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.     

The mechanism of elevated temperature intergranular cracking in heat-resistant alloys

Reheat or stress relief cracking phenomena have been reassessed in 2.25Cr1.5W heat-resistant alloys. During rupture test, time to intergranular failure increases with decreasing temperature and tensile stress and is shorter in the alloy containing a higher bulk content of phosphorus. Also the time to intergranular failure can be expressed by t = t₀·σ⁻ⁿ·exp(Q/RT) where t₀ is the proportional constant, n the stress exponent and Q the activation enthalpy. Matrix softening is accelerated under tensile stress and an active carbide growth occurs at grain boundaries oriented normal to the tensile stress direction. Because impurities segregate actively to dimples frequently observed at reheat intergranular fracture surfaces, the dimples are not micro-ductile fracture areas but the grain boundary carbide interfaces. The segregation concentration of the impurities is much higher at the grain boundary carbide interfaces than the carbide-free grain boundaries. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. The elevated temperature intergranular cracking under tensile stress occurs finally due to the carbide-free grain boundary cracking following the decohesion of the grain boundary carbide interfaces.     

Effects of vanadium additions on microstructure and hardness of hypereutectoid pearlitic steels

An investigation has been carried out on the effects of vanadium additions on the microstructure and hardness values of hyper-eutectoid pearlitic steels with carbon contents up to 1.15 wt.%, for a range of isothermal transformation conditions. As in the case of eutectoid steels with vanadium additions, the predominant carbide species at the grain boundaries is cementite, rather than vanadium carbide, even at the higher vanadium levels (0.2 wt.%). However, the extent of the pro-eutectoid grain boundary cementite network is progressively reduced as the vanadium content is increased, in spite of the hypereutectoid composition. According to SEM and TEM investigations, the morphology of the grain boundary carbide is less continuous in the higher vanadium alloys, and consequently less deleterious to mechanical properties. The microstructural investigation indicates that the cementite particles form independently at many different places on the grain boundary, consistent with vanadium addition increasing the driving force for nucleation. Vanadium additions also effectively decrease the austenite grain sizes. An increment in hardness values has been achieved because of vanadium additions, which has potential to improve other mechanical properties in hypereutectoid pearlitic steels, because of these modifications in microstructure.     

Alloy design of ductile phosphoric iron: Ideas from archaeometallurgy

Alloy design criteria to produce ductile phosphoric irons have been proposed based on a detailed microstructural study of ancient Indian irons. The alloy design aims at avoiding phosphorus segregation to the grain boundaries by (a) soaking the phosphoric iron at high temperatures within the ferrite + austenite region to precipitate austenite allotriomorphs, (b) utilizing a critical amount of carbon to segregate to grain boundaries, and (c) precipitation of some of the phosphorus in solid solution in the ferrite matrix as fine coherent phosphide precipitates.     

Segregation at the {013} symmetrical tilt grain boundary in dilute Fe-Si alloy bicrystals

The structure and segregation behaviour of the {013} symmetrical tilt grain boundary were studied in Fe-Si dilute alloy bicrystals. Structural features such as twins, cleavage tongues and steps at the grain boundary, revealed by brittle intercrystalline fracture, are described and the identity of the fracture surface with the grain boundary is discussed. The amounts of segregation of phosphorus and silicon obtained by Auger electron spectroscopy are correlated with the structure of the analysed regions. The scatter of measured values of concentration of phosphorus and silicon in different regions of the fracture surface is explained from a structural point of view. A repulsive interaction between phosphorus and silicon atoms was confirmed. Grain-boundary enrichment ratios for phosphorus of 185 and 278, and decreases of the silicon concentration to 63 and 60% of the bulk value, were found at the grain boundary in as-grown and annealed bicrystals, respectively.     

Effects of vanadium addition on nucleation and growth of pearlite in high carbon steel

Abstract

The influence of vanadium addition on the microstructure of high carbon steels has been investigated. A careful examination of the initial stages of austenite decomposi~ion has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanadium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferrite is the product of eutectoid transformation, and is not proeutectoid ferrite. This is because the first event is the nucleation of carbide particles along the grain boundaries. These carbides have been identified mainly as cementite. The presence of vanadium appears to change the morphology and distribution of the grain boundary cementite, so that rather than forming a grain boundary network, the cementite occurs in the form of a high density of small discrete particles along the boundaries. It is proposed that this occurs because vanadium increases the driving force for cementite nucleation. The formation of the grain boundary cementite depletes the surrounding region of carbon and encourages the formation of ferrite, but because of their discrete and fine dispersion, the cementite particles are engulfed by the more voluminous ferrite phase. In such regions, the onset of afully cooperative growth regime is delayed. Pearliteforms later at the ferrite/austenite interfaces.

MST/1923     

Structure and mechanical properties of Ni–Cr alloy produced by single roll strip casting

In the present investigation the microstructure and some mechanical properties of Ni–Cr alloy prepared by single roll strip casting (SRSC) were studied. The top surface (surface not in contact with the roll) of the as received sample was rough and lusterless. The grain size of the top surface was significantly larger compared to that of the bottom surface. Grain interior showed dendritic morphology. Etch-pits, formed by dislocation were observed on the top and bottom surfaces of the sample. Scanning electron microstructure revealed continuous corroded region along the grain boundaries. X-ray diffraction (XRD) study confirms the formation of chromium carbide at the grain boundary, which depletes Cr near the grain boundary. The hardness of the top surface is lower compared to the bottom surface. The cross-sectional surface shows much higher hardness compared to top and bottom surface, suggesting anisotropic nature of the alloy. The tensile properties of the sample were affected by chromium carbide precipitation at grain boundaries.     

Role of dislocation density and structure in grain boundary segregation of phosphorus and carbon in model Fe-P-C alloy

Abstract

The present study has investigated the effects of dislocation density, produced by tensile prestraining, on the grain boundary segregation of phosphorus and carbon in a Fe-0.06P-0.002C (wt-%) alloy during stress free isothermal annealing at 500°C for periods up to 1800 h. Changes in grain boundary segregation were followed using Auger spectroscopy, while changes in dislocation density and structure were observed using transmission electron microscopy techniques. The segregation of phosphorus (but not carbon) was enhanced, compared with unstrained specimens, during initial aging. Analysis of diffusion rates required to cause the observed increase in phosphorus segregation suggested that the kinetics of phosphorus segregation was enhanced by pipe diffusion. At intermediate aging times, desegregation of phosphorus was observed, an effect attributed to a reduction in intragranular solute levels resulting from phosphorus precipitation on dislocations. In the case of carbon this process continued to the longest aging times examined and, essentially, complete desegregation ensued. In contrast, at these long aging times, phosphorus segregation resumed, and this was associated with an increase in the binding energy of phosphorus to the carbon denuded grain boundary.     

Microstructural aspects of phosphorus grain boundary segregation in low alloy steels

The influence of microstructure on phosphorus grain boundary segregation in 2.6Cr–1Mo–0.3V low alloy steel was investigated. Measurement of the phosphorus grain boundary concentration and characterization of the steel microstructure (substructure) were performed by means of Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). It was found that the phosphorus grain boundary segregation during the aging of the steel at 773 K is faster in as-tempered microstructure than in as-quenched material. The lower dislocation density and the indirectly evidenced lower carbon content in ferrite matrix were considered to accelerate the kinetics of phosphorus segregation.     

Fracture of polycrystalline Fe−2.3%V−0.12%P alloy

Fracture experiments on a polycrystalline Fe-2.3mass%V-0.12mass%P alloy show that the fracture mode is directly related to the grain boundary segregation of phosphorus. Comparison of the experimental data with theoretical simulations showed that intergranular fracture spreads along those grain boundaries, which contain more than 17 at.% of segregated phosphorus.     

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     

Reverse temper embrittlement: a common source of perplexity in low alloy steels

The present paper attempts to assess the many aspects which are involved in the old, interesting and somewhat perplexing grain boundary segregation phenomenon commonly known as reverse temper embritlement (RTE). The actual mechanisms involving grain boundary failure, the mechanics of impurity segregation at grain boundaries and the speciality of grain boundaries are discussed at length . The thermodynamics of impurity segregation, which is treated as aneq uilibrium-type process, together with the segregation kinetics, which are required for an estimation of the extent of grain boundary impurity segregation in terms of time and temperature, are also considered. The various methods of portraying the effects of reverse temper embrittlement in terms of bulk chemistry, grain boundary chemistry, therm al history (tempera ture, time and impurity segregation characteristics) and material properties (micro structure, hardness and strength) are critically assessed and compared. Interms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very con sistent and small datascatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material properties such as hardness, tensile strength and grain size, and it is established that RT E effects can be adequately expressed in terms of (I) grain sizebulk phosphorus trends and/or (2) a grain boundary phosphorus factor.     

Evidence for Cr-carbide formation at the scale/metal interface during oxidation of FeCrAl alloys

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm, respectively) were investigated in respect of their oxidation behaviour at 1200 °C in air. Both alloys exhibited formation of grain boundary Cr-carbides after 1200 °C exposure, whereas the high-carbon content alloy additionally showed carbide formation at the interface between alloy and alumina surface scale. The extent of the carbides formed at both this interface and the alloy grain boundaries was dependent on the cooling rates of the oxidized samples. The presence of the carbides may decrease the adherence of the oxide scale to the metallic substrate and explain the decreased time to breakaway oxidation of the high-carbon content alloy.     

Effect of Rare-earth Element La on Co-base Superalloy

The effect of La element on the mechanicalproperties and constitution nearby grain boundaryin a Co-base superalloy Co-22Ni-22Cr-14k during760℃ ageing has been investigated by AES andmaterial testing. Results show that elementLa can be in a state of solid solution andsegregates mainly at the vicinity of grainboundaries. The prior segregation of La atomscan affect segregation behaviour of otherelements, so that exerts influence onmicrostructure of the alloy, Particularlyincreasing of La content can decrease Pconcentration at the vicinity of grain boundariesbut not decrease S concentration. S togetherwith La produce cosegregation effect. Thecontent of La has no obvious effect on hightemperature strength. When the content of Lacomes up to 0.02wt-％, it can obviously improvethe high temperature ductility.     

Effect of Phosphorus on Stress Rupture Properties of GH4133 Ni—Base Superalloy

The effect of phosphorus on the stress rupture property of GH4133 alloy has been investigated and is compared with that of IN718 alloy. The GH4133 alloy is crept by dislocation movement. Phosphorus has a tendency to prolong the rupture life of some wrought superalloys by inhibiting the dislocation movement. If the phosphorus addition is too high, its effect on impairing the grain boundary cohesion overwhelms that on inhibiting the dislocation movement,and the life of the GH4133 alloy can be shortened. The two functions of inhibiting the dislocation movement and impairing the grain boundary cohesion determine that the optimum phosphorus content in the GH4133 alloy is around 0.011 wt pct. Phosphorus exhibits a greater effect on prolonging the rupture life of IN718 alloy than that of GH4133 alloy. The two alloys are crept by different mechanisms. The intergranular phosphorus-bearing phase is precipitated in the IN718 alloy, while not in the GH4133 alloy. The precipitation of the phosphorus bearing phase can balance the phosphorus segregation at the grain boundaries and allows a more remarkable effect of phosphorus on extending the rupture life of IN718 alloy.     

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M₆C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M₂₃C₆ carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M₂₃C₆ carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.     

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.     

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.     

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.     

30CrMnSiA钢超塑性变形中的机理问题

利用扫描电镜和透射电镜研究了30CrMnSiA钢超塑性变形中组织结构变化。结果表明,变形中合金元素的扩散导致横向晶界的宽化,并且富集了Si、Cr、Mn元素。三角晶界上呈现的显微空洞宏观调节了晶粒的三维重排过程。未溶碳化物与晶格位错、晶界以及晶界位错之间有相互作用关系。扩散和位错运动微观调节了晶界滑动,并导致它的发展。