Addition of alloying elements can alter the properties of high-strength linepipe steel. Particularly the addition of Chromium and Molybdenum acts to suppress ferrite nucleation and promote the formation of acicular bainite microstructures and thereby increase the tensile properties of modern linepipe steel. While chemistry is a factor, welding parameters can also be influential and affect the HAZ toughness. The present work compares the effect of C, Cr, and Mo on the girth weld HAZ fracture toughness of X70 in identical welds.
Three pipes of size 48″ OD × 0.528″ WT with different combinations of C, Cr, and Mo were produced. Identical welding procedures were employed to produce two girth welds so that a low-C, Cr pipe (CE = 0.238) was joined to a high-C, Cr pipe (CE = 0.268) which in turn was joined to a low-C, Mo pipe (CE = 0.224). By evaluating the HAZ properties on either side of a weld, it was possible to accurately assess chemistry affects on HAZ properties.
These girth welds were subjected to different testing for the evaluation of girth weld HAZ impact and fracture toughness. These included all-weld metal and pipe body tensile testing, micro hardness testing of HAZ, microstructure analysis, Charpy V-notch testing of weld metal and HAZ, and CTOD testing of weld metal and HAZ at −5 °C and −20 °C. In addition, to investigate the transformation behaviour, Gleeble simulations of coarse-grain heat affected zone (CG-HAZ) were conducted using skelp samples, which were taken from the same coils as the pipe samples.
The results demonstrated that among the low and high carbon equivalent (CE) alloys, materials with low CE values showed better toughness properties. While among the low CE materials, the material with high Mo performed better in terms of toughness. No clear effect of weld position around the pipe circumference on the CTOD values was observed.