15NiCuMoNb5(WB36)

Nuclear Engineering and Design 206 (2001) 337– 350 www.elsevier.com/locate/nucengdes Copper precipitates in 15 NiCuMoNb 5 (WB 36) steel: material properties and microstructure, atomistic simulation, and micromagnetic NDE techniques I. Altpeter a,*, G. Dobmann a, K.-H. Katerbau b, M. Schick b, P. Binkele b, P. Kizler b, S. Schmauder b a Fraunhofer -Institut fu ¨ r Zersto ¨ rungsfreie Pru ¨ f6erfahren IZFP, Uni6ersita ¨ t, Geb. 37, 66123 Saarbru ¨ cken, Germany b MPA Stuttgart, Germany Accepted 24 November 2000 Abstract The material investigations presented confirm the results of earlier MPA investigations that the service-induced hardening and decrease in toughness in WB 36 materials are caused by the precipitation of copper. In the initial state of the material, generally only a part of the alloyed copper is precipitated. The other part is still in solution and can be precipitated during long-term operation at temperatures above 320– 350°C. The copper precipitation leads to a distortion of the crystal lattice surrounding the copper precipitates and yields internal micro-stresses. If the number and size of the copper precipitates change during operation of a component, a change of the residual-stress level occurs. Formation and growth of copper precipitates was simulated using atomistic calculations. In addition, it was possible to mathematically follow the movement of dislocations and their attachment to precipitates. In this way the nano-simulation was established as a scientific method for the numerically based understanding of precipitation hardening. The results obtained from load stress-related Barkhausen noise measurements demonstrated that these micro-magnetic procedures are generally suitable for the verification of copper precipitation. The goal of current research is to establish these findings statistically through further experimental measurements. In addition, the influence of different deformation states, macro residual stress, and thermal-induced residual stress have to be researched. This is important for future developments of non-destructive inspection techniques applied to inservice components. © 2001 Elsevier Science B.V. All rights reserved. 1. Introduction The low-alloy, heat-resistant steel 15 NiCuMoNb 5 (WB 36, material number 1.6368) is used * Corresponding author. Tel.: + 49-681-93023827; fax: + 49-681-93025920. as piping and vessel material in boiling water reactor (BWR) and pressurized water reactor (PWR) nuclear power plants in Germany. One reason for its wide application is the improved 0.2% yield strength at elevated temperatures. In addition, heat treatment is economical because a ferrite –bainite structure with relatively high 0029-5493/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 0 ) 0 0 4 2 0 - 9 a pressurizer in a PWR). projects have been and are being pursued with the goal to detect the generation of service-induced copper precipitation in WB 36 using micro-magnetic test procedures (Altpeter et al.. 1991. The feasibility study (Altpeter et al. Schick et al. This project (BMWi-Vorhaben. 1998). At IZFP Saarbru ¨ cken. and to understand the underlying microstructural processes.. In parallel to the above activities. “ Non-destructive testing using micro-magnetic techniques. 1995. Rau et al. starting from model calculations using the atomic length scale (‘nano-simulation’). 1998) showed that this process could generally be extended to the determination of high degree internal stress for copper precipitates. “ Simulation of the movement of dislocations and their pinning at precipitates.g. as follows: “ Material properties and microstructure. which showed a quantitative correlation between the number and size of the service-related generation of copper precipitates and the service-related increase of the elastic limit of WB 36 (Kizler et al. 1993). i. 1992. understanding of the mechanical behavior of WB 36. “ Results of the atomistic simulation. This resulted in obvious indications that these processes are caused by the copper precipitation that occurs during long-term operation at temperatures from 320°C upwards. 1995. 1998). the Materials Testing Institute (MPA) Stuttgart has performed several projects to shed light upon the operation-induced hardening and decrease in toughness of WB 36 (Ruoff and Katerbau. This paper outlines the current understanding of copper precipitation in WB 36.. The three topics mentioned above will be included. BMWi-Vorhaben. the operating temperature was between 320 and 350°C. 1998. 1997). MPA Stuttgart executed calculations using a mesoscopic theory of precipitation hardening and electronmicroscope data. 1998. Altpeter et al. Based on this level of information. According to Adamsky et al. 1996. 1998). which occurred during operation and in one case during in service hydrotesting. a government-supported project was started (BMWi-Vorhaben. 1998) included the statistical determination of the usability of the procedure to verify copper precipitates and to provide requirements for non-destructive determination and (possible) quantification of WB 36 material alterations such as service-induced hardening and decrease in toughness. 2000. The goal is to determine the mechanical properties of the material starting from its atomic structure. Willer and Katerbau. In all damage situations.e. theoretically improved. 1995)... whereas German nuclear power plants use the material mainly for pipelines at operating temperatures below 300°C and in some rare cases in pressure vessels up to 340°C (e. Jansky et al. Uhlmann et al. The following points are particularly important: “ Simulation of the formation and growth of copper precipitates in steel. Conventional power plants use this material at operating temperatures of up to 450°C. an operation-induced hardening associated with a decrease in toughness was seen in all cases. the processes that lead to the shift in the transition temperature of the notched-bar impact test in WB 36 are unknown. 1991. This is possible due to the nickel and molybdenum contents of the steel. / Nuclear Engineering and Design 206 (2001) 337–350 bainite content and without any pearlite also results from air cooling after austenitizing.. Following long hours of operation (90 000 –160 000) some damage was seen in piping systems and in one pressure vessel of conventional power plants during 1987 – 1992 (Adamsky et al. 1996.. Schick and Wiedemann. . A process is being used for the determination of the high degree of higher-order internal stresses that was developed and patented within the framework of a DFG-research project for steel containing cementite (Altpeter et al..338 I. Schick. The latter is mainly a shift in the transition temperature of the notched-bar impact test to higher temperatures. (1996). The objectives of this project are to describe quantitatively the alterations of the mechanical properties of WB 36 which are possible under light water reactor conditions.. The current project includes an in-depth. Since the beginning of the 1990s. Even though different factors played a role in causing the damage. 02%) were specified. which is significantly larger than the shift of 58 K measured by the notchedbar impact tests in this case.00 1.35 0.35 0. The explanation of the service-induced changes of the properties of the WB 36 material can be derived from the currently available knowledge on the iron –copper-phase diagram depicted in Fig. It is interesting that the KV – T curve of the initial state can. Kubaschewski.70 1.40 0. The copper. The first three components listed in Table 3 have shown damages (Adamsky et al. Rau et al. Material properties and microstructure Tables 1 and 2 show the specified data (status 1965) for the chemical composition and tensile tests of the WB 36 material in comparison to two other typical steel types used for vessel construction. and niobium contents. Min. Similar changes are Table 1 Chemical composition of vessel steels (status 1965)a Material 15 Mo 3 (TH 31) 13 CrMo 4 4 (TH 32) 15 NiCuMoNb 5 (WB 36) Min. Min. with a corresponding increase in hardness. / Nuclear Engineering and Design 206 (2001) 337–350 339 2. Max.. 1991. Max. 1982). 3. The upper shelf value also shows a noticeable reduction of the values of KIJ.35 0. explains the extensive application of the WB 36 material in the medium temperature range. although less relevant.15 0. is virtually lost when recovery annealing is performed at the temperature of the last stress-relief heat treatment of the component (in the present case at 580°C).20 0.40 0.80 a All values in mass %.18 0.50 0. Further investigations are being made in order to verify these results.17 Si 0.20 0.00 0. for the most part.80 1. . Further investigations have demonstrated that the difference between the initial state and the recovery annealed condition. in addition to the economical heat treatment..30 :0.050%) and nitrogen (max. 1996. of approximately 20% can be observed. Fig. This. A comparison is made between the initial state and the material condition after service (350°C. The service exposure results in a shift of the transition temperature of approximately 100 K. Jansky et al. The dependence of the stress intensity factor KIJ for crack initiation on the test temperature is shown in Fig. and the transition temperature for the notched-bar impact energy was elevated up to 70 K. nickel. It Mn 0. In addition to the shift of the transition temperature a reduction of the upper shelf energy. also the contents of aluminum (0. 57 000 h).70 0. obviously.40 Ni Nb Cu 0.25 0. Table 3 shows data on the change of material properties of WB 36 material components after long-term service temperatures ranging from 330 to 350°C. as well as the increased values of manganese along with decreased chromium content are typical for WB 36 material.I. 2. 1958.10 0.30 1.015 –0. 1.25 0. 0. 1986). Max.50 0.50 0. 1993).12 0. be restored by recovery annealing at 550°C for 3 h of the material state after long-term service.. as shown in Fig.15 0. which is still visible here. There are significant differences with respect to the strength values: the yield strength at 350°C for WB 36 is almost twice the value of the other two steels.50 found for the other components shown in Table 3. and it was assumed. The solubility of copper in steel at temperatures below approximately 650°C was unknown until the 1980s (Hansen and Anderko. Altpeter et al.70 0. 1 shows the respective notched-bar impact energy versus temperature curve (KV – T curve) for the initial state and after service at 350°C for 57 000 h. The strength parameters increased up to 140 MPa during operation.20 Cr Mo 0. 1991. In the meantime. C 0. 15 Mo 3 and 13 CrMo 44.25 0. that steel heat-treated at temperatures between 650 and 550°C would not contain any dissolved copper (Haarmann and Kalwa. visible under suitable TEM conditions. Copper particles ranging from 2 to 20 nm in size are present. the total number of particles after service is approximately twice as high as in the initial state. 1991. With the TEM investigations.. slowly precipitates during long-term operation at temperatures above 320°C and may lead to an undesirable increase in hardness and decrease in toughness. 4 shows an example of a transmission electron-microscope (TEM) image of service-exposed WB 36 material.. WB 36 — absorbed energy versus temperature curve. but rather for iron – copper model alloys only (Hornbogen and Glenn.. A5 (%) is now known. which is still in solid solution in the initial state of the material. however. Approximately the same volume of material was analyzed in both cases. . 1960. face-centered cubic like pure copper. Further investigations were carried out by Willer and Katerbau (1995) and Schick et al. a transition structure. (1998). the same as the surrounding matrix. In contrast. and between about 6 and 20 nm size. Goodman et al. Othen et al. 1994). by the way. Fig. well defined internal micro-stresses are present. / Nuclear Engineering and Design 206 (2001) 337–350 Table 2 Mechanical properties of vessel steels (status 1965) Material Yield strength. These precipitation processes have long been described. the desired increase in strength of the WB 36 material is caused by only a part of the alloyed copper. The results of the TEM investigations show that the number of particles increases during service for all of the three size groups. that WB 36 materials annealed in this temperature range still contain noticeable amounts of copper in solid solution (Sundman et al. Hornbogen et al. 1985). the material after service at the bottom.340 I. but not for steels. Rm (RT) (MPa) Uniform elongation. it was furthermore shown that the copper particles have three differing crystal structures depending on their size: up to 6 nm. body-centered cubic. above 20 nm. 1966. Some of the particles have a twin structure recognizable by a typical striping. The initial state of a WB 36 material is shown on top. 1. Therefore. Altpeter et al. 5 shows the number and size distribution of the precipitated copper particles visible in the TEM. 1973. The other part of the copper. ReH (MPa) at RT 15 Mo 3 (TH 31) 13 CrMo 4 4 (TH 32) 15 NiCuMoNb 5 (WB 36) ]265 ]295 ]430 350°C ]175 ]215 ]353 430–520 430–550 610–760 ]19 ]18 ]16 Ultimate tensile strength. In comple- Fig. Fig. Particles of the latter group make up about 50% of all particles visible by TEM and lead to a pronounced distortion of the crystal matrix in the regions around the particles. Therefore. The location of the maximum of the size distribution and the average diameter do not differ significantly.. First results of service-related copper precipitation in WB 36 materials were outlined by Ruoff and Katerbau (1992). DT (K) 55 40 50–60 45 70 Component Power plant T-Component Boiler drum Piping I. DHV – 28 – – 55 Transition temperature. DRp0. DRm (MPa) Vickers hardness.Table 3 WB 36-alteration of material properties during service Service conditions 330°C/91 600 h 48 330°C/163 000 h 110 338°C/128 000 h – 339°C/160 000 h 110 350°C/57 000 h 140 110 125 84 82 – Yield strength. 1992) NPP (1993) Coal-fired (1995) 341 . Altpeter et al. / Nuclear Engineering and Design 206 (2001) 337–350 Pressurizer Boiler drum Lignite (1987) Oil-fired (1990) Lignite (Greece.2 (MPa) Ultimate tensile strength. Fig. WB 36 — precipitations after long-term service. 4. Fig. Altpeter et al. WB 36 — fracture toughness at crack initiation. it was demonstrated by TEM that the number and size distribution of the copper precipitates are virtually the same for a material in the initial state and the material after service and an additional recovery annealing (at the temperature of the last stress-relief heat treatment). WB 36 — size distribution of the copper precipitates. . / Nuclear Engineering and Design 206 (2001) 337–350 Fig. Fig. Iron– copper phase diagram. Furthermore. 2.342 I. 3. mentary investigations using SANS (Small Angle Neutron Scattering) technique it was additionally found that the number of particles (not detectable in TEM) having a size of approximately 1 nm also increases significantly. 5. The diffusion of the atoms proceeds via vacancy jumps towards nearest neighbor atoms. Results concerning the thermal-induced hardening of copper alloyed steel and the possibility to pre-calculate this quantity were expected. which is located next to a vacancy. Simulation of formation and growth of copper precipitates in steel The objective of these investigations was to describe the formation and growth of copper precipitates on the atomic level. 3. 6.6 at.1. In the case of precipitates that can be cut.526 × 10 − 5 but in reality. Atomistic simulation (nanosimulation) of WB 36 material 3. called the critical  − DEi6 kT  . After t = 12.2 years. which was modified for the current investigation. temperature.% = 393 randomly distributed copper atoms.I. The attempt frequencies (wi ) to change the lattice location is dependent on the lattice constant a and the diffusion coefficient DO.1 years these have merged into four larger precipitates (approximately 2 nm in diameter).14 × 10 − 13. The simulation uses a body centered cubic crystal lattice. a characteristic angle is found between the dislocation segments at the moment of maximum stress. Therefore.i of iron or copper via wi = DO. the concentration is CREAL = 1. just hindered (Russell and Brown. The simulation has a vacancy concentration of CSIM = 1. Altpeter et al. / Nuclear Engineering and Design 206 (2001) 337–350 343 Fig. the movement of dislocations is not completely blocked. small precipitates (approximately 1 nm in diameter) have formed from the originally randomly distributed copper atoms. This type of position exchange is thermally activated and the jump frequency Y is given by: Gi6 = wi exp with k the Boltzmann’s constant and T. Calculations were done using the Monte-Carlo simulation program (Soisson et al. In the original state the lattice has 0.i /a 2. the lattice contains 2 × 323 = 65 536 lattice points. Simulation of the mo6ement of dislocations and their pinning at precipitates The pinning of dislocations at obstacles is the basic mechanism of precipitation strengthening (Nembach. 1997).. After t = 2. for example copper precipitates in WB 36 material. The activation energy DEiw is the difference in energy between the stable state and the saddle point position of a diffusing atom. 6. 1996). only the copper. a vacancy and 65 142 iron atoms. The activation energy depends on the local short-range-order and is determined separately for each position exchange. at 350°C. The following illustration shows the results of a simulation at a temperature of T = 350°C ( = 623 K) as displayed in Fig. Therefore. not the iron atoms are shown.2. During cutting of the precipitate by the moving dislocation. Results of a Monte Carlo simulation on formation and growth of copper precipitates. 1972). The model volume used was a cube having an edge length of 32 lattice constants and periodic boundary conditions. 3. the time scale of the simulation is multiplied by the appropriate factor. This stress is negligible compared to the stress caused by the dislocation. 1999a. it then provides the possibility to understand and pre-calculate the precipitation hardening starting from the atomistic properties (Russell and Brown. 7 shows a cross-section through the center of the model. the copper precipitate is indicated in light gray. Fig. consisting of 82 600 atoms. EAM. 2000).6 × 17. Fig. the lattice also relaxes slightly around the precipitate to account for the differing atomic diameters of iron and copper. The positions of the dislocation lines represent the maxima of the distribution. (0. 7. The model has internal stress due to the dislocation and it must relieve this stress using a relaxation algorithm.. Altpeter et al. angle. 1997).0 nm and consists of 1254 copper atoms. 8. 1989). This model was retained since the current simulation calculations are the first of this kind.. the copper precipitate has a diameter of 3. 7. The molecular dynamics program FEAt was used for the relaxation of the model (Kohlhoff and Schmauder. Further details are published separately (Nedelcu et al. measuring 7. In the present case. 7. Two edge dislocations were introduced into the model. / Nuclear Engineering and Design 206 (2001) 337–350 . To investigate this. If it can be determined using the nano-simulation. The inter-atomic forces are represented by the current Embedded-AtomModel.b. scale in 1 A nm).344 I. In addition to the movement of the dislocations. the movement of the dislocation is initiated without external force.. Using this relaxation algorithm.0 nm3. The model for the present simulation was a cuboid-shaped iron mono-crystal. . by calculating the Burgers vector density distribution (Nedelcu et al. Gliding plane and Burgers vector (b ) have the crystallographic label (1 – 10) and b = 9 Ž1 1 1. 1972. 1998). The EAM potentials for iron and copper can be found in the literature. 1999a.1 Fig. A cross-section is shown in Fig. It turned out that the internal stress was sufficient to initiate movement of the dislocations. Cross-section through the iron structure model used for the nano-simulation of the dislocation movement. Dislocation movement: movement to a precipitate (grey circle) and cutting of the precipitate.5 × 7. an algorithm was developed that recognized the changing position of the dislocation in the relaxing model. A potential developed by MPA Stuttgart was used for the iron –copper (Fe –Cu) interaction (Ludwig et al. Iron atoms were replaced with copper atoms in a spherical area representing a coherent precipitate. 2000). Nembach.b. located at the right and left of Fig. This is a key parameter for the understanding of precipitation hardening. rectified. . Previous investigations. the critical angle. The nano-simulation could therefore be established as a scientific method for the numerically based understanding of precipitation hardening using the performed calculations.. The second illustration shows the protuberance of the dislocation line indicating the attraction between the dislocation and the precipitate. 1999a. amplified.2. The Barkhausen noise signal is recorded using two differential air-core coils to separate the influence of the energizing magnetic field. The hardening of the investigated iron –copper system is therefore based on the modulus of transverse elasticity of matrix and precipitate. The magnetizing frequency ( fE) is 1 Hz and the energized field amplitude (HMAX) is 50 A cm − 1. 1998).2. 2000).2. Principles The micro-magnetic concept used is based on load –stress dependent Barkhausen noise measurements. This value corresponds to the value that can be calculated for copper precipitates of this size and is confirmed by continuum mechanics theory (Russell and Brown. Recording of the longitudinal magnetostriction cur6es All dimensional changes in the ferromagnetic material that result from changes in the magnetization state are called magnetostriction. jointly performed with PTB in Braunschweig.. 7. The angle between the dislocation segments. which is sufficient for these materials. Altpeter et al. The maximum of the Barkhausen noise amplitude is recorded as a function of the increasing load stress.1. While various theoretical approximations and parameters (that must be developed through experiments) are required for continuum mechanics theory. The last illustration shows the state of maximum stress. The shift of this maximum along the stress axis is a measurement of the change of the micro (or macro) residual stress condition. 4. the current calculations are based on nothing more than the elastic properties on the atomic length scale as represented by the EAM potentials.I. This curve runs through a magnetostrictive-related maximum. is approximately 140°. The first partial illustration shows the dislocation at a large distance from the precipitate (gray circle). 8 shows six different stages of the dislocation movement relative to a precipitate.2. This technique permits the collection of quantitative data of residual stress variations without the use of a reference method such as X-ray diffraction. The resulting so called Barkhausen noise profile is evaluated with respect to their maxima and their respective magnetic field positions by a computer (Altpeter et al. Integral load -stress related Barkhausen noise measurements To establish load –stress related Barkhausen noise measurements. Nedelcu et al. length variations parallel or perpendicular to the field direction. / Nuclear Engineering and Design 206 (2001) 337–350 345 Fig. appropriately filtered. The voltage induced in an encircling coil is used to calculate the respective induction B (t ) by integration.1. and displayed as function of the tangential field strength. and volumetric 4. The samples are magnetized in the longitudinal direction. Experimental set -up 4.3. The dislocations are perpendicular to the cross-section of the simulation model depicted in Fig. These magnetized samples are then used for the recording of ferro-magnetic hysteresis curves. The noise signal is recorded as inducted voltage.2. A differentiation is made between the longitudinal magnetostriction (uL) and the transverse magnetostriction (uT). have demonstrated the comparability of the magnetizing curves using the apparatus with a tolerance of B 10% of PTP’s calibration standard.b. 4. a measurement system was installed to record micromagnetic test values during concurrent tensile loading. The magnetic Barkhausen noise is triggered by an alternating magnetic field applied to the sample using an electromagnet. Recording of hysteresis cur6es Cylindrical samples are inserted into the Hystrometer (manufactured by List) for magnetization. 4. 1972. NDT using micro-magnetic techniques 4. under the influence of external stresses shows the following behavior: tensile stress results in the alignment of the magnetizing vectors of the individual domains in the direction of tension (Bozorth. The dependence of the longitudinal magnetostriction uL on the tangential component Ht of the exciting magnetic field is shown in Fig. In6estigation results Measurements were taken on two material states of WB 36 from the same melt. The change in the magnetic Barkhausen noise peak position between the service exposed and recovery annealed sample is within the error tolerance. three groups of copper precipitates are present whereby state B has more copper precipitates than state E in every group. The investigated samples consisted of rods with 8 mm diameter and 160 mm length. 1951). 1995) is much larger than the coercive field strength of the hysteresis curve. Barkhausen noise profile curves for the service-exposed material state and after recovery annealing. The measurement of the longitudinal magnetostriction under load stress is performed in a standard way using a strain gauge affixed to the samples and an amplifier (manufactured by HBM). the serviceexposed state (‘B’) and the recovery annealed state (‘E’). measured without load stress. The value recorded in the area of magnetic saturation is called saturation magnetostriction (uS). A difference of 140 MPa change in yield strength is not reflected in the B–H-loop. Altpeter et al. 11 without additional load. The strong localized residual stress of higher order. The longitudinal magnetostriction behavior of the two different material conditions was investigated in addition to the Barkhausen noise profile. 10. Both material conditions can be differentiated in the unstressed condition using the maximum Barkhausen noise amplitude MMAX. The material state after recovery annealing showing the same hysteresis curve.346 I.1 A cm − 1 in both microstructure states (service-exposed material state and recovery annealed state).3. 10). / Nuclear Engineering and Design 206 (2001) 337–350 Fig. is not reflected in the macroscopic hysteresis curve since this is averaged over volume. A positive magnetostrictive material. Fig. measured without load stress. Ferromagnetic hysteresis curve for the service-exposed material state. 1951). . like ironbased materials.5 9 0. The hysteresis curves of the service-exposed and recovery-annealed states are identical (Fig. 9. 9). This is a combination of longitudinal and transverse magnetostriction (Bozorth. This can be explained by the fact that the tension stress sensitivity (load and internal stress) of the test value MMAX (Altpeter et al.. The magnetic Barkhausen noise profile and the hysteresis curves were recorded using the encircling coil technique (Fig. The load stress applied to the sample is measured using a second amplifier connected to a load cell mounted on the tensile test machine. An obvious difference can be 4. TEM investigations have shown that in both conditions. The coercive strength (Hc) is 6. magnetostriction. due to copper precipitation. 12) for the recovery annealed material state. where the stress difference is also D| : 20 MPa. In this way the nano-simulation was established as a scientific method for the numerically based understanding of precipitation hardening. are reduced by recovery annealing. 1951). To quantify the degradation of the residual stress of a higher order. The stress difference of D| : 20 MPa indicates an integral value for the total sample volume. 5.05 mm m − 1).B (3. This is the case for the service-exposed material states starting at |B : 55 MPa (top right in Fig. it was possible to mathematically follow the movement of dislocations and their attachment to precipitates. Local stress concentrations can be much higher at the phase boundaries of copper and matrix. If internal stresses. In addition. uLMAX. the magnetic reversal process transforms into magnetostrictive negative rotation (Bozorth. load stress-related micro-magnetic tests were carried out. a change of the residual-stress level occurs.e. the superposition of macro stress and stress of a higher degree. The evaluation of the longitudinal magnetostriction curves therefore yield qualitatively and quantitatively the same result as the magnetic Barkhausen noise profile curves. The magnetic Barkhausen noise profile is influenced by the total stress state of the material. generally only a part of the alloyed copper is precipitated. where uLMAX is the maximum of the longitudinal magnetostriction. 12. 11. Variation of longitudinal magnetostriction uL with magnetic field Ht for the service-exposed material state and after recovery annealing. 12. The copper precipitation leads to a distortion of the crystal lattice surrounding the copper precipitates and yields internal micro-stresses. but starting at |E : 70 MPa. since the density of movements at the [100]-90° domain wall decreases in the direction of magnetization. With increasing tensile loads.I. bottom right of Fig. observed in the two material conditions. In the initial state of the material. i. 12 shows that the longitudinal magnetostrictive curve shifts to smaller values under increasing tensile load. Formation and growth of copper precipitates was simulated using atomistic calculations. The other part is still in solution and can be precipitated during long-term operation at temperatures above 320°C. from |B : 50 MPa to |E : 67 MPa. as shown in Fig.24 9 0.E (4. The copper-dependent residual stress reduced by recovery annealing leads to a change in the uL (Ht) curve for the no-load stress condition. This stress corresponds to the value on the MMAX(| ) curve where the maximum occurs. the total longitudinal magnetostrictive curve is in the negative area. . The curves were approximated using polynomials of the 6th order and the maxima of the respective approximation curves were determined analytically. measured without load stress. a characteristic for every type of steel. Altpeter et al. then a higher load must be applied to arrive at the same total stress condition. Fig.06 mm m − 1) and is much larger than uLMAX. due to copper precipitates. If the number and size of the copper precipitates change during operation of a component. The maximum of the MMAX(| ) curve is shifted by approximately D| : 20 MPa. / Nuclear Engineering and Design 206 (2001) 337–350 347 Fig.48 9 0. Above the tensile stress limit. Both material conditions ‘B’ and ‘E’ can be clearly differentiated in the load stress dependency of the magnetic Barkhausen noise profile. Conclusion The material investigations presented confirm the results of earlier MPA investigations that the service-induced hardening and decrease in toughness in WB 36 materials are caused by the precipitation of copper. / Nuclear Engineering and Design 206 (2001) 337–350 The results obtained from load stress-related Barkhausen noise measurements demonstrated that these micro-magnetic procedures are generally suitable for the verification of copper precipitation. the influence of different deformation states. and thermal-induced residual stress has to be researched.348 I. Altpeter et al. This is important for future devel- Fig. for material states after service (upper row) and after recovery annealing (lower row). Variation of the maximum MMAX of the Barkhausen noise amplitude and of longitudinal magnetostriction uL with applied load. macro residual stress. 12. In addition. The goal of current research is to establish these findings statistically through further experimental measurements. . 1986. (Eds. 19. R. Schmauder. McGraw-Hill. 1960..).. New York. G. vol. Laufzeit 1998 bis 2001. Iron — Binary Phase Diagrams. Metall. 1998. S. Jansky. Uhlmann. 2363– 2369. Nedelcu. Kizler. 1995. Betriebsbedingte Eigenschaftsa ¨ nderungen kupferhaltiger ferritischer Beha ¨ lter. 12. 22. G. 1998. Detemple. S. van Nostrand.L.R. M. D..J. pp. MPA-Seminar.. Phil. 10 – 11 Oct. Schmauder. Schmauder. Nucl. Schmauder. S.. 1991.. K. for Economic Affairs (BMWi) and for Environmental Protection..D. IZFP Saarbru ¨ cken.-G. Technischer Bericht. Eng. . F. 207– 214.. Untersuchung von thermisch induzierten phasenspezifischen Eigenspannungen in weißem Gußeisen und in Sta ¨ hlen mit unterschiedlicher Zementitmorphologie. Kalwa. Anderko.4 at pct copper... Stuttgart.C. Lu ¨ tjering. Modelling and Simulation in Materials Science and Technology.. Bozorth.W.. 1995. Adamsky. Goodman. Laufzeit 1997 bis 2000. 1996. Linking nanoscale and macroscale: calculation of the change in crack growth resistance of steels with different states of Cu precipitation using a modification of stress– strain curves owing to dislocation theory.. G... 1958.-J. S.und Rohrleitungsbausta ¨ hle. Andra ¨ . Part II: Atom probe analyses.. S175– S183. S. Beitrag Nr. M. Berichts-Nr. W. 64.. New York. Essen. In addition. Hansen. Beitrag Nr. New York. H. Lett.. Low. 2000. H. Faßbender. AIME 218. Abschlußbericht. BMWi-Vorhaben 150 1219: Nachweis von Cu-Ausscheidungen mittels mikromagnetischer Pru ¨ fverfahren. 1993... Ferromagnetism.. Glenn. Sci.J. P.. Feedwater piping guillotine breaks at 340°C operation temperature.. Nature Conservation.. N. Othen. E. 1999b. D. F. D. Kohlhoff. Hornbogen.M. Teichmann. S. p. Haarmann. J. Kizler.. S. 196. Schmauder. Des.. S. E. Transactions of the 12th International Conference on Structural Mechanics in Reactor Technology. Mater. Moldovan. Altpeter et al.. Raab F. Teichmann.. R. Farkas... S. and VGB Kraftwerkstechnik 77 H. Jenkins. Brenner. 1998. S. 523– 532. IZFP Saarbru ¨ cken. Eng... D. Eisenhu ¨ ttenwes. Part I: Field-iron microscopy. Smith. Mater. MPA Stuttgart. Kubaschewski. In: Kussmaul.. Transmission electron microscope investigation of the structure of copper precipitates in thermally-aged Fe – Cu and Fe– Cu– Ni. 950534-TW. 9 – 10 Jan.. / Nuclear Engineering and Design 206 (2001) 337–350 349 opments of non-destructive inspection techniques applied to inservice components. Stuttgart. 1991. North-Holland.I.. Hartmann. Trans. Berlin. P.. Model. Berichts-Nr. Amsterdam. M. References Adamsky. Moldovan. MPA Stuttgart. Ro ¨ sner.. S181– 191. Kizler. Koha ¨ rente Ausscheidung in Substitutionsmischkristallen des a-Eisens. Wiley.. I. Schmauder. Pedraza. Linking nanoscale and macroscale: Calculation of the change in crack growth resistance of steels with different states of Cu precipitation using a modification of stress– strain curves owing to dislocation theory... Tolksdorf. BMWi-Vorhaben 150 1087. Soisson and Professor Dr G. I. 1951.. A new method for coupled elastic-atomistic modelling. 1064– 1070. Altpeter. P. we thank Dr F. F. Pythian.... 980232-TW. Atomic scale modelling of edge dislocation movement in the a-Fe-Cu system. D. Hornbogen. K. Martin (both CEA Saclay) for providing the MonteCarlo software and their support during the further development of the software. M.. Kern. 1982.. A metallographic study of precipitation of copper from a-iron. E.. and Reactor Safety (BMU) for the promotion and sponsorship of the research work. N. Fu ¨ nf Jahrzehnte warmfeste Kupfer-Nickel-(Molybda ¨ n-)Sta ¨ hle. Betriebserfahrungen mit dem warmfesten Werkstoff 15 NiCuMoNb 5 in konventionellen Kraftwerksanlagen. J. Roth. VGB-Konferenz ‘Werkstoffe und Schweißtechnik im Kraftwerk 1991’. 50. 24th MPA-Seminar. 4. Lang. Embedded atom potential for Fe – Cu interactions and simulations of precipitate– matrix interfaces. To ¨ nnes. V. Schmauder. Atomistic Simulation of Materials: Beyond Pair Potentials. H. Simul.. K. M. E. Sci.. Nedelcu. Modelling simulation.. Srolovitz. 1996. Soc.. Nedelcu. 8 (1997) 667–675.. Trans. Ludwig.. eingereicht. 1991... Metall. P. N. Eng. R. P. BMBF-Vorhaben 150 1078. 1989. Constitution of Binary Alloys. 6.S. P. Springer. Kizler. 1998.. 2000. (Ed. 1973. K.-D. 37. I. Nachweis von Cu-Ausscheidungen mittels mikromagnetischer Pru ¨ fverfahren. 1966. T. 8. Arch. Modellierung der Versetzungsbewegung im a-Fe– Cu System auf atomarer La ¨ ngenskala. Schaden an der Eco-Falleitung eines 600 MW-Blocks bei der wiederkehrenden Wasserdruckpru ¨ fung. 8 – 9 Oktober 1998. Mag. 632.J.R. R. Uhlmann. S. 383– 391. New York. VGB Kraftwerkstechnik 66. Kizler. Moldovan. An FIMatom probe study of the precipitation of copper from iron-1. Acknowledgements The authors would like to thank the (German) Federal Ministry of Education and Research (BMBF).. 588– 598. Nembach. 1997.. S. Albrecht. IZFP Saarbru ¨ cken. In: Vitek. Kern. S. F.. O.. Altpeter.. S. Particle Strengthening of Metals and Alloys.-J.). 1999a. 2372– 2378.. S. Schick. M. K. Technischer Bericht. L. BMU-Vorhaben SR 2065. Rau. 1996... Schick. . Calphad 9. Schick. Untersuchungen zur sicherheitstechnischen Bewertung von geschweißten . Albert.. In: Christensen. Schmauder. 44. Essen. Kizler. 9–10 Jan. 1991. Helmrich. Smith. and VGB Kraftwerkstechnik 72 H. Ruoff. Katerbau.L.. BMU-Vorhaben SR 2239.. VGB-Konferenz ‘Werkstoffe und Schweißtechnik im Kraftwerk 1991’. 1–14. From nanoscale to macroscale: Study of damage in steels with Cu precipitates using micromechanically based stress– strain curves. MPA Stuttgart.. Acta Metall. 969. et al. 3898.-H. P.). (Eds. J. 1985. Orientierende TEM-Untersuchungen zur betrieblichen Za ¨ higkeitsabnahme des Werkstoffs WB 36.350 I. 20. 1994.. M. G... The Thermo-Calc databank system. Technischer Bericht 2... Roskilde. 1998.. 1995. M.-H. 1991.. 70.. Risø National Laboratory. H. Uhlmann.. Technischer Bericht. K. F.W. 1972.. Willer...1. A.. Willer. Soisson.. J. Denmark. Altpeter et al. Andersson. Barbu.. Mag. 1990 bis 1996. B. MPA Stuttgart. MPA Stuttgart. Technischer Bericht 2. Jansson.D. Brown. Russell.-O. 529– 534. Sundman. J.J. Auswertungen und Untersuchungen zur betriebsbedingten Za ¨ higkeitsabnahme des Werkstoffs 15 NiCuMoNb 5. Monte Carlo simulations of copper precipitates in dilute iron– copper alloys during thermal ageing and under electron irradiation. D. Wiedemann. Schick.V. A.2. D. D. Unvero ¨ ffentlichte Untersuchungen zum Werkstoff WB 36.. Jenkins. 1996. G. Phil.. 153– 190. P. Othen.. High-resolution electron microscopy studies of the structure of Cu precipitates in a-Fe. / Nuclear Engineering and Design 206 (2001) 337–350 Komponenten aus Werkstoff 15 NiCuMoNb 5 (WB 36) im Hinblick auf die Za ¨ higkeitsabnahme unter Betriebsbeanspruchung. B. M. J. 1998. Proceedings of the 19th Risø International Symposium on Materials Science: Modelling of Structure and Mechanics of Materials from Microscale to Product... A dispersion strengthening model based on differing elastic moduli applied to the iron– copper system. 1995.. S. Martin. 1992. 24. pp. Katerbau.. F.. Acta Mater. BMU-Vorhaben SR 2065. Langzeit-Glu ¨ hversuche an unterschiedlichen Schmelzen und Wa ¨ rmebehandlungszusta ¨ nden des Werkstoffs 15 NiCuMoNb 5. 5 (1992) 444–454. Wiedemann. MPA Stuttgart. BMU-Vorhaben SR 471. MPA Stuttgart. Schaden und Reparatur an einer Kesseltrommel aus 15 NiCuMoNb 5 (WSB 62). M. P. K.