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.     

Statistical analysis of phosphorus segregation on intergranular fracture facets in pressure vessel steels

Abstract

A533B and C–Mn steels, widely used as nuclear pressure vessel steels, have been aged at 520°C after tempering at 650°C for various periods of time to produce different levels of embrittlement resulting from the segregation of P to grain boundaries. Metallographic observation and tensile test results showed that the embrittlement heat treatment did not have significant influence on the microstructures or tensile properties of the steels. P segregation at grain boundaries and on intergranular facets was investigated using field emission gun transmission electron microscopy and Auger electron spectroscopy. After such treatment, enhanced segregation was found to be a linear function of the square root of embrittling time. Statistical analysis of the AES measurements indicated that there is a minimum segregation level for intergranular fracture to occur.     

Non-equilibrium grain-boundary segregation in austenitic alloys

The theory of non-equilibrium grain-boundary segregation is discussed with particular reference to recent ideas and data relating to boron grain-boundary segregation in Type 316 austenitic steel. The kinetics of the non-equilibrium grain-boundary segregation process are considered in depth and a model is developed which, it is hoped, will more realistically describe the magnitude and extent of the process. Reasonable agreement is found between the predictions of the model and experimental evidence for non-equilibrium boron, aluminium and titanium segregation to grain boundaries in austenitic steels. The model predicts, generally, that elements with large misfits with the matrix atoms will segregate most. Larger grain sizes lead to greater grain-boundary segregation. Also, the two critical heat-treatment parameters in non-equilibrium segregation are the solution-treatment temperature and the cooling rate from the solution-treatment temperature. Predictions of the worst combinations of these parameters for maximum non-equilibrium segregation to grain boundaries in austenitic steels are presented.     

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     

Method for determining the elastic modulus at grain boundaries for polycrystalline materials

Abstract

On the basis of the model of non-equilibrium grain boundary segregation induced by tensile stress, a set of kinetic equations is derived to formulate this process. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. In the present paper, based on such a widely approved model, a new approach is proposed to quantify the elastic modulus at grain boundaries for polycrystalline materials. Using the observation of Misra, the grain boundary elastic modulus E _gb = 2.03 × 10⁹ Pa at 883 K for Cr-Mo-V-2.6Ni steel is obtained for the first time. This result shows excellent agreement with the local elastic constants simulated theoretically by Kluge et al., and indicates that the grain boundary elastic modulus for a polycrystalline material is much lower than the commonly assumed value.     

Kinetic equations for non-equilibrium grain boundary segregation induced by applied tensile stress

Abstract

Based on an earlier model, a set of kinetic equations is derived to formulate the process of non-equilibrium grain boundary segregation induced by a low tensile stress. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. The simulation results justify both the earlier model and the present kinetic equations. They also show that an applied tensile stress can increase the diffusion rate of solute-vacancy complexes and decrease that of isolated solute atoms significantly, and can also bring forward the critical time of non-equilibrium segregation for phosphorus and delay that for sulphur.     

Crack propagation of secondary hardened alloy steels in gaseous hydrogen atmosphere

Abstract

The crack propagation behaviour of secondary hardened alloy steels having various Ni contents and a 18%Ni maraging steel was studied using modified compact tension specimens under 98–784 kPa hydrogen gas pressure p_H2 . The effect of Ni content and retained austenite was examined by comparing oil quenched specimens with those cooled to liquid nitrogen temperature. It was estimated from the effect of p_H2 on the crack propagation rate da/dt that the permeation of hydrogen from the crack tip surface decreased with increasing Ni content in the order 6 or 9, 13, then 18%Ni. It was also estimated that the hydrogen induced embrittlement of grain boundaries decreased with increasing Ni content in the order 6, 9, then 13%Ni and that the embrittlement was greater for the steel containing 18%Ni than for the steel containing 13%Ni. The effect of retained austenite was expected to suppress not the permeation of hydrogen, but the embrittlement of grain boundaries.

MST/757     

Kinetics of cold work embrittlement in rephosphorised,interstitial free steels

Abstract

The kinetics of cold work embrittlement, associated with grain boundary segregation of P in batch annealed interstitial free steels, was determined from the temperature–time cycle of the batch annealing process. The grain boundary segregation predictions were subsequently extended to a hypothetical optimised steel chemistry, where C effectively displaced P from the grain boundary through a site competitive interaction process. The P–C, site competitive process was expected to minimise the cold work embrittlement phenomena experienced in interstitial free steels. From a comparison of grain boundary P segregation, it was possible to draw conclusions concerning the minimum grain boundary P level that can be achieved through appropriate modification of the batch annealing cycle or by optimisation of the steel chemistry.     

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.     

Grain boundary segregation of phosphorus,manganese, and carbon in boiler shell weld material

Abstract

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life up to 50 years as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.     

Effect of phosphorus segregation and Ni2Cr formation on hydrogen embrittlement in 70Ni–30Cr alloys

Abstract

The effects of aging at 773 K on hydrogen embrittlement in Ni–30Cr (wt-%) alloys having two levels of P have been investigated by considering the grain-boundary segregation of impurity atoms and the Ni₂ Cr ordered-phase formation. Aging at 773K suppressed intergraular fracture and reduced the susceptibility to hydrogen embrittlement in the low-P alloy. Such behaviour can be explained in terms of the grain-boundary strengthening caused by the segregation of C atoms. During aging at 773 K, the Ni₂Cr ordered phase formed and the deformation mode changed from wavy slips to coplanar slip with paired dislocations, and then to coplanar slip with microtwins. In the low-P alloy, this change of deformation mode induced step-like cracks which may have occurred by the separation of either the {111} slip planes or the microtwin interfaces. In the high-P alloy, aging for short times caused C segregation to the grain boundaries which suppressed intergranular fracture. However, aging for longer times induced drastic intergranular hydrogen embrittlement because of the grain-boundary segregation of P atoms, which offset the effect of the boundary strengthening caused by C atoms.

MST/177     

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 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     

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.     

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

Abstract

A correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achieved have been considered. Some technological implications have been suggested.

MST/804     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

Influence of the structural embrittlement of metal alloys on their short-term crack resistance

We perform the comparative analysis of the sensitivity of short-term crack resistance and conventional mechanical characteristics of brittleness to typical symptoms of metallurgic and structural embrittlement of metal alloys. For this purpose, we studied various structural steels in the state of reversible or irreversible temper embrittlement, quenched steels with coarse original austenitic grain structure after overheating, steels susceptible to hydrogen embrittlement, high-chromium steels in the state of “475° brittleness,” and steam-pipe steels subjected to in-service thermal aging. Despite fairly high sensitivity of the parameterK _lc to these types of embrittlement, this parameter exhibits quite low sensitivity to the embrittlement of high-strength steels, in particular, in the case where it is caused by the processes running on grain boundaries. This phenomenon is explained by the difference between the sizes of the “fracture zone” and the original structure of embrittled cells. We formulate structural and mechanical conditions guaranteeing the required sensitivity of the characteristics of short-term crack resistance to possible metallurgic embrittlement. In the indicated anomalous cases, the desired correspondence can be attained by testing smaller specimens and, hence, by using milder loading modes, i.e., by passing to the determination of the parameterK _c and the characteristic of critical crack opening displacement δ_c introduced by V. V. Panasyuk. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv. Vol. 32, No. 1, pp. 9–21, January–February, 1996.     

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.     

High-temperature ductility of AISI 310 austenitic stainless steels

Abstract

The effects of deformation temperature and additions of impurity elements on the ductility of solution–treated 25Cr–20Ni steels have been examined at the relatively high strain rate of 0·11 ^s?1 by means of hot tensile tests. The ductility v. temperature curve can be divided into three regions: region I, 1000–1200 K, where there is a ductility minimum due to M₂₃C₆ precipitation on grain boundaries; region II, 1225–1400 K, which exhibits a plateau or a slight trough of ductility with corrugated boundaries as a result of dynamic recrystallization localized near the boundaries; and region III, 1450–1600 K, where recrystallization through the whole specimen leads to ductile fracture with complete necking. It was also confirmed that the grain boundary segregation of Sb and S and the sulphide precipitation on the grain boundaries accelerate intergranular cracking and reduce ductility in the range 1075–1300 K, and that, because sulphide particles melt on boundaries, the zero ductility temperature is markedly lowered by the addition of impurities such as S, P, or Sb.

MST/364     

Influence of phosphorus on weld heat affected zone toughness in niobium microalloyed steels

Abstract

Weld heat affected zone microstructures have been simulated in a niobium microalloyed C–Mn steel. These structures have been Charpy impact tested and examined metallographically. Troughs were observed in the plots of Charpy energy against post-weld heat treatment time at 600°C. The explanation for this behaviour is shown to be associated with non-equilibrium segregation of phosphorus to lath boundaries in the steel. Mathematical modelling of non-equilibrium segregation, together with high spatial resolution analytical electron microscopy results, support this contention. Using the predictions of the model, several suggestions are made concerning the alleviation of phosphorus segregation to boundaries and hence the reduction of cracking susceptibility during post-weld heat treatment of the steel.

MST/994