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Research Progress on stress corrosion crack propagation behavior of cold worked nuclear power structural materials in high temperature and high pressure water

Research Progress on stress corrosion crack propagation behavior of cold worked nuclear power structural materials in high temperature and high pressure water

In the early nuclear power plants, the pipes were mainly made of 304SS and 316SS austenitic stainless steel, and stress corrosion cracking (SCC) often occurred in the cold working area of steel pipes. Transgranular stress corrosion (TGSCC) of welded core shroud made of 316L occurred in a boiling water reactor (BWR) nuclear power plant in Japan. The results show that the sensitivity of Laves stainless steel is improved by the precipitation of Laves phase around the grain boundary.

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The bending, welding, grinding, cutting, drilling and other processes in the process of processing and assembling nuclear reactor structural materials will introduce plastic deformation, especially the shrinkage of welding heat affected zone is equivalent to 20% ~ 30% cold working of materials, which makes these parts produce microstructure similar to cold working deformation, resulting in the change of local mechanical properties and stress concentration Finally, it promotes the initiation of stress corrosion. From the microscopic point of view, the metal with fcc structure mainly relies on the lattice local slip to achieve the plastic deformation required by cold working. The slip will cause dislocation, and the movement of dislocation will produce a large number of point defects, which will lead to a large number of defects in the material, making the material brittle, and more likely to cause the initiation and propagation of SCC cracks.

At present, the main structural materials used in the third generation nuclear power plant in China are Austenitic 304, 316 stainless steel, nickel base 600, 690 alloy, welding metal nickel base 52 / 152 alloy and carbon steel, etc. in the process of production and assembly, local plastic deformation will inevitably occur in these structural materials, resulting in a large number of micro dislocations and vacancies, which makes their mechanical properties worse In addition, under the combined action of harsh water chemical environment, high temperature and high pressure, irradiation and other factors in the reactor primary circuit, SCC may be generated, which will threaten the safe operation of nuclear power plant. Therefore, it is of great significance to study the stress corrosion behavior of cold working materials for further exploring the mechanism and model of stress corrosion, and to develop the application technology to inhibit and slow down the stress corrosion of nuclear power structural materials.

Effect of cold working on SCC of nuclear power structure material

Effect of cold working on crack growth rate of SCC

A large number of studies have shown that cold working can improve the SCC sensitivity of 304 stainless steel, thus increasing the SCC crack growth rate. Kuniya et al. Studied the effect of cold working on SCC sensitivity of 304 stainless steel in high temperature water containing oxygen. It was found that the higher the cold working degree, the higher the SCC sensitivity of the material. Wang et al. considered that oxidation and local stress concentration of cold work hardening layer played an important role in the initiation and propagation of SCC.

Cold working results in the deformation of grains parallel to the cold rolling surface, and the grains become elongated along the cold rolling direction, resulting in continuous sheet residual stress area parallel to the cold rolling surface at the grain boundary, and a large number of defects appear at the grain boundary, which makes the grain boundary brittle. These grain boundaries with residual stress and a large number of defects are more conducive to the diffusion of oxygen, resulting in grain boundary oxidation parallel to the cold rolling surface, thus forming metal / oxide voids at the grain boundaries and accelerating the crack growth rate of SCC. Cold working can also form deformation bands in the grains, which will become the rapid diffusion channels of oxygen and ions. When the crack tip extends to the deformation zone, the oxidation of the deformation zone will lead to poor mechanical properties in the front of the crack, which is easy to fracture. At the same time, a local high stress zone will be formed to accelerate the crack propagation forward. The results show that the composition of diffusion in the grain boundary increases with the increase of diffusion rate.

In the process of cold working, dislocations are produced due to lattice slip, and a large number of vacancy defects are produced by dislocation movement. The higher the degree of cold working, the greater the density of defects in materials. Arioka et al. Carried out tensile tests on cold worked nickel based 690TT, nickel based 600Mt alloy, austenitic 316SS and carbon steel in high temperature pressurized water reactor (PWR) water environment, air and argon, and showed that holes would be formed in the front and surrounding area of the crack. This is because the vacancy defects generated in the cold working process will move towards the grain boundary under the effect of stress gradient, and then move along the grain boundary to the high stress area, forming high vacancy density locally and finally forming holes. Generally, the region at the front of the crack is a high stress area. However, due to the inhomogeneous microstructure of the material, the local high stress zone will be formed in other regions near the crack front end. In PWR high temperature water environment, the holes and higher vacancy density at the front of the crack will significantly reduce the mechanical properties of the grain boundary, weaken the binding capacity of the grain boundary, and increase the SCC crack growth rate of the material; the holes formed in the local high stress area nearby will also induce the SCC crack initiation, and form new stress corrosion cracks in some places. Therefore, the formation and movement rate of vacancies are important factors to control the crack growth rate of SCC. In addition, the vacancy defects produced by cold working can accelerate the diffusion of components in the material. The latest research shows that the diffusion rate of Ni in cold worked carbon steel is 4 times of that in non cold working materials. The vacancy movement provides activation energy for the movement of Ni and accelerates the diffusion rate of Ni, thus accelerating the dissolution of metal at the front of crack and increasing the crack growth rate of SCC.

According to Terachi et al., the vacancy and dislocation produced by cold working can also significantly increase the yield strength of 304SS and 316SS, and the CGR of the material increases with the increase of yield strength. In the materials with higher hardness, the plastic deformation zone is relatively small under the action of tensile stress, so there is a relatively large stress gradient in the stress zone of the material under the action of tensile stress, which leads to the larger stress corrosion CGR. The relationship between the yield strength (σ σ y) and the stress corrosion CGR of materials basically obeys the empirical formula

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The sampling direction also affects the crack growth rate. Fig. 1 is a schematic diagram of different sampling directions of one-dimensional rolling material. According to arioka et al., the SCC crack growth rate of T-L direction sample is higher than that of T-S direction sample. Moshier and brown believe that the SCC crack growth rate of S-T direction sample is about 10 times that of L-T direction sample. Although there is no comprehensive study on the effect of sampling direction on SCC crack growth, it can be seen from the existing literature that the SCC crack growth rate of samples in S-L, S-T and T-L directions is always higher than that of samples in T-S and L-S directions, which indicates that the crack growth rate of samples with crack propagation direction parallel to the cold rolling surface is higher than that of samples with crack propagation direction perpendicular to the cold rolling surface The crack growth rate of the sample with the surface parallel to the cold rolling surface is higher than that of the sample with the crack propagation plane perpendicular to the cold rolling surface. This may be related to the preferential oxidation in the high stress area parallel to the cold rolling surface and the vacancy diffusion caused by the stress gradient. Similarly, in the two samples with the crack propagation direction parallel to the cold rolling surface and perpendicular to the cold rolling surface, Chen et al. Observed that the SCC crack growth rate of the samples in the T-L direction was higher than that in the L-T direction, which may be related to the slender grains distributed parallel to the cold rolling surface during the cold rolling process. The direction of cold working deformation also affects the stress corrosion CGR. Hou et al. Carried out cold working in three directions of Ni based 600 alloy, which were marked as 1dcw (cold rolling direction 1-L), 2dcw (cold rolling direction 1-L, 2-T) and 3dcw (cold rolling direction 1-L, 2-T, 3-s), respectively. Then U-shaped bending SCC experiments were carried out on the materials. The results show that the influence degree of susceptibility to intergranular stress corrosion (IGSCC) is 1dcw > 3dcw > 2dcw. Compared with the other two samples, the residual stress of 1dcw sample is the largest, the local high stress zone at the grain boundary is the largest, and the crack growth rate is also the largest.

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Fig. 1 Schematic diagram of rolling and refrigeration processing

Effect of cold working on crack propagation direction of SCC

The local oxidation of the deformation band after cold working also affects the crack propagation direction, which may change the crack type. Garcı́a The results show that mixed SCC cracking occurs in boiling MgCl2 solution for 304 stainless steel as cold working state, and TGSCC gradually becomes the main cracking mode with the increase of cold working degree. Lu et al. Thought that 304lss had similar phenomenon in high temperature and high pressure oxygen containing water environment. The mode of crack propagation and the angle between grain boundary and slip band and load direction are related, as shown in Fig. 2. Among them, a is the cold working slip band, B is the grain boundary, the angle α is the angle between the cold working slip band and the load direction, and the angle β is the angle between the grain boundary and the load direction. When α > β, the crack propagates along the cold working slip band, which is TGSCC; when α < β, the crack extends along the grain boundary, and the crack propagation mode is IGSCC.

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Fig. 2 Schematic diagram of crack propagation direction of cold working 304L stainless steel

Yaguchi et al. Divided the IGSCC cracks observed in the previous stress corrosion research of cold working materials into two types: one type propagates along the direction of the pre cast crack, which is called type – I crack; the other is type – II crack which propagates along the direction perpendicular to the prefabricated crack and parallel to the cold rolling surface. The types of cracks in the samples are related to the degree of cold working, the stress field intensity factor and the water chemistry. The type – Ⅱ cracks generally appear in the materials with high degree of cold working. This is related to the preferential local oxidation of the cold rolled area through the formation of high stress area parallel to the cold rolling surface near the grain boundary. When the stress corrosion crack extends along the direction perpendicular to the cold rolling surface, the mechanical properties of the local oxidation zone are poor, which may lead to type – Ⅱ stress corrosion cracking. In the type – Ⅱ crack, oxidation occurs not only at the crack tip, but also at the front of the crack.

1.3 effect of temperature and hydrogen content on SCC crack growth behavior of cold worked materials

When SCC crack grows, high temperature can accelerate the diffusion of oxygen and metal ions in the crack, so the crack growth rate generally increases with the increase of temperature. In contrast, the mechanical properties of the materials can be improved at high temperature. The results show that the crack propagation rates of alloy 6903, 36 and ss3164 vary with the processing temperature of SCC. It can be seen from the figure that in PWR environment (500 mg / L B-2 mg / L li-30 ml H2 / kg H2O), the SCC crack growth rate first increases and then decreases with the increase of temperature between 280 ~ 360 ℃; there is a certain temperature between 320 ~ 340 ℃, under which the SCC crack growth rate reaches the maximum. This does not change the trend of dissolved oxygen at 3.5 kg / ml. However, figure 4 shows that the SCC crack growth rate of 20% cw690 nickel base alloy increases gradually with the increase of temperature in the range of 320 ~ 360 ℃ with the increase of hydrogen solubility (DH) to 45 ml / kg, and there is no trend of decrease. This indicates that the crack growth rate of Alloy 690 is affected not only by temperature, but also by the content of dissolved hydrogen in solution.

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Fig. 3 temperature dependence of crack growth rate on cold worked 316SS SCC 

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Fig. 4 temperature dependence of SCC crack growth rate of cold worked 690TT alloy

The results show that the plastic zone has relatively large dislocation density due to the cold working of the material. Due to the poor plasticity of the material at low temperature, there is a relatively high stress gradient at the crack tip, and a large number of defects move to the crack tip, which further increases the local defect density at the crack tip. In the process of dislocation moving towards the crack tip, a large number of vacancies will be formed in the metal matrix in front of the crack, which will increase the brittleness of the metal matrix in front of the crack, and make it easier to fracture. When the temperature is higher, the defects move faster and slip is more likely to occur. Therefore, the defect density at the crack tip is smaller and the stress gradient in the front area of the crack is small, which will not cause the defect to move to the crack tip. At this time, the crack tip region of the material is more flexible and not easy to fracture. Therefore, in the case of higher temperature, the crack CGR decreases with the increase of temperature. On the other hand, with the increase of experimental temperature, the metal atoms are more likely to diffuse to the crack surface and react with high temperature and high pressure water solution, which improves the oxidation rate of crack surface, so the crack growth rate is faster at higher temperature. At the same time, high temperature can accelerate the local oxidation of deformation zone in cold working zone and promote the crack propagation forward. Therefore, the crack CGR increases with the increase of temperature at low temperature.

Figure 4 shows that the SCC crack growth rate of 20% cw690 alloy increases with the increase of temperature in the range of 320 ~ 360 ℃ in the water environment with 45 ml / kg DH, cgrdh = 45 ml / kg < cgrdh = 30 ml / kg in the range of 320 ~ 340 ℃, and cgrdh = 45 ml / kg > cgrdh = 30 ml / kg in the range of 350 ~ 360 ℃. The hydrogen absorption rate of 20% cold worked carbon steel was measured after crack propagation experiment in 360 ℃ reducing environment. The test results showed that there was a minimum value of hydrogen absorption rate near 360 ℃, which indicated that h in reducing environment combined with vacancy in the material near 360 ℃. Fukai et al. [39,40] believe that in Fe and Ni, vacancy density increases with the increase of H2 content; in Nb, Au and Fe, hydrogen induced vacancy can increase the lattice diffusion rate. At the same time, it is possible that the crack propagation rate increases along the crack tip with the increase of CRH = 350 ~ kg / ml in the region of high CRH = 350 ~ kg / ml.

Stress corrosion mechanism of cold working materials

Many scholars have done a lot of research on the stress corrosion process and mechanism of cold working materials, and put forward some theories, but there is no complete and unified model to explain the stress corrosion behavior of cold working materials. The slip dissolution model, which was proposed by Ford and Andresen, is a generally accepted stress corrosion model. In this model, a dense oxide film will be formed on the surface of the alloy where stress corrosion occurs. The oxide film will break due to plastic deformation under the action of tensile stress. The exposed metal will dissolve metal ions in the corrosive environment, and at the same time, it will be renewed under the action of self passivation The formation of oxide film, through the process of slip film rupture metal dissolution and re passivation, finally makes SCC crack continue to expand forward. According to the slip dissolution model, Farady established a semi empirical expression of CGR

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Where is the atomic weight of the crack (M / g), where is the length of the electron crack (2 or 3); ρ is metal density (g / cm3); F is Faraday constant (9.65 × 104c / mol); ε f is rupture strain of oxide film; I0, t0 and N are constants, n is related to corrosion potential, solution conductivity, sulfur content of alloy, sensitization degree and alloy type; i0t0i0t0n is charge density (C / cm2) involved in dissolution / oxidation process; ε; CT, ε; CT is crack tip strain rate (CTSR). The crack tip strain is caused by creep, applied strain or crack propagation through the plastic deformation zone.

Some studies combine the slip dissolution model with the change of material mechanical properties to explain the stress corrosion behavior of cold working materials. In conclusion, cold working mainly affects stress corrosion CGR from two aspects: (1) cold working mainly changes the stress corrosion CTSR. During cold working process, plastic deformation and a large amount of residual stress will be introduced into the material, resulting in hardening and stress concentration of the material. At the same time, a large number of dislocations and vacancy defects will be produced. All of these will make the mechanical properties of materials in high stress zone worse, make them brittle and easy to fracture, increase stress corrosion cracking CTSR and stress corrosion CGR. (2) Cold working can also change the oxidation rate of stress corrosion crack tip, increase the charge density i0tn0i0t0n involved in dissolution / oxidation, and accelerate corrosion. There are a lot of dislocations and vacancy defects in the sheet high stress zone and deformation band formed at the grain boundary by cold working, which makes the anion and o more easily diffuse, which leads to the preferential local oxidation. At the same time, the cations in the material are easier to diffuse into the solution, promote the dissolution of metal ions at the crack tip, accelerate the corrosion at the crack tip, and accelerate the crack growth rate.

Problems to be solved and future research trends and directions

  • (1) The relationship between DH and stress corrosion crack growth rate in cold worked samples. It can reduce the oxidation potential of the water in the primary circuit of the PWR, which can reduce the oxidation potential of the water in the primary circuit of the power plant. However, some researchers have observed that the stress corrosion CGR increases with the increase of DH in water under certain conditions, which is different from the conventional cognition. It is necessary to further study the relationship between DH and CGR and explore its mechanism.
  • (2) There are many cold working forms of structural materials in nuclear power plant, including bending, welding, grinding, cutting, drilling and other operations. The cold working effect is not the same as the cold rolling process in the experiment, and the research results may be different from the actual situation. In the future, it is necessary to further study the influence of cold working effect on stress corrosion sensitivity caused by different machining forms.
  • (3) At present, the effect of cold working on Microstructure and stress corrosion process of materials has been studied. However, a systematic theory has not been formed, and the formation and movement process of dislocations and vacancies at grain boundaries and grain boundaries of cold worked materials have not been thoroughly explored. The change of microstructure in cold working materials is also little known. The role of cold working in the formation of stress corrosion passivation film and crack tip dissolution is not clear, which needs to be studied in the future In the process of research, it gradually improved and formed a systematic and clear theory.

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