High Chromium Ferritic Steels Research Articles

On the Possibility of Improving the Oxidation Resistance of High-Chromium Ferritic Stainless Steel Using Reactive Element Oxide Nanoparticles

High-chromium ferritic steels are current the only viable candidates for cheap interconnect materials for application in high-temperature solid oxide fuel and electrolyzer cells (HT-SOFCs/SOECs). The durability and operating characteristics of interconnects manufactured using these materials may be improved significantly by applying a protective-conducting MoCo2O4 coating and depositing an intermediate layer consisting of nanoparticles of Gd2O3—a reactive element oxide—on the surface of the steel substrate. The study demonstrated that the conditions of the thermal treatment of this layered system determine the efficacy of the applied modification with the reactive element. The persistence of this effect was tested over 7000 hours of quasi-isothermal oxidation in air at 800 °C.

Corrosion Resistance of AISI 442 and AISI 446 Ferritic Stainless Steels as a Support for PEMWE Bipolar Plates

Cost reduction in bipolar plates in proton exchange membrane water electrolyzers has previously been attempted by substituting bulk titanium with austenitic stainless steels protected with highly conductive and corrosion-resistant coatings. However, austenitic steels are more expensive than ferritic steels due to their high nickel content. Herein we report on the corrosion resistance of two high chromium ferritic stainless steels, AISI 442 and AISI 446, as an alternative material to manufacture bipolar plates. Electrochemical corrosion tests have shown that AISI 442 and AISI 446 have similar corrosion resistance, while AISI 446 reveals more noble corrosion potential and performs better during potentiostatic stress tests. The current density obtained during polarization at 2 V versus the standard hydrogen electrode (SHE) is 3.3 mA cm-2, which is more than two times lower than on AISI 442. Additionally, surface morphology characterization demonstrates that in contrast to AISI 442, AISI 446 is not sensitive to intercrystalline or pitting corrosion. Moreover, EDX energy dispersion analysis of AISI 446 reveals no differences in the chemical composition of the surface layer compared to the base material, as a confirmation of its high corrosion resistance. The results of this work open up the perspective of replacing austenitic stainless steels with less expensive ferritic stainless steels for the production of components such as bipolar plates in proton exchange membrane water electrolyzers.

Laves Phase Precipitation Behavior in HiperFer (High Performance Ferritic) Steel with and without Boron Alloying

High-chromium ferritic stainless HiperFer steels were developed for high-temperature applications in power conversion equipment. The presented research describes the precipitation behavior of the Laves phase after the thermomechanical treatment of Fe-17Cr-0.6Nb-2.4W HiperFer alloys with and without the addition of 55 ppm boron. The boron-alloyed variant was produced with the aim of enhancing grain boundary strengthening and consequently increasing creep resistance. The focus is set on the effect of boron on the thermomechanically induced precipitation of (Fe,Cr,Si)2(Nb,W) Laves phase at grain boundaries. The addition of boron modifies the diffusion conditions in the area of grain boundaries. Consequently, the formation of Laves phase is promoted and the particle growth and coarsening process are suppressed. The impact of boron addition was validated by performing creep and thermomechanical fatigue testing in the standard processing state of HiperFer steel. In the B-alloyed variant, increased creep ductility through the modification of the particle-free zone widths at high-angle grain boundaries was encountered. Nevertheless, an optimized thermomechanical treatment is necessary to fully utilize the increased ductility effect for the creep strength optimization of the B-alloyed grade.

NDT study of precipitation processes in thermally aged Fe-20Cr alloy

Thermal embrittlement of high-chromium ferritic and martensitic steels at intermediate temperatures has been investigated in PM2000 (Fe20Cr) steel exposed to 475 °C by various non-destructive techniques, including magnetic Barkhausen noise (MBN), positron annihilation spectroscopy (PAS) and Mössbauer spectroscopy (MS). These techniques were used in a complementary manner to characterize the phase separation of the Fe-Cr ferrite phase into Fe-rich (α) and Cr-rich ferrite (α') phases, the phenomenon known as 475 °C embrittlement. Together with a reference steel T91, samples were subsequently isothermally annealed up to 750 h. While the MS technique clearly confirmed the formation of bcc Cr-rich precipitates in PM2000 steel with annealing time, the MBN and PAS techniques only indicated the formation of new pinning sites for magnetic domains and trapping sites for positrons in PM2000, up to 500 h annealing time, respectively. This observation was attributed to the precipitation process of Al-rich oxides in the oxide-dispersion strengthened PM2000 steel. This process, competitive to the chromium-rich α' phase precipitation, was found to be dominating the micromagnetic and positron-annihilation characteristics during the early stages of the annealing.

Comparison of electromagnetic inspection methods for creep-degraded high chromium ferritic steels

Nondestructive testing (NDT) techniques are used to evaluate the material degradation of ferromagnetic materials, for example, in sensitive environments, such as thermal power plants, where the materials are subjected to creep damage. There is no consensus on the use of an electromagnetic NDT technique to characterize the evolution of creep damage in high-chromium ferritic steels. In this work, an overview and comparison of three different electromagnetic NDT techniques that were applied to high-chromium steels is provided to understand creep evolution in terms of microstructural changes, such as precipitation, dislocation and grain size. To quantify the empirical measurements, a modelling technique was proposed for each applied method. The model parameters were optimized for each NDT technique and tested material. Depending on the model parameters, the accuracy of the parameter determination depends strongly on the NDT technique, which indicates its correlation with the microstructural information.

Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

Impact of Tungsten on Thermomechanically Induced Precipitation of Laves Phase in High Performance Ferritic (HiperFer) Stainless Steels

High-chromium ferritic stainless steels strengthened by Laves phase precipitates were developed for a high-temperature application in steam power plants. The impact of tungsten content on the precipitation of the intermetallic Laves phase during the newly developed thermomechanical process route was investigated. Due to rapid thermomechanically induced precipitation, a considerable reduction in processing time in comparison to the conventional solely thermal two-step processing of high chromium ferritic steels was achieved. Nevertheless, comparable mechanical properties at room temperature, i.e., the ultimate tensile strength of 712 MPa and the yield strength of 434 MPa, were obtained. The microstructure was analyzed by scanning electron microscopy (SEM) in combination with digital particle analysis, to estimate the particle size and the phase fraction of the Laves phase. The mean particle size of 52 nm and the volume fraction of 4.11% were achieved. Due to the tungsten content, an increase in the volume fraction and particle size was observed, giving rise to the higher strengthening effect.

Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar–H2–H2O

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H2–4%H2O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr2O4 spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O2 oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

3D multiphysics modeling aided APU development for vehicle applications: A thermo-structural investigation

Solid oxide fuel cells (SOFC) are suitable for on-board electricity generation as Auxiliary Power Unit (APU) to support the electric power supply in heavy-duty vehicles. In order to satisfy the requirements of a lightweight fuel cell stack for mobile applications, thin-walled components must be used for the stack structure. This necessity is associated with material, process and design difficulties that must be solved in order to achieve a successful utilization. In this work a novel lightweight SOFC stack design with metal-supported cell was studied both numerically and experimentally. The metallic components are made from the Intermediate Temperature Metal (ITM), a high performance, high chromium ferritic stainless steels alloy. The multiphysics modeling approach (fluid dynamics, heat transfer, structural mechanics) was utilized in this work to predict the temperature distribution and the thermo-structural behavior of the new developed design. Geometric details of the fuel cell stack components as well as appropriate nonlinear, temperature and time-dependent constitutive models were developed to describe the material behavior. Experimental data were used to determine the material model parameters and validated the simulation results. The three-dimensional stress and deformation distributions in the individual stack components were evaluated and their maximum values for elements at risk were identified. Thus, the developed model enables the investigation of sustainability and serviceability of the structural elements to ensure a reliable operation of the stack. The developed computational model can be used as a design tool for parametric studies and optimization analysis to investigate the effects of process boundary conditions, material properties as well as geometrical design parameters and their variation on the induced thermal stresses.

Effect of Prehot Corrosion on Erosion Behavior of High Chromium Ferritic Steel for Heat Exchangers

This study presents the prehot corrosion effect on erosion behavior of AISI 446 SS in simulated heat exchanger environment at elevated temperature. Samples were spray deposited using two salt mixture (Na2SO4/NaCl). Subsequently, low-temperature hot corrosion tests were carried out at 550, 650, and 750 °C for 20 h. Chlorination followed by sulfidation was mainly responsible for the passive layer formation during the process of hot corrosion. The prehot corroded samples were subjected to air-jet erosion test using alumina as the erodent, at impact velocity of 100 m/s and flux rate of 4.2 g/min, with variable impingement angles of 30 deg, 60 deg, and 90 deg. The passive layer formed during corrosion underwent detachment of metallic flakes through cracking during the impact of erodent, and was responsible for a significant change in erosion rate. Cutting, plowing, lip formation, and particle embedment were identified as the operative mechanisms during erosion.

Small Punch Creep Life Prediction from Steady-State Deflection Rate in High-Chromium Ferritic Heat-Resistant Steels

The small punch (SP) creep deformation and rupture behavior of different high-chromium ferritic heat-resistant steels were studied at various temperatures in the range from 923 K to 1073 K (650 °C to 800 °C) and loads in the range from 65 to 400 N. The creep deflection rate, after an initial abrupt decrease, showed no significant variation during most of the creep time and exhibited a steady-state creep stage. The time to steady-state creep stage accounted for less than 30 pct of the creep rupture life. A close relationship was found to exist between steady-state deflection rate and time to rupture, resembling Monkman–Grant relation. This relationship was independent of the type of steels studied, resulting in a single master curve. This relationship was used for predicting SP creep rupture life. The predicted rupture life was within an accuracy factor of 1.2. The study revealed that the creep rupture life can be reasonably well predicted by evaluating steady-state deflection rate, without waiting for the fracture of specimen to occur. This approach of prediction using steady-state deflection rate was found to be more advantageous than that using minimum deflection rate as the latter appeared relatively close to the onset of acceleration creep.

Microstructure, mechanical properties and strengthening mechanisms of friction stir welded Kanthal APMT™ steel

Kanthal APMT™ steel (Fe-22Cr-5Al-3Mo) is being investigated for possible use as an accident-tolerant fuel cladding material in advanced light water reactors. Generally, high-chromium ferritic steels do not have good weldability because of a variety of metallurgical issues. In the present study, friction stir welding (FSW), a solid-state joining process, was applied to a Kanthal APMT™ plate in a bead-on-plate configuration using a tool rotation rate of 600 RPM and a traverse speed of 25.4 mm/min. Microstructure and mechanical properties were evaluated to determine the weld quality and examine feasibility of applying FSW as a joining technique for this steel. Microstructural characteristics were examined by optical microscopy and electron microscopy. The stir zone (SZ) contained equiaxed grain structure with an average grain size of 13.7 μm. Interestingly, Vickers microhardness profile across the processed zone revealed no significant change in microhardness. Mini-tensile testing, however, showed a marked improvement in mechanical properties of the SZ in the longitudinal direction compared to the base metal. The results are discussed in the light of fundamental strengthening mechanisms.

Development of High-Chromium Ferritic Heat-Resistant Steels with High Nitrogen Content

Development of High-Chromium Ferritic Heat-Resistant Steels with High Nitrogen Content

Temperature dependence of phase composition in W and Si-alloyed high chromium ferritic steels for SOFC interconnect applications

Tungsten alloyed high-chromium ferritic steels such as the commercially available alloy Crofer 22 H have been proposed as construction materials for interconnects in Solid Oxide Fuel Cells. The background of the present study relates to the qualification of such alloys, especially with respect to a possible optimization of the tungsten concentration, aiming at formation of the strengthening intermetallic phases. For this purpose the chemical composition of intermetallic phases in a number of Fe-Cr-W-base model alloys after exposure at temperatures between 600 °C and 900 °C was measured by SEM/EDX and TEM/EDX. The obtained chemical and phase compositions were used for estimation of the iron-rich corner of the Fe-Cr-W system in the temperature range 600–900 °C. Finally, the experimental results were compared with values calculated using the Thermocalc software and the database TCFE 7. This comparison showed that in the temperature range 800 °C–900 °C the calculations gave a qualitatively correct description of phases present in the microstructure, however, substantial differences between calculations and experiments existed in the temperature range 600 °C–700 °C. Moreover, it was found that the solubility of silicon in the C14 Laves phase of the type Fe2W is substantially smaller than that in the Fe2Nb based laves phase present in Crofer 22 H.

高窒素添加した高Crフェライト系耐熱鋼の開発

高窒素添加した高Crフェライト系耐熱鋼の開発

EBSD and electron channeling study of anomalous slip in oligocrystals of high chromium ferritic stainless steel

In the present study, electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques were applied to investigate the deformation pattern of coarse ferrite grains after being subjected to 3%, 6%, and 10% tensile deformation. Oligocrystals of Crofer® 22H ferritic steel were obtained as experimental material at 1075°C for 22min annealing. Using kernel average misorientation (KAM) mapping obtained from EBSD, possible slip planes are (110), (101), (12–1) and (32-1) in grain A; (0–11), (−101), (−112), (1–21) in grain B; and (0–11), (1–21) and (11–2) in grain C. Combining ECCI and EBSD techniques enables us to identify two a0[11¯1]/2 edge dislocations that occur on the (110)[1–11] and (32–1)[1–11] slip systems for grain A, thereby breaking down Schmid’s law.

Creep-fatigue life evaluation of high chromium ferritic steel under non-proportional loading

T Multiaxial creep-fatigue tests under non-proportional loading conditions with various strain rates were carried out using a hollow cylinder specimen of a high chromium ferritic steel at 823K in air to discuss the influence of non-proportional loading on failure life. Strain paths employed were a push-pull loading and a circle loading. The push-pull loading test is proportional strain path test. The circle loading test is non-proportional strain path test in which sinusoidal waveforms of axial and shear strains have 90 degree phase difference. The failure life is affected largely by the strain rate and the non-proportional loading. This paper presents a modified strain range for life evaluation considering the strain rate based on a non-proportional strain parameter proposed by authors. The strain range is a suitable parameter for life evaluation of tested material under non-proportional loading at high temperature. KEYWORDS. Life evaluation; Creep-fatigue; Multiaxial strain; Nonproportional loading; High chromium ferritic steel.

Theoretical Investigation of Stabilizing Mechanism by Boron in Body-Centered Cubic Iron Through (Fe,Cr)23(C,B)6 Precipitates

Small amounts of boron improve the mechanical properties in high-chromium ferritic heat-resistant steels. In this work, the stabilizing mechanism by boron in body-centered cubic iron (bcc Fe) through (Fe,Cr)23(C,B)6 precipitates was investigated by first-principles calculations. Formation energy analysis of (Fe,Cr)23(C,B)6 reveals that the compounds become more stable to elemental solids as the boron concentration increases. Furthermore, the interface energy of bcc Fe(110) || Fe23(C,B)6(111) also decreases with boron concentration in the compounds. The decreased interface energy caused by boron addition is explained by the balance between the change in the phase stability of the precipitates and the change in the misfit parameter for the bcc Fe matrix and the precipitates. These results show that boron stabilizes the microstructure of heat-resistant steels, which is important for understanding the origins of the creep strength in ferritic steels.

Temperature Dependence of Laves Phase Composition in Nb, W and Si-Alloyed High Chromium Ferritic Steels for SOFC Interconnect Applications

The Laves phase strengthened ferritic steel Crofer 22 H has been proposed as construction material for interconnects in solid oxide fuel cells. The background of the present study relates to the further qualification of this steel, especially with respect to a possible optimization of amount and composition of the strengthening Laves phase precipitates. For this purpose the chemical composition of the Laves phase in a number of high purity model alloys as well as in Crofer 22 H equilibrated at temperatures between 700 and 1100 °C was measured by EDX/WDX and atom probe tomography (APT). The obtained chemical compositions were used for a qualitative estimation of the site occupancy for Fe, Cr, Nb, W and Si in the Laves phase unit cell. Additionally, the results from APT measurements indicate the important role of impurities such as e.g. titanium in the Laves phase formation. Finally, the experimental results were compared with Thermocalc calculations using the database TCFE 7. This revealed that within the temperature range 800-900 °C a qualitative description of phases is possible, however, substantial differences existed particularly for the steel Crofer 22 H at and below 700 °C and above 950 °C.