| Most structural weld procedures involve multipass welding. Multipass Heat affected zone (HAZ) consists of a variety of sub-regions, which form due to the overlap of different passes. This results in toughness degradation in small regions within the HAZ. These regions have been found to influence the overall HAZ toughness, even though they are surrounded by relatively higher toughness regions. These are termed as local brittle zones (LBZs). Their presence has been highlighted by Crack tip opening displacement (CTOD) tests.;This work involved studies on LBZ presence in two structural steel weldments, used in bridges (A514 and A588) and their influence on their overall HAZ toughness. The alloys were submerged arc welded with a special groove geometry to maximize the desired HAZ microstructures. Charpy impact and CTOD tests were conducted for the base metal, weld center line, and HAZ.;In order to detect low toughness HAZ microstructures, HAZ CTOD specimens were further analyzed through fractographic studies in determining the basic fracture type, locating unstable fracture initiation sites, the HAZ regions at these sites and their quantification expressed as percentage HAZ microstructure sampled by fatigue precracks. Weld thermal simulation was carried out to recreate the most frequently observed HAZ unstable fracture initiation site microstructures. These simulated HAZ microstructures were studied for predominant microstructures and their individual impact toughness behavior.;For A514, upper bainite formation in CGHAZ was found to degrade the HAZ toughness significantly. For A588, though upper bainite was observed in CGHAZ, the overall HAZ toughness was not significantly affected during CTOD testing. The surrounding high toughness HAZ microstructure seemed to have offset LBZ influence for the test conditions used for A588. However, thermal simulation studies showed that there are A588 CGHAZ regions with poor toughness for the welding conditions used.;Design stresses and flaw sizes under LBZ presence, were computed using available numerical equations and CTOD values. This gave an insight into whether high factor of safety is needed in the presence of these microstructures and whether the test used was sufficient to establish LBZ significance. |
