Day Three (July 15, 2010)

[E-5] Structure and Material Intensity

E-5-1

NDE of ferromagnetic steel using spectrum analysis of changes in induced voltage and magnetic flux density

Hiroaki KIKUCHI, Katsuyuki ARA, Yasuhiro KAMADA and Satoru KOBAYASHI (Iwate University)

Abstract
Magnetic NDE of ferromagnetic steel using spectrum analysis of induced voltage at pick-up coil and magnetic induction has been studied. Magnetic yoke, having an excitation coil and a pick-up coil, was located on a specimen, and a sinusoidal current was applied to the excitation coil for magnetizing the specimen. A voltage induced at the pick-up coil was measured, and the corresponding magnetic induction was calculated. Then, their frequency spectrums were obtained thorough Fourier analysis. For deformed specimen, higher harmonic components decrease as compared with undeformed specimen. Klirrfactors for the induced voltage and the magnetic induction were defined and evaluated. Those values decrease with increasing reduction ratio of the specimen; this means the waveforms of induced voltage and magnetic induction for a less deformed specimen has a large distortion in those waves. Those changes reflect nonlinearity of the magnetization curve of specimen. The results in this study indicate a capability of Magnetic NDE using spectrum analysis of measurement wave forms.

Keywords
NDE, magnetic yoke, spectrum analysis, magnetic induction, induced voltage, Klirrfactor

E-5-2

Effect of Chromium Content on Stress Corrosion Cracking of Shielded Metal Arc Weld Metal for 600 Type Alloy in High Temperature Pressurized Pure Water

Satoru NISHIKAWA, Shigeru OKITA, Atsunori YAMAGUCHI (Japan Power Engineering and Inspection Corporation) and Kenji IKEUCHI (OSAKA UNIVERSITY)

Abstract
When their Cr contents were increased to the same level as those of the alloy 82(18-22mass%Cr), the weld metals of alloy 182(13-17mass%Cr) sustained only slight SCCs in the as-welded state, and no crack was detected after the post weld heat treatment (SR+LTA) of stress relief annealing at 893 K followed by aging at 673 K. These results suggest that the higher Cr content of the alloy 82 is responsible for its higher resistance to SCC than that of the alloy 182. The carbide, Ni₁₆(Mn, Cr)₆Si₇ (G phase) was precipitated at the grain boundary in the alloy 182 containing 18.5mass% Cr when the SR+LTA treatment was applied. TEM-EDS analyses suggested that the G phase was enriched in P, and so could decrease the P content in the grain boundary region. Probably, the decreased P content at the grain boundary due to the precipitation of G phase contributed to the enhancement of the SCC resistance of the Cr-added alloy 182 by the SR+LTA treatment.

Keywords
Stress corrosion cracking, Alloy 600 weld metal, Cr content, G phase, Phosphorus, SR+LTA

E-5-3

Effects of C and Nb Contents on Stress Corrosion Cracking of Shielded Metal Arc Weld Metal for 600 Type Alloy in High Temperature Pressurized Pure Water

Satoru NISHIKAWA, Shigeru OKITA, Atsunori YAMAGUCHI (Japan Power Engineering and Inspection Corporation), Keisuke SOGABE, Yoshiki MIKAMI and Masahito MOCHIZUKI (Osaka University)

Abstract
The SCC susceptibility of the weld metal was evaluated from the maximum depth and number of cracks occurring during the CBB (Creviced Bent Beam) test in high temperature pressurized water using plate specimens cut from shielded metal arc weld metals. When the specimen received a heat treatment consisting of stress relief annealing (SR) for 72 ks at 893 K and subsequent ageing (LTA) for 720 ks at 673 K, however, the significant influences of the increases in C and Nb contents on the SCC susceptibility were observed; the susceptibility of weld metals with higher C contents (~0.07mass%) decreased with increasing the Nb content up to ~2.6mass%, but a further increase in the Nb content enhanced the SCC susceptibility remarkably. The hardness increased remarkably with the Nb content, suggesting that the higher stress was applied to the specimen during the CBB test, as the Nb content was increased. Also, the weld metal that yield stress is high could be confirmed to high stress value at grain boundary by FEM analysis. This increase in the applied stress is a possible factor that contributes to the increase in the SCC susceptibility with the Nb content in the weld metal free from the grain boundary depletion of Cr.

Keywords
Stress corrosion cracking, Alloy 600 weld metal, C and Nb contents, γ’ phase, stress, FEM analysis

E-5-4

Fabrication of imitative stress corrosion cracking specimens suitable for　electromagnetic nondestructive evaluations using solid state bonding

Noritaka YUSA, Tetsuya UCHIMOTO, Toshiyuki TAKAGI and Hidetoshi HASHIZUME (Tohoku University)

Abstract
This study proposes a method to fabricate artificial defects that is almost identical to stress corrosion cracking from the viewpoint of electromagnetic nondestructive evaluations. The key idea is to realize a region having electrical resistance embedded inside a conductive materials using solid state bonding. A rough region is introduced into the surface of the materials so that the region is partially bonded to realize electrical resistance The validity of the method is demonstrated using type 316L austenitic stainless steels. Eddy current tests and subsequent destructive tests confirm that signals due to the fabricated specimens are very similar to those due to stress corrosion cracks.

Keywords
stress corrosion cracking, performance demonstration, nondestructive evaluation, electrical resistance, diffusion bonding

E-5-5

Characteristics of microstructure and SCC behavior near the fusion boundary of Ni-base weld metal and low alloy steel

Hiroshi ABE, Makoto ISHIZAWA and Yutaka WATANABE (Tohoku University)

Abstract
SCC behavior near the fusion boundary of dissimilar weld joint with Ni-based weld metal and low alloy steel in high temperature water has been investigated with special attention on relation to the characteristics of microstructure. From the results of CBB test (1.0 ppm-Na2SO4 and 8ppm dissolved oxygen), it has been obvious that the LAS has no SCC susceptibility. On the other hand, Alloy182 in the weld samples have SCC susceptibility. Most of crack tips have located at the fusion boundary, and the penetration of oxide into LAS has been observed. It has been thought that crack become obtuse when crack reach at fusion boundary. It has been suggested that the dominant factor of crack retardation or arrest is low SCC susceptibility of LAS in high temperature water.

Keywords
Stress Corrosion Cracking (SCC), Dissimilar Weld, Ni-based Weld, Low Alloy Steel

E-5-6

Residual Microstress in Austenitic Stainless Steel due to High Strain Rate

Kenji SUZUKI (NIIGATA UNIVERSITY) and Takahisa SHOBU (Japan Atomic Energy Agency)

Abstract
Austenitic stainless steel (SUS316L) was used as the specimen material. The specimens were deformed plastically with a wide range (6.7 × 10⁻⁵ ~ 670 s⁻¹). The residual micro-stresses were measured using hard synchrotron X-ray. The residual micro-stress for each lattice plane was arranged with Young’s modulus, which was calculated by Kröner model. In the high strain rate, the change of the residual micro-stress became small. The residual micro-stresses peened with laser and water-jet were examined as well as the tensile specimens.

Keywords
Residual Stress, Synchrotron, X-ray, Austenitic Stainless Steel, Strain Rate, Peening

E-5-7

Study on mitigation of stress corrosion cracking by peening

Takahru MAEGUCHI, Kazuya TSUTSUMI, Masahiko TOYODA, Takahiro OHTA, Taketoshi OKABE and Tomonobu SATO (Mitsubishi Heavy Industries, LTD.)

Abstract
In order to verify stability of residual stress improvement effect of peeing for mitigation of stress corrosion cracking in components of PWR plant, relaxation behavior of residual stress induced by water jet peening(WJP) and ultrasonic shot peening(USP) on surface of alloy 600 and its weld metal was investigated under various thermal aging and stress condition considered for actual plant operation. In the case of thermal aging at 320-380°C, surface residual stress relaxation was observed at the early stage of thermal aging, but no significant stress relaxation was observed after that. Applied stress below yield stress does not significantly affect stress relaxation behavior of surface residual stress. Furthermore, it was confirmed that cyclic stress does not accelerate stress relaxation.

Keywords
Residual stress, Stress corrosion cracking, Water jet peening, Ultrasonic shot peening

E-5-8

Application of Ultrasonic Shot Peening for Internals of Boiled Water Reactor Power Plants

Ikuko KAMEYAMA, Toru OHTSUBO, Kazuo SUDO, Yuuki MASUDA and Satoshi YAMAMOTO (TOSHIBA CORPORATION)

Abstract
Toshiba have developed a method of reactor internals replacement and also applied some of methods to improve the residual stress for internals that can’t be replaced easily. Ultrasonic shot peening (USP) is a stress improvement method that improves the surface residual stress of heat affected zone by using the vibration of Ultrasonic wave. As the system of USP is simpler and more compact than the other methods, USP is applicable to welds on the bottom of reactor vessel during reactor internals replacement. In this paper, the efficiency of USP is verified as a stress improvement method and also usefulness is shown by the simulated results of the process of reactor internals replacement.

Keywords
stress improvement, USP, internals replacement, residual stress, SCC

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