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.
Formation Mechanism of Lack of Fusion in MAG Welding
This paper deals with the reason and the prevention of the lack of fusion formation in MAG welding. The results obtained are as follows: (1) The lack of fusion which is the plane defect appears in the weld metal near weld bond, and the inside of the lack of fusion consists of oxides of Mn, Si. (2) The lack of fusion is formed when the solid surface appears frequently in molten pool. This solid surface becomes wider with an increase in welding speed, and the humping bead is formed when the solid surface spreads out all over the molten pool. (3) The motive force which forms the solid surface is the arc pressure. Addition of helium into the shield-
Ultrasonic detection and sizing of complex defects and implications for structural assessment procedures
Ultrasonic inspections of superheater header closure welds at Wylfa Power station revealed numerous, closely-spaced defect indications which required careful interpretation and analysis. To accommodate the multiplicity ofindications the inspection procedure included the use of focussed 0Υ probes, which enabled detailed maps to be prepared for fracture assessment. The ultrasonic procedure provided a conservative estimate of the defective areas without requiring excessive time for the data analysis. The degree of conservatism has been investigated both using a mock-up with representative defects and by local grinding of a near-surface defect on the plant. The fracture assessment employed a two-stage approach, firstly using a standard analysis to identify those headers for which good reserve factors could readily he demonstrated. Subsequently, a more refined analysis of the more defective headers was performed which included the results from material sampling and fracture toughness testing. This paper demonstrates that there are several significant conservatisms implicit in the ultrasonic data analysis and the subsequent defect characterisation needed to make the fracture analysis tractable. Notwithstanding these conservatisms, all the welds were demonstrated to be fit for service without repair.