High-chromium Ferritic Stainless 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.

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.

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.

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.

窒素含有ステンレス鋼の耐孔食性に及ぼす微量元素の影響

To improve the performance of stainless steel, we subjected solid-state steel to a nitrogen absorption treatment. In the fabrication process, a commercially available high-chromium ferritic stainless steel (Fe-22Cr-1Mo) was heat-treated at 1423 K in a nitrogen atmosphere. The heat-treatment transformed the ferric phase into the austenite phase. This process loaded over 1 mass% of nitrogen into the steel material. Most of the added nitrogen formed a solid solution in the matrix, but a minor portion formed nitrides with the very small quantities of elements such as titanium and aluminum that pre-existed in the steel. The nitrogen-containing steels were then analyzed by pitting potential measurements and ferric chloride corrosion examination. The pitting corrosion resistance of Fe-22Cr-1Mo-1N exceeded that of conventional materials such as Fe-18Cr-12Ni and Fe-22Cr-1Mo. However in the ferric chloride corrosion tests, pits developed in Fe-22Cr-1Mo-1N at temperatures above 323 K. These pits were possibly initiated at the sites of minute nitride resulting from the nitrogen absorption process.

The Effect of Niobium Carbides and Laves Phase on the Yielding Behaviour of a Stabilized Ferritic Stainless Steel

High-chromium ferritic stainless steels have been developed for applications such as exhaust systems that require good formability. To improve formability, continuous yielding is preferred. However, in high-chromium ferritic stainless steels an upper yield point is often present as a result of free interstitials and Cottrell atmospheres. The upper yield point can be removed by temper rolling but it would be better to avoid it via a suitable heat treatment. This paper describes how this can be done in the case of a ferritic stainless steel containing 0.011%C, 0.012%N, 18%Cr, 2,1%Mo, 0.33%Nb, 0.15Ti%. Despite the presence of Nb and Ti, which should bind the free carbon and nitrogen as carbides and nitrides, an upper yield point was still observed. Previously it has been suspected that this is due to an intermetallic Laves phase present in this steel depleting the Nb in the matrix so that some carbon remains free. A series of short-term annealing experiments showed that the upper yield point diminishes, when the annealing temperature increases above 550 °C, finally disappearing after a heat treatment at 750 °C. On the basis of Thermo-Calc calculations and EDS analyses, free interstitials in the matrix could be related to depletion of MX or insufficient time to reach the equilibrium state.

Microstructural and Texture Development during Multi-Pass Hot Deformation of a Stabilized High-Chromium Ferritic Stainless Steel

Microstructural and Texture Development during Multi-Pass Hot Deformation of a Stabilized High-Chromium Ferritic Stainless Steel

Effect of σ-phase Embrittlement on Fatigue Behavior in High-Chromium Ferritic Stainless Steel

The purpose of the present study is to evaluate the effect of σ-phase embrittlement on fatigue behavior in high-chromium ferritic stainless steels. When ferritic stainless steels are exposed to the temperature range of 700∼ 800°C, the precipitation of σ-phase occurs. The precipitation brings about the embrittlement of materials which is known as the σ-phase embrittlement. Vickers hardness of a high-chromium ferritic stainless steel, type 447 with a chromium content of 30.7% increased significantly by the aging at 750°C due to the precipitation of hard σ-phases which are Cr- and Mo-rich inter-metallic compounds. On the other hand, in type 444 whose chromium content was 18.7%, the precipitation of σ-phase and the increase of hardness were not recognized by the aging at 750°C. Axial fatigue tests were performed using type 447 aged at 750°C for two different aging periods of 150h and 300h, and the effect of σ-phase embrittlement on the fatigue behavior was discussed. The aged type 447 exhibited much lower fatigue strengths than the un-aged one, where fatigue strengths decreased with increasing aging time. The brittle fracture of σ-phase and softening of ferritic phase reduced both crack initiation and growth resistances of the aged specimens, resulting in the lower fatigue strengths.

Microstructural Changes of High-Chromium Ferritic Stainless Steel Subjected to Cyclic Loading in 475°C Embrittlement Region

When ferritic stainless steels are exposed to the temperature range of 400-550°C, they undergo 475°C embrittlement that could be attributed to the spinodal decomposition. In the present study, high-chromium ferritic stainless steel, type 447, was aged at 520°C under both static and cyclic loading conditions and microstructure was analyzed using a transmission electron microscope (TEM). By aging under static condition, hardness and tensile strength were largely increased. TEM observation revealed the mottled contrast due to the typical spinodal decomposition of Cr phase. Under cyclic loading condition, increases in hardness and tensile strength were smaller than those under static condition. TEM micrographs of the specimens subjected to cyclic loading exhibited a lower contrast of the mottled pattern than those of the specimens aged under static condition, indicating that cyclic loading suppressed the spinodal decomposition, thus resulting in the lower increases in hardness and tensile strength.

Oxidation of ferritic stainless steel interconnects: Thermodynamic and kinetic assessment

Planar solid oxide fuel cells with yttria-stabilized zirconia electrolytes typically operate at temperatures in the range of 700–850 °C. The maximum temperature is limited by the use of ferritic stainless steel interconnects which offer significant advantages such as low cost and high thermal and electronic conductivity over traditional ceramic interconnects. However, these alloys rely on the formation of a protective chromia scale for oxidation resistance that results in an increase in ohmic resistance and can volatilize leading to a loss of cathode catalytic activity. To better understand the oxidation behavior of chromia forming ferritic stainless steels, thermodynamic modeling was performed in conjunction with experimental oxidation testing and empirical kinetic evaluation. The phase stability and oxidation behavior of the Fe–Cr–O ternary system were assessed using thermodynamic calculations. Calculated ternary phase diagrams were validated against the experimental oxidation data of Fe–20Cr and Fe–18Cr ferritic stainless steels, GE-13L and AL-441HP, respectively. Results indicate that the use of accelerated testing, such as exposing the system to higher temperatures, can lead to changes in phase equilibria and the oxidation kinetics of the alloys. Through combined thermodynamic assessment and controlled oxidation experiments, the oxidation behavior of high-chromium ferritic stainless steels is presented and discussed.

Effect of [formula omitted]-phase embrittlement on fatigue behaviour in high-chromium ferritic stainless steel

When ferritic stainless steels are exposed to the temperature range of 700–800 °C, embrittlement takes place, which is known as the σ-phase embrittlement. In the present study, axial fatigue tests were conducted using a high-chromium ferritic stainless steel, type 447 with the chromium content of 30.7%, which was aged at 750 °C for 150h and 300h, and the effect of σ-phase embrittlement on the fatigue behaviour was discussed. Vickers hardness increased significantly by aging due to the precipitation of hard σ-phase. The aged specimens exhibited much lower fatigue strengths than the un-aged one, where fatigue strength decreased with increasing aging time. The precipitation of brittle σ-phase reduced the crack initiation and growth resistances, resulting in the lower fatigue strengths of the aged specimens.

756 高Crフェライト系ステンレス鋼の疲労挙動に及ぼすσ相ぜい化時間の影響(OS9-4 材料の疲労と破壊におけるミクロ計測と損傷評価4,OS9 材料の疲労と破壊におけるミクロ計測と損傷評価)

756 高Crフェライト系ステンレス鋼の疲労挙動に及ぼすσ相ぜい化時間の影響(OS9-4 材料の疲労と破壊におけるミクロ計測と損傷評価4,OS9 材料の疲労と破壊におけるミクロ計測と損傷評価)

OS0726 高Crフェライト系ステンレス鋼の時効による組織変化に及ぼす繰返し応力負荷の影響(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

The present paper describes the effect of cyclic stressing on aged microstructure in high-chromium ferritic stainless steel, Type 447. Hardness and tensile strength were largely increased by aging at 520℃ due to the 475℃ embrittlement. TEM observation revealed that the embrittlement could be attributed to the spinodal decomposition of Cr phase. Cyclic stressing during aging at 520℃ suppressed the spinodal decomposition and resulted in less hardness increase than that under static aging.

스테인리스강의 국부부식 저항성 연구에 미세방울셀의 응용

Micro-droplet cell with free droplet as a micro-electrochemical technique has been limited to apply to electrochemical systems with high wetting properties such as an acidic solution and low grade stainless steels(Type 316L). By loading negative pressure to a droplet, control of droplet size, and use of hydrophobic gasket, the cell is modified to be allowed to use for electrochemical systems with high wetting properties. For giving the reliability of new cell, studies on local corrosion were conducted for three different systems-an acidic chloride solution and high chromium ferritic stainless steel, the other acidic chloride solution and type 316, and a neutral chloride solution and type 316. stainless steel. Firstly, the modified micro-droplet cell allows the anodic polarization curves in an acidic chloride solution to show the fact that the local corrosion of high chromium stainless steel near the interface is due to the Cr depleted zone. Secondly, the local anodic polarization test of type 316 L in the other acidic chloride solution can be successfully conducted in the cell. Furthermore, the local polarization curves help elucidating the corrosion of type 316 with phase. Finally, the polarization curves of type 316 L in a neutral chloride solution indicates that the factor affecting the pitting corrosion resistance was inclusions rather than .

Biomaterials: Past Successes and Future Problems

The practice of using metals and alloys to repair or replace human body parts is now well established. This paper will briefly review the successes and, in reality, the remarkably few failures of the traditional materials-mainly titanium alloys, cobalt-chrome alloys, amalgams and stainless steels. The paper will then concentrate on the problems likely to be encountered before new advanced, but not necessarily corrosion resistant, materials can be successfully used for in-vivo purposes. Two of the most important parameters in determining the suitability of a material for biomedical applications are its biocompatibility and corrosion resistance. With regard to these two parameters pure titanium appears to be almost the perfect biomaterial. Unfortunately, many surgical and dental applications require materials with specific mechanical properties, such as high strength or ductility, which pure titanium is unable to provide. Hence materials such as the Ti6A14V alloy and 316L stainless steels are used despite their poorer corrosion resistance, occasionally with undesirable consequences. Furthermore, there is a desire to make use of a number of advanced materials, such as: memory-shaped alloys, porous materials and composites, low precious metal amalgams and rare earth magnets. Unfortunately, nearly all of these materials have inadequate corrosion resistances to be used directly in-vivo without some form of protection. However, it is obvious that most of the traditional techniques for reducing corrosion rates, such as controlling the environment and cathodic protection, cannot be applied to biomaterials, even coatings are of only limited use since many orthopaedic and dental devices are subjected to wearing and abrasion processes. As a result it has traditionally been considered that the only successful method of reducing corrosion within the human body is to fabricate the implants from a corrosion resistant alloy. However, although this approach may eventually be successful in some areas, for example it may be possible to use alloying additions to design a highly corrosion resistant memory-shape alloy; it is highly unlikely that such an approach will be successful with respect to rare earth magnets. One challenge therefore appears to be to design biocompatible coatings that can resist both the chemical and mechanical environments, which are often poorly defined for both orthopaedic and dental applications, without degrading the very property that is required in the advance material. Potential examples of these applications are hydroxyapatite (HA) coatings on porous titanium, titanium or titanium nitride films on Ni-Ti memory-shaped alloys and very high chromium ferritic stainless steel claddings on rare earth magnets.

J & L TYPE 446 MOD-1

Abstract J and L Type 446 MOD-1 stainless steel is a high-chromium ferritic stainless steel with molybdenum and titanium. The titanium is for weld stabilization. A titanium plus niobium stabilized grade (MOD-2) is also available. This datasheet provides information on composition, physical properties, hardness, elasticity, tensile properties, and shear strength. It also includes information on forming, heat treating, and joining. Filing Code: SS-739. Producer or source: J & L Specialty Steel Inc.

Fretting corrosion resistance and fretting corrosion product cytocompatibility of ferritic stainless steel.

To avoid nickel ion release from SUS317L as an implant material, a new type of nickel, commercially free, of high purity, and high chromium ferritic stainless steel, was developed. The new stainless steel (FJ) was studied for aspects of fretting corrosion and cytocompatibility compared with SUS317L. A pin-on-plate fretting corrosion test in an artificial physiologic solution, and cell culture in media with the addition of the artificial physiologic solution used for fretting was conducted. Resistance to the fretting induced crevice corrosion of FJ was higher than that of SUS317L because of the favorable electrochemical stability of the FJ alloy. The amount of iron ion or colloidal fine particles released from FJ was about a quarter of that from SUS317L, although the weight loss of a pin of FJ was almost 5/3 that of SUS317L. The artificial physiologic solution used for SUS317L fretting was more harmful to the growth of L929 and MC3T3-E1 cells than that used for FJ fretting. FJ was therefore superior to SUS317L as a biomaterial, judging from the resistance to fretting-induced crevice corrosion, electrochemical stability, and the cytocompatibility of fretting corrosion products.

Effects of Ni on Stress Corrosion Susceptibility of High-Cr Ferritic Stainless Steels in Hot Cl−Solution

Abstract Effects of nickel (Ni) on the stress corrosion susceptibility of high-chromium (Cr) ferritic stainless steels (SS) in boiling solution of 42% lithium chloride (LiCl) plus thiourea (NH2CSNH2) were investigated using a uniaxial tension-constant load tester. The influence of Ni on film breakdown potential, repassivation rate, creep behavior, and susceptibility to hydrogen (H)-induced failure of the alloys was evaluated. Commercial alloys of 29% Cr- 4% molybdenum (Mo) (UNS S44700) and 29% Cr-4% Mo-2% Ni (UNS S44800) ferritic SS were compared. The 29% Cr-4% Mo alloy was immune to stress corrosion cracking (SCC) at opencircuit potential (OCP) but became susceptible when polarized anodically to critical cracking potential (Ecc, −450 mVSCE) which was 20 mVSCE more noble to the stabilized corrosion potential (Ecorr, −470 mVSCE). Addition of 2% Ni to the 26% Cr-4% Mo alloy did not affect Ecc, but did induce susceptibility to cracking by increasing Ecorr to a value 80 mVSCE noble to Ecc. The 2% Ni addition ...