Optical and Electron Micrographic Studies of Two Failed Fillet-Welded Nozzle Joints in a 316L Stainless Cylindrical Tank

摘要

An analysis of the possible causes of the development of cracks in the stainless steel tank body at the weld boundaries for two long 316L nozzles connected to a cylindrical 316L tank by fillet weld is described in this paper. The cracks were discovered only after the tank was brought to site after a road journey of about 899 K.M. The experimental methods employed were : (a) visual examination, (b) optical photomicrography, and (c) microfractography by a transmission electron microscope. The experimental data indicated positively that the failure was due to fatigue. Ubiquitious ratchet marks characteristic of fatigue failure under rotating beam bending were observed on the fracture surfaces. Typical fatigue striation marks were seen in the microfractographs of the fracture surfaces. The necessary cyclic loading for fatigue failures arose during the road transport. The long nozzles penetrated nearly 3000 m.m. inside the tank without any support at the free end. The situation aggravated because the nozzles were kept at an angle of 45 deg to the vertical during the road transport, instead of being kept vertically, thus acting as cantelevers under cyclic loading. The workmanship was also very poor for the welds, with a large number of grind marks forming easy paths for the cracks. (Atomindex citation 09:376330)