Al Steel Research Articles

Oxidative passivation of Fe–Cr–Al steels in lead-bismuth eutectic under oxygen-controlled static conditions at 700° and 800 °C

Lead-bismuth eutectic (LBE) is a candidate heat transfer fluid for fast nuclear reactors and for concentrating solar power energy systems, due to its low melting point and beneficial thermophysical properties. Increasing the maximum operating temperature beyond 600 °C, can increase thermal efficiency of power conversion. However, LBE causes liquid-metal corrosion in many solids used as structural materials at high temperatures, due to increasing solubility of their constituents. A way to mitigate this issue is to establish a controlled oxidative passivation, by maintaining a desirable oxygen concentration in liquid LBE to promote formation of protective oxide scales on structural materials. In this study, the corrosion behavior of three Fe–Cr–Al alloys in LBE at 700 and 800 °C has been tested in two different oxygen concentrations. It was found that oxygen concentration of 1 × 10−6 wt% in LBE at both temperatures promotes formation of protective scales in all tested candidates. At 700 °C, these scales are more compact and well-adhered to bulk in all candidates. At 800 °C, lower-Al-containing candidates develop a thinner but adherent scale containing various oxides, while scales in higher-Al-containing candidates contain mostly Al-oxide and suffer more breaking and spalling off. However, oxygen concentration of 2 × 10−7 wt% proves to be insufficient for protective scales formation in all three Fe–Cr–Al alloys.

Study on Bainitic Transformation by Dilatometer and In Situ LSCM.

This study investigates the bainitic transformation kinetics of carbide-free bainitic steel with Si + Al and carbide-bearing bainitic steel without Si + Al, as well as the phase transformation and microstructure through in situ high-temperature laser scanning confocal microscopy. Results show that bainitic ferrite plates preferentially nucleate at the grain boundary. New plates nucleate on previously formed ones, including two dimensions which appear on a plane where a three-dimensional space of bainitic ferrite forms. Nucleation on the formed bainitic ferrite is faster than that at the grain boundary in some grains. The bainitic ferrite growth at the austenite grain boundary is longer and has a faster transformation rate. The bainitic ferrite growth on the formed bainitic ferrite plate is shorter and has a slower transformation rate. The location and number of nucleation sites influence the thickness of the bainitic ferrite. The higher the number of plates preferentially nucleating at the original austenite grain boundary, the greater the thickness of the bainitic ferrite.

Electrolysis-assisted recovery of superhydrophilicity from superhydrophobized surfaces

Superhydrophilicity has been widely used in industrial applications. Superhydrophilic metal surfaces are subject to becoming superhydrophobic when exposed in air, which is due to the accumulation of airborne organics on the surfaces. This wettability transition disables the desired functions and causes dramatic inconvenience in industrial applications. Therefore, a nondestructive treatment method is needed to recover the original superhydrophilicity. Herein, an electrolysis-assisted method is proposed to realize the recovery of wettability in which the metal surfaces are used as cathode in the electrolysis processing system. Bubbles resulted from electrolysis can effectively and efficiently remove the airborne organics absorbed on the surfaces, resulting in the recovery of superhydrophilicity. In addition, it is experimentally verified that this method can also be used to get rid of the low energy substances such as fluoroalkylsilaneis, from the chemically-treated superhydrophobic surfaces. What's more, the hierarchical microstructures and substrates are free of damage. The method can be repeatedly employed for the recovery of superhydrphilicity whenever the superhydrophilic surfaces are superhydrophobized. Notably, as far as we have investigated, this approach can be applied to metal materials such as Al, Ni, W, Ti, Cu and stainless steel.

Corrosion of low alloy steel in stagnant artificial or stirred natural seawater: The role of Al and Cr

AbstractCarbon steel and low alloy (containing Al and Cr) steel coupons were immersed for 6–7 months in stagnant artificial seawater or in natural marine site. The corrosion processes were studied via the combination of electrochemical techniques and the analysis of corrosion product layers by µ‐Raman spectroscopy and X‐ray diffraction. In natural seawater, the low alloy steel showed better resistance to corrosion and the best results were obtained when the mill scale was removed from the steel surface. This shows that improved corrosion resistance is due to a protective layer that forms spontaneously in the environment. In stagnant artificial seawater, the behavior of low alloy steel coupons (without mill scale) was compared with that of carbon steel coupons. The polarization resistance of the carbon steel coupons remained approximately constant over time. The corrosion product layer proved to be mainly composed of magnetite, an electronic conductor that does not hinder oxygen reduction. In contrast, the polarization resistance of the low alloy steel coupons increased with time, as the growing corrosion product layer, enriched with insulating FeOOH compounds, hindered oxygen diffusion.

Study on Micro Segregation of High Alloy Fe–Mn–C–Al Steel

The segregation patterns of solute elements in the Fe–17.1Mn–0.24C–1.38Al steel are systemically investigated by experiments and simulations. The electron probe microanalyzer (EPMA) is conducted to assess possible micro segregation. The result indicates that with the increase of solid fraction in dendrite arms, Mn shows a positive segregation tendency, C uniformly distributes, and Al content slightly fluctuates. Compared with micro segregation models, experimental results of C match well with the Brody‐Fleming model. The segregation ratios of Mn are in good agreement with the lever‐rule. There is no certain regularity displayed between the experimental and calculation results about the segregation ratios of Al. The matrix of the steel is inhomogeneous with severe Mn and C micro segregation in the inter dendritic zone, whereas Al shows the opposite trend. A multicomponent phase field method coupled to thermodynamic calculations is used to simulate the concentration profiles of solute elements in the dendrite arms, and the numerical results are in favorable agreement with the experimental ones. Finally, the roles of undercooling and cooling rate on dendrite morphology and micro segregation of Mn are investigated. It reveals that large undercoolings could improve Mn segregation, while cooling rates have little effect on the maximum Mn segregation concentration but contribute to reduce the total inter dendritic Mn amount. They both have refinement effects on the dendritic structure. Those are helpful to understand and improve the segregations in Fe–Mn–C–Al steels.

Application of (polyaniline/zeolite X) composite as anticorrosion coating for energy recovery devices in RO desalination water plants

Reverse osmosis desalination water plants have several operational issues such as corrosion in many areas like energy recovery devices that are accustomed to restore energy and scale back prices. Thus, there is a necessity to reduce the corrosion rate of the devices utilized in energy recovery. This study focuses on protection of some metals and alloys that are utilized in this industry by introducing a new anticorrosion coating. The new anticorrosion coating was formulated by the interaction of polyaniline and X-type zeolite that is employed as hosting material. Polyaniline was first formulated by conventional oxidation polymerization method of aniline in acidic medium at temperature below 5 °C by using ice bath. Then, ammonium persulphate was used as oxidizer. Numerous characterization techniques were used to demonstrate the encapsulation of polyaniline in X-type zeolite frames such as IR, UV–visible spectroscopy, scanning electron microscopy, transmission electron microscope, energy-dispersive X-ray and X-ray diffraction. The new anticorrosion coating was evaluated by using weight loss technique, Tafel polarization and electrochemical impedance spectroscopy; all of these techniques showed the effective anticorrosion properties of the prepared coating in which the corrosion rate from the polarization curves results, for all coated samples, was less than the bare ones as it recorded 2.403, 1.094, 23.48, 35.09 MPY for bare 304 and 316 stainless steel, Al and carbon steel, respectively, and 0.3132, 0.2733, 0.2506, 10.81 MPY for the coated samples. Corrosion results showed a noteworthy protection of the tested metals and alloys in saline water coated with the polyaniline/zeolite X.

Ab initio thermodynamics of complex alloys: The case of Al- and Mn-doped ferritic steels

In the context of Al and Mn-doped ferritic steels, we progressively elaborate an atomic-scale energy model to reproduce the thermodynamic behaviour of quaternary FeAlMnC on a bcc lattice. This model is built on physical concepts: DFT calculations, pair Hamiltonians, non-configurational thermal effects, these elements being combined in a reasoned way to lead to a mastered and predictive formulation. In particular, this approach allows to explore the correlation between ordering in substitutional ternary FeAlMn and interstitial carbon, which brings new elements to the metallurgy of carbon in ferritic steels.

Stabilizing austenite via a core-shell structure in the medium Mn steels

Retained austenite plays a significant role in the strength-ductility balance of medium Mn steels. In this contribution, a new processing route (Flash-Austenite Reversion Treatment, Flash-ART) was proposed to design retained austenite with a compositional core-shell structure in an Fe-0.20C-7.76Mn-1.99Al (wt%) steel. It was interestingly found that the austenite core, which should transform into martensite based on its composition and size, was stabilized by the Mn enriched shell. Flash-ART allows us to obtain more retained austenite compared with the conventional ART.

Effect of substituting Na2O for SiO2 on the non-isothermal crystallization behavior of CaO-BaO-Al2O3 based mold fluxes for casting high Al steels

The non-isothermal crystallization behaviors of CaO-BaO-Al2O3-based mold fluxes in which SiO2 had been replaced by Na2O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction. The initial crystallization temperatures, crystalline phases generated at different cooling rates, crystallization rates, Ozawa indices NO (as determined using Mo's equation) and effective crystallization activation energies (as determined using the Friedman equation) were evaluated. The initial crystallization temperature increased along with the Na2O content in the flux. In addition, the precipitation of CaF2 crystals was inhibited while the growth of CaAl2O4 crystals was enhanced at Na2O concentrations over 4 wt%. Increasing the cooling rate inhibited the precipitation of MgAl6O10, and of CaSiO3, MgAl2SiO6 and NaAlSi3O8, respectively, in fluxes containing 0, 4 and 8 wt% Na2O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na2O level was raised, as a direct result of changes in their NO values. The NS1 flux consistently had the lowest NO value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the NO values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na2O levels, and this was evidently an important factor affecting the initial crystallization temperature.

Dynamic Strain Aging and Serration Behavior of Three High-Manganese Austenitic Steels

The strain-rate dependence of the flow stress of three high-manganese austenitic steels with different compositions was studied in terms of dynamic strain aging and serration behavior at room temperature under quasi-static strain-rate ranges from 10−4 to 10−1 s−1. Tensile stress–strain curves showed that serrated flow occasionally appeared and the strain-rate dependence of flow stress was measured to be negative at all strains for Fe-22Mn-0.3C and Fe-30Mn-0.2C steels and at higher strain for Fe-30Mn-0.2C-1.5Al steel. Based on the tendency for a critical strain corresponding to the onset strain of serrated flow and electron back-scattered diffraction analysis, it was found that serrated flow was not induced by deformation twinning and $$ \gamma \to \varepsilon $$ martensitic transformation, but by dynamic strain aging associated with the interaction between partial dislocation and solute carbon. Also, the addition of Al content increased the critical strain, meaning that it effectively prohibits dynamic strain aging. In addition, dynamic strain aging promoted deformation twinning and/or $$ \gamma \to \varepsilon $$ martensitic transformation and resulted in negative strain-rate dependence of flow stress in the high-manganese austenitic steels.

Particle and microstructural characteristics in the coarse-grained heat-affected zone of Al–Ti–Ca complex deoxidized steels

Abstract The particles and microstructures in the coarse-grained heat-affected zones of Al–Ti–Ca complex deoxidized steels with different Al content were investigated by in-situ observation, various analytical techniques and calculations. The results show that the inclusions in high Al steel consist of Al–Ca oxides containing small amounts of TiO x surrounded by an outer layer of MnS. In low Al steel, the inclusions consist of Al–Ca–Ti oxides with high TiO x content covered by an outer layer of MnS, and the high TiO x content in the inclusions can remarkably enhance the ability of the inclusions to promote acicular ferrite nucleation. The fine-grained microstructure obtained in the coarse-grained heat-affected zones of low Al steel is attributed to the pinning effect of the finer particles and the formation of a high density of acicular ferrite.

Integrity of 5052 Al-mild steel dissimilar welds fabricated using MIG-brazing and cold metal transfer in nitric acid medium

Comparitive analysis of the corrosion resistance of H32 5052 Al alloy and galvanized mild steel lap joints fabricated by a) Metal Inert Gas (MIG) Welding-brazing and b) Cold metal transfer (CMT) was done by keeping wire feed rate and welding speed constant. The weld interfaces of 5052 Al and mild steel were characterized using Field emission scanning electron microscopy, X-ray diffraction and nano-indentation measurements. Weight loss tests were done as per ASTM G 67-04 on the lap joints. Results indicated formation of Al-Fe-Si layer at the weld bead-mild steel interface in both the techniques. The layer was found to be Aluminium rich in MIG –braze sample whereas it was iron rich in the CMT sample. The layer thickness was found to be higher in the toe region than the head region in both samples. The Al-Fe-Si layer preferentially dissolved during the weight loss test of the welds. The thickness of Al-Fe-Si layer for MIG-braze welds was greater resulting in higher intergranular corrosion.

Effect of atmospheric corrosion on Al and Ti thin film deposited on steel surface in urban region

Pure metals Aluminum (Al) and Titanium (Ti) thin films have been deposited on steel by using physical vapor deposition. The effect of these protective layers on atmospheric corrosion steel surface have been investigated after exposure the samples for one year to urban atmosphere. The rust morphology observed using Scanning Electronic Microscope (SEM) analysis consists of a cotton ball structure for the steel samples coated with Al, and flowery structure for samples coated with Ti. The Raman spectroscopy (RS) used in this study consolidates deeply the information about the structure of the rust layer. The Al coating shows the predominance growth of iron oxide as goethite. However in case of uncoated steel or Ti coating, the oxide structure was principally lepidocrocite. Raman shifting to high wavenumber values gave stretching corresponding to diaspore. The comparison of Raman bands for the Ti coating on steel to the literature suggests that in our case the Ti films are not converted to a protective layer of anatase or rutile during exposure. This result can explain the important growth of the rust layer in atmosphere and suggest that Ti acting as a getter for atmospheric pollutant. However, the Al steel coating present an interesting resistant to atmospheric corrosion.

Relationship between the microstructure and properties of thermomechanically processed Fe−17Mn and Fe−17Mn−3Al steels

Two austenitic Mn steels (Fe−17Mn and Fe−17Mn−3Al (wt%, so as the follows)) were subjected to thermomechanical processing (TMP) consisting of forging followed by solutionization and hot rolling. The rolled samples were annealed at 650 and 800°C to relieve the internal stress and to induce recrystallization. The application of TMP and heat treatment to the Fe−17Mn/Fe−17Mn−3Al steels refined the austenite grain size from 169 μm in the as-solutionized state to 9–13 μm, resulting in a substantial increase in hardness from HV 213 to HV 410 for the Fe−17Mn steel and from HV 210 to HV 387 for the Fe−17Mn−3Al steel. The elastic modulus values, as evaluated by the nanoindentation technique, increased from (175 ± 11) to (220 ± 12) GPa and from (163 ± 15) to (205 ± 13) GPa for the Fe−17Mn and Fe−17Mn−3Al steels, respectively. The impact energy of the thermomechanically processed austenitic Mn steels was lower than that of the steels in their as-solutionized state. The addition of Al to the Fe−17Mn steel decreased the hardness and elastic modulus but increased the impact energy.

Grain refinement in coarse-grained heat-affected zone of Al–Ti–Mg complex deoxidised steel

ABSTRACTThe particles and microstructure characteristics of coarse-grained heat-affected zone (CGHAZ) in Al–Ti–Mg killed steels with different Al contents were investigated. The results show that inclusion in high Al steel consists of Al–Mg oxide surrounded with a layer of MnS. However, inclusion in low Al steel is Al–Ti–Mg oxide covered with a layer of MnS, effectively promoting the formation of acicular ferrite. The precipitates of both steels are (Ti, Nb)N, the finer and more dispersed inclusions and precipitates in low Al steel can effectively inhibit austenite grain growth by grain boundary pinning during the thermal cycle. The fine-grained microstructure is obtained in CGHAZ of the low Al steel due to the pinning effect of finer particles and the high density of acicular ferrite.

Corrosion/Passivation Behavior of Concentrated Ionic Liquid Electrolytes and Its Impact on the Li-Ion Battery Performance

The corrosion/passivation phenomenon of Al and stainless steel (SS) with an ionic liquid-based electrolyte N-methyl-N-propyl-piperidinium bis(fluorosulfonyl)imide (PMpipFSI)-LiFSI was systematically studied by cyclic voltammetry, and its impact on the battery performance was evaluated in NMC532/Li cell. Our results showed that these electrolytes could eliminate the Al corrosion due to their capability of forming a passivation layer during the first anodic scan after 3 V vs Li+/Li. In contrast, no passivation behavior was observed for SS electrode, and the corrosion reaction was kinetically suppressed when the lithium salt concentration increases. We demonstrated that while 1 M LiFSI-PMpipFSI extensively corrodes SS in a NMC532/Li cell, the 5 M electrolyte does not and actually enables the normal cycling of the cell both using SS-2032-coin cells. Further, when an Al-coated coin cells were used, the Coulombic efficiency dramatically improved from 90% to >99.9% with superior capacity retention at both room temperature and 55°C.

Deformation Behavior of Al alloy and Steel Dissimilar Joints by Friction Stir Welding under Cyclic loading

Deformation Behavior of Al alloy and Steel Dissimilar Joints by Friction Stir Welding under Cyclic loading

Mechanical Behavior of High-Mn Steels Processed by Hot Rolling

The mechanical properties of Fe-28%Mn-1.5%Al and Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb (all in wt.%) steels subjected to hot plate rolling at a temperature of 1423 K with a total reduction of 60% were studied. The steels exhibited quite different mechanical properties in spite of almost the same original microstructures and similar stacking fault energies. The yield strength and total elongation of the Fe-28%Mn-1.5%Al steel are about 260 MPa and 45%, respectively, whereas those properties in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel comprise 350 MPa and 53%, respectively. The tensile flow stress vs strain curves of the hot rolled steel samples can be described by Ludwigson-type relations with parameters being dependent on the strengthening mechanisms. Frequent deformation twinning in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel promoted the strain hardening and improved the strength and ductility.

A Mechanochemical Route to Cutting Highly Strain-Hardening Metals

Highly strain-hardening metals such as Al, Ni, and stainless steels, although relatively soft, are well known as being difficult to cut, because of an unsteady and highly redundant mode of plastic deformation—sinuous flow—prevailing during chip formation. This difficulty in cutting is greatly ameliorated, if the workpiece surface ahead of the chip formation region is coated with certain chemical media such as glues, inks, and alcohols that are quite benign. High-speed imaging shows that the media effect a change in the local plastic deformation mode, from sinuous flow to one characterized by periodic fracture—segmented flow. This flow transition, due to a mechanochemical effect, results in significant reduction of deformation forces and energy, often > 50%, thus facilitating the cutting. The effect is mostly pronounced at smaller undeformed chip thickness, typical of finish and semi-finish machining regimes. The quality of the cut surface, as measured by defect density and surface roughness, improves by an order of magnitude, when the media are applied. Furthermore, this surface is relatively strain free in contrast to conventionally machined surfaces. The mechanochemical effect, with a strong coupling to the flow mode, is controllable, with the media showing similar efficacy across different metal systems. The results suggest opportunities for improving performance of machining processes for many difficult-to-cut gummy metals.

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Detailed studies on microstructure–property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel—17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase.