Development of Evaluation Procedures for the Fracture Mecharical Characterization of Welds at Cryogenic Temperatures

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The fabrication of load-bearing members of large superconducting magnet support structures requires the welding of thick section. These weldments must be capable of withstanding high stresses at liquid helium temperature (4K). Therefor, the cryogenic fracture toughness of the weldments is important design parameter.In this research project, evalustion procedures for the fracture mechanical characterization of thick section weldments are investigated.1.Elastic-plastic fracture toughness (JIc) tests were performed with 25-mm-thick compact tension (1TCT) specimens at 4K evaluate the fracture toughness and the temperature rise of thick section weldment in forged JN1 type austenitic stainless steel plate for the fusion reactor magnets of the next generation. The weld was produced by tungsten inertgas (TIG) are welding using Inconel 625 type filler metals. Au/0.07%Fe-Chromel thermocouples were used to measure the temperature rise near the crack tip. The JN1 type austenitic stainless steel we … More ld was tested and analyzed at five locations through the depth of 200-mm-thick welding. The effect of nitrogen content on J_<IC> is examined, and the excellent correlation between temperature rise and increase in plastic component of J-integral is found.2.J_<IC> tests were carried out with 1TCT specimens at 4K to establish the effects of nitrogen content.inclusion content and secondary cracking on J_<IC> of thick section weldment in forged JJ1 type austenitic stainless steel plate. The test specimes were machined from TIG weldment in 200-mm-thick forged plate. Temperatures of base metal specimen were also measured during straining at 4K,and a two dimensional finite element analysis was used to interpret the experimental measurements.3.J_<IC> tests were carried out with 0.5TCT and 1TCT specimens at 4K to evaluate the fracture toughness of SUS316 type austenitic stainless steel slectron-beam weld for the superconducting coil systems in the Large Helical Device. The SUS316 type austenitic stainless steel weld was tested and analyzed at three locations through the depth of 75-mm-thick welding. The effects of specimen size, precracking temperature and ferrite content on J_<IC> are examined.4.J_<IC> tests supplemented with acoustic emission measurement were conducted to characterize austenitic stainless steel welds at 4K.5.Small punch tests were performed to estimate the fracture toughness of austenitic stainless steel welds at 4K.The effect of loading rate to small punch test results is examined. Less