Tracking local brittle zone in the heat affected zone of girth-welded API 5L x46 pipe

Abstract

In this study, microhardness variation as well as macro and micro structural examination of the heat affected zone (HAZ) of a girth welded API 5L X46 pipeline material were conducted as a means of tracking local brittle zone (LBZ) in the HAZ region. The weldment analysed were built from heat input range of 695 J/mm – 2567 J/mm. Analysis of the results revealed that the HAZ profile changes with variation in the heat input and becomes shallow but wider as the heat input increases. Defects free welds were achieved under the heat input range of 1650 J/mm – 2017 J/mm welding condition. Localized high hardness values were obtained at certain locations within the HAZ of intermediate heat input welds produced at 1467 J/mm due to thermal stresses induced strains at this heat input in the resolidified weld. Other than this, non-equilibrium rapid heating/cooling that is common during welding as well as the magnitude of mechanical strain generated on cooling vary with heat input and was attributed to the development of high hardness value at localized region within the HAZ of the welds in low heat input welding condition. The macrographic profile at these locations, contrasted against that of a failed pipeline material of similar specification obtained from typical oil and gas infrastructure, established that crack initiation and propagation followed the trend of microhardness variations in the girth welded pipe. The crack initiates at specific location in the HAZ with very high hardness in the range 186-216 Hv within a radius of about 3-5 mm from the edge of the fusion zone.

Keywords: API 5L X46; Girth welding; Heat input; Heat affected zone; Local brittle zones