High tensile strength steel such as 950 MPa class is used in hydropower plant components due to its lower cost and higher reliability. This steel is already used for penstocks in the Kanna-gawa and Omaru-gawa hydropower plants in Japan. Proper welding is one of the most critical factors to ensure integrity of the plant components against brittle fracture from weld defects. For example, preheat- and interpass-temperature controls are essential to avoid weld defects and to ensure strength, since the weld process induces residual stress and possible weld deformations such as angular distortion or unevenness. Undermatching (lower strength in weld metal than in the base plate) makes high strength steel plate become easier to use for welding because restrictions in the welding process can be reduced by using softer welding consumables. Tensile strength in welded joints increases near the base plate when the undermatched zone is not much wider than the thickness. When the appropriate welding conditions for assuring strength and preventing brittle fracture are clarified, it becomes clear to be possible to use undermatched joints for penstock.

In this paper, numerical simulation of stress-strain behavior during the weld process is performed by considering the microstructural effect due to phase transformation. Phase transformation data is obtained from measured dilatometric curves in continuous cooling transformation (CCT) diagrams. Phase transformation data involves many parameters, including not only the maximum temperature, cooling rate and heat input, but also the superposition of multi heat cycles. Then, the characteristics of deformation and strength in a welded joint of 950 MPa class steel plate for penstock with undermatched region, which is equivalent to using weld material with lower strength level, are discussed to expand the allowable welding conditions in penstock fabrication. Finally, brittle fracture behaviors are discussed by using standard crack-tip opening displacement (CTOD) tests and wide plate tests with through-thickness notch by the welded joint.

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