Seiichi Kawaguchi

Microstructural changes and fracture behavior of CF8M duplex stainless steels after long-term aging

Microstructural changes and fracture behavior in cast CF8M duplex stainless steel after aging at 300-450°C for 300-10000 h have been investigated. Both, optical microscopical and transmission electron microscopical analyses, hardness and ferrite content measurements have been carried out in this study. Strengthening and aging phenomena of the ferrite phase have been identified by hardness measurements. Spinodal decomposition and heterogeneous precipitation of G-phase were found to be responsible for strengthening of the ferrite phase after aging with a temper parameter [P = log(t) + 0.4343 O/R(1/673.2-1/4)] (see Appendix A) in the range ca. 1.8-4.5. Three different fracture modes, dimples, cleavage and α/γ grain boundary separation, have been observed for Charpy V-notch and CT test specimens fractured at 20°C.

Reconstitution of Fracture Toughness Specimen for Surveillance Test

Fracture toughness is one of the most important material properties that influences the integrity of reactor pressure vessels (RPVs). Methods for reconstitution of fracture toughness specimen have been developed preliminarily in order to increase the number of fracture toughness data of irradiated RPV steels. The specimen adopted in this study was a 12.5mm thick compact specimen (½TCT). The size of insert material, which is removed from the assumed broken specimen, is 13 × 15 × 12.5mm 3 . Materials used were two kinds of unirradiated RPV steels. Welding procedures investigated were CO 2 laser beam welding(LBW) and electron beam welding(EBW). Several welding parameters, such as welding current, speed of travel, etc. were varied parametrically. Then optimum welding conditions of both weldings for reconstitution of CT specimen were investigated. It was found that fracture toughness values obtained from the reconstituted CT specimens of two steels are almost the same as those from original specimens in both the transition and the upper shelf temperature ranges. It turns out that the above methods can be applied for the reconstitution of irradiated fracture toughness specimen in hot cell.

APPLICATION OF NEW WELDING TECHNOLOGY FOR THE MANUFACTURING OF NUCLEAR PRESSURE VESSELS

ABSTRACT The welding technology applicable for the manufacturing of pressure vessels is being improved. One particular improvement is aimed at using a low heat-input for joint welding. This paper describes the current status of application of new welding technology for the manufacturing of pressure vessels in nuclear power plant.

Prediction Method of Tensile Properties and Fracture Toughness of Thermal Aged Cast Duplex Stainless Steel Piping

Cast duplex stainless steels of CF8M and CF8 are used in major components because of their superior characteristics, such as corrosion resistance, weldability, and so on. However, these stainless steels are known to have tendency of thermal aging embrittlement after long term service. Therefore, the mechanical properties have been investigated using tensile test specimens and fracture toughness specimens aged at 300 to 450°C for up to 40,000 hours. From the results, the effects of thermal aging on the mechanical properties of these cast duplex stainless steels were identified. The true stress-true strain curve prediction method (TSS model) and fracture toughness prediction method (H3T model) after long term service were established. These prediction methods are used for the evaluation on the plant life management of nuclear power plants in Japan.Copyright

Fatigue strength of dissimilar welded joints for the main vessel of an LMFBR

Abstract The employment of welded joints composed of dissimilar metals is one simple and inexpensive way to connect a main vessel made of austenitic stainless steel and a roof slab constructed of ferritic steel in the design of liquid metal fast reactors. Since dissimilar-metal welded joints have not been used for such large structures so far in Japan, the structural integrity of this type of joint should be carefully examined for such a design option to be selected. Here various kinds of tests were conducted for eleven types of welded joints of 50 mm thickness to obtain this fundamental strength characteristics. Type 304 stainless steel was used as one of the parent metals in all the joints. They differ from each other in regard to the type of ferritic steel, welding metal and welding procedure. Low-cycle fatigue tests were conducted for round-bar specimens made from these welded joints at room temperature. Fatigue crack-propagation tests were also conducted for some of the joints. Tests after manufacturing a large-scale shell model were also conducted. The results of these tests demonstrated that the present manufacturing technique can, produce welded joints of high quality and reliability. A trial calculation for actual design conditions showed the existence of large margins against fatigue failure or fatigue crack-propagation of a significant amount.