Segregation In Steels Research Articles

Probing rare earth segregation in steels

The challenge of characterizing the precise sites of rare earth (RE) atoms has limited our understanding of microalloying and the design of RE steels with superior performance in recent decades. Electron microscopy with near-atomic resolution can be used to identify RE retention or trapping in alloys but fails to provide reliable results in steel due to the extremely low solid solubility of RE atoms. Here, we probed the segregation and clustering of RE atoms by identifying the location of RE oxides via the in situ oxidation of these extremely active atoms in dendritic regions, ultimately providing direct visualization of the distribution of RE atoms. First-principles calculations confirmed that dendritic interfaces can serve as trapping sites for solute RE atoms. These findings effectively solve the problems caused by the use of traditional techniques to characterize RE atoms in steels and provide the first evidence for a solid solution of RE atoms.

Numerical study on effect of inclusions on hydrogen segregation in steel under stress conditions

It is generally believed that hydrogen embrittlement (HE) of metallic material is closely related to the presence of hydrogen traps such as inclusions inside. Therefore, the study of hydrogen segregation in the inclusions is essential for the further understanding of the mechanism of HE phenomena. In this study, a numerical model for exploring the hydrogen transport and hydrogen segregation in inclusions under stress conditions was developed, and the effect of inclusion's shape and orientation, micro-cracks and stress magnitude on hydrogen segregation was investigated. Results showed that, inclusion's shape made a great effect on the hydrogen concentration at the inclusions/matrix interface, which became higher as inclusion closed to a globular shape. The hydrogen concentration inside the inclusions was affected by both the shape of inclusions and the angle between the orientation of inclusions and the hydrogen diffusion direction. The hydrogen segregation in inclusions with micro-cracks was much greater than that without micro-cracks, which caused 3–4 times local hydrogen concentration increase in inclusions, leading to the enhancement of HE susceptibility of the steel. The magnitude of stress did not change the regions of hydrogen segregation, but the hydrogen concentration at each region increased with the increasing stress.

Grain boundary segregation in steels: Towards engineering the design of internal interfaces

Grain boundary segregation in steels: Towards engineering the design of internal interfaces

Multiscale thermodynamic analysis on fracture toughness loss induced by solute segregation in steel

A significant loss of fracture toughness () is induced by intergranular (grain boundary; GB) segregation of metalloid solute in alloy steels. Yet, the mechanism has not been clarified from a multiscale point of view. From a thermodynamic approach aided by first-principles calculations, it is shown that segregated solute with higher energetic stability on fracture surfaces causes a larger linear reduction in the ideal work to intergranular fracture (); i.e. the energy difference between a GB and its two fracture surfaces. Remarkably, the combined analysis with first-principles calculations and fracture mechanics experiments found several orders of magnitude more energy loss in for a specific range in the within only a few tenths of . These results illustrate that the GB of steel has the threshold energy of atomic cohesion under which catastrophic failure occurs. Our research scheme would play a key role in identifying specific solute elements to toughen the GB in metallic systems used under aggressive environments.

Kinetics of phosphorus segregation at grain boundaries of low-alloy low-carbon steel

In a temperature range of 280–320°C, the mechanism and kinetics of segregation of impurities in steels have yet remained insufficiently studied. Under these conditions diffusion of impurities in the bulk of steel grains practically ceases, and for describing the kinetics of the process it is incorrect to use the Langmuir-McLean equation. In this work we put forward two new approaches to describe the mechanism and kinetics of phosphorus segregation in steels: a model of sequential changes in the state of phosphorus based on first-order reactions and a model of diffusion redistribution of phosphorus between boundaries of carbide precipitates, structure defects, and boundaries of steel grains. A comparative analysis of the suggested models has been conducted, and estimates of the kinetics of segregation based on them have been made; these estimates have been compared with the experimental results obtained in the temperature range of 280–320°C for test times to ∼20 years. It has been shown that these models fairly well describe the experimental kinetics of phosphorus segregation in boundaries of steel grains.

Crack Failure Analysis on Kitchen Knife of 5Cr15MoV Steel

The cracking failure phenomenon of kitchen knife made by 5Cr15MoV steel was analyzed by chemical composition detection, hardness measurement, metallographic examination and SEM analysis methods. The results showed that the crack was mainly due to the organizational stress caused by carbide segregation in steel. Overhigh hardness and substandard chemical composition were the secondary cause of cracking.

First-principle Study of the Solution Type of Boron in γ-iron

The solution of boron in γ-iron is the foundation of the study of boron segregation in steel. There are three different viewpoints on boron solution in γ-iron: (i) substitutional type, (ii) interstitial type, and (iii) both types. In this paper, the solution type and the diffusion coefficient of boron atom in γ-iron are studied using the first principle. The results show that the formation energy of boron atom at octahedral site is smaller than that of boron atom at substitution site, and boron atoms prefer to occupy the octahedral sites in γ-iron. The boron atoms diffuse through interstitial sites and the diffusion coefficient is 2.3×10-6exp[-1.35(eV)/kT](m2/s).

Irradiation Assisted Grain Boundary Segregation in Steels

AbstractThe understanding of radiation-induced grain boundary segregation (RIS) has considerably improved over the past decade. New models have been introduced and much effort has been devoted to obtaining comprehensive information on segregation from the literature. Analytical techniques have also improved so that chemical analysis of layers 1 nm thick is almost routine. This invited paper will review the major methods used currently for RIS prediction: namely, Rate Theory, Inverse Kirkendall, and Solute Drag approaches. A summary is made of the available data on phosphorus RIS in reactor pressure vessel (RPV) steels. This will be discussed in the light of the predictions of the various models in an effort to show which models are the most reliable and easy to use for forecasting P segregation behaviour in steels. A consequence of RIS in RPV steels is a radiation induced shift in the ductile to brittle transition temperature (DBTT). It will be shown how it is possible to relate radiation-induced P segregation levels to DBTT shift. Examples of this exercise will be given for RPV steels and for ferritic steels being considered for first wall fusion applications. Cr RIS in high alloy stainless steels and associated irradiation-assisted stress corrosion cracking (IASCC) will be briefly discussed.

Phosphorus grain boundary segregation in steel 17-4 PH

A correlation between grain boundary etching of a martensitic steel and phosphorus segregation was established. Using metallographic experiments, the phosphorus diffusivity D in the steel and the Gibbs free energy Δ G P of phosphorus grain boundary segregation were determined: D=0.183 −0.174 +3.475×10 −4 exp(−229±19 kJ mol −1/RT) m 2 s −1, Δ G P=−43.1 kJ mol −1.

Kinetics of non-equilibrium grain-boundary segregation induced by applied tensile stress and its computer simulation

A set of kinetic equations is offered for non-equilibrium grain-boundary segregation induced by applied tensile stress. The simulation using the equations is in good accordance with the observation of sulfur grain-boundary segregation in steel.

Phosphorus segregation in CrMoV low-alloy steels

The phosphorus segregation at temperatures of 773 and 853 K has been studied in two low-alloy Cr–0.4 Mo–0.5 V steels with different chromium contents (0.85 and 2.5 mass%). Segregation of elements on grain boundaries was measured by means of Auger electron spectroscopy. Transmission electron microscopy techniques were used for the identification of secondary phases in the steels investigated. A significant influence of aging temperature on the kinetics and equilibrium value of phosphorus grain-boundary concentration was observed. Both lower temperature and higher bulk chromium content were found to increase the grain-boundary phosphorus segregation. This is in agreement with the Langmuir–McLean theory, the model of phosphorus–carbon site competition, and also confirms the significant effect of carbide formation on phosphorus segregation in steels.

Surface segregation of antimony on ferritic single crystals

Antimony has frequently been reported to cause temper embrittlement induced by grain boundary segregation in steels containing both nickel and chromium. In order to study the interdependent segregation behaviour of antimony and nickel and the effect of chromium on segregation of these elements, surface segregation was studied on Fe–25% Cr–2%Ni–0.15%Sb and Fe–2.5%Si–2%Ni–0.15%Sb single crystals. Segregation was investigated by means of Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) in the temperature range between 500 and 650°C. On Fe–25%Cr–2%Ni–0.15%Sb, segregation of antimony leading to surface saturation and cosegregation of nickel are observed on (100), (110) and (111) oriented surfaces. LEED investigations yield a complex domain structure which can be derived from a c(2×2) pattern for the (100) oriented surface and a (1×1) pattern for the (111) oriented surface. On the (110) oriented surface, reconstruction occurs. On Fe–2.5%Si–2%Ni–0.15%Sb, the investigated (100) oriented surface is also saturated with segregating antimony. Cosegregation of nickel occurs, but is weaker than on the crystal containing chromium; thus, chromium intensifies cosegregation of nickel with antimony. The LEED pattern exhibits a complex domain structure which is very similar to the structure observed on the (100) oriented surface of the crystal containing chromium.

Reducing axial segregation in steel with turbulent feed of the metal into the mold of a continuous caster

Reducing axial segregation in steel with turbulent feed of the metal into the mold of a continuous caster

Effect of niobium and titanium on tin precipitation in Fe alloys

In an attempt to manage tin segregation in steel, the formation of intermetallic compounds of tin with niobium, titanium, and aluminium was investigated. Two series of alloys based on Fe, Sn, and Al with either Nb or Ti additions were prepared by arc melting under argon. After cold rolling, the alloys were annealed at 850°C for 96 h. The microstructures were studied using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and microprobe analysis. The level of Nb addition was found to affect the formation of Sn rich compounds. For small Nb additions, no Sn containing compounds were found, whereas the higher Nb alloy exhibited a Fe2 Nb based Laves phase containing about 1 at.-%Sn. Some η-Fe2 Nb3 , which was associated with the alumina formation, was also found but did not contain tin. No Sn compounds were found with Ti additions. The experimental observations were compared with thermodynamic calculations performed using MTDATA software.

New developments in data processing of noisy images

In some of the methods used in metallography, the signal-noise ratio is so low that direct interpretation is not possible. With regard to microanalysis, the segregation in steels is currently being studied using quantitative X-ray mapping. The acquisition time and counting statistics are limiting factors, particularly for elements in low concentration that present low count rates compared with the background level. Noise from the useful information can be filtered out by geostatistical techniques. A new commercial software, IMAGEO, has been developed by the Ecole des Mines de Paris for noise filtering and signal deconvolution. The main features of this method and the software are described with detailed application examples. With this technique, it is now possible not only to get information from images that would be unusable, but, in the case of microanalysis, to save time for elements in higher concentration by using less counting time.

Measurement of Macrosegregations of Steels by X-Ray Microfluorescence

AbstractAn apparatus based on X-ray microfluorescence has been developed at IRSID in order to routinely and quantitatively determine the state of segregation in steels. The equipment consists principally of an iron anticathode X-Ray tube, a Si(Li) energy - dispersive detector and a “on the fly” operation in a multi-scale analysis mode. The acquisition of a chromium or manganese mapping in a (150 × 150 mm2) steel sample is possible over a total time of about one hour. The calibration curves for chromium or manganese are linear for the usual compositions of steels between 0.3% to 3%.

Macroprobe Mode for the Study of Segregation in Steels

During these last 10 years, high performance correction procedures have been developed for classical EPMA, and it is nowadays possible to obtain accurate quantitative analysis even for soft X-ray radiations. It is also possible to perform EPMA by adapting this accurate quantitative procedures to unusual applications such as the measurement of the segregation on wide areas in as-cast and sheet steel products.The main objection for analysis of segregation in steel by means of a line-scan mode is that it requires a very heavy sampling plan to make sure that the most significant points are analyzed. Moreover only local chemical information is obtained whereas mechanical properties are also dependant on the volume fraction and the spatial distribution of highly segregated zones. For these reasons we have chosen to systematically acquire X-ray calibrated mappings which give pictures similar to optical micrographs. Although mapping requires lengthy acquisition time there is a corresponding increase in the information given by image anlysis.

Comparative STEM and FEGSTEM analysis of grain boundaries in steels

A survey is presented of recent trends in high resolution analytical electron microscopy (AEM) as applied to grain boundary segregation in metals. Experimental data highlighting the spatial resolution and sensitivity of AEM for Si, P, and Mo interfacial segregation in steels are given. Confirmatory theoretical estimates of the limits of sensitivity and spatial resolution for solute enrichment produced by the nonequilibrium segregation mechanism are also derived. Practical and theoretical comparisons between thermionic gun and field emission gun scanning transmission electron microscopes are made. It is shown that significantly higher spatial resolution and sensitivity are achievable with the field emission gun systems. The use of higher accelerating voltages and thinner specimens should also be beneficial, although only the latter appears to be borne out in practice.

A new computer model of grain boundary segregation

Improvements have been made to an earlier model of grain boundary segregation. The new model includes an influx of vacancies into the region of the material considered by the model, and accounts for the effect of a grain boundary on defect-binding energies. The model predictions for boron segregation in steel are compared with the original model.

Cleaning effect of rare earths on grain boundary segregation in steels

Cleaning effect of rare earths on grain boundary segregation in steels