and the acting potentials.

Evolution of the Non-metallic Inclusions Influenced by Slag-Metal Reactions in Ti-Containing Ferritic Stainless Steel

Laboratory experiment and thermodynamic calculation for the Ti-containing 24 mass pct Cr ferritic stainless steel with a CaO-SiO2-Al2O3-MgO system slag were performed to investigate the effect of slag addition on the inclusion characteristics in molten steel. The morphology, composition, and size evolution of inclusions in steel samples were analyzed in three-dimensional by the electrolytic extraction method and in two-dimensional by the automatic analysis method. The results showed that the Ti content significantly decreased after the slag addition. However, the change of the Si content showed an opposite tendency. The decrease of the Ti content in steel was due to the reduction of SiO2 and Al2O3 in the slag by dissolved Ti in steel. An increase of the TiO2 content in the slag can decrease the Ti loss in steel based on the slag-steel kinetic analysis. The total O content in the steel melt decreased from 62 to 26 ppm, and the steel cleanliness was improved, since the number density of inclusions decreased after the slag refining. The results of a kinetic analysis showed that the rate-determining step of the oxidation of Ti in the steel and the reduction of SiO2 in the slag were the mass transfer on the slag side. In addition, high Ti2O3-containing inclusions were found to be transformed to Cr2O3-Ti2O3-Al2O3 and Cr2O3-Ti2O3-SiO2 system inclusions after the slag addition. The Al2O3 contents in inclusions increased while the Ti2O3 contents decreased with time. However, there were some amount of high melting point inclusions with high Al2O3 content, which were not what we expected. When plotted on logarcxithmic scales, the mole ratio X_{{{text{Al}}_{2} {text{O}}_{3} }} /(X_{{{text{Ti}}_2 {text{O}}_{3} }} cdot X_{{{text{Cr}}_{2} {text{O}}_{3} }} ) values of the inclusions were expressed as a linear function of the a_{text{Al}}^{2} /(a_{Ti}^{2} cdot a_{text{Cr}}^{2} cdot a_{text{O}}^{3} ) values of the steel melts with a slope of unity, which was theoretically expected.

Effect of added stabilizing elements on thermal activation process of plastic deformation in 18Cr ferritic stainless steel

The effects of a trace amount (~0.1 at.%) of added stabilizing elements (Nb, Ti, and Zr) on the tensile properties of 18Cr ferritic stainless steel sheets at strain rates ranging from 10−3–102 s−1 (from ordinary quasi-static deformation to dynamic deformation) were investigated. The additional Ti or Zr elements formed coarse carbide or nitride precipitates, which consumed interstitial solute elements (C and/or N) in the ferrite matrix. In the Nb-added steel, however, only fine NbN precipitates with a few hundred nanometers in size were observed locally, implying a small amount of solute Nb in the ferrite matrix. Ti or Zr additions had only a slight effect on the deformation resistance of 18Cr ferritic steel sheets and strain rate sensitivity (m) of the flow stress, however, the Nb addition increased the deformation resistance and significantly reduced the strain rate sensitivity. The measured activation volume (v*) values of the Ti- and Zr-added steels (62 b3 and 66 b3) were almost equivalent to one of the 18Cr base steel (64 b3). This result suggested that a general thermally activated process for screw dislocation motion is controlled by double-kink nucleation, which is slightly affected by Ti or Zr additions. The high v* value (106 b3) of the Nb-added steel indicated that the thermally activated process of plastic deformation would be associated with the substitutional solute Nb atoms (act as short-range obstacles) interacting with the moving dislocations. These results showed that the Nb element played a distinct role in plastic deformation from the other stabilizing elements of Ti and Zr.

High Temperature, High Cycle Fatigue Demeanor of 10wt% cr Steel and Unlike Metal Weld

The present study primarily focuses on understanding the high cycle fatigue behavior (HCF) of alloy 10wt%Cr Ferritic Steel and Dissimilar metal weld (DMW) joint between 617M and 10wt%Cr Ferritic Steel. For assessing the HCF behavior, tests were conducted under stress controlled cycling, by employing dissimilar stress ratios (R) and wide range of temperatures (300 K – 853 K). The S-N curves plotted at R= -1 and temperatures (300 K,673 K,853 K) for 10wt%Cr Ferritic Steel shows that fatigue life decreased with increase in stress amplitude. It is also observed that fatigue life of 10wt%Cr Ferritic Steel falls with increase in the temperature regardless of the stress amplitude, clearly showing the strong dependence of fatigue life on the temperature. An effort has been made to find out the fatigue parameters at 300 K and 853 K using Basquin equation. These fatigue parameters were used for life prediction, showed that predicted life is in good agreement with experimental life with in a scatter band of 2. At 853 K, Goodman diagram shows that limiting alternating stress decreases with increase in the mean stress. The results were linked with the detailed scanning electron microscope investigation where it is analyzed that at 300 K, the fatigue failure was by trans-granular mode, characterized by striations while at 673 K and 853 K, intergranular mode and strong oxidation is seen, thus lowering the life at said temperatures. The standard S-N behavior for DMW at R= -1 and at temperature of 853 K showed that the welding reduces the number of cycles to failure. Vicker’s hardness measurements show that there is softening in the 10wt%Cr side & hardening in the butter layer resulting in failure of all non-defective samples on the 10wt%Cr side. HCF test was also showed on damaged samples at 230 MPa and 200 MPa, found, that crack initiates & propagates near the damage at 230 MPa while at 200MPa crack initiates and propagates in the 10wt%Cr side irrespective of the damage.