Properties Of Ferritic Stainless Steel Research Articles

Effect of cold single-roll drive rolling on the microstructural evolution and mechanical properties of ferritic stainless steel

In this study, the microstructural evolution and mechanical behavior of AISI 430 ferritic stainless steel produced by cold single-roll drive rolling were investigated. By increasing the rolling deformation, the width of the grains decreased, and after 60% rolling, highly elongated ferrite grains and shear bands occurred in the microstructure. With the increase in the strain, first, the shear texture and then the deformation texture were strengthened in the samples. Dislocation density increased with increasing strain, which led to increasing the macro and microhardness. Also, the up and down surfaces of the sample had different hardness values due to being exposed to two rolls with different speeds. The 60% rolled sample had the highest yield strength (823 MPa), the highest ultimate tensile strength (833 MPa), and the lowest ductility (10.7%), which was due to increasing dislocation density and shear bands. After 60% rolling, the number, size, and depth of dimples decreased compared to the initial steel. However, the fracture mode of all deformed samples was ductile.

Effect of Induced Martensite Content on Microstructure Evolution and Mechanical Properties of Ferritic Stainless Steel

Effect of Induced Martensite Content on Microstructure Evolution and Mechanical Properties of Ferritic Stainless Steel

The effect of cold rolling reduction ratio on the microstructure and mechanical properties of ferritic stainless steel 430

The effect of cold rolling reduction ratio on the microstructure and mechanical properties of ferritic stainless steel 430

An analysis of microstructure and mechanical properties of ferritic stainless steel 430 during cold rolling and subsequent annealing

Systematic study on the microstructural evolution and mechanical properties of ferritic stainless steel (FSS) 430 with different annealing processes was carried out in the present work. The results show that microstructural refinement can be achieved by optimization of annealing processes, improving elongation, yield strength, and tensile strength of FSS 430. An optimal annealing temperature of 950 ℃ is found with better homogeneity and enhanced mechanical properties among the cold-rolled and annealed FSSs. During annealing processes, the fraction of high-angle grain boundaries of FSS 430 annealed at 950 ℃ is found to be the highest, indicating that a homogeneous microstructure with high recrystallization rate is formed inside the FSS 430 strips. In addition, high fraction of the hard grains (SF < 0.4) and low fraction of the soft grains are found inside FSS 430 annealed at 950 ℃, improving the plasticity of material. Overall, the optimization of annealing processes benefits the microstructural refinement of FSS and thereby improving the mechanical properties of materials.

Modeling the Chemical Composition of Ferritic Stainless Steels with the Use of Artificial Neural Networks

The aim of this paper is an attempt to answer the question of whether, on the basis of the values of the mechanical properties of ferritic stainless steels, it is possible to predict the chemical concentration of carbon and nine of the other most common alloying elements in these steels. The author believes that the relationships between the properties are more complicated and depend on a greater number of factors, such as heat and mechanical treatment conditions, but in this paper, they were not taken into account due to the uniform treatment of the tested steels. The modeling results proved to be very promising and indicate that for some elements, this is possible with high accuracy. Artificial neural networks with radial basis functions (RBF), multilayer perceptron with one and two hidden layers (MLP) and generalized regression neural networks (GRNN) were used for modeling. In order to minimize the manufacturing cost of products, developed artificial neural networks can be used in industry. They may also simplify the selection of materials if the engineer has to correctly select chemical components and appropriate plastic and/or heat treatments of stainless steel with the necessary mechanical properties.

Microstructural Refinement and Mechanical Properties of Ferritic Stainless Steel Processed by Equal-Channel Angular Pressing

Ferritic stainless steel (FSS 430) has been deformed by equal-channel angular pressing (ECAP) at room temperature at an equivalent strain (evm) of 1.2 by adopting the route Bc. Microstructural developments were studied by optical microscopy, transmission electron microscopy, and microtexture by electron backscattered diffraction. Typical ultrafine-grains of less 0.2 µm could be identified. The XRD patterns confirm the presence of a BCC phase alone. An increase in peak broadening is attributed to the reduction of crystallite size and increase of lattice strain arising from deformation during ECAP. The grain refinement takes place through simple shear deformation by elongation of grains, splitting of grains into bands, the subdivision of bands into subgrains by dynamic recovery, and conversion of subgrains into grains by progressive lattice rotation (PLR). Weakly textured rolled ferritic stainless steel is strongly textured at an imposed equivalent strain of 0.6 with the development of ideal shear components of D2θ and $$\bar{E}_{\theta }$$ for bcc materials. On the second pass of ECAP texture is partially randomized and the intensity of components was decreased and new components of D1θ and Fθ are developed due to subdivision of grains into bands, bands into subgrains, and conversion of subgrains into grains of high angle of misorientation by PLR. The yield strength, tensile strength, and hardness are increased almost 1.5 times that of as-received coarse-grained material due to a high degree of grain refinement and an increase in defect density or strain. The yield strength of 687 MPa of as-received material was enhanced to 1093 MPa of ECAPed ferritic stainless steel. However, the material has lost its ductility significantly due to high defect density and a significant amount of non-equilibrium nature of grain boundaries.

Comparison in magnetic properties of ferritic stainless steels measured using bar and ring samples

Comparison in magnetic properties of ferritic stainless steels measured using bar and ring samples

Texture and anisotropic mechanical properties of ferritic stainless steel stabilized with Ti and Nb

Textures and anisotropic mechanical properties of the annealed Fe17Cr ferritic stainless steel stabilized with Ti and Nb were examined using scanning electron microscopy, electron backscatter diffraction and tensile tests. The average size of the ferrite grains in the annealed steel sheets is about 36 μm. Pole figure, inverse pole figure, and orientation distribution function by EBSD demonstrate that {111} annealing γ-fiber texture, which is beneficial to the deep drawing, develops after rolling and annealing. Furthermore, in the γ-fiber {111} texture, {111} 〈112〉 component is stronger than the {111} 〈110〉 component. The produced texture leads to the anisotropic mechanical properties in the annealed steel sheets. The yield strength and ultimate tensile strength in the transverse direction is higher than that in the rolling direction, and the elongation in the transverse direction is lower than that in the rolling direction. Moreover, strain rate has little effect on the yield strength, ultimate tensile strength, and elongation in both transverse direction and rolling direction.

Effect of Microstructural Characteristics on Mechanical Properties of Ferritic Stainless Steel

To improve the mechanical properties of hot-rolled ferritic stainless steel (SUS430), the microstructural characteristics of SUS430 were changed using a heating process under various conditions. The hardness of SUS430 decreased upon the increase in the heating temperature to 900 °C, and the hardness increased when the sample was heated to temperatures greater than 900 °C. The high hardness of the sample heated at 1000 °C (H1000 °C) is attributed to the heating time: A high hardness was obtained for a H1000 °C sample that was heated for 1 h (H1000 °C-1h), but this decreased when the heating time was increased to more than 1 h. The high hardness of H1000 °C-1h is caused by the fine Cr23C6 precipitates that are distributed in the sample around the grain boundaries. On the other hand, the large precipitates of Cr23C6 in H1000 °C-12h decrease the hardness. The hardness value of SUS430 is directly attributed to the mechanical properties and the ultimate tensile strength. The tensile strength of H1000 °C-1h was found to be about 200% and 20% higher than the as-received and H1000 °C-12h samples, respectively. Despite the increase in the tensile strength of the H1000 °C-1h sample, the ductility was not found to decrease significantly, for example, the fracture strain was approximately 25%. This occurrence is affected by a severe slip in the ferrite base grain, and the high strength of H1000 °C-1h is influenced by the interruption of the slip by the Cr23C6 precipitates. Unlike the tensile strength, similar fatigue properties were observed for both H1000 °C-1h and H1000 °C-12h, which is associated with the low crack driving force of H1000 °C-12h, caused by the roughness-induced crack closure arising from the large Cr23C6 precipitates.

An Investigation on Base Metal Block Shear Strength of Ferritic Stainless Steel Welded Connection

This study develops a finite element analysis model to predict the ultimate strength of the base metal block shear fracture based on previous experimental results and compares the experimental results with the analysis results to verify the effectiveness of the analysis model. This study also analyzed additional variables of the welding direction and weld length on the applied load to investigate the structural behaviors and fracture conditions. In addition, predicted strength according to the analysis results were compared with those by the current design equations, and the equations proposed by previous researchers. As a result, the design formula by the current design equations, such as Korea Building Code (KBC)/American Institute of Steel Construction (AISC) and European Code (EC3), and the equations proposed by Oosterhof and Driver underestimated the base metal block shear strength of ferritic stainless steel by up to 42%. Equations suggested by Topkaya and Lee et al. for carbon steel and austenitic stainless steel welded connections provided more accurate strength predictions, while they did not reflect the difference of material properties. Therefore, this study proposed a modified strength equation for ferritic stainless steel welded connection with base metal block shear fracture considering the stress triaxiality effect of the welded connection and the material properties of ferritic stainless steel.

Effect of additive particle size on the CuO-accelerated formation of LaFeO3 perovskite conversion coatings in molten carbonate baths

An original iron conversion coating process has been recently developed to enhance the functional properties of ferritic stainless steels for use as Solid Oxide Fuel Cell interconnects. The conversion process produces a dense LaFeO3 perovskite layer grown above a spinel oxide underlayer by an immersion treatment in specially-formulated molten carbonate baths at around 600 °C. Galvanic coupling with a coarse CuO powder added to the bath has been earlier proved to be an effective approach to significantly reduce conversion times and coating thicknesses onto a 18Cr Type K41 ferritic stainless steel substrate. Further investigations on the CuO acceleration effects on the K41 steel are reported in the present work focusing on the effect of CuO particle size on the conversion times and coating structure. Studies carried out with different concentrations and particle sizes of the CuO additive have indicated that conversion coating kinetics is strongly affected by the CuO particle size highlighting thus the fact that CuO did not fully dissolve in the carbonate bath and that galvanic coupling effects took place prevalently with CuO particles dispersed in the molten bath. Dramatic reduction in conversion times could be obtained through CuO nanoparticle additions to the bath. The shortest conversion time of <3 h was achieved by adding 6 mol% nano CuO, at 610 °C. As consequence, perovskite coatings with thicknesses well below 10 μm could be produced due to minimal substrate corrosion and spinel underlayer growth during the short conversion processes in the nano CuO-containing salt baths. This structural refinement could play an important role for improving dimensional stability and functional properties of perovskite coatings in SOFC interconnect applications.

Post-fire behaviour of ferritic stainless steel material

Ferritic stainless steel has a high potential to be used as structural material, as it has a desirable mechanical properties and a relatively low price compared with other stainless steel materials. However, the post-fire mechanical properties of ferritic stainless steel have not been investigated up to now. This paper presents an experimental investigation on mechanical properties of ferritic stainless steel after exposed to high temperatures up to 1000°C. Residual mechanical properties and microstructure of the specimens are examined. The ferritic stainless steel specimens were cooled down by four different cooling methods, namely cool-in-chamber, cool-in-air, cool-in-air-with-fan and cool-in-water. It is shown that different cooling rates have minor effects on the strengths of ferritic stainless steel, but it affects the strain and Ramberg-Osgood parameter (n) in a certain temperature range. It is shown that the design equations in literature cannot provide accurate predictions for the post-fire mechanical properties of ferritic stainless steel materials. Therefore, a unified design equation is proposed to predict the post-fire mechanical properties, and a stress-strain model is also proposed to predict the post-fire stress-strain relationship of specimens which are cooled down in chamber from up to 1000°C. Reliability analysis has also conducted to assess the reliability of the proposed design rules. It is shown that the modified stress-strain models compare well with the experimental results throughout the full range of stress-strain curves.

Improving the Oxidation Resistance and Electrical Properties of Ferritic Stainless Steels for Application in SOFC Interconnects

The paper presents the results of cyclic oxidation studies performed for two ferritic stainless steels: Crofer 22 APU and H25T. The surface of the steels was coated with particles of oxides of the following active elements: yttrium, cerium, samarium, and gadolinium. Two coating methods were used: electrolysis and dip-coating. In addition, yttrium ion implantation was carried out for the purpose of reference. Cyclic oxidation tests were carried out for ca. 1200 h in air at 900 °C. It was determined that particles of oxides of active elements reduce the rate of oxidation, improve the adhesion of the Cr2O3 scale to the substrate, and decrease area specific resistance.

Effect of tin addition on the microstructure and properties of ferritic stainless steel

This article reports the effects of Sn on the inclusions as well as the mechanical properties and hot workability of ferritic stainless steel. Precipitation phases and inclusions in Sn-bearing ferritic stainless steel were observed, and the relationship between the workability and the microstructure of the steel was established. Energy-dispersive X-ray spectroscopic analysis of the steel reveals that an almost pure Sn phase forms and MnS-Sn compound inclusions appear in the steel with a higher Sn content. Little Sn segregation was observed in grain boundaries and in the areas around sulfide inclusions; however, the presence of Sn does not adversely affect the workability of the steel containing 0.4wt% Sn. When the Sn content is 0.1wt%–0.4wt%, Sn improves the tensile strength and the plastic strain ratio and also improves the plasticity with increasing temperature. A mechanism of improving the workability of ferritic stainless steel induced by Sn addition was discussed: the presence of Sn lowers the defect concentration in the ultra-pure ferritic lattice and the good distribution of tin in the lattice overcomes the problem of hot brittleness that occurs in low-carbon steel as a result of Sn segregation.

Oxidation properties of ferritic stainless steel in dual Ar–H2–H2O/air atmosphere exposure with regard to SOFC interconnect application

The oxidation kinetics and microstructure of oxide products formed on the DIN 50049 ferritic stainless steel were studied during exposure to a dual Ar–H2–H2O/air atmosphere, i.e. under conditions simulating SOFC operation at 1073K. The parabolic rate constants of the scales on the air and Ar–H2–H2O sides of the steel were almost identical to those observed for scales oxidized in single atmospheres. The dense oxide scales that developed on the DIN 50049 steel during oxidation in a dual atmosphere were composed of Cr2O3 and the Mn1.5Cr1.5O4 spinel.

Inclusions and solidification structures of high pure ferritic stainless steels dual stabilized by niobium and titanium

As the raw materials in the post process of rolling and heat treatment, ingots have great effects on the properties of the final products. Inclusions and solidification structures are the most important aspects of the quality of ingots. Niobium and titanium are usually used to react with carbon and nitrogen to improve the properties of ferritic stainless steels. In this research, combined with thermodynamic calculation, effects of niobium and titanium on the inclusions and solidification structures in three kinds of high pure ferritic stainless steels with different titanium additions were investigated by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy disperse spectrometer (EDS). Results show that Al2O3 and a few (Nb,Ti)N particles form when titanium addition is 0.01 %. Furthermore, inclusions are mainly TiN and Al2O3–TiOx–TiN duplex inclusions when titanium addition is more than 0.10 %. Those two types of inclusions are in well distribution, and can afford nuclei to the solidification process. Therefore, the ratio of equiaxed zone increases with the increase of titanium addition. The ratio increases from 42.1 % to 64.0 % with the titanium addition increasing from 0.01 % to 0.10 %, and it increases to 85.7 % when the titanium addition reaches 0.34 %.

The Effect of Finish Rolling Temperature on Microstructure and Mechanical Properties of Vanadium Containing Ferritic Stainless Steel

In this paper, the influence of finish rolling temperature during hot rolling on microstructure and mechanical properties of ferritic stainless steel containing vanadium was investigated. It showed that with the rolling temperature increasing, the tensile strength and hardness value fell down, while the value of ductility rising and less precipitates appearing. If the finish rolling temperature was set too high, the grain would get coarsened and the α-fiber get strengthened, which had an adverse effect on the formability and wrinkle resistance of the production sheet. 850°C was a reasonable finish rolling temperature.

Effects of pre-oxidation on the microstructural and electrical properties of La0.67Sr0.33MnO3−δ coated ferritic stainless steels

LaxSr1−xMnO3 (LSM) is commonly used as a protective layer on the metallic interconnects of solid oxide fuel cells (SOFCs) to prevent surface oxidation and chromium poisoning. However, the volume shrinkage at elevated temperatures causes the LSM coatings to crack, resulting in chromium diffusion. Therefore, this paper investigates the effects of pre-oxidation on the microstructure and electrical properties of ferritic stainless steels coated with La0.67Sr0.33MnO3 (LSM). Four ferritic stainless steels were selected for use as interconnect substrates: Crofer22APU, Crofer22H, ss441, and ZMG232L. The candidate materials were pre-oxidised at 850°C for 25 and 50h, respectively. After the pre-oxidation process, the LSM films with a thickness of 3–4μm were deposited on the surface of samples by using Pulsed DC magnetron sputtering. After aging the coated specimens at elevated temperatures, the morphologies and crystalline structures were examined using SEM/EDX and XRD, respectively. The results indicated that the pre-oxidised layer, (Mn, Cr)3O4, could significantly suppress chromium penetration from the interior to the surface of the specimens. Moreover, the area specific resistance (ASR) values for the 25-h pre-oxidised specimens were 2.24, 12.21, 2.30, and 6.77mΩcm2 for Crofer22APU, Crofer22H, ss441, and ZMG232L, respectively, at 800°C for 500h in an air atmosphere.

Effects of Nb and W additions on high-temperature creep properties of ferritic stainless steels for solid oxide fuel cell interconnect

Tensile and creep properties of a newly developed ferritic stainless steel (Crofer 22 H) with additions of Nb and W for planar solid oxide fuel cell (pSOFC) interconnect are investigated at 25–800 °C. These properties are compared with those of another ferritic stainless steel (Crofer 22 APU) without Nb or W. A significantly improved tensile and creep strength of Crofer 22 H over Crofer 22 APU for pSOFC interconnect is found and attributed to a precipitation strengthening effect of the Laves phase. According to the creep stress exponent, activation energy, and microstructural observations, the superiority of Crofer 22 H over Crofer 22 APU in creep resistance at 650–800 °C involves a power-law dislocation creep mechanism interacting with an in situ precipitation strengthening mechanism. The relation between creep rupture time and applied stress for Crofer 22 H is well described by a simple power law. Larson–Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 H steel. A significant coarsening of the Laves phase is responsible for the reduced improvement of creep resistance in Crofer 22 H at the low-stress, long-term region of 800 °C.

Effect of Autogenous Arc Welding Processes on Tensile and Impact Properties of Ferritic Stainless Steel Joints

The effect of autogeneous arc welding processes on tensile and impact properties of ferritic stainless steel conformed to AISI 409M grade is studied. Rolled plates of 4 mm thickness have been used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure, and fracture surface morphology of continuous current gas tungsten arc welding (CCGTAW), pulsed current gas tungsten arc welding (PCGTAW), and plasma arc welding (PAW) joints are evaluated and the results are compared. It is found that the PAW joints of ferritic stainless steel show superior tensile and impact properties when compared with CCGTAW and PCGTAW joints, and this is mainly due to lower heat input, finer fusion zone grain diameter, and higher fusion zone hardness.