Yohei Sakakibara

The Effect of Cold Work on Microstructure and SCC Susceptibility in Simulated BWR Environment for Non‐Sensitized Austenitic Stainless Steels

To reveal the effect of cold work on SCC susceptibility in simulated BWR environment for low carbon austenitic stainless steels, a CBB test was performed where the microstructures were analyzed by EBSD. In this study, two of type 316NG and one of type 304NG stainless steel were tested and analyzed. Type 316NG #1 which has relatively higher Ni and Mo content than type 316NG #2 showed higher SCC susceptibility and type 316NG #2 having lower Ni and Mo content than type 316 #1 showed lower susceptibility. But the number of cracks doesn’t change dramatically, then it implies that the cold work prior to CBB test doesn’t increase the SCC initiation sites. Type 304NG has no cracks by CBB test. EBSD parameters concerned with plastic strain within grains or random grain boundaries have no significant difference. ∑3 ratios were also analyzed by EBSD and it was found that the deviation of ∑3 boundaries increases and ∑3 ratio decreases with the applied strain. ∑3 boundaries show the good resistance to intergranular corrosion and cracking so that cold work might accelerate the micro-cracks propagation and coalescence.

Novel Fractgraphy of Ni-Based Alloy by SEM/EBSD Method

Alloy 718 superalloy has been used for blades of air craft jet engine and gas turbine. The failures of these parts derive from fatigue and creep, but it is difficult to estimate applied stress from observation of samples from damaged parts. Oxidation and crush on fracture surfaces prevent from the estimation by fractgraphy especially. Therefore, we established the novel observation of the microstructure on the cross section beneath cracks by using EBSD. Tensile test, fatigue test and creep test were performed, and the change in dislocation density and the difference of strain contour of these specimens were specified by the new method. Finally the applied stress could be estimated and the test temperature could be also estimated.

Monte Carlo Simulation Based on SCC Test Results in Hydrogenated Steam Environment for Alloy 600

We investigated the applicability of a stress corrosion cracking (SCC) engineering model and simulation method developed on the basis of the SCC of sensitized 304 stainless steel in a simulated BWR environment to the primary water stress corrosion cracking (PWSCC). We conducted a uniaxial constant loading test on Alloy 600 in a 400 °C hydrogenated steam environment and found that the number of cracks observed on a specimen surface after every passage of 450 h could be approximated to Poisson distribution, indicating that a Poisson random process model is applicable to the SCC in this system. By applying the engineering model, we statistically processed experimental data by assuming that the time distribution of occurrence of microcracks follows exponential distribution, and then obtained input data for the SCC simulation. Using coalescence coefficient, k, as a fitting parameter to obtain a reasonable k-value, it was found that the best agreement between the experimental and simulation results for the number of microcracks and the maximum crack length at k = 0.15. This is about one third the k-value of 0.5 found in sensitized 304 stainless steel in the BWR environment, indicating that coalescence is more subdued in PWSCC than in SCC in the BWR environment.

Stress Corrosion Cracking Behavior Around Interface Between Nickel Base Alloy (Alloy182) and Low-Alloy Steel

A modified middle scaled creviced bent beam (CBB) test was conducted in order to investigate stress corrosion cracking (SCC) behavior around fusion boundary between nickel base alloy (Alloy82) and low-alloy steel (LAS) under high stress conditions in light water reactor (LWR) coolant environment.An Alloy182/LAS clad plate specimen was placed between the CBB jigs so that the tensile stress occurs in Alloy182 and the SCC cracks propagate from the Alloy182 surface to the fusion boundary. In pure water, only oxides and cracks propagating along the fusion boundary were observed. Meanwhile in Na2SO4 injected water with electrical conductivity 0.3μS/cm, deep cracks propagated into the LAS. The higher conductivity enhanced the driving force of the SCC cracks.A notched specimen was also tested to investigate SCC propagating behavior in more severe stress conditions than those in a smooth surface specimen. When a notched specimen was used, even in pure water condition, some cracks penetrated the fusion boundary and oxides were observed. In the notched specimen, crack nucleation points are limited in the notch root and the stress relaxation hardly occurs due to multiple crack nucleations. Such severe stress conditions also contribute the cracks to propagate into the LAS.To discuss the crack propagation behavior in the vicinity of the fusion boundary, the cross section was observed with an optical microscope. The observation indicates that the crack propagating depth into the LAS and the diameter of oxides increase with increasing the Alloy182 thickness. Although an effect of applied strain and welding direction of specimen to SCC behavior was investigated, there was no significant difference in the morphology by them.Since 1% strain was applied to CBB specimen surface and much greater strain was observed at a notched specimen, elasto-plasticity should be considered to stress condition of a SCC crack tip. An ‘equivalent stress intensity factor, KJ’ was, therefore, proposed to describe a stress condition in elasto-plastic area. A good relationship was observed between depth of SCC crack in LAS and KJ value. It was found that the proposed parameter KJ was effective to describe the stress state at the crack tip from the elastic region to the plastic region up to approximately 1% strain.Copyright

Residual Stress Evaluation of Hydraulically Expanded Tube-to-Tubesheet Joint

An evaluation of a residual stress of hydraulically expanded tube-to-tubesheet joints is important in strength evaluation of certain devices such as heat exchangers, steam generators, etc. In this study, we carried out the analytical and experimental investigation in order to develop a model for the prediction of residual stress.The experiment of the tube expansion was carried out with nickel-base alloy tubes and a low-alloy steel tubesheet. The residual stress of each tube was evaluated by sectioning method where the released strain was measured after each tube was cut into thin strips. As a result, the tensile residual stress was detected at the transition zone between the expanded zone and the non-expanded zone on each tube. A finite element analysis was also performed with a three dimensional model for the tube-to-tubesheet joint. The accuracy of the analysis, however, needed to be improved. Therefore FEM models were developed where the following factors were considered: (i) work-hardened layer on the tube before expansion, (ii) friction between the inner surface of the tube and the mandrel used for hydraulic expansion. The accuracy improvement of the analysis was confirmed through the comparison of the numerical and experimental results.Copyright