Cr Ferritic Steel Research Articles

The Cr-Nb-Si system: Improved thermodynamic modelling and its use in simulation of Laves phase in steel

Thermodynamic modelling of the Cr-Nb-Si ternary system is revised considering new findings in the binary Cr-Nb and extension to multicomponent systems. Thermodynamic model parameters of intermetallic phases are re-optimized based on density functional theory (DFT) calculations and experimental data. Particular attention was given to the transformations of Laves phase polytypes (i.e. C14 and C15). The calculated phase stabilities of technologically relevant Laves phase polytypes C14 and C15 are discussed. Their extension to multicomponent systems is tested for typical steel grades used in high-temperature applications. C14 is the dominant phase in high Cr ferritic steel.

Interatomic potential to study the formation of NiCr clusters in high Cr ferritic steels

Under irradiation NiSiPCr clusters are formed in high-Cr ferritic martensitic steels as well as in FeCr model alloys. In the literature little is known about the origin and contribution to the hardening of these clusters. In this work we performed density functional theory (DFT) calculations to study the stability of small substitutional NiCr-vacancy clusters and interstitial configurations in bcc Fe. Based on DFT data and experimental considerations a ternary potential for the ferritic FeNiCr system was developed. The potential was applied to study the thermodynamic stability of NiCr clusters by means of Metropolis Monte Carlo (MMC) simulations. The results of our simulations show that Cr and Ni precipitate as separate fractions and suggest only a limited synergetic effect between Ni and Cr. Therefore our results suggest that the NiCrSiP clusters observed in experiments must be the result of other mechanisms than the synergy of Cr and Ni at thermal equilibrium.

Effect of Sn on Corrosion Behavior of Ultra-pure 17 mass% Cr Ferritic Stainless Steels in Sulphuric Acid

The effect of Sn on corrosion behavior of ferritic stainless steels in 20 mass% H2SO4 was investigated by alternating current and direct current electrochemical methods and gravimetric tests at 25 °C. The results show that Sn can effectively improve general corrosion resistance of ferritic stainless steels in H2SO4, mainly due to highly raised hydrogen evolution overpotential which is in favour of strong suppression of hydrogen evolution reaction, and inhibitive effect of dissolved Sn2+ on elementary anodic reactions. With increasing Sn content, the better corrosion resistance can be reached. The Electrochemical Impedance Spectroscopy (EIS) includes four time constants and can be perfectly fitted by equivalent circuit: Rs(CdlRct)(QadsRadS(RL1 (L1 (RL2L2)))).

Influence of Nb content on grain size and mechanical properties of 18 wt% Cr ferritic stainless steel

The influence of Nb contents between 0.20 and 1.20wt% on the grain size and mechanical properties of 18wt% Cr ferritic stainless steel produced by investment casting was investigated. The average grain sizes of the three steels decreased apparently with increasing Nb content mainly due to the increasing number of pre-existing oxides formed at higher temperature, which were more likely to be the nuclei of heterogeneous nucleation. The thermodynamic analysis of Nb(C,N) formation was in conformity to the experimental result that the Nb(C,N) precipitates became larger with increasing Nb content. The as-cast specimen with the smallest grain size of steel C had the worse tensile strength and elongation in comparison with the as-cast specimens of steels A and B, mostly owing to the catenarian and dendritic Nb(C,N) particles distributed densely at the grain boundaries. The mechanical properties of specimens were not improved remarkably through high temperature solid-solution, whereas the mechanical properties of normalized specimens in the three steels were improved to different degrees. The coalescence and sparse distribution of smaller precipitates at grain boundaries after normalizing effectively weakened the local stress concentration arising from the reticular distribution of particles. The normalized specimen of steel A with 0.24wt% Nb still showed good mechanical properties. Normalizing at 850°C for 2h is the appropriate heat treatment for the 18wt% Cr ferritic stainless steel. The comparatively rational Nb content of the ferritic stainless steel is between 0.20 and 0.40wt% for investment casting production.

Creep Damage and Fracture in Advanced Tungsten Modified 9%Cr Ferritic Steel

Advanced tungsten modified 9%Cr ferritic steel (ASTM Grade P92) is a promising material for the next generation of fossil and nuclear power plants. Unfortunately, there are rather few published reports on damage processes in P92 steel during high temperature creep and the effect of damage evolution on the creep strength is not fully understood. In this work, the creep behaviour of P92 steel in as-received condition and after long-term isothermal ageing was investigated at 600 and 650°C using uniaxial tension creep tests. To quantify the effect of each damage process on the loss of creep strength, most of creep tests were followed by microstructural and fractographic investigations. It was found that the large Laves phase particles, which coarsened during creep exposure, served as preferential sites for creep cavity nucleation.

Composite Cu-LaFeO3 Coating on High Cr Ferritic Stainless Steels for IT-SOFC Interconnects

Lowering the operating temperatures of Solid Oxide Fuel Cells (SOFCs) at Intermediate Temperature (IT) range makes it possible to use cheap and robust metallic interconnects in place of the traditional ceramic parts. Ferritic stainless steels are ideal choice for SOFC interconnects due to mechanical properties and Thermal Expansion Coefficients (CTE) compatibility with the other parts of the cells, although they are notoriously subjected to various degradation processes in the operating environment. The growth of insulating chromia scales and the evaporation of chromium towards the cathode material are unacceptable issues that usually requires the application of protective coatings to ensure long-term stability of the stack performances. In last years several ceramic materials, particularly spinels and perovskites, have been investigated as protective layers, by applying different techniques for coating layer formation (1). Apart from conventional coating methods, a novel passivation technique based on spontaneous chemical reactions occurring in alkaline molten carbonate baths has been recently proposed by present authors (2,3). Dense and well-adhering La-Fe perovskite layers on austenitic and low chromium ferritic stainless steels have been obtained with this method. Evidence has been also provided that chemical composition of the salt bath may strongly affect the coating growth mechanism, allowing a morphology-tuned synthesis of the passivation layer. In this work, the influence of copper oxide addition to the molten carbonate bath is evaluated, with the aim of enhancing the electronic conductivity of the coating by producing a copper-perovskite composite layer. The passivation of the steel during the formation process is evaluated by means of in-situ electrochemical methods. Morphology and phase composition of the coatings are studied with Scanning Electron Microscopy and X-Ray diffraction analysis. The experimental results show that, under specific molten salt bath compositions, reduction of copper oxide and deposition of rounded metallic copper particles occurs on the coating surface simultaneously with the growth of the typical cubic-shaped LaFeO3 perovskite crystals. The Figure shows a typical evolution of the coating structure over the synthesis time. For over-exposed synthesis (i.e. 48h) the initially deposited metallic copper particles dissolve again in the bath leaving hollow perovskite grains. Bath chemical composition and synthesis time can therefore be tailored to obtain an effective Cu decorated LaFeO3 perovskite coating surface completely covering the steel substrate and with homogeneous dispersion of the Cu particles. The effect of adding copper on the LaFeO3coating will be examined for IT-SOFC interconnect applications. Acknowledgements This work was partially supported by the European Commission through the FP7 Fuel Cells and Hydrogen Joint Undertaking, Grant Agreement 325331 (Steel Coatings For Reducing Degradation in SOFC, SCORED 2.0). References Shaigan N, Qu W, Ivey DG, Chen W. A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources. 2010 Mar;195(6):1529–42.Frangini S, Masci A, Zaza F. Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells. Corros Sci. Elsevier Ltd; 2011 Aug;53(8):2539–48.Frangini S, Zaza F, Masci A. Molten carbonate corrosion of a 13-Cr ferritic stainless steel protected by a perovskite conversion treatment: Relationship with the coating microstructure and formation mechanism. Corros Sci. Elsevier Ltd; 2012 Sep;62:136–46. Figure 1

Methodology for corrosion evaluation in HAZ of 11%-Cr ferritic stainless steel

A novel methodology is proposed for corrosion-wear measurement in the Heat affected zone (HAZ) of 11%-Cr ferritic stainless steel. Weld beads with different stress-concentration were manufactured by using MIG/MAG process. After, the welded sample is extracted from the plate, the beads were bended and external stress was applied. Finally, they were inserted in ferric-chloride solution. Corrosive wear were assessed by means of optical microscopy in the HAZ by using polymeric resin mask and comparing profiles before and after inserting the sample into the solution. The results demonstrate the feasibility of the proposed methodology for assessing corrosive wear in the HAZ.

Long term high temperature oxidation characteristics of La and Cu alloyed ferritic stainless steels for solid oxide fuel cell interconnects

To ensure the best performance of solid oxide fuel cell metallic interconnects, the Fe-22 wt.% Cr ferritic stainless steels with various La contents (0.006–0.6 wt.%) and Cu addition (1.57 wt.%), are developed. Long-term isothermal oxidation behavior of these steels is investigated in air at 800 °C, for 2700 h. Chemistry, morphology, and microstructure of the thermally grown oxide scale are examined using XPS, SEM-EDX, and XRD techniques. Broadly, all the steels show a double layer consisting of an inner Cr2O3 and outer (Mn, Cr)3O4. Distinctly, in the La-added steels, binary oxides of Cr, Mn and Ti are found at the oxide scale surface together with (Mn, Cr)3O4. Furthermore, all La-varied steels possess the metallic Fe protrusions along with discontinuous (Mn, Cr)3O4 spinel zones at the oxide scale/metal interface and isolated precipitates of Ti-oxides in the underlying matrix. Increase of La content to 0.6 wt.% is detrimental to the oxidation resistance. For the Cu-added steel, Cu is found to segregate strongly at the oxide scale/metal interface which inhibits the ingress of oxygen thereby suppressing the subscale formation of (Mn, Cr)3O4. Thus, Cu addition to the Fe-22Cr ferritic stainless steels benefits the oxidation resistance.

Non-instantaneous growth characteristics of martensitic transformation in high Cr ferritic creep-resistant steel

Microstructural observation and high-resolution dilatometry were employed to investigate kinetics of martensitic transformation in high Cr ferritic creep-resistant steel upon different quenching/cooling rates. By incorporating the classical athermal nucleation and impingement correction, a non-instantaneous growth model for martensitic transformation has been developed. The developed model describes austenite/martensite interface mobility during martensite growth. The growth rate of martensite is found to be varied from 1 × 10−6 to 3 × 10−6 m/s. The low interface mobility suggests that it is not appropriate to presume the instantaneous growth behavior of martensite. Moreover, based on the proposed model, nucleation rate of martensite under different cooling rates is found to be nearly the same, while the growth rate of martensite is promoted by increasing the cooling rate.

Effect of Process Parameters on Microstructure and Hardness of Oxide Dispersion Strengthened 18Cr Ferritic Steel

Pre-alloyed ferritic 18Cr steel (Fe-18Cr-2.3W-0.3Ti) powder was milled with and without nano-yttria in high-energy ball mill for varying times until steady-state is reached. The milled powders were consolidated by upset forging followed by hot extrusion. Microstructural changes were examined at all stages of processing (milling, upset forging, and extrusion). In milled powders, crystallite size decreases and hardness increases with increasing milling time reaching a steady-state beyond 5 hours. The size of Y2O3 particles in powders decreases with milling time and under steady-state milling conditions; the particles either dissolve in matrix or form atomic clusters. Upset forged sample consists of unrecrystallized grain structure with few pockets of fine recrystallized grains and dispersoids of 2 to 4 nm. In extruded and annealed rods, the particles are of cuboidal Y2Ti2O7 at all sizes and their size decreased from 15 nm to 5 nm along with significant increase in number density. The oxide particles in ODS6 are of cuboidal Y2Ti2O7 with diamond cubic crystal structure (Fd \( \bar{3} \) m) having a lattice parameter of 10.1 A and are semicoherent with the matrix. The hardness values of extruded and annealed samples predicted by linear summation model compare well with measured values.

Effect of Sn on the Pitting Corrosion Behavior of Ultra‐Purified 17% Cr Ferritic Stainless Steel

The influence of Sn on pitting corrosion of ferritic stainless steels has been investigated by AC and DC electrochemical methods in 3.5 wt% NaCl and gravimetric tests in 6 wt% FeCl3 at 25 °C. 0.14 wt% Sn addition can improve pitting potential, while excessive addition (about 0.33 wt%) can deteriorate pitting corrosion resistance. Sn‐bearing steels have higher passive film resistance and faster growth rate of more compact passive film, in which the charge carrier density is reduced probably due to the presence of SnO2. Dissolved Sn2+ can inhibit elementary anodic reactions. Therefore, the content of Sn should be optimized within 0.1–0.23 wt% to obtain better corrosion resistance.

Effect of Tin on The Corrosion Resistance of 16 Cr Ferritic Stainless Steel in Acidic Solution and Chloride-Containing Media

Effect of Tin on The Corrosion Resistance of 16 Cr Ferritic Stainless Steel in Acidic Solution and Chloride-Containing Media

Improvement of Surface Ridging Resistance of an Ultra-purified Ferritic Stainless Steel by Optimizing Hot Rolling Condition

The effects of the finisher entry temperatures (FETs) on the surface ridging behavior for an ultra-purified 21%Cr ferritic stainless steel have been investigated. The results indicate that decreasing the FET facilitates the formation of in-grain shear bands in the hot rolled slab. The in-grain shear bands supplied recrystallization nucleation sites in grains during subsequent annealing through coalescence of subgrains, which is beneficial to refine the microstructures and intensify the {111}//ND textures. This effect will evolve to the final cold rolled and annealed sheet. The micro-texture analysis indicates that the formation of grain colony in the final sheet is weakened by decreasing the FET. Then, the surface ridging resistance of FSS is enhanced due to the optimizing of micro-texture distribution.

Development of a small punch testing method to evaluate the creep property of high Cr ferritic steel: Part I—Effect of atmosphere on creep deformation behavior

In a preliminary study carried out for evaluating creep properties with the small punch (SP) creep testing method, creep tests were performed using reduced activation ferritic/martensitic steel (RAFMs) F82H specimens both in an argon atmosphere and in vacuum (less than 10−3Pa). The results showed that the rupture time was approximately 2.5 times longer in the argon atmosphere compared to that in the vacuum. Changes in the deflection rate with the deflection and time and changes in the minimum creep deflection rate with the rupture time were almost independent of the test atmosphere, although the minimum creep deflection rate decreased as the rupture time increased. After comparing the SP creep test results, which were sorted by using the Larson-Miller parameter, and the standard creep test results, the ratio (F/σ) between the load (F) in the SP creep test and the stress (σ) in the standard creep test was determined to be 2.1 and 2.3 for the argon atmosphere and vacuum, respectively. Consequently, it was found that the ratio, namely, the load/stress conversion coefficient, depended on the test atmosphere. This result implied that the stress and/or strain distribution on the SP specimen varied with the type of atmosphere. This change with the test atmosphere seemed to be closely associated with changes in the friction between the specimen and the ball.

Development of a small punch testing method to evaluate the creep property of high Cr ferritic steel: Part II – Stress analysis of small punch test specimen by finite element method

From the creep rupture data for the reduced activation ferritic/martensitic steel F82H, the ratio between the load in the small punch (SP) creep test and the stress in the standard creep test is estimated to be 2.1 in an argon atmosphere and 2.3 in a vacuum. However, the reasons for the difference between the values of stress conversion coefficients and the physical meaning of these values have not been clarified. In this study, stress analyses of SP creep specimens were carried out for F82H by using finite element analysis to clarify the physical meaning of the stress conversion coefficient. The friction coefficient between the ball and specimen affected the SP creep properties, and an increase in the friction coefficient resulted in an increase in the rupture time. The applied load (F) and the averaged equivalent stress in the steady state (σs) correlated linearly, and the increasing friction coefficient tended to decrease σs. It was possible to replicate the test results obtained in the argon atmosphere with a friction coefficient of 0.39 and the results obtained in the vacuum with a friction coefficient of 1.3. It was also found that the SP creep test results, which were determined by σs, agree well with the standard creep test results.

Role of hard martensite phase prior to cold-rolling on microstructure evolution after annealing in ferritic stainless steel

To improve the surface quality of 16mass%Cr ferritic stainless steel sheets processed by cold-rolling and subsequent annealing, the role of the hard martensite phase, in particular, the hardness ratio k between martensite and ferrite prior to cold-rolling on microstructure evolution was investigated by varying the k value through tempering specimens at different temperatures after intercritical annealing. In the specimen with the high ratio k (k>2.0), the heterogeneous deformation in ferrite around the hard martensite becomes prominent and the areas with the high KAM value are locally limited with less development of cold-rolling texture. The specimen with the small ratio k (k<2.0), however, shows rather uniform deformation structure, because tempered martensite deforms easily. Therefore, the recrystallized grain with the specimen k>2.0 becomes fine, because the presence of hard martensite provides the nucleation site of recrystallization leading to fragmenting the elongated {100}<011>colony structure, and the orientation is randomized. Consequently, the presence of the hard martensite phase prior to cold-rolling is effective for preventing the ridging of ferritic stainless steel sheets.

Effect of Annealing Time on the Formability of a Ultra Purified Ferritic Stainless Steel Containing Copper

For developing of a ultra-purified ferritic anti-bacterial stainless steel, 1.4 Cu % was added in 21% Cr ferritic stainless steel. The effect of annealing time on the formability was analyzed. The micro texture of steel was measured by using electron back scattering diffraction analysis. The Cu precipitates through different annealing periods were investigated by means of transmission electron microscopy. Average plastic strain ratio was calculated by the r values at 0, 45°and 90° directions measured by tensile testing. The results shows that with the extension of annealing time, the morphology of the Cu precipitates became into rod from spherical and the size increased. Average plastic strain ratio increased first and then decreased, while the change trend of anisotropy index was opposite. The higher formability is due to the {111} texture components increase significantly, while more {001}&lt;110&gt; component at 60 min was adverse for the formability. And anisotropy index was determined by γ components and {110}&lt;110&gt; components.

Effect of aging temperature on the microstructures and mechanical properties of ZG12Cr9Mo1Co1NiVNbNB ferritic heat-resistant steel

The effect of aging on the mechanical properties and microstructures of a new ZG12Cr9Mo1Co1NiVNbNB ferritic heat resistant steel was investigated in this work to satisfy the high steam parameters of the ultra-supercritical power plant. The results show that the main precipitates during aging are Fe(Cr, Mo)23C6, V(Nb)C, and (Fe2Mo) Laves in the steel. The amounts of the precipitated phases increase during aging, and correspondingly, the morphologies of phases are similar to be round. Fe(Cr, Mo)23C6 appears along boundaries and grows with increasing temperature. In addition, it is revealed that the martensitic laths are coarsened and eventually happen to be polygonization. The hardness and strength decrease gradually, whereas the plasticity of the steel increases. What’s more, the hardness of this steel after creep is similar to that of other 9%–12%Cr ferritic steels. Thus, ZG12Cr9Mo1Co1NiVNbNB can be used in the project.

Effect of Temperature on Mechanical Properties of 9%Cr Ferritic Steel

Financial support from CICYT Projects MAT2012-39124 and PET2007-0475 is gratefully acknowledged.

Development of 9Cr Ferritic Steel Tube and Pipe SAVE12AD for Advanced Power Boilers

Development of 9Cr Ferritic Steel Tube and Pipe SAVE12AD for Advanced Power Boilers