Segregation of phosphorus and boron in austenite in Fe–10%Mn–P–B-alloys

Equilibrium segregation of phosphorus at the grain boundaries of austenite has been studied in Fe–10%Mn–P–B-alloys. The samples were equilibrated at temperatures of 750–1100°C and analysed after rapid quenching using Auger-electron spectroscopy. The results show that boron markedly reduces segregation of phosphorus in austenite. Boron was also found to be segregated at the prior austenite grain boundaries. The intergranular boron concentration increases slightly with a rising austenitising temperature, but does not show any dependence on boron or phosphorus content of the alloy. The results can be explained by assuming segregation equilibria and mutual displacement between B and P in austenite, a value of ?97 kJ/mole for the free energy of boron segregation and ?47 kJ/mole for phosphorus segregation.     

Effects of boron on the creep properties,phosphorus segregation behaviour,and stress relief cracking susceptibility of 1.5% Cr–0.5% Mo steels

The effects of 25 ppm boron and of 0.05% phosphorus on the creep behaviour at 550 °C and on the fracture temperature in the constant load fracture test were tested for a 1.5% Cr–0.5% Mo steel. B and P decrease the creep strength, the rupture elongation is increased by B. B and P lower stress relief cracking susceptibility, however, it is increased at low stress. In the B doped material the rate of P grain boundary segregation is accelerated and the level of equilibrium segregation is somewhat higher, the equilibrium segregation is somewhat lower in the bainitic than in the martensitic structure.     

Effect of vanadium on grain boundary segregation of phosphorus in low alloy steels

The phosphorus grain boundary segregation at 853 K was investigated in three low alloy steels with different vanadium content. Kinetic dependence of the phosphorus grain boundary concentration was determined experimentally by means of AES and described theoretically, as well. To assess the influence of the individual alloying elements on the phosphorus segregation, the metal composition of carbide phases at 853 K was predicted by means of thermodynamic calculations and confirmed by experimental measurements (TEM + EDX). The vanadium was found to enhance the phosphorus grain boundary segregation by reducing the amount of dissolved and segregated carbon. Thereby, the equilibrium of mutual displacement C (segregated) ? P (segregated) was shifted to more phosphorus segregation. The results achieved indicate that vanadium indirectly increases sensitivity of low alloy steels to intergranular embrittlement.     

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.     

Nb的非平衡晶界偏聚动力学模拟与实验研究

摘要：Nb是现代高性能钢铁材料中重要的微合金化元素,其在晶界有强偏聚特性。采用3种Nb 空位复合体扩散系数分别对非平衡晶界偏聚进行拟合,根据铁 铌二元合金中Nb在晶界偏聚实验的EPMA测量结果筛选出最终的复合体扩散系数,并据此讨论了低温恒温温度,基体Nb含量,原奥氏体晶粒尺寸对非平衡晶界偏聚动力学的影响,得出了最符合实验结果的铌 空位复合体扩散系数公式。结果表明,在1000℃恒温过程,Nb非平衡晶界偏聚的临界时间在15min左右,临界时间常数为6.57×105。从1200℃固溶态冷却至某低温等温时,随着等温温度的升高临界时间迅速减小,Nb在晶界的最大偏聚量逐渐越小;随着基体Nb含量增加晶界Nb的最大偏聚量线性增加;随着原奥晶粒尺寸的增加临界时间逐渐增大。     

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

The phosphorus grain boundary segregation kinetics during tempering at 680°C and aging at 500°C of 2.7Cr-0.7Mo-0.3V steels with phosphorus mass contents of 0.004, 0.014, and 0.027 % was investigated. To determine the grain boundary concentrations of phosphorus the Auger electron spectroscopy was used. Chemical compositions of carbide particles were determined by means of EDX/STEM. Xu Tingdong's and McLean's models of non-equilibrium and equilibrium segregations, respectively, were used to analyze experimental data. It was shown that a phosphorus grain boundary enrichment during tempering was mainly caused by non-equilibrium segregation. During aging the mechanism of the equilibrium grain boundary segregation was prevalent. Slow phosphorus segregation kinetics was observed in the experimental steels during aging.     

Effect of titanium on the grain boundary segregation of phosphorus in Fe-Ti-P and Fe-Ti-C-P alloys and on the embritflement of steels

Equilibrium segregation of phosphorus at grain boundaries of Fe-Ti-P and Fe-Ti-C-P alloys, reduction of the grain boundary segregation of P by binding with Ti, by displacement from the grain boundaries by C and by trapping at TiC precipitates. Grain boundary segregation of P in deep drawing steels, effects of the carbon and titanium content.     

A microstructural characterization of scale formed on austenitic Fe–29.7Mn–8.7Al–1.04C and duplex Fe–29Mn–10Al–0.4C alloys in oxidizing-sulfidizing environments

Austenitic Fe–29.7Mn–8.7Al–1.04C and duplex Fe–29Mn–10Al–0.4C alloys were exposed to the SO₂–O₂ mixed gas environments at 900 °C. The morphology of the scales formed on these alloys under oxidizing-sulfidizing conditions were examined by SEM and X-ray mapping techniques. The results showed that the addition of carbon up to 1 wt.% to the Fe–29.7Mn–8.7Al–1.04C alloy led to the formation of internal sulfidation and intergranular oxidation in the mixed oxidant gas environments. On the other hand, no evidence of intergranular oxidation could be detected in the duplex Fe–29Mn–10Al–0.4C alloy with lower carbon concentration.     

Strong and Ductile Martensitic Steels for Automotive Applications

The limits of strength and ductility of a medium‐carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without prior austenite deformation. The minimum in ductility found after tempering at 350°C is explained by the formation of cementite and grain boundary segregation of phosphorus. Two thermomechanical treatments were tested involving different austenite conditions produced by variation of the deformation temperature. The deformed conditions, recrystallized or work‐hardened, exhibit higher ductility at all tempering temperatures tested. A combined thermomechanical treatment is proposed that provides the highest ductility after tempering at 300°C independent of the phosphorus content. All thermomechanical treatments described in this study refine or eliminate carbide films at prior austenite grain boundaries. It was found possible to increase the tensile strength and the fatigue limit by deformation of austenite prior to quenching while maintaining or increasing the ductility level.     

Phosphorus and carbon segregation: Effects on fatigue and fracture of gas-carburized modified 4320 steel

Phosphorus and carbon segregation to austenite grain boundaries and its effects on fatigue and fracture were studied in carburized modified 4320 steel with systematic variations, 0.005, 0.017, and 0.031 wt pct, in alloy phosphorus concentration. Specimens subjected to bending fatigue were characterized by light metallography, X-ray analyses for retained austenite and residual stress measurements, and scanning electron microscopy (SEM) of fracture surfaces. Scanning Auger electron spectroscopy (AES) was used to determine intergranular concentrations of phosphorus and carbon. The degree of phosphorus segregation is directly dependent on alloy phosphorus and carbon content. The degree of carbon segregation, in the form of cementite, at austenite grain boundaries was found to be a function of alloy phosphorus concentration. The endurance limit and fracture toughness decreased slightly when alloy phosphorus concentration was increased from 0.005 to 0.017 wt pct. Between 0.017 and 0.031 wt pct phosphorus, the endurance limit and fracture toughness decreased substantially. Other effects related to increasing alloy phosphorus concentration include increased case carbon concentration, decreased case retained austenite, increased case compressive residual stresses, and increased case hardness. All of these results are consistent with the phosphorus-enhanced formation of intergranular cementite and a decrease in carbon solubility in intragranular austenite with increasing phosphorus concentration. Differences in fatigue and fracture correlate with the degree of cementite coverage on the austenite grain boundaries and the buildup of phosphorus at cementite/matrix interfaces because of the insolubility of phosphorus in cementite.     

First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part I. Iron (Fe)-Solute (B,C, P,and S) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by solute segregation has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy (=2·surface energy − GB energy) of bcc Fe Σ3(111) symmetrical tilt grain boundary (STGB) is reduced by the segregation of sulfur (S) and phosphorous (P) while it is increased by the segregation of boron (B) and carbon (C). The rate of the decrease/increase in the cohesive energy is proportional to the experimental shift in the DBTT of high-purity iron with increasing segregation. This indicates that the change in the cohesive energy of GB plays a key role in the GB embrittlement of metals.     

Equilibrium grain-boundary segregation and the effect of boron in B-doped Fe−3wt%Ni austenitic alloy

The equilibrium segregation of boron at grain boundaries and the relation between grain boundary hardening and segregation in B-doped Fe−30 wt%Ni austenitic alloy were studied with the methods of microhardness measurement and Particle Tracking Autoradiography (PTA). It was found that equilibrium segregation of boron at grain boundaries appeared in the alloy as annealed between 650 and 960°C and no segregation of boron appeared above 1050°C. It could be concluded that an excess grain boundary hardening by addition of boron to the alloy was caused by the grain boundary segregation of boron.     

超低碳贝氏体钢快速加热奥氏体长大过程中移动晶界上硼的反常偏聚

利用径迹显微照相技术研究了超低碳贝氏体钢焊接热影响区在焊接热循环快速加热过程中硼在奥氏体晶界上的偏聚行为。发现以高密度位贝氏体为原始组织的材料进行快速加热时，新形成的奥氏体晶粒边界上在很高温度下仍会出现反常的晶界硼偏聚。用晶界位错驰原制对这种新的非平衡现象进行了讨论。     

The influence of grain boundary precipitation on the measurement of chromium redistribution and phosphorus segregation in Ni-16Cr-9Fe

The redistribution of chromium at the grain boundary and the segregation of phosphorus to the grain boundary in Ni-16Cr-9Fe is measured following thermal treatment at 700 °C for 1 to 100 hours. The addition of carbon to the base alloy results in the formation of Cr₇C₃ precipitates at the grain boundary and the formation of a chromium depleted zone in the adjacent matrix. Measurement of the Cr concentration is affected by the presence of Cr-rich carbides, and a technique of ratioing the Auger signal of the element of interest to a sum of the signals of elements present in the carbide and the matrix is required to minimize the scatter in the data. The presence of carbides does not affect the kinetics or extent of phosphorus segregation to the grain boundary, and there is no evidence of co-segregation of phosphorus with any major alloying element. The free energy of segregation of phosphorus is determined to be 46.2 KJ/mole at 1100 °C and 40.8 KJ/mole at 700 °C. Results show that the intergranular fracture path is along the carbide-matrix interface as opposed to through the carbides or some distance into the matrix. These results permit the calculation of the coverage of the grain boundary with carbides.     

Embrittlement of austempered nodular irons: Grain boundary phosphorus enrichment resulting from precipitate decomposition

The microstructures, mechanical properties, and fracture behavior were characterized for a series of Mg treated nodular cast iron specimens austenitized at 1170, 1255, and 1340 K and subsequently austempered at 640 K. The ductility and toughness of the alloy decreased as austenitization temperatures were increased, which is contrary to the behavior anticipated from the observed micro-structural evolution. Fractographic and surface chemical analyses demonstrated that the mechanical property degradation was associated with embrittlement of the austenite grain boundaries by phosphorus. The primary mechanism of grain boundary phosphorus enrichment does not appear to be equilibrium segregation, and an alternative mechanism based on the decomposition of P rich precipitates is proposed and discussed. Formerly with Michigan Technological University, Houghton, MI     

Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute. Formerly a Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA15213 Formerly with Oak Ridge National Laboratory, Oak Ridge, TN 37830     

Pb在GH871合金晶界的偏聚

利用PERKIN-ELMERPHI595型俄歇谱仪研究了新型含铌铁基高温合金GH871的晶界上各元素的含量。结果表明Pb含量在5×10     

Quench embrittlement of hardened 5160 steel as a function of austenitizing temperature

Charpy V-notch (CVN) specimens from experimental heats of 5160 steel containing 0.001 and 0.034 mass pct phosphorus were austenitized at temperatures between 830 °C and 1100 °C, quenched to martensite, and tempered at temperatures between 100 °C and 500 °C. Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of tested CVN specimens and carbide formation on prior austenite grain boundaries. Quench embrittlement, the susceptibility to intergranular fracture in as-quenched and low-temperature tempered high-carbon steels due to cementite formation as affected by phosphorus segregation on austenite grain boundaries, developed readily in specimens of the high phosphorus steel austenitized at all temperatures. The low phosphorus steel developed quench embrittlement only after austenitizing at 1100 °C. Intergranular fractures correlated with low room-temperature CVN impact toughness. The results are discussed with respect to the dissolution of carbides during austenitizing and the effect of phosphorus on grain boundary, carbide formation, and stability.     

Effects of Thermal History and Microstructure on Segregation of Phosphorus and Alloying Elements in the Heat-Affected Zone of a Low Alloy Steel

The grain boundary segregation of phosphorus and alloying elements in the heat-affected zone (HAZ) of a low alloy steel was studied quantitatively with atom probe tomography. Non-equilibrium segregation mainly occurred during welding and subsequent fast cooling, leading to remarkable segregation of P, C, Mn, and Mo. The segregation of these four types of solutes showed similar microstructure-dependence at this stage, in which the segregation levels are higher in coarse-grained HAZ and intercritically reheated coarse-grained HAZ than in fine-grained HAZ. After simulated aging, P and Mn showed further enrichment at grain boundaries through equilibrium segregation, while desegregation was observed for C and Mo. In addition, it seems that precipitation of Mo at dislocations was greatly promoted during aging, which probably also contributed to the increase of P and Mn at grain boundaries.