Boundary Phosphorus Segregation Research Articles

Molecular Dynamics Study of Phosphorus Migration in Σ3(111) and Σ5(0-13) Grain Boundaries of α-Iron

Phosphorus atoms in steels accumulate at grain boundaries via thermal and irradiation effects and induce grain boundary embrittlement, which is experimentally confirmed by an increase in the ductile-brittle transition temperature. Quantitative prediction of phosphorus segregation at grain boundaries under various temperature and irradiation conditions is essential for preventing embrittlement. To develop a model of grain boundary phosphorus segregation in α-iron, we studied the migration of a phosphorus atom in two types of symmetrical tilt grain boundaries (Σ3[1-10](111) and Σ5[100](0-13) grain boundaries) using molecular dynamics simulations with an embedded atom method potential. The results revealed that, in the Σ3 grain boundary, phosphorus atoms migrate three-dimensionally mainly in the form of interstitial atoms, whereas in the Σ5 grain boundary, these atoms migrate one-dimensionally mainly via vacancy-atom exchanges. Moreover, de-trapping of phosphorus atoms and vacancies was investigated.

Effect of grain boundary character distribution and grain boundary phosphorus segregation on the brittleness of an interstitial-free steel

Grain boundary character distribution and grain boundary P segregation in an interstitial-free steel annealed for up to 4h at 700°C after cold rolling are investigated using electron backscatter diffraction and Auger electron spectroscopy. During annealing, the recrystallized grain size and hardness of the steel do not change apparently, being ~10µm and 90HV, respectively, but the number percentage of random high-angle grain boundaries initially increases with increasing annealing time until reaching a maximum at 3h and then decreases slightly after that. The random high-angle grain boundaries play an important role in the steel embrittlement. Both P grain boundary segregation and ductile-to-brittle transition temperature (DBTT) of the steel increase in the same trend with increasing number percentage of random high-angle grain boundaries. The relationship between DBTT and P boundary concentration (Cp) is linear, being expressed as DBTT(°C)=3.12CP−118.87 (Cp, at%), showing that the annealing treatment of cold-rolled interstitial-free steel should be precisely designed to avoid embrittlement.

Grain Boundary Phosphorus Segregation in Thermally Aged Low Alloy Steels

Intergranular embrittlement due to grain boundary segregation of phosphorus is recognized as one of the potential degradation factors in irradiated reactor low alloy steels at high neutron fluence. In this study, low alloy steels thermally aged at 400-500°C were investigated to evaluate the correlation between phosphorus segregation and intergranular embrittlement. Phosphorus segregation determined using Auger electron spectroscopy increased after thermal aging above 450°C and was in good agreement with the calculated value based on McLean's model. No influence of thermal aging on tensile properties or hardness was observed. The ductile brittle transition temperature determined using a one-third size Charpy impact test increased at a P/Fe peak ratio of 0.14. These results indicated that there is a threshold level of phosphorus segregation for non-hardening embrittlement. The ductile to brittle transition temperature (DBTT) increased with the proportion of intergranular fracture, so this result shows that there is a relationship between DBTT and the proportion of intergranular fracture. The fracture stress decreases due to non-hardening embrittlement on the thermally aged material with high proportion of intergranular fracture.

Grain Boundary Phosphorus Segregation in Thermally Aged Low Alloy Steels

Grain Boundary Phosphorus Segregation in Thermally Aged Low Alloy Steels

Control of phosphorus inter-granular segregation in ferritic steels

Low- and high-alloy ferritic steels, 2.25Cr1Mo and E911, were irradiated by 250 keV Ni + ions at 300 °C to study radiation-induced inter-granular phosphorus segregation behaviour and the effect of hafnium on radiation-induced segregation. Irradiations were carried out in an ion accelerator to a dose of 0.452 dpa for 2.25Cr1Mo and 0.305 dpa for E911 and E911+1%Hf. Grain boundary phosphorus segregation was detected by using field emission gun transmission electron microscopy with an energy dispersive X-ray analyzer. The phosphorus segregation at boundaries after irradiation in the high-alloy steel is higher than that in the low-alloy steel. Hafnium can suppress radiation-induced segregation to approximately one-sixth of the level in hafnium-free materials. A model to predict radiation-induced segregation in foil-samples is presented. Site competition between carbon and phosphorus is taken into consideration. The predicted results show good agreement with experimental data.

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.

Influence of residuals resulting from scrap use in the electric arc furnace process on the properties of hot-work tool steels

Hot-work tool steels are produced by the electric arc furnace route applying scrap as the main raw material. Besides iron being the most important alloying element for the steel production, a number of generally undesired residual elements are contained in scrap. Due to the continuous scrap recycling, some residuals that normally cannot be removed from the melt such as copper and nickel are in the future expected to enrich in scrap as well as in the steel produced with consequences for the material properties not yet known [1, 2]. To extend the knowledge, it was the objective of a project financially sponsored by the ECSC to determine the impact of scrap use on the properties of hot-work tool steels by investigating the influences of the residuals nickel, copper, phosphorus and aluminum. For the investigation, 13 heats of steel H11 (X38CrMoV5-1) were produced via vacuum induction melting followed by electroslagor vacuum arc remelting. While the alloying elements were kept on a constant level in the admissible range according to German standard (SEL), the residuals were adjusted according to Table I. From the remelted billets with a weight of 1 ton forged bars with a dimension of 90 mm square were produced for the tests. Besides of the working properties, the mechanical properties and the microstructure were investigated. Details concerning the material production and testing are included in [3]. The residual phosphorus was investigated within the scope of 0.002% to 0.021%. It most notably had a deteriorating impact on the mechanical properties, e.g., toughness and ductility. In impact bending tests at room temperature and elevated temperatures, the impact bending energy was reduced with increasing phosphorus content (Fig. 1). The effect was noticed for unnotched and notched specimens likewise. The reason for this behavior most likely is the segregation of phosphorus at grain boundaries and at interfaces between martensite laths and carbides, a phenomenon that has been discussed in connection with the role of phosphorus in temper embrittlement [4]. Segregation of phosphorus leads to a reduction of cohesion in the microstructure and allows easy crack initiation, thus reducing toughness. For the alloys tested, a noticeable reduction of toughness occurred with 0.021% P although the impact bending values were still high enough to meet the demands of various customer specifications. Also, ductility determined in tensile tests at room temperature was reduced with increasing phosphorus content. A content of 0.021% P was found to increase the yield strength by 100 MPa while it did not have a bearing on tensile strength. The effect on yield strength results from the integration of phosphorus atoms in the iron-solid solution and has already been described for engineering steel grades. Due to the different atom radii the crystal lattice is distorted and thus a hardening effect and an influence on yield strength is evoked. The properties determined in creep tests such as creep strength and yield strength were significantly reduced by phosphorus (Fig. 2). This effect on the creep resistance can be explained by time-dependent grain boundary phosphorus segregation reducing toughness. Also it is known that phosphorus accelerates the velocity in primary and secondary creep regions as it facilitates the formation of coarse carbides that reduce creep resistance [5]. The influence of copper was determined for contents between 0.05% and 0.38%. Copper as a residual in steel H11 did not show any effect on the processing properties although this element is always mentioned in connection with defects during casting and hot-forming operations [6]. Obviously, the effects of

Intergranular fracture stress and phosphorus grain boundary segregation of a Mn-Ni-Mo steel

*EDF, Direction des Etudes et Recherches, Dpt. Etude des Mate´riaux, Ecuelles, BP1,77283 Moreˆt sur Loing, France**Ecole des Mines de Paris, Centre des Mate´riaux, UMR CNRS 7633, BP87,91003 Evry Ce´dex, France(Received February 3, 1999)(Accepted February 18, 1999)IntroductionNuclear Reactor Pressure Vessel (RPV) steel A508 class 3 which is a low alloyed steel is not usuallysensitive to reversible temper embrittlement when properly heat treated. However heterogeneous zonesmay be present in particular near the inner side of the vessel. These zones result from the segregationof the alloying elements (C, Mn, Ni, Mo) and impurities (S, P) taking place during solidification of thematerial. They are called segregated zones (or ghost lines). They can reach 2 mm thick along the radiusand 30 mm long through the circumferential direction. Their susceptibility to reversible temperembrittlement is mainly due to grain boundary phosphorus segregation triggering brittle intergranularfracture when the material is tested at low temperature. In this material like in other steels the influenceof some other alloying elements (Mo, Mn...)isclearly significant (1) and should also be taken intoaccount. But phosphorus effect has proved to be predominant.The effect of ghost lines on A508 class 3 toughness properties has been pointed out (2) but has neverbeen quantitatively studied. The aim of the present study is therefore to find out a quantitativerelationship between grain boundary phosphorus segregation and critical intergranular fracture stress. Asynthetic steel with a chemical composition representative of an average segregated zone was preparedfor the present study. A number of heat treatments were applied to reach different embrittlementconditions. Then brittle fracture properties were obtained by performing cryogenic fracture tests onnotched tensile specimens while the corresponding grain boundary phosphorus levels were measured byAuger electron spectroscopy. Systematic fractographic observations were carried out. Moreover anattempt to determine the influence of temperature on the critical intergranular fracture stress was made.Materials and Heat TreatmentsThe intrinsic mechanical characteristics of segregated zones cannot be easily determined because oftheir very small dimensions (;2 mm width). A material with a chemical composition representative ofan average segregated zone was therefore prepared. Another plate was also elaborated with the nominalcomposition of A508 class 3 (3). A comparison between both materials can be made on Table 1 whereit is noted that the synthetic steel is enriched with most of the alloying elements. In particular

The embrittlement characteristics of large turbine casing bolts: the influence of service induced strain

The present article attempts to describe an assessment of the effects of accumulated permanent service strain on the degree of toughness losses, caused by reverse temper embrittlement (RTE) experienced by large turbine casing bolts during high temperature service. Effectively, it was established that (a) reported studies in this area were scarce, (b) the early work exhibited somewhat contradictory trends while (c) the post 1990 studies indicated that the presence of strain exacerbated the extent of RTE. The few modern day studies suggested that the effects of strain in low alloy steel bolts were greatest at low levels of up to around % ϵ∽0.5% and the trend could be described by the expression: C N= A(% ε) −0.8, where C N was the normalised ambient Charpy energy and A was a scaling coefficient. Also the Charpy fracture appearance transition temperature (FATT) was also shown to increase in proportion to (% ϵ) −0.8. One study has reported an embrittlement constant, termed C RTE, which was a function of grain size and bulk %P in the steel, and whose value determined the likelihood of bolt embrittlement during service, i.e., low values predicted embrittlement with high values indicating non-embrittlement during service. It was reported that strain decreased the value of C RTE. Within the strain range 0.2% to 0.6% one study established that the effects could be described as follows: C RTE =0.35−0.44×(% ε), while at lower strain values another study reported that C RTE=0.0772×(% ε)−0.64. Note that the exponent in the latter expression was similar to that which described the normalised Charpy energy trends. It has been suggested that the total RTE effects, or toughness losses, were the summation of two separate processes, viz., a temperature-time controlled grain boundary phosphorus segregation component and a strain induced grain boundary phosphorus segregation component. The latter was thought to be related to enhanced phosphorus movement along certain dislocation networks, caused by plasticity, to grain boundary locations. Although the data was scarce, it was suggested that the degree of grain boundary segregation by phosphorus during high temperature service increased with increasing levels of strain. This trend between the atomic fraction of grain boundary phosphorus C GB P and accumulated strain was simply described numerically as C GB P=10.54×(% ε) 0.35. Finally, actual bolt strain distributions with service times in real working situations are discussed together with considering both embrittlement and creep damage degradation acting together with time, or accumulated service strain.

Periodic oxide cracking on Fe2.25Cr1Mo produced by high-temperature fatigue tests with a compression hold: Hecht, R.L. and Weertman, J.R. Metall. Trans. A (Feb. 1993) 24A (2), 327–333

Low- and high-alloy ferritic steels, 2.25Cr1Mo and E911, were irradiated by 250 keV Ni+ ions at 300 °C to study radiation-induced inter-granular phosphorus segregation behaviour and the effect of hafnium on radiation-induced segregation. Irradiations were carried out in an ion accelerator to a dose of 0.452 dpa for 2.25Cr1Mo and 0.305 dpa for E911 and E911+1%Hf. Grain boundary phosphorus segregation was detected by using field emission gun transmission electron microscopy with an energy dispersive X-ray analyzer. The phosphorus segregation at boundaries after irradiation in the high-alloy steel is higher than that in the low-alloy steel. Hafnium can suppress radiation-induced segregation to approximately one-sixth of the level in hafnium-free materials. A model to predict radiation-induced segregation in foil-samples is presented. Site competition between carbon and phosphorus is taken into consideration. The predicted results show good agreement with experimental data.

A probabilistic model for fatigue crack growth: Liao, M. and Yang, Q.-X. Eng. Fract. Mech. (Nov. 1992) 43 (4), 651–655

Low- and high-alloy ferritic steels, 2.25Cr1Mo and E911, were irradiated by 250 keV Ni+ ions at 300 °C to study radiation-induced inter-granular phosphorus segregation behaviour and the effect of hafnium on radiation-induced segregation. Irradiations were carried out in an ion accelerator to a dose of 0.452 dpa for 2.25Cr1Mo and 0.305 dpa for E911 and E911+1%Hf. Grain boundary phosphorus segregation was detected by using field emission gun transmission electron microscopy with an energy dispersive X-ray analyzer. The phosphorus segregation at boundaries after irradiation in the high-alloy steel is higher than that in the low-alloy steel. Hafnium can suppress radiation-induced segregation to approximately one-sixth of the level in hafnium-free materials. A model to predict radiation-induced segregation in foil-samples is presented. Site competition between carbon and phosphorus is taken into consideration. The predicted results show good agreement with experimental data.

Experimental observations of fatigue crack growth in IS-1020 steel under constant amplitude loading: Singh, S.B. and Kumar, R. Int. J. Pressure Vessels and Piping (1993) 53 (2), 217–227

Low- and high-alloy ferritic steels, 2.25Cr1Mo and E911, were irradiated by 250 keV Ni+ ions at 300 °C to study radiation-induced inter-granular phosphorus segregation behaviour and the effect of hafnium on radiation-induced segregation. Irradiations were carried out in an ion accelerator to a dose of 0.452 dpa for 2.25Cr1Mo and 0.305 dpa for E911 and E911+1%Hf. Grain boundary phosphorus segregation was detected by using field emission gun transmission electron microscopy with an energy dispersive X-ray analyzer. The phosphorus segregation at boundaries after irradiation in the high-alloy steel is higher than that in the low-alloy steel. Hafnium can suppress radiation-induced segregation to approximately one-sixth of the level in hafnium-free materials. A model to predict radiation-induced segregation in foil-samples is presented. Site competition between carbon and phosphorus is taken into consideration. The predicted results show good agreement with experimental data.

A microcalorimetric study of fatigue crack propagation in precipitation-harnened CuCoSi alloys: Varschavsky, A. and Donoso, E. Mater. Lett. (Nov. 1992) 15 (3), 207–211

Low- and high-alloy ferritic steels, 2.25Cr1Mo and E911, were irradiated by 250 keV Ni+ ions at 300 °C to study radiation-induced inter-granular phosphorus segregation behaviour and the effect of hafnium on radiation-induced segregation. Irradiations were carried out in an ion accelerator to a dose of 0.452 dpa for 2.25Cr1Mo and 0.305 dpa for E911 and E911+1%Hf. Grain boundary phosphorus segregation was detected by using field emission gun transmission electron microscopy with an energy dispersive X-ray analyzer. The phosphorus segregation at boundaries after irradiation in the high-alloy steel is higher than that in the low-alloy steel. Hafnium can suppress radiation-induced segregation to approximately one-sixth of the level in hafnium-free materials. A model to predict radiation-induced segregation in foil-samples is presented. Site competition between carbon and phosphorus is taken into consideration. The predicted results show good agreement with experimental data.

High-resolution auger electron spectroscopy of grain boundary phosphorus segregation in NiCrMoV and NiCr steels

This communication presents some AES data which compare average measurements of grain boundary phosphorus segregation to those from specific microstructural features such as the prior austenite grain boundary carbide/ferrite interface

The effects of grain boundary phosphorus segregation and heat treatment on toughness of 9% Ni steel and its welded joint.

Effects of phosphorus (P) and heat treatment on toughness have been investigated on a quenched and tempered 9% Ni steel base metal and its welded joint simulated by a thermal cycle simulator. The mechanism of toughness improvement by the diminution of P component was examined by mechanical tests, microstructure observation and computer simulation of the grain boundary segregation. The critical CTOD value and Charpy absorbed energy in the base metal showed remarkable deterioration as increasing P content, when the cooling rate after tempering was lower than 1×10-2 K/s. Simulated bonds showed reduction of toughness when the P level was higher than 0.008% and/or the cooling rate after post weld heat treatment (PWHT) was lower than 1×10-2 K/s. The Charpy absorbed energy of base metal was higher than that of PWHT welded joint when they showed the same critical CTOD value. Intergranular ductile fracture was occurred in the base metal, while intergranular brittle fracture in PWHT welded joint was observed.The effect of cooling rate following the tempering on the grain boundary segregation of P was estimated by computer simulation, based on Guttmann-McLean theory. The computed amount of P segregation in grain boundary was closely related to the toughness both for a base metal and a simulated bond. It was also suggested that when the grain boundary segregation of P was lower than 0.1, high toughness level was obtained in the base metal and the simulated bond. The difference of toughness between the base metal and the simulated bond was attributed to the coarsening of grain size by the welding thermal cycle. The 9% Ni steel with P level lower than 0.005% shows excellent toughness in the base metal and the welded joint.