Grade Ferritic Stainless Steel Research Articles

Dislocation Hardening in a New Manufacturing Route of Ferritic Oxide Dispersion-Strengthened Fe-14Cr Cladding Tube.

The microstructure evolution associated with the cold forming sequence of an Fe-14Cr-1W-0.3Ti-0.3Y2O3 grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material, initially hot extruded at 1100 °C and then shaped into cladding tube geometry via HPTR cold pilgering, shows a high microstructure stability that affects stress release heat treatment efficiency. Each step of the process was analyzed to better understand the microstructure stability of the material. Despite high levels of stored energy, heat treatments, up to 1350 °C, do not allow for recrystallization of the material. The Vickers hardness shows significant variations along the manufacturing steps. Thanks to a combination of EBSD and X-ray diffraction measurements, this study gives a new insight into the contribution of statistically stored dislocation (SSD) recovery on the hardness evolution during an ODS steel cold forming sequence. SSD density, close to 4.1015 m-2 after cold rolling, drops by only an order of magnitude during heat treatment, while geometrically necessary dislocation (GND) density, close to 1.1015 m-2, remains stable. Hardness decrease during heat treatments appears to be controlled only by the evolution of SSD.

Post-fire stress-strain response of structural ferritic stainless steels

In this study, the effects of heating temperature and heat soak time on the post-fire stress-strain behaviour of three ferritic stainless steel grades (i.e., EN 1.4003, 1.4016, 1.4509) are experimentally investigated. The test results revealed that the three grades behaved very differently after exposure to elevated temperatures because of microstructure changes during heating and cooling down to room temperature. The heating temperature affected not only the residual strength and deformation capacity but also the shape of the stress-strain curve. A prolonged heat soak had no noticeable effect on the steel grade 1.4509 but increased strength loss of grades 1.4003 and 1.4016. Two stress-strain models, i.e., one exhibiting continuous yielding (without yield plateau) and the other with yield plateau, were proposed to use jointly to capture the post-fire stress-strain response of the three ferritic grades.

Effect of hot rolling parameters on recovery mechanism in 436 (17%Cr, Nb-Mo) ferritic stainless steel

Ferritic stainless steel (FSS) grades are widely used for applications that require high strength and corrosion resistance. Their cost and versatility in the steel market have attracted a lot of interest from both industry and academic research. Despite their desirable properties, these steels grades experience surface defects as a result of microstructural evolution that evolves due to softening mechanism. The current study simulated the roughing hot rolling processes of AISI 436 (17%Cr, Nb-Mo) FSS to study the effects of inter-pass time and strain on the precipitation behaviour and the softening mechanisms in 436 FSS. The softening mechanisms and the resulting microstructures were investigated using SEM-EBSD technique. The results revealed Particle Stimulated Nucleation of new grains during the simulated roughing rolling which promoted recrystallisation due to strain accumulation. Stored deformation energy was found to increase with an increase in interpass time and strain.

OUTOKUMPU MODA 430/4016

Abstract OUTOKUMPU MODA 430/4016 is a 16% chromium ferritic stainless steel that combines good mechanical properties with good corrosion resistance and heat and oxidation resistance up to 815 °C (1500 °F). It is the most commonly used ferritic stainless steel grade and can be used to replace type 304 austenitic stainless steel in certain applications. It is best suited for mildly corrosive environments. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1340. Producer or source: Outokumpu Oyj.

Behaviour of Ferritic Stainless Steel Bolted T‐stubs Under Tension‐Part 1: Experimental Investigations

AbstractResearch on stainless steel structures has primarily focused on the structural response and design of individual members, whilst the response of joints has received far less attention to date. Recently, a series of experimental tests has been conducted on austenitic and duplex stainless steel moment resisting connections which highlighted both the excellent ductility and significant overstrength exhibited by such connections as well as the severe conservatism of current design rules specified in EN 1993‐1‐8 when applied to stainless steel joints. A comprehensive experimental study on the structural behaviour of stainless steel T‐stubs fabricated from EN 1.4003 ferritic stainless steel grade is reported in this paper. The tested configurations included both single and double bolt rows and have been assembled with A4‐80 stainless steel bolts. In order to determine the material characteristics of the T‐stubs and the connecting bolts, tensile coupon tests were performed and the material anisotropy exhibited by ferritic stainless steel has been quantified. The ultimate resistance, plastic deformation capacity and failure mode of each T‐stub were obtained and are reported herein. The obtained results are utilised to assess the accuracy of existing design rules and to validate the numerical model discussed in the companion paper.

High-Temperature Oxidation Resistance of PDC Coatings in Synthetic Air and Water Vapor Atmospheres.

This work is aimed at the development and investigation of the oxidation behavior of ferritic stainless-steel grade AISI 441 and polymer-derived ceramic (PDC) protective coatings. Double-layer coatings of a PDC bond coat below a PDC top coat with glass and ceramic passive fillers’ oxidative resistance were studied at temperatures up to 1000 °C in a flow-through atmosphere of synthetic air and in air saturated with water vapor. Investigation of the oxide products formed at the surface of the samples in synthetic air and water vapor atmospheres, at different temperatures (900, 950, 1000 °C) and exposure times (24, 96 h) was carried out on both uncoated steel and steel coated with selected coatings by scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The Fe, Cr2O3, TiO2, and spinel (Mn,Cr)3O4 phases were identified by XRD on oxidized steel substrates in both atmospheres. In the cases of the coated samples, m- ZrO2, c- ZrO2, YAG, and crystalline phases (Ba(AlSiO4)2–hexacelsian, celsian) were identified. Scratch tests performed on both coating compositions revealed strong adhesion after pyrolysis as well as after oxidation tests in both atmospheres. After testing in the water vapor atmosphere, Cr ions diffused through the bond coat, but no delamination of the coatings was observed.

Buckling of stainless steel welded I-section columns

This paper studies the buckling behaviour and design of welded I-section stainless steel columns. Experimental and numerical structural performance data together with the design methods for stainless steel welded I-section columns available in the literature have been collated and reviewed. A numerical modelling programme including validation and parametric studies has been carried out to supplement the literature experimental and numerical data for the assessment of the existing codified and literature proposed flexural buckling design formulations for stainless steel welded I-section columns. Columns of austenitic, duplex and ferritic stainless steel grades undergoing major axis and minor axis flexural buckling have been investigated. From comparisons with the EN 1993–1-4 (EC3) flexural buckling capacity predictions, it was found that (1) for the austenitic welded I-section columns, the EC3 buckling curve (α = 0.76 and λ¯0 = 0.2) is suitable for both axes, (2) for the duplex and ferritic grades, the EC3 buckling curve (α = 0.76 and λ¯0 = 0.2) is conservative, and a higher buckling curve with (i) α = 0.49 and λ¯0 = 0.2 for both axes or (ii) α = 0.49 and λ¯0 = 0.2 for minor axis and α = 0.34 and λ¯0 = 0.2 for major axis may be adopted. In addition, comparisons with the recently proposed Continuous Strength Method showed marginally improved strength predictions but with slightly higher scatter.

Microstructure and mechanical properties of gas metal arc welded AISI 430/AISI 304 dissimilar stainless steels butt joints

Nowadays, there is an increasing demand for the use of joining dissimilar metals in various industrial applications owing to the economic benefits and better joint performance. Ferritic stainless steel grades is a good alternative for austenitic ones in several applications as they display higher ductility and strength as well as better corrosion resistance in chloride environments. However, the poor ductility and low impact toughness of ferritic stainless-steel welded joints limit their wide-spread application. Thus, the demand for joining dissimilar ferritic and austenitic stainless steels is ever increasing in several engineering applications in various industries such as nuclear power plants, coal fired boilers, automobile manufacturing industry, chemistry and petro-chemistry industries, etc. Therefore, successful joining of these two different types of stainless steel grades using conventional fusion welding methods is rather important. In this study, the weldability of AISI 430 ferritic and AISI 304 austenitic steel plates using gas metal arc welding and the effect of heat input on microstructure and the mechanical properties were investigated. For this purpose, AISI 430 and AISI 304 plates with a thickness of 5 mm were joined using different heat input values. The microstructures evolved in the weld region of the joints and their mechanical properties were determined by detailed optical microscopy investigations, micro-hardness measurements and tensile tests. Thus, the influence of the heat input on the weldability of these two different stainless-steel plates using gas metal arc welding was also studied.

Design of stainless steel cross-sections with outstand elements under stress gradients

A significant amount of research has been reported on stainless steel tubular sections, while studies on I- and C-sections remain relatively limited. This paper presents a comprehensive numerical study on the response of stainless steel I- and C-sections subjected to minor axis bending, with outstand flanges subjected to stress gradients. Numerical models are developed and validated against reported test data on austenitic stainless steel sections under minor axis bending. Subsequently, parametric studies using standardised material properties on austenitic, duplex and ferritic stainless steel grades, covering a wide variety of cross-section slendernesses, are carried out to expand the structural performance data. The results are used to assess the applicability of the Eurocode slenderness limits, revealing that the Class limit 3 for outstand flanges under stress gradient is overly conservative. Moreover, Eurocode underestimates the predicted bending strengths, whereas the level of accuracy and consistency improves for stocky sections, when the Continuous Strength Method is used. Aiming to address the lack of accuracy and consistency in the design predictions of slender sections, particular focus is placed on their performance. It is demonstrated that outstand elements under stress gradients exhibit significant inelastic behaviour after the compression flanges have locally buckled. Inelastic buckling behaviour is not considered in current design guidance, thus resulting in overly conservative and fundamentally incorrect strength predictions. An alternative design method based on the plastic effective width concept is proposed for slender stainless steel I- and C-sections in minor axis bending, which leads to more favourable and less scattered strength predictions.

High-Temperature Oxidation of AISI441 Ferritic Stainless Steel for Solid Oxide Fuel Cells

Several ferritic stainless steel grades are widely studied and used in solid oxide fuel cells (SOFCs) technology as interconnect materials. Their high-temperature oxidation behavior is interesting to evaluate their applicability at SOFCs operating conditions and to design degradation tests and models predicting the lifetime of a SOFC stack. In this work the AISI441 grade was oxidized in static air at 850°C to study its oxidation kinetic by weight gain measurements. It was found a parabolic growth with a rate constant of 9.42 x 10-14 g2cm-4s-1. Data calculated using the diffusion coefficients of the species involved in the oxidation process resulted in higher weight gain. Discrepancies between the measurements and the model were corrected taking into account the chromium volatilization.

Passive filler loaded polysilazane‐derived glass/ceramic coating system applied to AISI 441 stainless steel, part 2: Oxidation behavior in synthetic air

AbstractThis article features the oxidation behavior of ferritic stainless steel grade AISI 441 coated with protective polymer‐derived ceramics (PDC). Two PDC compositions are studied with respect to their oxidation resistance in a flow‐through atmosphere of synthetic air at temperatures of up to 1000°C. The coatings contain a combination of six passive fillers: Y‐containing ZrO2, glass microspheres, alumina‐yttria‐zirconia (AYZ) powder, and three commercial glasses. They are pyrolyzed in air for 1 hour at 800°C with heating and cooling rates of 3 K/min. Detailed microstructural examination of the oxide products formed at the surface of samples after exposure to air at 900°C, 950°C, and 1000°C for 1‐48 hours is analyzed. Both uncoated steel and steel coated with two of the protective systems described in part 1 of this article are investigated. Fe, Cr2O3, TiO2, and a spinel of the composition (Mn,Cr)3O4 are identified at the oxidized surface of the steel substrate using X‐ray diffraction. A significant weight gain of the unprotected steel is measured after all experiments, while oxidation tests of the coated steel show a negligible weight gain after 900°C and 950°C. During the early stages of coating oxidation, the monoclinic‐to‐tetragonal ratio in the zirconia filler is shifted toward the monoclinic modification. Longer exposures and higher temperatures lead to the formation of yttrium aluminum garnet (YAG) due to glass microsphere crystallization and solid state reactions in the AYZ powder. The crystallization of the three commercial glasses functioning as sealants leads to the formation of Ba(AlSiO4)2 also known as hexacelsian, which subsequently transforms to celsian. YZr8O14 is also formed. The protective effect of the PDC coatings applied to the stainless steel is demonstrated up to 950°C.

An Investigation on Corrosion and Stress Corrosion Cracking initiation of a Ferritic Stainless Steel in a Tertiary Amine Solution

The present study focused on stress corrosion cracking (SCC) and corrosion behavior of ferritic stainless steel (grade 430) in activated methyl diethanolamine (aMDEA) solution, which is classified as a tertiary amine. In this regard, cyclic polarization and U-bend tests were performed in CO2 loaded aMDEA with different concentrations at 25 and 70 °C to observe corrosion behavior and also the possibility of crack initiation. Based on the obtained results, it was found that the corrosion rate increased in concentrated amine solutions. Also, by increasing temperature from 25 to 70 °C both corrosion rate and susceptibility to SCC initiation were intensified. Increment of amine concentration and also increase in temperature led to more absorption of CO2, generating a more acidic solution. Overall it could be stated that while for the grade 430 stainless steel investigated in this study corrosion and cracking was observed Therefore it could be concluded that in amine-containing environments this steel is not a very suitable alternative for carbon steels, which are commonly used in these environments.

Electrical Conductivity Behaviour of Ferritic Steel Interconnect in Function of Spinel Composition, Electrode Material and Thermal Cycles

Conductivity of interconnects has been measured in function of different parameters. Two ferritic stainless steel grades were used, Crofer 22H and 22APU. Two different spinels composition were deposited, CuMn2O4 and Cu1.35Mn1.65O4. Their thicknesses were varied from 4 to 16 µm to observe the influence on conductivity and its evolution over time. Measurements were done with direct contact between spinel coatings or by interposing an electrode material. For this purpose, LSC (La0.5Sr0.5CoO3) or LSCF (La0.6Sr0.4Co0.2Fe0.8O3) were directly screen printed on the spinel layer. All these different layers (CuxMnyO4, LSC, LSCF) were deposited on small Crofer coupons (40x20x0.3 mm) and M_grid™. Experiments were performed under oxidizing atmosphere at 800°C under air flow during hundreds of hours. Several thermal cycles were done to assess the stability of the coatings.

Optimizing the Plasma Arc Welding Process Parameters to Attain the Minimum Corrosion Rate in the AISI 409M grade Ferritic Stainless Steel Autogenous Joints

Abstract The combination with good mechanical properties and manufacturing characteristics, makes stainless steel an indispensable tool for the designer. Stainless steels become susceptible to localized intergranular corrosion when chromium carbides form at the grain boundaries during high temperature exposure. This depletion of chromium at the grain boundaries is termed ‘sensitization’, because the alloys become more sensitive to localized attack in corrosive environments. Stainless steels like AISI 409M grade, which are having low chromium content (approximately 11%), are susceptible to localized corrosion. The susceptibility of these alloys are strongly affected by welding processes and filler metals, which change the microstructure of the alloy in order to have optimum mechanical properties. Hence, the present investigation has been carried out to understand the effect of Plasma Arc Welding (PAW) process on pitting corrosion behavior of AISI 409M grade stainless steel joints. Also, the PAW process parameter such as welding speed, voltage, heat input were optimized with help of Response Surface Methodology (RSM) to attain the maximum tensile strength and minimum corrosion rate in the welded joints.

Effect of heat input on microstructure and mechanical properties in gas metal arc welding of ferritic stainless steel

The variation of heat input on microstructure and mechanical properties in gas metal arc welding of ferritic stainless steel (grade 409 L) using two different austenitic filler wires ER304 L and ER308 L have been discussed in this work. Three different heat input parameters of 0.3, 0.4 and 0.5 kJ mm−1 are used in this experimental work and the effect of heat input on the properties of weldment were investigated using metallographic study and mechanical testing. Percentage dilution, chromium nickel ratio, microstructure, microhardness and tensile properties were all found to be affected by the heat input. It was observed that welded sample with heat input of 0.4 kJ mm−1 possesses superior mechanical properties irrespective of type of filler wire used. Mechanical properties obtained by using 304 L filler wire were found to be better than those obtained using 308 L filler wire, irrespective of heat input provided during welding.

Electrochemical, surface analytical, and spectroscopic study of passive film and pits formation on food grade ferritic stainless steel AISI‐430 in aqueous acetic acid containing chloride ions

Pitting corrosion of food grade ferritic stainless steel AISI‐430 was studied in 20% aqueous acetic acid solution containing 0.5 M NaCl. Potentiodynamic polarization measurements at a slow scan rate reveal formation of metastable pits and repassivation. Surface analysis reveals remnants of the passive oxide layer adhered along the circumference of the pits, evidences of loss of microcrystallites, no significant loss in crystallinity during corrosion process and formation of mixed oxides of iron and chromium. The pitting of ferritic steel is an autocatalytic process, and the proposed mechanism suggests that both chloride and acetate ions play important role.

Effect of Starting As-cast Structure on the Microstructure–Texture Evolution During Subsequent Processing and Finally Ridging Behavior of Ferritic Stainless Steel

Effect of the initial as-cast structure on the microstructure–texture evolution during thermomechanical processing of 409L grade ferritic stainless steel was studied. Samples from the regions of cast slab having ‘columnar,’ ‘equiaxed,’ and a mixture of ‘columnar’ and ‘equiaxed’ grains were subjected to two different processing schedules: one with intermediate hot-band annealing before cold-rolling followed by final annealing, and another without any hot-band annealing. EBSD study reveals that large columnar crystals with cube orientation are very difficult to deform and recrystallize uniformly. Resultant variations in ferrite grain structure and retention of cube-textured band in cold-rolled and annealed sheet contribute to ridging behavior during stretch forming. Initial equiaxed grain structure is certainly beneficial to reduce or even eliminate ridging defect by producing uniform ferrite grain structure, free from any texture banding. Application of hot-band annealing treatment is also advantageous as it can maximize the evolution of beneficial gamma-fiber texture and eliminate the ridging defect in case of completely ‘equiaxed’ starting structure. Such treatment reduces the severity of ridging even if the initial structure contains typically mixed ‘columnar-equiaxed’ grains.

Influence of heat treatments on the microstructure and degree of sensitization of base metal and weld of AISI 430 stainless steel

ABSTRACT AISI 430 is a non-stabilized ferritic stainless steel grade with carbon content lower than 0.12%.After hot and cold rolling this material is annealed. The slow cooling after soaking at temperatures between 900oC and 1000oC promotes the formation of a high quantity of carbides and nitrides, while the rapid cooling partially suppresses the formation of these precipitates, but introduces martensite in the microstructure. Intergranular martensite can also be produced in the weld metal and in the heat affected zone (HAZ) of welds of non-stabilized ferritic stainless steels. In this work, several heat treatments between 900oC and 1000oC, with different cooling rates, were performed in a commercial sheet of AISI 430 grade. Also, an autogenous welding was produced with GTAW process, and post weld heat treatment at 700°C was carried out. The different microstructures produced were analyzed by optical and scanning electron microscopy (SEM). The degree of sensitization was measured by double loop electrochemical potentiodynamic tests (DL-EPR). The pitting corrosion resistance was evaluated by cyclic polarization tests in 3.5%NaCl solution. Hardness and toughness tests were also performed in selected heat treatment conditions. The results indicate that the slow cooling results in a higher degree of sensitization than observed in the material rapid cooled from the annealing temperature. The ferritic martensitic structure produced by water cooling has higher pitting potential and lower degree of sensitization, but is brittle at room temperature. A subsequent tempering treatment between 600 and 800oC can increase the toughness, but the corrosion resistance may decrease due to carbides precipitation.The heat affected zone of AISI 430 welds contains intergranular martensite, which is brittle and susceptible to corrosion attack. Post weld heat treatment at 700oC decomposed the martensite into ferrite and carbides and improved the corrosion resistance.

Characterization and modelling of both high-cycle hightemperature fatigue behaviour of Stainless Steel Grades

In the same context of thermo-mechanical fatigue and high temperature applications of stainless steel, high-frequency vibration fatigue at high temperatures should be considered, in particular for automotive exhaust gas applications. In fact one of the most frequent incidents that can happen on exhaust components is an accumulation of low-cycle thermal fatigue and high-cycle fatigue. The prediction of the lifetime of a structure under such complex thermal and mechanical loading is therefore a constant challenge at high temperature due to the coupling of metallurgical, oxidation or creep effects. In order to better understand in a first approach, the high cycle fatigue of stainless steels at high temperatures, tractioncompression tests were performed on flat specimens at 25Hz, under air and in isothermal conditions from ambient temperature to 850°C. Two different stress ratios, R=-1 and 0.1, are characterized with the objective to assess a multiaxial model for high temperature. Different criteria are used to predict the ruin of a structure under high-cycle fatigue but in general for ambient-around temperatures. In particular, multiaxial and stress-based DangVan criterion is today widely used to evaluate the risk of fatigue cracks initiation and it has been implemented recently in our fatigue life processor Xhaust_Life®. Therefore the Dang Van criterion was identified from the isothermal high cycle fatigue using the 2 stress ratio and its validity is discussed especially for temperatures higher than 500°C where strain rate and creep effects have increasing influence. Results are presented for two ferritic stainless steel grades used in high temperature exhaust applications: K41X (AISI 441, EN 1.4509) and K44X (AISI 444Nb, EN 1.4521).

Thermomechanical fatigue behaviour of ferritic stainless steel grades for high temperatures applications

Nowadays high temperatures resistant materials are needed to resist to high temperature applications (up to 1000°C), such as automotive exhaust gas manifolds. Some developed stainless steel grades, including ferritic grades or austenitic refractory grades, can be used in this temperature range and both in continuous or cyclic thermal conditions. In order to predict the thermomechanical fatigue damage of stainless steel parts submitted to cyclic thermal loading and constrained bonding conditions, the elastoviscoplastic model by Chaboche is determined for a wide range of temperatures, of strain amplitudes and strain rate levels thanks to isothermal traction-compression tests. The validation procedure is performed afterward by comparison with stabilized behavior under non isothermal conditions on a dedicated thermal fatigue test performed on V-shape specimens. Results of simulation show very good fitting with the experimental curves which would lead to a more accurate fatigue life prediction. A damage model was derived from Taira’s thermal low-cycle fatigue model to include dwell-time period at high temperature and creep-oxidation effect. In this paper the example of K44X, a dedicated grade for high temperatures applications, is presented.