DEFORMATION LOCALISATION AND EAC IN INHOMOGENEOUS MICROSTRUCTURES OF AUSTENITIC STAINLESS STEELS

Transcription

1 DEFORMATION LOCALISATION AND EAC IN INHOMOGENEOUS MICROSTRUCTURES OF AUSTENITIC STAINLESS STEELS Ulla Ehrnstén*, Tapio Saukkonen**, Wade Karlsen*, and Hannu Hänninen** *VTT Materials for Power Engineering, Kemistintie 3, PO Box 1000, VTT, Espoo, Finland, ** Helsinki University of Technology, Engineering Materials, Puumiehenkuja 3, PO Box 4200 TKK, Espoo, Finland Inhomogeneous microstructures, e.g. grain size, dislocation density etc., always occur in welded structures. Varying manufacturing methods leading to complex strain paths result in highly varying cold work and consequent residual strains. The role of strain localization is not widely familiar as a precursor for failure and its mechanisms are still not fully known. If strain localization occurs by a creep mechanism, the incubation time for crack initiation can be very long, as frequently observed in NPPs. EBSD employed to measure strain distributions in a Type 304 austenitic stainless steel weld shows a high variation in residual strain distribution, which was verified by hardness measurements as well as with residual stress measurements. Strain localization investigations are also performed on specimens from Super Slow Strain Rate Test using a very slow strain rate of s 1. This is in the creep strain rate range, where diffusion along dislocation cores and grain boundaries occur together with grain boundary sliding. SSSRT s were performed on sensitized Type 304 austenitic stainless steel either with or without cold deformation in simulated BWR environment. To increase the understanding of IGSCC and the role of localization of deformation, versatile techniques like FE SEM/EBSD and TEM were used to characterize the materials after the extremely slow strain rate tests. Local variation in the amount of surface cold work seems to affect crack initiation. During crack growth, strain localization occurs at grain boundaries ahead of the crack tips. Local variations of grain size also affect strain localization. I. INTRODUCTION Intergranular stress corrosion cracking (IGSCC) of sensitized stainless steels in oxidizing boiling water reactor (BWR) environments caused major capacity factor losses in the 1970 and 1980 s. Large research programs to solve the problem were launched, major factors contributing to the problem were identified and quantified, and remedial actions were taken. 1,2 Consequently, the amount of IGSCC in sensitized stainless steels decreased remarkably, and the capacity factor of the BWRs has increased. The remedial actions included development and employment of low carbon stainless steels, new welding techniques decreasing the residual stresses and eliminating sensitization, as well as improvement of the water chemistry with different measures. More recently, the role of cold work in promoting IGSCC has gained more broad attention. This is due to the observation that non sensitized, low carbon stainless steels can also suffer from IGSCC, and that this type of cracking is connected to cold working. 1 8 Also, the observed risk for IGSCC in off normal PWR conditions is considered to be largely connected to cold deformation. 9,10 Localization of plastic deformation and the interactions between oxidation and strain localization are most probably playing a key role in the cracking of sensitized as well as non sensitized, cold worked stainless steels. Localization of deformation can be affected by several phenomena, such as dynamic strain ageing, environmentally enhanced creep, dynamic recovery and relaxation. All these can also be influenced by cold deformation. Cold deformation, i.e., deformation below the recrystallisation temperature occurs due to welding, grinding, machining, forging etc. Some degree of cold deformation is unavoidable in e.g. HAZ s, as manifested by increased hardness and dislocation density as well as residual stresses and strains. 2 The influence of cold deformation is, on the other hand, dependent on factors such as the degree of deformation, the deformation temperature, strain rate, strain path, alloy composition, etc. 11 The particular deformation temperature is important because it dictates the means of transmitting strain, and thereby the predominant deformation mechanism during dynamic loading. Low temperatures typically promote displacive transformation since dislocation motion is a thermally activated process, but higher temperatures clearly promote slip. Higher temperatures also increase the vacancy concentration and diffusion rates of vacancies, and, thus vacancy accelerated dislocation climb is more prominent with increasing temperature. Likewise, dislocation movement leads to dynamic

2 recovery in the material by the annihilation of dislocations of opposite sign. That has an effect of reducing the number of dislocations, which counteracts strain hardening. 12 Time affects the also kinetics of dislocation related deformation mechanisms. For that reason, the rate of loading can have a significant effect on the consequent mode of strain transmission, and/or on the stress at which straining occurs. 13 Of particular interest to crack initiation is the heterogeneity of deformation within a material. The most obvious case is the onset of necking in a tensile test, when the strain localizes on a macro level at the maximum engineering stress. However, as a consequence of processes such as post weld material shrinkage or surface machining or grinding, local cold work can alter the local material condition. This can lead to a non uniform deformation response on a local scale. On a still finer scale, heterogeneous deformation can occur in a multigrain material due to the randomness of the lattice orientation with respect to the critical resolved shear stress. As some grains are more suitably oriented for deformation, they undergo more deformation than their neighboring grains, resulting in a heterogeneous distribution of local strain. Strain may also be localized within a single grain, as a consequence of the formation of shear bands. A mechanistic understanding of the effects of plastic deformation on IGSCC is important, especially as the trend in NDE inspection strategy is towards risk informed inspection. A mechanistic understanding is also important in order to increase the understanding of the possible risk for IGSCC in PWR plants, and for increased understanding of the reasons of the very long incubation times for crack initiation. II. MATERIAL AND EXPERIMENTAL METHODS The investigations focused on studies of localization of deformation and comprised of characterization of residual strains and stresses in a nuclear power plant weld and characterization of specimens made of sensitized Type 304 stainless steel after super slow strain rate testing in simulated BWR environment. II. A Characterization of a nuclear weld The strain distribution in a nuclear pipe weld was determined using Electron Back Scattered Diffraction, EBSD. Further, measurements of residual stresses were made of the weld using the Contour method. Microhardness measurements using HV1 were performed in the characterized region, and the results were converted to local amount of strain, using the calibration curves for hardness and mis orientation versus plastic strain. 14 EBSD is a very versatile method for characterizing crystalline materials. The local plastic strains in the nuclear power plant weld were measured using a calibration curve developed for quantitative measurement of local plastic strain using intra grain mis orientations. 14 In the Contour method, used for the residual stress measurements, the specimen is cut using electric discharge machining, EDM, along a selected plane. The released residual stresses will result in displacements along the cutting plane. These displacements are measured with high accuracy and the numbers were here used as input values for calculating the residual stresses using the Elmer FE model. A pipe section from a 275 x 22 mm pipe made of Type 304 stainless steel, which had been manufactured according to nuclear specifications and welded using tungsten inert gas welding, TIG, was used in these investigations. II. B Super slow strain rate tests in simulated BWR environment Material from the same high carbon austenitic stainless steel pipe of Type 304 as used for the residual strain and stress measurements was used for the super slow strain tests, SSSRT. The chemical composition of the stainless steel material is presented in Table I. The material was given a solution annealing heat treatment at 1100 C/1 h followed by a sensitization heat treatment at 620 C for 24 h. Two material conditions were tested: with and without 10% cold work. Cold work was applied by tension at room temperature after the sensitization treatment. Super slow strain tests were performed using a bellows loading system, which enables very slow strain rates with truly constant extension rate and measurement of the strain from the specimen itself. 15 The materials were tested in simulated BWR environment (400 ppb O 2, 300 C) using two strain rates: s 1 and s 1. The latter is regarded to be in the range where environmentally assisted creep may occur, Figure Round tensile type specimens 4 mm with a gauge length of 12 mm were manufactured using machining, and they were tested in as machined condition. TABLE I. Chemical composition of the Type 304 stainless steel. Material C Si Mn P S Cr Mo Ni Co N Type

3 localization of strain at the grain boundaries was also observed. The amount of plastic strain in the base material decreased towards the outer surface of the pipe, Figure 2. Plastic strain was also measured in the weld metal, where it varied in the different directions of solidification, being highest in the areas of weld bead boundaries. The strain values obtained by hardness measurements were in fairly good agreement with the EBSD results, Figure 3. The results from the residual stress measurements using the Contour method showed the highest tensile stresses in the middle of the weld, and extending to the base material on both sides of the weld on the inner surface of the pipe, Figure 4. The stress at the outer surface of the pipe is mainly compressive. The residual stresses are higher on one side of the weld compared to the other. The EBSD measurements also showed different amounts of plastic strain on each side of the weld. Fig. 1. Deformation mechanism map for Type 304 SS. At very slow strain rates, deformation occurs as diffusion creep along grain boundaries and through dislocation motion. 16 Environment can further enhance creep. The SSSRTs were performed in four parallel autoclaves connected to a re circulating water loop. The loading of each specimen was individually controlled, and the strain was measured using a LVDT from the specimens. On line measurements were performed of conductivity (inlet and outlet), oxygen concentration (inlet and outlet), ph, ECP, temperature and pressure. The specimens were investigated after the SSSRTs using versatile methods with the focus on strain localization. The methods included optical microscopy (OM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). In addition to intra grain mis orientation EBSD maps, also pattern quality maps and local mis orientation maps were used to characterize strain localization in the SSRT specimens. Specimens for TEM investigations were prepared from the deformed gauge region of each bar, from adjacent to the fracture surface and from about 3 mm away from it. III. EXPERIMENTAL RESULTS III. A. Results from measurements of local strain distribution in a Type 304 nuclear weld III.B Super slow strain rate test results The heat treatment (650 C/24 h) resulted in a degree of sensitization of 11.8% (I r /I a ), measured using DL EPR. The grain boundary chromium content was measured using TEM/EDS to be as low as 11%. The 10% cold deformation resulted in a hardness increase of the material from 153 HV5 to 190 HV5. Intergranular cracking (IG) was obtained in all eight tested specimens. The results showed, as expected, an increased sensitivity of the slow strain rate test with decreasing strain rate, manifested by a higher amount of intergranular cracking (46% vs. 36% for sensitized material) and smaller strain to fracture (3% vs. 7% for sensitized material) with decreasing strain rate, Figure 5. The results also showed the detrimental influence of deformation manifested by a higher amount of intergranular cracking in specimens that had been cold deformed 10% before the SSSR testing (90% IG in cold deformed specimen and 46% IG in non cold worked specimen). The specimens tested at the super slow strain rate of s 1 showed very little reduction in area, Figure 6. Both crack initiation and growth was intergranular, Figure 7, and small steps were typically observed at the crack mouths, indicating a shear strain component, Figure 8. Secondary cracks were observed in all specimens, but the amount and depth of these did not consistently correspond to the elongation to fracture or amount of intergranular fracture. The EBSD measurements across a whole weld revealed highest degrees of plastic strain (10 20%) in the heat affected zone (HAZ) close to the root area of the weld, Figure 1. This highly deformed zone extended 5 7 mm from the fusion line to the base material. In the HAZ,

4 (a) Fig. 1. Pattern quality map (a) showing the microstructure of the fusion line area at the weld root, and local mis orientation map from the same area of the investigated weld. The weld is on the right hand side in the pictures. (a) Fig. 2. Mis orientation maps close to the weld root (a) and weld crown. The weld is on the left hand side in the pictures. (a) Fig. 3. Measured values of plastic strain (%) in different areas of the cross section of the weld using EBSD (a) and hardness measurements.

5 (a) Fig. 4. Residual stresses in a nuclear pipe weld of Type 304 austenitic stainless steel. The shape of the weld is indicated in the Figure with a dotted line. Type 304, Solution annealed and sensitized Stress, MPa Strain rate s 1 elongation to fracture ~7% Strain rate s 1 elongation to fracture 3% Strain, % Fig. 6. Macrographs of SSSRT specimens of sensitized and 10% cold worked Type 304 stainless steel tested in simulated BWR environment using s 1 (a) and s 1. Almost no reduction of area is seen in. Fig. 5. Stress vs. strain for solution annealed and sensitized Type 304 austenitic stainless steel specimens tested using different strain rates, i.e., s 1 and s 1, in simulated BWR environment at 300 C. Fig. 7. Fully intergranular cracking obtained in sensitized and 10% cold worked Type 304 stainless steel after SSSR testing in simulated BWR environment using a strain rate of s 1.

6 III.A.I Results from electron backscattered diffraction, EBSD, investigations The results from the EBSD investigations show nonuniform distribution of plastic strains in the specimens, with higher plastic strains in the vicinity of grain boundaries than inside the grains, Figure 9. Non uniform distribution of the plastic strains was also observed at the surface of the specimens, indicating local strain gradients. It seems that local strain distributions at the surface may affect the location of crack initiation, which would occur at the location with the smallest degree of local strain, Figure 10. Also the local differences in grain sizes affected the strain distribution, being higher in the region with smaller grain size compared to a neighboring area with larger grain size, Figure 11. Fig. 8. SEM picture of a non cold worked, sensitized specimen surface after SSSRT using a strain rate of s 1, showing a small step at the crack mouth of a small, intergranular secondary crack. (a) 50 µm =50 µm; BC; Step=0.3 µm; Grid606x µm Fig. 9. Pattern quality map (a) showing the microstructure of a crack tip area in non cold worked sensitized Type 304 stainless steel after SSSR testing in simulated BWR environment and the local mis orientation map in the same area. Blue, through green to red color correspond to the range from smallest to highest degree of mis orientation, i.e., from lowest to highest amount of plastic strain.

7 =20 µm; LocalM; Step=0.15 µm; Grid606x441 =20 µm; BC; Step=0.15 µm; Grid606x441 20µm 20µm (a) Fig. 10. Pattern quality map (a) and local mis-orientation map close to the surface of an non-cold-worked SSSRT specimen showing less strain at the location of the crack initiation site compared to the immediate surrounding. =500 µm; LocalM; Step=1 µm; Grid1212x882 =500 µm; BC; Step=1 µm; Grid1212x µm 500 µm (a) Fig. 11. Pattern quality map (a) and local mis-orientation map close to the surface of a non-cold-worked SSSRT specimen showing more strain in the area with smaller grain size compared to that with larger. The strain is localized at the grain boundaries. III.A.II. Transmission electron microscopy results The TEM-results revealed several differences between the materials, depending upon the prior coldwork, strain rate and proximity to the fracture surface. The most obvious differences were evident between the specimens which had been cold-worked and those which were not. The cold-worked material had a much higher density of dislocations, arranged in a cellular structure, while the lower density of dislocations in the non-coldworked material was arranged in a planar structure, Figure 12. The materials strained at s-1 differed in a similar way. The uniformly-elongated region of the materials did not exhibit much difference as a function of strain rate. Close to the fracture surface, the accumulation of further strain had promoted cell size reduction in the cold-worked material. Of particular interest was that, at the lower strain rate, strain was localized to shear bands, and when those shear bands impinged on a grain boundary, a bloom of strain was formed in the adjacent grain, Figure 13.

8 (a) Fig. 12. The microstructures of the non cold worked (a) and cold worked Type 304 stainless steel after straining at s 1 show clear differences. The dislocations in the cold worked material are arranged in a cellular structure, while the lower density of dislocations in the non cold worked material is arranged in a planar structure. Fig. 13. TEM picture showing shear bands impinging on a grain boundary, producing a bloom of strain in the adjacent grain. The specimen is sensitized and 10% cold worked Type 304 stainless steel and strained at s 1 in simulated BWR environment. IV. DISCUSSION The results from the super slow strain rate tests on deformed and non deformed, sensitized Type 304 austenitic stainless steel showed an increased sensitivity of the test with decreasing strain rate, manifested by a higher amount of intergranular cracking and smaller strain to fracture with decreasing strain rate. It also revealed a detrimental influence of prior deformation. Therefore, when using very slow strain rates, SSRT seems to be a suitable method for investigating materials also having lower SCC susceptibility. The investigations of a typical nuclear power plant weld showed inhomogeneous distribution of residual strains and stresses both at a macroscopic and microscopic level. On a macroscopic level, higher residual stresses and strains were measured in one of the two HAZ s. The evolution of residual stresses and strains during welding is a result of several parameters such as the position of the welding torch, the stiffness of the component (which changes with the amount of weld passes), the interpass temperature, etc. A non homogeneous distribution of the residual stresses and strains in the HAZ s on each side of a weld is typical. On a microscopic level strain localization at grain boundaries was observed. The EBSDmeasurements on sensitized Type 304 stainless steel SSSRT specimens revealed non uniform deformation, with higher plastic strains in the vicinity of grain boundaries than inside the grains and non homogeneous strain distribution both next to the surface of the specimens and inside the material. The EBSD method is a very powerful tool to measure local residual plastic strains and localization of deformation. The TEM results showed differences between the materials, depending on prior cold work, strain rate and proximity to the fracture surface. Localization of deformation was observed through observations of shear bands. When shear bands terminate at a grain boundary,

9 and the orientation of the neighboring grain restricts effective strain transmission, an increase in the local stress occurs, as indicated by the strain blooms observed in the adjacent grain in a specimen tested in simulated BWR environment at the super slow strain rate of s 1. This can also be the reason for the EBSD observations showing higher strains in one of two neighboring grains. Considering a material comprised of grains of random orientation, there is always a level of heterogeneity imposed by the differences in the local critical resolved shear stress superimposed on the macroscopic strain distribution within the material. The combination of strain heterogeneity both at the multigrain level, and via shear bands within the grains, would most likely enhance the local residual stresses. Thus, the residual stresses arising locally from pre straining of the materials of real components can be expected to be an important precursor to crack initiation. In the presence of an aqueous environment there is additionally the possibility for corrosion processes to take place. From a deformation standpoint, of particular interest is the possibility that corrosion processes at the material surface can result in the injection of additional vacancies into the material locally. 17,18 These corrosionproduced vacancies interact with dislocations, enhance dislocation mobility and can produce increased creep rates locally and, thus, further enhance localization of deformation. 19,20,21 Non homogeneous microstructures always occur in welded structures and materials fabricated by different methods and subjected to complex strain path. This nonhomogeneity results in inhomogeneous strain and stress distribution, which obviously affect both crack initiation and growth. ACKNOWLEDGMENTS This work has been performed within the national nuclear safety program SAFIR2010, within the DEFSPEED project, which is financed by VYR (State Waste Management Fund), VTT (Technical Research Centre of Finland) and the Swedish Radiation Safety Authority. The Contour measurements were performed in a project financed by TVO. REFERENCES 1. J. DANKO (ed.). Proceedings on Seminar on Countermeasures for Pipe Cracking in BWRs. EPRI Report NP 3684 S4 (1984). 2. J. HAKALA, H. HÄNNINEN, P. AALTONEN. Stress Corrosion and Thermal Fatigue Experiences and Countermeasures in Austenitic SS Pipings of Finnish BWR Plants. Nuclear Engineering and Design, 119, (1990). 3. U. EHRNSTÉN, P. AALTONEN, P. NENONEN, H. HÄNNINEN, C. JANSSON, T. ANGELIU. Intergranular Cracking of AISI 316NG Stainless Steel in BWR Environment, 10th Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors. Nevada, USA, 5 9 August, 2001, ANS, NACE, TMS, 10 p (2001). 4. S. TÄHTINEN, H. HÄNNINEN, M. TROLLE. Stress Corrosion Cracking of Cold Worked Austenitic Stainless Steel Pipes in BWR Reactor Water, 6th Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors. San Diego, CA, USA, August 1 5, 1993, TMS, 265 (1993). 5. R. HORN, G. GORDON, P. FORD, R. COWAN. Experience and Assessment of Stress Corrosion Cracking in L Grade Stainless Steel BWR Internals, Nuclear Engineering and Design, 174, 313 (1997). 6. T. ANGELIU. Microstructural Characterization of L Grade Stainless Steels Relative to the IGSCC Behavior in BWR Environments, Proc. of Corrosion 2001, NACE, Houston, Texas, March 11 16, 2001, Paper No , 14 p (2001). 7. P. ANDRESEN, P. EMIGH, M. MORRA, R. HORN. Effects of Yield Strength, Corrosion Potential, Stress Intensity Factor, Silicon and Grain Boundary Character on the SCC of Stainless Steels, 11th Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Stevenson, WA, Aug , 2003, pp (2003). 8. S. OOKI, Y. TANAKA, K. TAKAMORI, S. SUZUKI. Study on SCC Growth Behavior of BWR Core Shroud, 12th International Conference on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, TMS, 2005, pp (2005). 9. T. COUVANT, P. MOULART, L. LEGRAS, P. BORDES, J. CAPELLE, Y. ROUILLON, T. BALON. PWSCC of Austenitic Stainless Steels of Heaters of Pressurizers, Proceedings of the 6th International Symposium on Contribution of Materials Investigations to Improve the Safety and Performance of LWRs. French Nuclear Society, Fontevraud, September, Paper A100 TO3, 12 p (2006). 10. B. TIMOFEEV, V. FEDEROVA, A. BUCHATSKII. Intercrystalline Corrosion Cracking of Power Equipment Made of Austenitic Stainless Steels, Materials Science, 40 1, 48 (2004). 11. T. COUVANT, F. VALLIANT, J M. BOURSIER, D. DELAFOSSE. Effect of Strain Path on Stress Corrosion Cracking of AISI 304L Stainless Steel in PWR Primary Environment at 360 C, Proc. of Eurocorr 2004, Nice, France, , 2004, Event No. 226, 11 p (2004).

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