In order to obtain a predictive equation of Charpy impact absorbed energy of austenitic weld metals at 4.2K, a quantitative relation between absorbed energy at 4.2K and influencing factors, such as ferrite number, carbon content and oxygen content was studied experimentally. These factors were selected based on the fact that the cryogenic toughness of austenitic weld metal is influenced by δ ferrite, chromium carbide and silicon-mangannese type oxide. The absorbed energy at 4.2K was obtained by a method of converting the lateral expansion at 4.2K into the absorbed energy by using the lateral expansion vs. absorbed energy diagram of each weld metal at 77K. The lateral expansion at 4.2K was obtained by testing the impact specimen in the glass vessel filled with liquid helium. In weld metals containing δ ferrite, the ferrite number was measured by a ferrite indicator. On the other hand, in weld metals containing no δ ferrite, the ferrite number was obtained with the help of the modified Szumachowski's constitution diagram. Following experimental equation relating the absorbed energy at 4.2K (vE) to the ferrite number (FN), the carbon content (%C) and the oxygen content (%O) was developed by regression analysis of the experimental data: vE[J]=90.6-4.56(FN)-44.2(%C)-824(%O) It is expected that the above equation is applicable to predict the absorbed energy of the austenitic weld metal in the as-welded condition at 4.2K.
BENKEI, which was a large window frame conventional magnet at KEK, has been converted to a superconducting magnet. By the conversion, the pole gap has been doubled from 0.5m to 1.0m retaining an analyzing power of 2Tm. Several new techniques were adapted to coil windings and cryostat fabrication. The superconducting BENKEI has shown satisfactory performances for long term operation.