Influence of lack-of-fusion defects on load capacity of MAG welded joints
Pressure vessels operating at elevated temperatures and pressures are made of creep resisting plate and welded from individual components using the MAG welding process. The paper proposed focuses on the occurrence of lack-of-fusion defects in MAG welded plate components of pressure vessels. Lack of fusion, which is a planar defect and is, therefore, difficult to detect by common NDT methods, can result in catastrophic consequences since the latter do not only damage a pressure vessel but also jeopardize the environment and workers' health. The present paper deals with a research on the influence of a welding speed in welding of plate components made of S235JR steel of 8 mm in thickness, special care being taken to deliberately produce lack-of-fusion defects and possibly other weld defects. Test pieces prepared so as to contain various lack-of-fusion defects were subjected to destructive tensile testing. The force required for a weld failure was compared to actual weld defects. In the test pieces produced with lower welding speeds, a failure occurred in the welded joint and along a lack-of-fusion defect. In the test pieces produced with higher welding speeds, however, a fracture occurred outside a weld, although lack of fusion was present in the weld. Parallel to the tensile tests, radiographic examinations of lack-of-fusion defects were conducted and relevant micro sections were analysed. The aim of the study conducted was to elaborate a guideline for quality welding of plate components of pressure vessels.
Analysis of Ultrasonic Indications in Lack of Fusion Occurring in Welds
Lack of fusion is a very dangerous weld defect in a welded structure. Because of the notch effect, a crack may further propagate under the smallest load applied. This type of defect is very difficult to detect with non-destructive testing methods. For the purpose of the present study, lack-of-fusion defects were artificially produced in a multi-pass welded joint. The aim was to find out how ultrasonic devices can sense this type of defect in a butt weld. In addition radiography was used to detect the occurrence of lack of fusion in the welded joint. In the ultrasonic examination 60° and 70° angle probes were applied. They showed lack of fusion at the opposite side-walls of the groove. A normal probe was then used to detect the occurrence of inter-run lack of fusion. The weld defects provided a multiple reflection of ultrasonic waves, which confirmed the presence of lack of fusion in a certain depth. This was additionally confirmed by an ultrasonic thickness measuring gage. Finally, micro sections of the weld were made to establish the actual location and shape of lack-of-fusion defects.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.