1. Project special reports

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1 1. Project special reports 1.1 Deformation localisation and EAC in inhomogeneous microstructures of austenitic stainless steels Ulla Ehrnstén 1, Wade Karlsen 1, Janne Pakarinen 1, Tapio Saukkonen 2 Hänninen 2 and Hannu 1 VTT Technical Research Centre of Finland 2 Aalto University School of Engineering Abstract 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 localisation is probably playing a key role as a precursor for crack initiation but its mechanisms are still not fully known. If strain localisation 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 localisation investigations were also performed on specimens from Super Slow Strain Rate Test (SSSRT) 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 in simulated BWR environment on sensitized Type 304 and non-sensitised Type 316L austenitic stainless steel either with or without cold deformation. Local variation in the amount of surface cold-work affects crack initiation. Local variations of grain size also affect strain localisation. During crack growth, strain localisation occurs at grain boundaries ahead of the crack tips. Introduction Intergranular stress corrosion cracking (IGSCC) of sensitized stainless steels in oxidising 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. Consequently, the amount of IGSCC in sensitised stainless

2 steels decreased remarkably. The remedial actions included development and employment of low-carbon stainless steels, new welding techniques decreasing the residual stresses and eliminating sensitisation, 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 nonsensitised, low carbon stainless steels can also suffer from IGSCC, and that this type of cracking is connected to cold working. Also, the observed risk for IGSCC in PWR conditions has added to the importance of understanding EAC initiation better. Localisation of plastic deformation and the interactions between oxidation and strain localisation are most probably playing a key role in the cracking of sensitised as well as non-sensitised, cold-worked stainless steels. Localisation 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. 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. 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, but higher temperatures clearly promote slip, since dislocation motion is a thermally activated process. 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 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. Time also affects the 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. Of particular interest to crack initiation is the heterogeneity of deformation within a material. As a consequence of processes such as post-weld material shrinkage or surface machining or grinding, local cold work alters 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 multi-grain 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 neighbouring grains, resulting in a heterogeneous distribution of local strain. Strain may

3 also be localised 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 typically very long incubation times for crack initiation. Material and experimental methods The investigations focused on studies of localisation of deformation was comprised of characterisation of residual strains using electron back-scattered diffraction, EBSD, of specimens made of sensitised Type 304 and non-sensitised Type 316L stainless steels after super-slow strain rate testing in simulated BWR environment. The results were compared to the results from residual strain and stress measurements on a nuclear weld. The deformation structures were also investigated using transmission electron microscopy, TEM. Super-slow strain rate tests in simulated BWR environment The chemical composition of the two stainless steel materials used for SSSRT s is presented in Table 1. The tests used round tensile type specimens with a 4 mm gauge region 12 mm in length. Table 1. Chemical composition of the stainless steels. Material C Si Mn P Type Type 316L S Cr Mo Ni N The sensitised material was from the same high carbon material as used for the residual strain and stress measurements. It was tested with and without pre-deformation, at two strain rates, i.e., s -1 and s -1. The non-sensitised Type 316L material was pre-deformed to four different levels, i.e., 5, 15, 20 and 28%, and was strained in simulated BWR environment at the slower strain rate only. All the specimens made from sensitised stainless steel were strained to fracture, while the non-sensitised SSSRT specimens were mainly strained to ~5% in BWR environment. One set of specimens

4 was tested in two phases, first 5% and if no macroscopic cracking was observed, additional straining of <4%, was applied. The non-sensitised specimens were tested in either as-machined or in polished condition. Pre-deformation was made by tension at room temperature. Experimental results Results from measurements of local strain distribution in a Type 304 nuclear weld The EBSD-measurements across a whole weld revealed that the highest degrees of plastic strain (10 20%) were found in the heat affected zone (HAZ), close to the root area of the weld, Figure 1a. This highly deformed zone extended 5-7 mm from the fusion line to the base material. In the HAZ, localisation of strain at the grain boundaries was also observed. In the weld, strain localisation at the weld bead and dendrite boundaries was observed. The amount of plastic strain in the base material decreased towards the outer surface of the pipe. The strain values obtained by hardness measurements, which are easy to perform on e.g. field failure components, were in fairly good agreement with the EBSD results. The residual strain measurements correlate well with results of residual stress measurements made using the Contour method, Figure 1b. The residual stress decreases towards the outer surface of the pipe, where it is mainly compressive. Super-slow strain rate test results Intergranular cracking (IG) was obtained in all SSSRT specimens made of sensitised type 304 SS. The results showed 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 sensitised material) and smaller strain to fracture (3% vs. 7% for sensitised material) with decreasing strain rate. 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-deformed specimen). Both crack initiation and growth was intergranular, and small steps were typically observed at the crack mouths, indicating a shear strain component, Figure 2. Of a total of 16 specimens made of non-sensitised Type 316L material, macroscopic cracking was observed in four specimens. Of those, intergranular cracking was observed in one specimen, while in the other three specimens the cracking mode was transgranular environmentally assisted cracking. The macroscopic intergranular

cracking occurred in a specimen having 15% pre-deformation that was further strained 5.6 % at the super-slow strain rate in simulated BWR environment.

gradient at the surface. Once initiated, the crack continued to grow intergranularly in the less deformed material.

This finding further emphasises the roles that local deformation and a strain gradient have on intergranular crack initiation susceptibility.

Local lattice mis-orientation map from the fusion line area of the weld root in a nuclear weld made of Type 304 austenitic stainless steel (a) and the residual

5 cracking occurred in a specimen having 15% pre-deformation that was further strained 5.6 % at the super-slow strain rate in simulated BWR environment. The intergranular crack had occurred at the location of strain gauge contact, Figure 2b, where the local degree of deformation was obviously high, as was the strain gradient at the surface. Once initiated, the crack continued to grow intergranularly in the less deformed material. All the other specimens with macroscopic cracking had been pre-deformed to a higher degree of cold work, i.e., 20 or 28% and further strained at least 5% at the super-slow strain rate. This finding further emphasises the roles that local deformation and a strain gradient have on intergranular crack initiation susceptibility. This behaviour has also been frequently observed in field failures. (a) (b) Figure 1. Local lattice mis-orientation map from the fusion line area of the weld root in a nuclear weld made of Type 304 austenitic stainless steel (a) and the residual stresses in the same weld (b). (a) (b) Figure 2. Intergranular crack initiation was observed in sensitised Type 304 SS, with indications of a shear strain component (a). Intergranular cracking was also observed in non-sensitised Type 316L SS (b). The local deformation from the strain gauge has obviously affected initiation.

Oxide layer cracking is considered to be a prerequisite for crack initiation. Surface cracking was found in all the investigated SSSRT specimens when examined by SEM.

As seen in Figure 3, the cracks can be restricted only to the oxide layer, and not necessarily result in EAC crack initiation in the substrate (alloy) below. Figure 3. FIB milled micro cross section imaged by using FEG-SEM.

$Results from electron backscattered diffraction, EBSD, investigations The results from the EBSD investigations show non-uniform distribution of plastic strains in specimens made from sensitised and$ Nonuniform distribution of the plastic strains was also observed at the surface of the specimens, indicating local strain gradients.

Nonuniform distribution of the plastic strains was also observed at the surface of the specimens, indicating local strain gradients.

6 Oxide layer cracking is considered to be a prerequisite for crack initiation. Surface cracking was found in all the investigated SSSRT specimens when examined by SEM. Such small features are challenging to investigate further by SEM, but additional information can be gained by utilizing focussed ion beam, FIB, to make a micro-section of the crack. As seen in Figure 3, the cracks can be restricted only to the oxide layer, and not necessarily result in EAC crack initiation in the substrate (alloy) below. Figure 3. FIB milled micro cross section imaged by using FEG-SEM. The surface crack in the oxide seems to stop at the oxide-metal interface. Results from electron backscattered diffraction, EBSD, investigations The results from the EBSD investigations show non-uniform distribution of plastic strains in specimens made from sensitised and non-sensitised stainless steel, with higher plastic strains in the vicinity of grain boundaries than inside the grains, Figure 4. Nonuniform distribution of the plastic strains was also observed at the surface of the specimens, indicating local strain gradients. The local strain distributions at the surface seem to affect the location of crack initiation, which would occur at the location with the smallest degree of local strain. Also the local differences in grain sizes in the sensitised stainless steel affected the strain distribution, being higher in the region with smaller grain size compared to a neighbouring area with larger grain size, Figure 5. Intergranular cracks with a length on the order of one grain, without further crack growth, were seen in some specimens, Figure 4b. Thus, very short crack formation, although being a prerequisite for cracking, does not necessarily lead to crack growth in all cases.

=5 µm; LocalM5; Step=0,08 µm; Grid303x220 =20 µm; LocalM; Step=0.15 µm; Grid606x441 20µm (a) 20µm (b) Figure 4.

preferential strain at the grain boundaries in both sensitised (a) and non-sensitised stainless steel (b).

Pattern quality map (a) and local mis-orientation map (b) close to the surface of a noncold-worked SSSRT specimen, showing more strain in the area with smaller grain size

$the fracture surface. The most obvious differences were evident between the specimens which had been cold-worked and those which were not.$

7 =5 µm; LocalM5; Step=0,08 µm; Grid303x220 =20 µm; LocalM; Step=0.15 µm; Grid606x441 20µm (a) 20µm (b) Figure 4. Mis-orientation maps close to the surface of SSSRT specimens showing less strain at the location of the crack initiation site compared to the immediate surrounding, and preferential strain at the grain boundaries in both sensitised (a) and non-sensitised stainless steel (b). =500 µm; BC; Step=1 µm; Grid1212x882 =500 µm; LocalM; Step=1 µm; Grid1212x µm 500 µm (b) (a) Figure 5. Pattern quality map (a) and local mis-orientation map (b) close to the surface of a noncold-worked SSSRT specimen, showing more strain in the area with smaller grain size compared to that with larger. The strain is localised at the grain boundaries. Transmission electron microscopy results The TEM results revealed several differences between the materials, depending upon the prior cold-work, 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 6. The materials strained at s-1 differed in a similar way. The uniformly-elongated region of the materials

$Close to the fracture surface, the accumulation of further strain had promoted cell size reduction in the cold-worked material.$ Of particular interest was that, at the lower strain rate, strain was localised to shear bands, and when those shear bands impinged on a grain boundary, a bloom of

Of particular interest was that, at the lower strain rate, strain was localised to shear bands, and when those shear bands impinged on a grain boundary, a bloom of

The microstructures of the non-cold-worked (a) and cold-worked (b) Type 304 stainless steel after straining at 1 10-8 s -1 show clear differences.

8 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 localised to shear bands, and when those shear bands impinged on a grain boundary, a bloom of strain was formed in the adjacent grain, Figure 7. (a) (b) Figure 6. The microstructures of the non-cold-worked (a) and cold-worked (b) Type 304 stainless steel after straining at s -1 show clear differences. The dislocations in the coldworked 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. Figure 7. 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.

9 Discussion The results from the super-slow strain rate tests on deformed and non-deformed sensitised 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. Crack initiation is affected by local inhomogenities in the material, e.g. differences in grain size and surface deformation resulting in residual strain gradients. This was also observed in non-sensitised and pre-deformed Type 316L stainless steel, where macroscopic intergranular cracking was observed in a specimen where the strain gauge had resulted in heavy local deformation at the location of initiation. The detrimental effect of cold-work is well known. What is less known is the influence of strain gradients and its influence on crack initiation through localisation of deformation. Further investigations are needed e.g. to understand the importance of absolute levels of deformation compared to gradients, e.g., the influence of a scratch in a polished surface compared to one in a moderately machined surface. 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, including 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 localisation at grain boundaries was observed. Residual strains were also observed in the weld metal, indicating that a driving force for crack growth exists in the weld, although weld cracking is not typically observed in NPPs. The EBSD-measurements 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 nonhomogeneous strain distribution both next to the surface of the specimens and inside the material. The TEM results showed differences between the materials, depending on prior coldwork, strain rate and proximity to the fracture surface. Localisation of deformation was observed through observations of shear bands. When shear bands terminate at a grain boundary, and the orientation of the neighbouring 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

10 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 neighbouring 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 multi-grain 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. These corrosion-produced vacancies interact with dislocations, enhance dislocation mobility and can produce increased creep rates locally and, thus, further enhance localisation of deformation. 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 The DEFSPEED project was financed by VYR (State Waste Management Fund), VTT and the Swedish Radiation Safety Authority. The Contour measurements were performed in PULU project financed by Tekes and TVO. References Ehrnstén, U., Saukkonen, T., Karlsen, W., Hänninen, H. Deformation localisation and EAC in inhomogenenous microstructures of austenitic stainless steels. 14 th International Conference on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Virginia Beach, Virginia, USA, August 23-27, American Nuclear Society. Vol. 2 (2009), Pakarinen, J. TEM study of the effect of prior cold work on the deformation microstructures of SSSRT tested AISI 316 stainless steel. Report VTT-R

DEFORMATION LOCALISATION AND EAC IN INHOMOGENEOUS MICROSTRUCTURES OF AUSTENITIC STAINLESS STEELS

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,