Thermal simulation of HAZ regions in modern high strength steel

Abstract

Thermal simulation of the heat affected zone (HAZ) resulting from single and dual torch mechanised pulsed gas metal arc welding (P-GMAW) has been achieved in an X80 line pipe. The continuous cooling of an X80 steel with initial acicular microstructure over Δt8-5 from 2 to 50 s results in mixed bainite and martensite microstructure at fast cooling times, bainite at intermediate cooling times and granular bainite with bainite at longer cooling times. These phases were confirmed through optical and electron microscopy. The resulting transformation temperatures have been assembled and presented as a continuous cooling transformation diagram. The toughness of the steel was examined with Charpy impact specimens for three simulated HAZ cooling cycles, grain coarsened (GC HAZ), intercritically reheated grain coarsened (ICR GC HAZ) and an interrupted intercritically reheated grain coarsened (NTR ICR GC HAZ). The NTR ICR GC HAZ is a novel HAZ that can result from dual torch welding at fast travel speed and close torch spacing. All of the thermally simulated HAZ regions showed reduced toughness that was attributed to bainitic microstructure and large effective grain sizes.