An Appraisal on Assessment of HAZ Toughness

Abstract

Advances in welding technology and materials science have resulted in great improvements in the reliability of welded structures. However, catastrophic failures have not been uncommon. Most of the weld related failures have been attributed to have originated from the heat affected zone (HAZ), which is believed to be the weakest link in the heterogeneous welded joint comprising of weld metal, HAZ and the unaffected base material. One of the key requirements in the integrity assessment of welded joints is the availability of representative property data for the HAZ, which can be used for comparing with the applied stress and predict critical stress or the remaining life. The difficulties in estimating the properties of the HAZ are compounded by a microstructural gradient within a narrow zone. In this investigation, detailed experimental studies were carried out on the HAZ obtained in manual metal arc, submerged arc and gas metal arc welding processes. The properties evaluated include hardness, tensile data, CVN impact and fracture toughness. Inspite of placing the notch close to the fusion boundary, CVN impact tests do not provide the correct estimation of toughness because of irregular HAZ boundary and the finite ischolar_main radius of the CVN notch (0.25mm) which entails the crack to sample several heterogeneous grains. On the other hand, a sharp fatigue precrack narrows down the microstructural heterogeneity in the fracture toughness tests. Even so, the variation of the fracture toughness (CTOD) within the HAZ is as unpredictable as in CVN toughness. This depends on the crack tip encountering either a local brittle microstructure indicating pop-ins or the deviation of crack tip into adjacent softer microstructural regions resulting in an apparent increase of fracture toughness. Literature is lacking in proper validation criteria before the HAZ toughness test results can be applied for integrity assessment. The present paper systematically investigates the problems associated with evaluation of the properties of HAZ in typical C-Mn-Nb micro alloyed steel.