Combined quenching and tempering induced phosphorus segregation to grain boundaries in 2·25Cr–1 Mo steel

Abstract

Combined quenching and tempering induced phosphorus segregation to prior austenite grain boundaries in α 0·077 wt-%P doped 2·25Cr–1Mo steel was examined using field emission gun scanning transmission electron microscopy. The results indicate that combined equilibrium and non-equilibrium phosphorus segregation may play an important part in temper embrittlement of the steel caused by direct tempering after quenching. Non-equilibrium segregation requires the formation of sufficient quantities of vacancy–impurity complexes and their migration to grain boundaries is of great importance in the segregation. For this reason, the mechanism for migration of the complexes is discussed in detail.

MST/3419     

The critical time in temper embrittlement isotherms of phosphorus in steels

The critical time in non-equilibrium segregation isotherm will induce a critical time in relative temper embrittlement isotherm, at which when a steel is held, a maximum in the extent of embrittlement will occur. This suggestion has been confirmed in present paper for phosphorus non-equilibrium segregation into grain boundaries in some steels.     

Critical time and temper embrittlement isotherms

Abstract

The critical time in a non-equilibrium segregation isotherm will induce a critical time in the relative temper embrittlement isotherm, at which when a steel is held, a maximum extent of embrittlement will occur. This suggestion has been confirmed for the non-equilibrium cosegregation to grain boundaries of Ti, Sb, and Ni in low carbon Ni–Cr steels and for phosphorus non-equilibrium segregation to grain boundaries in some steels.     

Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

Non-equilibrium intergranular segregation in ultra low carbon steel

Abstract

Impurity segregation to grain boundaries in ultra low carbon steel was investigated by Auger electron spectroscopy and SEM during isothermal annealing at 900°C and continuous cooling. The results of isothermal annealing at 900°C show that a concentration peak appears at different times for phosphorus, sulphur, and boron, which is contrary to the equilibrium segregation theory of McLean. The phenomena could be satisfactorily explained by the non-equilibrium segregation theory based on the impurity–vacancy complex mechanism. Under continuous cooling, the segregation concentration at the grain boundary largely depends on the cooling rate. At a low cooling rate the concentration of phosphorus and boron at the grain boundary is higher than that of sulphur, while at the higher cooling rate the concentration of sulphur is higher.     

磷的非平衡晶界偏聚现象及对回火脆性的影响

为确定304L钢中P的偏聚特征,分析了Briant在俄歇能谱(AES)下得到的磷晶界偏聚浓度.研究表明,经1100℃固溶1h、水淬后的304L不锈钢,不仅在700℃恒温时效过程中出现磷的晶界偏聚浓度峰值,并在500、550、600、650、700℃分别恒温时效100h后,600℃处发现磷的晶界偏聚浓度峰值.分析发现,磷在304L钢中发生了非平衡晶界偏聚.通过对磷非平衡晶界偏聚恒温动力学特征的讨论,提出了回火脆性的非平衡晶界偏聚机理.     

Sulphur segregation and precipitation in HAZ of boiler steel thick section welds

Abstract

Stress relief cracking can occur in weld heat affected zones (HAZ) after post-weld heat treatment (PWHT) and periods of service at elevated temperatures. Stress relief cracking is generally believed to occur by sulphur induced decohesion ahead of a growing sharp crack. The impurity segregation behaviour in a microalloyed steel, typical of that used in the construction of a power station boiler where intermittent cracks were observed along the weld fusion boundaries, has been assessed. In particular the type and amount of segregation in the coarse grained HAZ (CGHAZ) before and after PWHT has been determined. It was found that significant sulphur segregation occurred during the CGHAZ thermal cycle resulting in elemental sulphur on the prior austenite grain boundaries. Following PWHT some desegregation of sulphur, coupled with the formation of sulphides and carbides on the prior austenite grain boundaries, was observed; in addition, significant phosphorus segregation to the prior austenite grain boundaries and grain boundary precipitate/matrix interfaces was seen.     

Combined equilibrium and non-equilibrium segregation treatment of temper embrittlement in low alloy steels

Abstract

Embrittlement is an important factor for low alloy ferritic steels used for components and structures in the power and petrochemical industries when exposed to a higher temperature. The embrittlement may be classified into non-hardening embrittlement and hardening embrittlement. The non-hardening embrittlement, for example temper embrittlement, originates from grain boundary segregation of impurity elements such as phosphorus. To predict this segregation behaviour, a model is established by simplifying a low alloy steel as a dilute Fe–C–Mo–P quaternary alloy and modifying previous models. This model is applied to segregation predictions in a 2.25Cr–1Mo steel subjected to a complex heat treatment cycle.     

Temper embrittlement of hot work die steel

Abstract

The influence of the parameters of an additional tempering at 600°C on temper embrittlement in quenched and previously double tempered 5 wt-%Cr tool steel having a post-martensitic microstructure was investigated. A detailed examination of associated changes of fracture mechanism, microstructure, and precipitate was carried out. The dominant factor controlling the evolution of brittleness was the segregation of phosphorus to prior grain and lath boundaries as well as to martensite lath/carbide interfaces. The segregation effect of phosphorus at these interfaces was accompanied by cosegregation of sulphur and silicon together with simultaneous carbide precipitation.

MST/1132     

Interfacial segregation of competitive impurities in embrittled and de-embrittled Fe-3%Ni

Abstract

The kinetics of surface segregation in embrittled and de-embrittled Fe-3 at.-%Ni-0.0ll at.-%Sb containing 20 at.-ppm S was studied by Auger electron spectroscopy at temperatures ranging from 550 to 630°C. In addition, information on the grain boundary segregation in the embrittled material has been obtained from analysing intercrystalline fracture surfaces. Grain boundary segregation is characterised by cosegregation of Sb and Ni, which is increasingly promoted as the grain boundary concentration of C decreases. On the surface of the embrittled material, segregation competition of S and Sb is observed. This is stimulated by Sb enrichment via grain boundaries. In the de-embrittled material, Sb surface segregation is inhibited so that S remains the dominant segregant. These differences in the kinetic behaviour are consistent with the demixing model of temper embrittlement. Further, the S-Sb segregation competition leads to an inhomogeneous surface chemistry; some of the individual facets are enriched with Sb, others with S. Thus, the S-Sb surface segregation competition is not only characterised by the presence of fast diffusion paths but also by anisotropy. The conditions for the segregation dominance are analysed using Guttmann's equilibrium segregation modelfor multicomponent systems.     

Interfacial segregation and grain boundary embrittlement: An overview and critical assessment of experimental data and calculated results

One of the most dangerous technical failures of materials is intergranular brittle fracture (temper embrittlement) as it proceeds very quickly and its appearance is often hardly predictable. It is known that this phenomenon is closely related to the chemistry of grain boundaries and to the difference of the segregation energies of the grain boundaries and the free surfaces (Rice–Wang model). To elucidate the effect of individual solutes on embrittlement of various materials such as steels and nickel-base superalloys, grain boundary and surface segregation was extensively studied in many laboratories. As a result, numerous data on surface and grain boundary segregation have been gathered in literature. They were obtained in two main ways, by computer simulations and from experiments. Consequently, these results are frequently applied to quantify the embrittling potency of individual solutes. Unfortunately, the values of the segregation energy of a solute at grain boundaries as well as at the surfaces obtained by various authors sometimes differ by more than one order of magnitude: such a difference is unacceptable as it cannot provide us with representative view on the problem of material temper embrittlement. In some cases it seems that these values do not properly reflect physical reality or are incorrectly interpreted. Due to the above mentioned large scatter of the segregation and embrittlement data a critical assessment of the literature results is highly needed which would enable the reader to avoid both the well known and less well known pitfalls in this field. Here we summarize the available data on interfacial segregation and embrittlement of various solutes in nickel and bcc iron and critically discuss their reliability, assessing also limitations of individual approaches employed to determine the values of segregation and strengthening/embrittling energies, such as density functional theory, Monte Carlo method, molecular statics and dynamics and tight binding on the theoretical side, and Auger electron spectroscopy, 3D tomographic atom probe, and electron microscopy techniques on the experimental side. We show that experimental methods have serious limitations which can be overcome by accepting reasonable assumptions and models. On the other hand, the theoretical approaches are limited by the size of the computational repeat cell used for the calculations of the segregation energy. In both cases, a careful critical analysis of the available segregation energy and/or enthalpy reflecting physical reality allows to assess the reliability of these values and their applicability in analysis of intergranular brittle fracture in steels and nickel-base alloys.     

Influence of thermal histories on intermediate temperature embrittlement of an Fe-17Cr alloy

Intermediate temperature embrittlement is found in an Fe-17Cr alloy. The embrittlement is due to void formation on and near grain boundaries during tensile deformation. On the other hand, the segregation of phosphorus and sulphur impurity atoms to grain boundaries enhances intermediate temperature embrittlement.     

Grain boundary chemistry

Recent advances in experimental measurements of grain-boundary segregation are reviewed. It is now feasible to measure segregation and relate it to boundary structure at the atomic level. It is also possible to measure nanometre-level segregation at dozens of grain boundaries, so a better statistical picture of the distribution of segregant is available. There is strong experimental evidence emerging for an electronic explanation of the role of segregation in controlling bulk mechanical properties, such as temper embrittlement.     

Determination of Neutron Irradiation-Induced Phosphorus Segregation on Grain Boundaries in a P-doped 2.25Cr1Mo Steel

Irradiation-induced impurity segregation to grain boundaries is one of the important radiation effects on materials.For this reason, phosphorus segregation to prior austenite grain boundaries in a P-doped 2.25Cr1Mo steel subjected to neutron irradiation is examined using field emission gun scanning transmission electron microscopy (FEGSTEM) with energy dispersive X-ray microanalysis (EDX). The steel samples are irradiated around 270 and 400℃, respectively.The irradiation dose rate and dose are ～1.05×10-8 dpa/s and ～0.042 dpa respectively for 270℃ irradiation, and 1.7×10-8 dpa/s and 0.13 dpa respectively for 400℃ irradiation. The FEGSTEM results indicate that there is no apparent phosphorus segregation during 270℃ irradiation but there is some during 400℃ irradiation.     

Influence of precipitation and segregation on hydrogen embrittlement in aged 9Cr–1Mo steel

Abstract

A study is reported of temper embrittlement and hydrogen embrittlement in a series of model 9Cr–1Mo steel alloys in which the levels of silicon and phosphorus have been varied to separate the formation of the brittle intermetallic (Laves) phase from the segregation of phosphorus during aging. Phosphorus segregation was mildly detrimental to ductility properties, Laves phase formation was more detrimental, and their effects combined produced the most severe loss in ductility. Hydrogen effects were additive to those of aging. In unaged material without silicon enrichment, only M₂₃C₆ precipitates were detected, with little phosphorus segregation. With silicon enrichment, phosphorus segregation to lath and grain boundaries was enhanced. This enhancement increased the susceptibility of the materials to hydrogen embrittlement, promoting transgranular cleavage and chisel fracture. In aged material, the high phosphorus alloys showed some grain boundary segregation, but only limited interaction with hydrogen. In the high silicon alloys, the formation of Laves phase was most evident. This enhanced hydrogen embrittlement resulted in extensive chisel, transgranular cleavage, and some intergranular fracture. In the high silicon high phosphorus alloy, both Laves phase formation and phosphorus segregation were evident. This resulted in enhanced susceptibility to hydrogen embrittlement, producing intergranular fracture. Thus, silicon controls the susceptibility to hydrogen embrittlement in unaged alloy by promoting phosphorus segregation and in aged alloy by promoting Laves phase formation. In the aged alloy, segregation of phosphorus can enhance the effect of silicon.

MST/1785     

大塑性变形制备超细晶/纳米晶Al-Mg铝合金的研究进展

近年来,大塑性变形(SPD)制备具有先进结构和功能的超细晶和纳米晶Al-Mg铝合金的研究取得了很大进展。SPD后,合金的晶粒显著细化、位错密度提高及有非平衡晶界和晶界偏析形成,这些微观结构导致合金的强度、硬度大幅提高。然而,SPD合金的塑性普遍较低。综述了SPD制备的Al-Mg铝合金在结构和性能方面的一些最新研究成果。     

A model for intergranular segregation/dilution induced by applied stress

A model for the effects of low applied stress on grain boundary segregation/dilution of solute has been suggested in the present paper. This model is based on the following assumptions: (1) The grain boundary is a weaker region on strength than the perfect crystalline in the interior of gain and will preferentially be deformed when a polycrystalline is exerted by an low applied stress. (2) Grain boundaries will work as sources of vacancies to emit vacancies when a compression stress is exerted on them and as sinks to absorb vacancies when a tension stress is exerted; (3) Oversaturated vacancies induced by the applied stress will be combined with the solute atoms to form vacancy-solute atom complexes, the diffusion rate of which is far greater than that of solute atoms in matrix; (4) The effects of applied stress on grain boundary segregation/dilution of solute will be controlled by the balance between the complex diffusion and the reverse solute atom diffusion. According to this model, there will be a critical time during stress aging, at which a maximum level of grain-boundary segregation/dilution will occur. This model can be corroborated by Shinoda and Nakamura's observation for phosphorus and Misra's observation for sulfur in steels. It can be expected that a new basis for understanding the low ductility intergranular fracture induced by applied stress will result from this new model.     

Y对耐热铝导体材料铸态组织和性能的影响

随着特高压输电技术在我国的大力发展,铝合金导体材料作为特高压输电线路的主要组成部分,受到业内的广泛关注.本文采用电导率测试、硬度测试、金相显微镜和扫描电镜观察等手段,研究添加不同含量稀土Y对铸态Al-Zr耐热铝导体材料的影响.研究结果表明:Y元素和Fe、Si等杂质元素形成金属间化合物,可净化基体,改变杂质相的形态和分布,使其粒子化、球化和细化.Y元素在枝晶网络和晶界分布,从而细化晶粒和枝晶组织,但添加量达到0.5%时晶粒细化不均匀.当Y含量为0.2%时,电导率达到60%IACS;当Y含量为0.3%时,硬度达到最高值20.9HBS,且电导率并无明显下降.加入0.3%Y可使耐热铝导体材料获得较好的综合性能.     

On the cleavage strength of grain boundaries in Ni3Al: Effect of impurity segregation

The cleavage strength of grain boundaries in Ll₂ type of intermetallic compounds (Ni₃Al) is investigated using the tight-binding (TB) electronic theory of s, p and d basis orbitals. It is shown that cleavage strength of the grain boundaries depends strongly on the segregation of impurity atoms at the grain boundaries, in agreement with experimental results. The stoichiometry effect of the Ll₂ compound is also discussed.     

An FEGSTEM Study of Grain Boundary Segregation of Phosphorus during Quenching in a 2.25Cr-1Mo Steel

Quenching-induced phosphorus segregation to prior austenite grain boundaries in a 0.077 wt pct P-doped 2.25Cr-1Mo steel is examined using field emission gun scanning transmission electron microscopy (FEGSTEM). A phosphorus level of around 1.56 at. pct is observed for the water-quenched sample. In recognition of insufficiently high spatial resolution of the technique for grain boundary composition analysis, the measured results are corrected by an analytical convolution method. The corrected phosphorus segregation level may be up to about 4.7 at. pct. The quenchinginduced phosphorus segregation is nonequilibrium segregation and the migration of vacancy-phosphorus complexes plays an important role in the kinetic process. For such a reason, the mechanism for migration of the complexes is discussed in some detail.