Recently, high strength steel plate such as 950MPa class strength is focused as material of hydropower plant due to its effect on cost reduction and it has been already used to penstocks in Kanna-gawa and Omaru-gawa hydropower plants in Japan. However, it comes to issue that welding restrictions for preventing welding cracks and keeping strength are much severer than ordinary strength steel, for example, preheat condition or inter-pass-temperature range. If under-matching (lower strength in weld metal than base plate) is applied, it is possible that high strength steel plate becomes easier to use because restrictions in welding process may decrease by application of softer welding materials. By applying softer welding, it is assumed that mechanical performances of the structure changes due to existence of heterogeneous region. However, quantitative effect of application of under-matching for high strength steel on deformation behavior and fracture assessment is not certain.
In this report, in order to evaluate the effect of under-matching design on brittle fracture behavior, standard CTOD tests and wide plate tests with through-thickness notch by the welded joint using actual combination of 950MPa steel plate for penstock and various welding materials (950-570MPa class), were performed. As a result, welded joint with softer welding materials showed higher resistance to brittle fracture. Furthermore, taking into consideration of allowable stress, sufficient load capacity was observed even in case of application of 570MPa class welding consumables.
