Satoru Aoike

SCC Test Verification of Effect of Moving Heat Source on FEM Evaluation for Weld Residual Stress of Butt-Weld Pipe

Weld residual stress of a small bore pipe was evaluated using three-dimensional thermal elasto-plastic analysis, which was based on the finite element method (FEM), taking into consideration the effect of a moving heat source during the butt-weld process. A moving heat source during the weld process complicates analysis process due to the necessity of considering the set welding conditions at each time step and this increases computation time. Therefore, there are remarkably few studies on the effect of a moving heat source during the weld process on the analytical results of weld residual stress for small bore pipes. We used this analysis to find the effect of the welding start/end on weld residual stress. Weld residual stress is not a circumferentially uniform state. Increased tensile and compressive residual stress occurred near the end position of the final welding pass. In addition, an accelerated stress corrosion cracking (SCC) test was carried out using a boiling 42% magnesium chloride (MgCl2 ) solution. In particular, butt-weld joints of low-carbon austenitic stainless steel (Type 316L) pipe, which is often used in boiling water reactors (BWRs), were examined. Comparison between three-dimensional thermal elasto-plastic analysis and accelerated SCC testing showed the tensile stress zone indicated by analysis agreed well with SCC occurrence indicated by examination.Copyright

Technical Basis of Fatigue Crack Growth Rate Curve for Ni-Base Alloy in BWR Water Environment

When the flaws are detected in Japanese nuclear power components by in-service inspection, structural integrity assessment are performed in the technical judgment on continuous service. If cyclic loading is assumed, fatigue crack growth analysis should be conducted based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). However, fatigue crack growth analysis for BWR components consisting of Ni-base alloy is currently impossible, since the reference curve of fatigue crack growth rate for Ni-base alloy in BWR water environment is not yet prescribed in the JSME FFS Code.In this study, fatigue crack growth behavior of Ni-base alloy used for Japanese BWR plants in BWR water environment was investigated. Based on the experimental data, the fatigue crack growth rate curve was evaluated. Four test parameters of material, corrosion potential, stress ratio and load rising time were considered. As a result of fatigue crack growth tests, the effects of all test parameters on the fatigue crack growth behavior were found. A Mean curve of fatigue crack growth rate in Paris law format, which was a function of stress ratio and rising time, was formulated based on crack growth data in normal water chemistry (corrosion potential was over 150 mVSHE) for weld metal and heat affected zone (HAZ), respectively. A reference curve of fatigue crack growth rate was also formulated by the statistical treatment considering the scatter of crack growth rate.Further, in order to determine the threshold stress intensity factor range ΔKth of reference curve of fatigue crack growth, ΔK decreasing tests were conducted under the test condition of 1 second of rising time. As a result, the threshold value of ΔK was evaluated based on the ASTM E 647, and the ΔKth of the reference curve was conservatively determined considering the margin.© 2014 ASME

Underwater Remote Microscopic Observation Technique for 3D Curved Surface of LWR Structures

To survey core internals simply, new microscopic observation techniques were developed. These techniques were involved the use of “Gel electrode” and “underwater microscope”. A Gel electrode can etch the surface of core internals without a watertight reservoir that makes an etching environment. The underwater microscope can be used to observe surface figures of core internals directly. These techniques were applied to the cracks detected at the shroud support of Tokai II Power Station for investigation of the cause of crack initiation. It became clear that initiation points of the detected cracks were in a nickel-base weld metal.© 2012 ASME

Development of Stress Reduction Method in the Pipe Welded Zone

Stress Corrosion Cracking (SCC) failures have occurred in the vicinity of austenitic stainless steel pipe welds used in boiling water reactors, since the late 70s. One of the initial countermeasures against SCC has been to use low-carbon stainless steel. However, in older plants, SCC failures in low-carbon stainless steel pipe were still observed in recent years. It is well understood that residual tensile stress due to welding largely affects occurrence and growth of SCC in low-carbon stainless steel. Because of this, it is important to reduce the residual tensile stress in the welded zone in addition to utilizing less susceptible material. However, a countermeasure to reduce residual tensile stress in small-bore pipe has not been established. In this study, a new stress reduction method is developed in order to reduce residual tensile stress in the welded zone, even for small-bore pipe. This method is applied to the butt-welded zone while the pipe is filled with water. First, the pipe is frozen at two points centering around the welded zone by cooling the outer surface of the pipe to transform the water to plugs of ice. The volume inside the pipe between the ice plugs becomes watertight and pressurized by the expansion of the plugs up-on freezing. The pipe expands near the zone of the weld groove, because the wall thickness of the butt-welded zone is thinner locally because of the weld counter bore. In this process, tensile strain occurs near the welded zone due to the pipe expansion beyond its plastic range, and then hoop tensile plastic strain of inner surface is larger than that of outer surface. Once the pressure load is removed, the difference in these tensile strains induces the hoop residual tensile stress on the inner surface to reverse to a compressive stress. On the other hand, the local bending deformation that occurs in the welded zone results in an axial residual compressive stress on inner surface. Both experimental studies and finite element analysis confirmed that the hoop and axial residual tensile stress at the inner surface of the welded zone are reversed to compressive stress by the pipe expansion.© 2009 ASME