Mechanical Properties Of Ferritic Steels Research Articles

Mechanical Behavior of Oxide Dispersion Strengthened Steel Directly Consolidated by Rotary Swaging.

Among the main benefits of powder-based materials is the possibility of combining different constituents to achieve enhanced properties of the fabricated bulk material. The presented study characterizes the micro- and sub-structures and related mechanical properties of ferritic steel strengthened with a fine dispersion of nano-sized Y2O3 oxide particles. Unlike the typical method of preparation via rolling, the material presented herein was fabricated by direct consolidation from a mixture of powders using the versatile method of hot rotary swaging. The mechanical properties were evaluated at room temperature and also at 1300 °C to document the suitability of the prepared steel for high-temperature applications. The results showed that the imposed shear strain, i.e., swaging ratio, is a crucial parameter influencing the microstructure and, thus, material behavior. The workpiece subjected to the swaging ratio of 1.4 already exhibited a sufficiently consolidated structure with ultra-fine grains and featured high room-temperature microhardness values (up to 690 HV0.5), as well as a relatively high maximum flow stress (~88 MPa) when deformed at the temperature of 1300 °C with the strain rate of 0.5 s-1. However, the dispersion of oxides within this sample exhibited local inhomogeneities. Increasing the swaging ratio to 2.5 substantially contributed to the homogenization of the distribution of the Y2O3 oxide particles, which resulted in increased homogeneity of mechanical properties (lower deviations from the average values), but their lower absolute values due to the occurrence of nucleating nano-sized recrystallized grains.

Improving mechanical properties of P91 and IN617 dissimilar weld joints for advanced ultra super critical power plants by buttering technique

• Enhancement of mechanical properties using buttering layer for dissimilar weld. • Buttering helps in achieving favorable ductility for dissimilar welds. • With buttering creep rupture life of dissimilar welds has increased. • Buttered dissimilar welds can be implemented for very high-temperature boilers. To improve the mechanical properties of ferritic steel and nickel superalloy, while maintaining economic symmetry, this investigation is performed on fabricating dissimilar metal welds for advanced ultra-supercritical (AUSC) boilers, with a buttering technique. The present work compares a dissimilar weld of IN617 alloy and P91 steel with and without buttering by multipass gas tungsten arc welding for mechanical properties variations, creep strength at elevated temperature, and microstructural evolution. Thin layers of buttering material are applied at the ferritic end, followed by the closure weld consisting of welding a second member to a butter layered component. Further, significant improvement in mechanical properties (tensile strength, toughness, and hardness) and creep rupture life (though the failure remains ductile characterized by dimples variations) 2 to 3 times longer has been found with buttering in contrast with no buttering.

Hydrogen Diffusivity in Different Microstructures of 42CrMo4 Steel

It is well known that the presence of hydrogen decreases the mechanical properties of ferritic steels, giving rise to the phenomenon known as hydrogen embrittlement (HE). The sensitivity to HE increases with the strength of the steel due to the increase of its microstructural defects (hydrogen traps), which eventually increase hydrogen solubility and decrease hydrogen diffusivity in the steel. The aim of this work is to study hydrogen diffusivity in a 42CrMo4 steel submitted to different heat treatments—annealing, normalizing and quench and tempering—to obtain different microstructures, with a broad range of hardness levels. Electrochemical hydrogen permeation tests were performed in a modified Devanathan and Stachursky double-cell. The build-up transient methodology allowed the determination of the apparent hydrogen diffusion coefficient, Dapp, and assessment of its evolution during the progressive filling of the microstructural hydrogen traps. Consequently, the lattice hydrogen diffusion coefficient, DL, was determined. Optical and scanning electron microscopy (SEM) were employed to examine the steel microstructures in order to understand their interaction with hydrogen atoms. In general, the results show that the permeation parameters are strongly related to the steel hardness, being less affected by the type of microstructure.

Influences of interstitial and extrusion temperature on grain boundary segregation, Y−Ti−O nanofeatures, and mechanical properties of ferritic steels

Doping with interstitials influences the grain boundary (GB) composition of metallic alloys, enabling changes in elemental GB enrichment, grain size, and mechanical properties or even promoting nanoparticle formation. Yet, little efforts on these doping effects have been made in oxide dispersion-strengthened (ODS) steels. Here, by combining advanced microscopy techniques, we studied the impact of interstitial concentration and extrusion temperature on the GB structure-dependent solute segregation, Y−Ti−O nanofeatures, and mechanical properties of ferritic Fe–14Cr (wt%) ODS steels fabricated by ball milling. We found that doping with high carbon and oxygen contents causes the GB to be decorated with the interstitials and promotes nanoparticle formation along the GBs, thereby retarding capillary-driven grain coarsening. This effect performs twofold, through grain size refinement and particle hardening. For samples with low interstitial contents, altering the extrusion temperature does not significantly change the material's mechanical properties and microstructure or the nonstoichiometric chemistry of nanoparticles, which are highly stable at high temperatures. Further, for all the samples, Y–Al oxides in the initial precipitation stages rapidly become coarsened at high temperatures, as Al weakens the thermal stability of nanoparticles, thereby transforming them to core-shell structures with Y−Al-rich cores and Ti−O-rich shells in the later precipitation stages.

Effect of carbon and alloying solute atoms on helium behaviors in α-Fe

Helium bubbles could strongly degrade the mechanical properties of ferritic steels in fission and fusion systems. The formation of helium bubble is directly affected by the interactions between helium and the compositions in steels, such as solute atoms, carbon and irradiation defects. We thereby performed systematical first-principles calculations to investigate the interactions of solute-helium and carbon-solute-helium. It is found that substitutional helium is more attractive than interstitial helium to all the considered 3p, 4p, 5p and 6p solutes. The attraction between carbon and substitutional helium suggests the carbon-solute-helium complex can be formed stably. By examining the charge density difference and thermal stability, it is found that the ternary complex shows stronger attraction with He than that of solute-helium pair for some solutes (S, Se, In, Te, Pb and Bi) and the complex could existed in iron stably at 700 K. The present theoretical results may be helpful for exploring alloy additions to mitigate the formation of large helium bubbles.

Effect of In Situ Nano-Particles on the Microstructure and Mechanical Properties of Ferritic Steel

Transmission electron microscopy (TEM), optic microscopy (OM), and small-angle- X-ray-scattering(SAXS)are combined to comprehensively characterize the effect of endogenous nano-particles obtained by a new method, on microstructure and mechanical properties of the ferritic steel. Morphology of nano-particles, the change of microstructure, and the performance of steel are systematically studied. Results indicate that the endogenous nano-particles are titanium oxide which not only has a positive effect on refining the structure of ferrite steel, but also can significantly improve its strength without much losing its plasticity. Tensile tests reveal that tensile strength and yield strength of the investigated steel are increased from the original 392 and 247 MPa to 524 and 430 MPa, respectively. However, the value of the elongation-to-failure is only decreased by 3%, reached to 25% ultimately.

Effect of preliminary diffusion oxidation on mechanical properties of ferritic steel in oxygen-containing lead

The influence of preliminary diffusion oxidation on mechanical properties of ferritic steel SUH409L (Fe-11Cr) in oxygen containing stagnant lead melt (10−6–5×10−8wt.%) in the temperature range 400–600°C has been studied. Oxidation in the temperature range 600–800°C for 24–150h results the formation of thin oxide films (1–4μm). The thickness and composition of films depend on temperature and duration of oxidation. In particular, the increase in temperature leads to the evolution of oxide's phase composition from magnetite to chromium-containing spinel and discontinuous formation of chromium oxide Cr2O3. Another effect of heat treatment during oxidation is the growth of grain size of steel with increasing temperature and duration. The oxidation leads to the decrease of tensile strength and ductility of steel that correlates with increasing grain size. In lead the strength of pre-oxidized steel reduces. It manifests more significant for oxidation modes which lead to maximum growth of grain sizes. It was proved that the presence of oxide layer limits direct contact between metal and lead that consequently prevents steel's embrittlement. The oxidation which minimally affects on the grain sizes of steel (600°C and 800°C for 24h) provides the minimal losses in plasticity in lead.

Effects of Mn partitioning on nanoscale precipitation and mechanical properties of ferritic steels strengthened by NiAl nanoparticles

The critical role of Mn partitioning in the formation of ordered NiAl nanoparticles in ferritic steels has been examined through a combination of atom probe tomography (APT) and thermodynamic and first-principles calculations. Our APT study reveals that Mn partitions to the NiAl nanoparticles, and dramatically increases the particle number density by more than an order of magnitude, leading to a threefold enhancement in strengthening. Atomistic structural analyses reveal that Mn is energetically favored to partition to the NiAl nanoparticles by preferentially occupying the Al sublattice, which not only increases the driving force, but also reduces the strain energy for nucleation, thereby significantly decreasing the critical energy for formation of the NiAl nanoparticles in ferritic steels. In addition, the effects of Mn on the precipitation strengthening mechanisms were quantitatively evaluated in terms of chemical strengthening, coherency strengthening, modulus strengthening and order strengthening.

Synergistic effects of Cu and Ni on nanoscale precipitation and mechanical properties of high-strength steels

There is an increasing demand for ultrahigh-strength ferritic steels strengthened by nanoprecipitates. Improvement of the precipitation strengthening response requires an understanding of the nanoscale precipitation mechanisms. In this study, the synergistic effects of Cu and Ni on nanoscale precipitation and mechanical properties of ferritic steels were thoroughly investigated, and new steels with ultrahigh strength and high ductility have been developed. Our results indicate that Ni effectively increases the number density of Cu-rich nanoprecipitates by more than an order of magnitude, leading to a substantial increase in yield strength. It appears that Ni decreases both the strain energy for nucleation and the interfacial energy between the nucleus and the matrix, thereby decreasing the critical energy for nucleation of Cu-rich nanoprecipitates. Cu and Ni are also found to be beneficial to grain-size refinement, resulting from lowering the austenite-to-ferrite transformation temperature, as determined from thermodynamic calculations. In addition, the strengthening mechanisms of Cu and Ni were quantitatively evaluated in terms of precipitation strengthening, grain refinement strengthening and solid-solution strengthening. The current findings shed light on the composition–microstructure–property relationships in nanoprecipitate-strengthened ferritic steels.

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Effect of Decarburization on Structural and Mechanical Properties of Ferritic Steels

The consequences of decarburization on the creep properties of 2.25 Cr–1 Mo steel have been analyzed. First, a relationship between the creep rate and the carbon concentration as a function of temp...