Grain boundary segregation of phosphorus and

This paper reports a study of grain boundary segregation of phosphorus and sulfur in types 304L and 316L stainless steel. Phosphorus was found to segregate in both alloys, but the amount of segregation depended on several factors. These included the time and temperature of the aging heat treatment, the total composition of the steel, and compositional banding. Sulfur was usually precipitated as chromium-rich sulfides and the amount of elemental segregation was low. The correlation between segregation of phosphorus and corrosion in the Huey test was not exact. This fact appeared to result from the contribution of other microstructural elements to the corrosion process.     

The effect of phosphorus segregation on the intermediate-temperature embrittlement of ferritic,spheroidal graphite cast iron

The objective of this article is to study the effect of phosphorus segregation on the fracture modes of the intermediate-temperature intergranular embrittlement which occur in ferritic, spheroidal graphite cast iron. The specimens were quenched from 820 °C and 500 °C during the furnace-cooling period of ferritization annealing in order to vary the degree of phosphorus segregation, then deformed in tension at various temperatures between 20 °C and 520 °C with a constant crosshead speed of 0.01 mm/s. These two kinds of specimens were also fractured by impact at about -50 °C in the vacuum chamber of a scanning Auger microscope in order to analyze the phosphorus segregation and compare the fracture modes. The results show that the fracture mode of the intermediate-temperature embrittlement is influenced by the history of heat treatment prior to tension. When the specimens were held at 500 °C and quenched from this temperature, the fracture was intergranular. However, the specimens quenched from 820 °C revealed cleavage fracture with cracks propagating radially from a central region with magnesium-rich particles. Identified by transmission electron microscopy (TEM), the particles were MgO. Grain-boundary segregation of phosphorus in the specimen held at 500 °C was confirmed by Auger analysis of the impact fracture surface. Segregation of phosphorus must play an important role in the fracture mode of the intermediate-temperature intergranular embrittlement.     

Application of a thermodynamic database to the calculation of surface tension for iron-base liquid alloys

Thermodynamic models based on Butler's equation for surface tension of liquid alloys has been discussed. In alloys, in which activities of components deviate largely from Raoult's law, the calculated surface tensions are found to be affected by the selection of the ratio of the coordination number in the surface phase to that in the bulk phase. Then, the surface tension of liquid Fe-Al, Fe-Co, Fe-Cr, Fe-Mn, Fe-Mo, Fe-Ni, Fe-Si, Fe-Ti and Fe-W binary alloys and liquid Fe-Cr-Ni ternary alloys have been calculated from thermodynamic data in a database constructed by Kaufman. The calculated results reproduce the concentration dependence of the surface tension in those alloys reported so far, but their absolute values are dependent upon the selection of surface tension values for pure elements.     

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between A_cm and A₁. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.     

Surface tension and wettability studies of liquid Fe-Ni-O alloys

Surface tensions of iron-nickel alloys were measured as a function of oxygen potential at 1550 °C using the sessile drop technique. The surface tension of pure liquid nickel and iron-nickel alloys was measured at a total pressure of 1 atmosphere under varying CO₂/CO ratios. An increase in the oxygen potential in the gas phase was found to correspond to a decrease in surface tension of pure nickel and iron-nickel alloys, indicating that oxygen is surface active in both liquid nickel and iron-nickel alloys. At low oxygen potentials, nickel additions to liquid iron were found to cause small decreases in alloy surface tensions; however, at higher oxygen potentials, the surface tension of the alloy exhibited a minimum value as nickel was added to iron. The adsorption coefficients of oxygen in liquid iron-nickel alloys and pure liquid nickel were determined from the surface-tension data using Belton’s analysis, and were found to be similar to those calculated from kinetic studies. Wettability of iron-nickel alloys on an alumina substrate was studied through contact-angle measurements. At a constant alloy nickel content, the contact angle between the alloy and alumina decreased with increased oxygen potential in the gas phase.     

Effect of phosphorus on solidification process and segregation of directionally solidified IN738 superalloy

The effect of phosphorus on the solidification and the solute segregation of a directionally solidified IN738 Ni-based superalloy was investigated experimentally employing a method of partially directional solidification and subsequent quick quenching. It was found that the phosphorus addition widened the solidus-liquidus temperature interval of the alloy. Both the P content and the solidification rate affected the morphology of the solid-liquid interface and the precipitation of phosphides in the alloy. Phosphorus segregated largely in the intercellular/interdendritic regions and promoted the segregation of other alloying elements in the solidified structure. Formerly Graduate Student, Institute of Metal Research, Academia Sinica     

Effect of dissolved oxygen on the surface tension of liquid iron

Abstract

The oscillating drop technique was used to measure the surface tension of liquid iron-oxygen alloys in the temperature range 1560–1645°C. No noticeable temperature dependence of the surface tension was observed. The surface activity of oxygen in iron was determined to be 2.1 × 10⁶ dynes/cm. This value is twice as large as the accepted value in the literature. According to the monolayer approximation, the liquid iron surface becomes saturated with oxygen when each adsorbed oxygen atom occupies an area of 11.4 Ų. Data-analysis suggests that saturation of the surface of liquid iron with oxygen results from the formation of a two-dimensional ionic compound. This compound seems to correspond to Fe₂O or FeVO, where V stands for a vacancy.

Résumé

La technique de la goutte oscillante a ét é employ ée pour mesurer la tension superficielle des alliages liquides fer-oxyg ène entre 1560°C et 1645°C. Aucune variation de la tension superficielle avec la temperature n'a été observée. L'activité suferficielle de l'oxygéne dans le fer a été établie comme étant 2.1 × 10⁶ dynes/cm. Cette valeur est deux fois celle admise dans la littéature. Selon l'approximation d'une monocouche, la surface du fer liquide sera saturée d'oxygé ne quand chaque at orne d'oxygene absorbe occupe une superficie de 11.4 A². L'analyse des données indique que la saturation de la surface du fer liquide par l'oxygéne résulte de la formation d'un composé ionique bi-dimensionnel. Il semble que ce compose corresponde à Fe₂O ou FeVO, ou V est une lacune dans le réseau.     

Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

Effect of alloying on the morphological and technological characteristics of eutectic alloys based on iron

The effect of alloying with aluminum and cerium on morphological features and production characteristics of eutectic alloys based on iron is studied. An aluminum content from 0.5 to 3.0% modifies particle shape from compact and rounded to particles with a very developed surface. A reduction in the segregation point of impurity elements, primarily sulfur and phosphorus, is observed on adding 0.05–0.16% of cerium. Cerium also promotes formation of spherical particles which provides good production characteristics.     

Influence of sulfur,phosphorus, and antimony segregation on the intergranular hydrogen embrittlement of nickel

The effectiveness of sulfur, phosphorus, and antimony in promoting the intergranular embrittlement of nickel was investigated using straining electrode tests in IN H₂SO₄ at cathodic potentials. Sulfur was found to be the critical grain boundary segregant due to its large enrichment at grain boundaries (10⁴ to 10⁵ times the bulk content) and the direct relationship between sulfur coverage and hydrogen-induced intergranular failure. Phosphorus was shown to be significantly less effective than sulfur or antimony in inducing the intergranular hydrogen embrittlement of nickel. The addition of phosphorus to nickel reduced the tendency for intergranular fracture and improved ductility because phosphorus segregated strongly to grain interfaces and limited sulfur enrichment. The hydrogen embrittling potency of antimony was also less than that of sulfur while its segregation propensity was considerably less. It was found that the effectiveness of segregated phosphorus and antimony in prompting intergranular embrittlementvs that of sulfur could be expressed in terms of an equivalent grain boundary sulfur coverage. The relative hydrogen embrittling potencies of sulfur, phosphorus, and antimony are discussed in reference to general mechanisms for the effect of impurity segregation on hydrogen-induced intergranular fracture.     

Time Dependent Effects in Surface Tension Measurements of an Industrial Fe‐alloy

The surface tension of an industrial FeC‐alloy containing Si, Mn and S in the few atomic per cent range was measured by the oscillating drop method in ground based electromagnetic levitation and under reduced gravity conditions on board a parabolic flight. The results from the parabolic flight exhibited a large variation between different heating cycles and a discrepancy with the ground based experiments with regard to the value of the surface tension at the liquidus temperature and the temperature coefficient. The variation of the surface tension temperature coefficient is interpreted as resulting from the loss of volatile surface active components such as S and Si in subsequent heating cycles. The results demonstrate the importance of the time scale of the measurement and ambient atmosphere for surface tension measurements of industrial alloys containing volatile surface active components. For comparison with the experimental values the surface tension and segregation were modelled using different models. As a result of the analysis, the surface tension as a function of temperature best representing the starting industrial alloy is obtained as σ(T) = 1.52–1.53 10^‐4 (T–1752K) Nm^‐1.     

Grain boundary segregation in austenitic stainless steels and its effect on intergranular corrosion and stress corrosion cracking

This paper reports a study of grain boundary segregation, intergranular corrosion, and intergranular stress corrosion cracking in austenitic stainless steels. The results show that phosphorus, nitrogen, and sulfur all segregate to grain boundaries in these materials and that they can affect one another's segregation through site compctition. In particular, the results demonstrate that phosphorus segregation can be lowered by the presence of nitrogen and sulfur in the steel. Also, if manganese is present in the steel, sulfur segregation will be greatly decreased as a result of formation of manganese sulfides. Phosphorus, sulfur, and nitrogen will not initiate intergranular corrosion in the modified Strauss test, although if corrosion is initiated by chromium depletion, these elements might enhance the corrosion process. Phosphorus segregation does enhance corrosion in the Huey test, even in steels that have not undergone grain boundary chromium depletion, although there does not appear to be a precise correlation between the depth of corrosion penetration and phosphorus segregation. Intergranular stress corrosion cracking in 288 °C water at a pH of 2.5 and electrochemical potential of OV_SHE can occur in these steels even in the absence of chromium depletion if sulfur is present on the grain boundaries. Phosphorus segregation appears to have very little effect.     

The density and surface tension of dilute liquid Na-In alloys and comparison with liquid Na-Cd alloys

The density and surface tension of five liquid Na-ln alloys, containing between 0.5 and 7 at. pct In, have been measured in the temperature range 170° to 400°C using a maximum bubble pressure technique which incorporates an automatic pressure measuring and recording device. The results are compared with corresponding data reported previously for Na-Cd alloys. The gram-atomic volumes of the Na-ln alloys, calculated from the densities, indicate a substantial contraction on alloying which is, on average, about double that for the Na-Cd alloys and qualitatively consistent with thermodynamic data for the two systems. The surface tension of liquid sodium is increased slightly on adding indium, indicating a lower indium concentration in the surface than in the bulk, in contrast to the marked surface active behavior of cadmium. The surface excess concentrations of indium and cadmium are calculated using Gibbs’ adsorption equation. The surface excess entropy, estimated from the temperature dependence of the surface tension, is compared and briefly discussed for the two systems.     

Fundamental research on phosphorus behaviour in the smelting reduction process

The behaviour of phosphorus in the smelting reduction process was investigated in this paper. Experiments were carried out under the conditions of smelting reduction: the phosphate reduction in molten slag by solid carbon, liquid iron saturated with carbon and CO gas, respectively. Furthermore, experiments on the phosphate reduction in pre-reduced iron ore by liquid iron saturated with carbon were carried out respectively when liquid iron was in permanent contact with molten pre-reduced iron ore, and when powders of the pre-reduced iron ore were injected into the iron bath from the bottom. The results showed that there was considerable unreduced P₂O₅ in the slag. Part of the reduction product of phosphorus escaped from the bath, while the other part was absorbed by liquid iron. The pig iron with low content of phosphorus can be obtained by smelting reduction in comparison with blast furnace. In the process of liquid iron bath with coal-oxygen-ore injection, the phosphorus content in liquid iron could be kept constant in 100 kg scale tests.     

The surface tension of Fe-Y and Fe-C-Y alloys at 1550°C

The surface tensions of liquid Fe-Y and Fe-C-Y alloys containing 0 to 2 wt pct C and up to 0.8 wt pct Y were measured at 1550°C with a sessile drop technique. Yttrium was found to be surface active in both Fe-Y and Fe-C-Y alloys in the composition ranges studied. The addition of carbon reduces the surface tension of Fe-Y alloys. This work is based on a portion of a thesis submitted by R. MRDJENOVICH to Wayne State University in partial fulfillment of the requirements for the degree Master of Science in Metallurgical Engineering.     

Surface segregation in austenitic stainless steel

We report in this paper a study of surface segregation in austenitic stainless steel. Auger electron spectroscopy was used to measure segregation as a function of time and temperature. We have found that P, N, S, Cr, and Ni will all segregate to the surface. However, their presence on the surface often depends on the competitive and attractive interactions between the various elements. We show that thermodynamic data on ternary liquid iron alloys are quite valuable in predicting these interactions. We also discuss possible applications of these studies.     

From viscosity and surface tension to marangoni flow in melts

This article covers some of our recent work on slag viscosity, the surface tension of liquid Cu-O alloys, and the relative role of Marangoni and bulk flow on refractory wear in iron-silicate slags. A viscosity model developed for slags containing SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, MnO, FeO, PbO, NiO, Cu₂O, ZnO, CoO, and TiO₂ is capable of representing the effects of temperature, silica, and network-modifier cations within a wide range of temperatures and compositions. It forms a useful part of a computational package for multiphase-equilibrium (MPE) calculations and for predicting slag viscosities. The models are well applicable to a range of industrial slags (blast furnace, new iron making, base-metal and Platinum Group Metals (PGM) smelting, and coal-ash slags). The package has also some capability of predicting the viscosity of slags containing suspended solids. The surface tension of liquid copper-oxygen alloys has also been analyzed. The adsorption behavior of oxygen in liquid copper is well represented by the combined Langmuir-Gibbs isotherm. According to the rate data for silica-rod dissolution in liquid iron-silicate slags at 1573 K, the preferential attack at the slag line diminishes as the linear velocity of flow at the surface of the rotating silica rod reaches 9 to 16 cm/s. A tentative analysis gives the critical condition, that relates the critical Reynolds (Re) and Marangoni (Ma) number by the equation Re*²=0.13 Ma*. This article is dedicated to Professor Ken Mills. The authors have been inspired by Ken and have benefited greatly from his encouragement in their research in a number of areas. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Phosphorus interdiffusivity inα-Fe binary and alloy systems

The isothermal segregation kinetics of phosphorus to Fe and Fe-based alloy surfaces have been monitored from 783 to 923 K using Auger electron spectroscopy. The P segregation kinetics are consistent with a model which assumes bulk diffusion of P to be the rate controlling mechanism in the segregation process. The activation energy and preexponential for P diffusion in Fe calculated from the diffusion data are 314 ± 30 kJ · mol⁻¹ and 8 × 10⁵ cm²s in the temperature range studied. Similar results are found for the carbon free Fe-based alloys studied. Phosphorus segregation kinetics for a carbon containing alloy are more rapid, with evidence of a dependence on carbon concentration. The activation energy and pre-exponential calculated in this study for carbon-free alloys are considerably higher than published values measured at higher temperatures by conventional techniques. It is pointed out that an increase in activation energy with decreasing temperature is consistent with observations in several studies of Fe and other ferromagnetic alloys of an increase in activation energy for diffusion below the ferromagnetic transition temperature.     

The role of interphase boundary adsorption in the formation of spheroidal graphite in cast iron

The interphase boundaries in gray and ductile cast iron were studied with a scanning Auger microprobe (SAM). Sulfur and oxygen were found to be adsorbed at the flake/ metal interfaces in the gray iron, while the nodule/metal and intercrystalline graphite interfaces in the ductile iron were free of foreign elements. The only magnesium detected in the magnesium modified ductile iron was combined with phosphorus and sulfur as a compound. A model is presented which proposes that Fe−C−Si eutectic alloys in the absence of surface active impurities (such as in vacuum casting of high purity materials) produce nodular graphite due to the inherent instability of the graphite/melt interface. The sulfur and oxygen always present in commercial alloys adsorb at the graphite/melt interface, effectively “stabilizing” the active sites on the graphite basal planes, and preventing spherulitic growth. The purpose of modifiers is to getter these impurities.     

Effect of carbon and sulphur on the surface tension of molten iron

Surface tensions of Fe‐4%C‐S alloys were measured at 1623 and 1823 K using the sessile drop technique. Thermodynamic models based on Butler's equation for surface tension of liquid alloys have been compared with experimental results. Calculated values are found to be in good agreement with those of the experimental data of the system. At the same sulphur activity, the effect of carbon on the surface tension of Fe‐C‐S alloys was found to extrude only when the sulphur content was less than 0.005 %.