Field Girth Weld HAZ Toughness Improvement: X80/Grade 550

Abstract

Field welding and field weld rework can be a significant cost in the construction of pipelines. Heat affected zone (HAZ) material adjacent to a weld is of particular concern because the base material microstructure has been altered significantly. In instances where Engineering Critical Assessment (ECA) is used for defect acceptance, optimizing and/or improving the base material for field weldability will reduce repair welding rates, which in turn improves project economics. Several alloys of X80/Grade 550 were assessed. All materials were robotically welded to simulate a typical mechanized field weld. Two of the alloys were also welded using a field mechanized welding system. These welds were subjected to tests assessing field weldability. Weldability is a broad term used to summarize various material properties related to the level of conduciveness to welding. For the purposes of this paper, the term field weldability is used to describe the level of HAZ toughness of a material subjected to field welding conditions. Charpy V-notch (CVN) and crack tip opening displacement (CTOD) tests were utilized to assess the toughness of the welded material. Optical microscopy was employed to characterize the HAZ microstructures. In addition, all materials were subjected to HAZ thermal processing in a Gleeble thermo-mechanical simulator. Gleeble dilatometry curves were constructed to characterize phase transformation behavior, and tested materials were used to characterize HAZ microstructures using optical microscopy. Gleeble HAZ CVN specimens were processed in order to assess the toughness of a uniform, idealized HAZ microstructure. It was found that HAZ toughness was better for material chemistries that promote lower phase transformation temperatures. Lower phase transformation temperatures caused the formation of favorable microstructural phases, with finer coarse grain HAZ (CGHAZ) prior austenite grain size, as well as fine packet size. Phase transformation temperature and prior austenite grain size were found to be most dependant on the carbon and carbon equivalent content of the material. The steel containing the lowest amount of carbon displayed the highest phase transformation temperature, coarsest CGHAZ prior austenite grain size, and lowest HAZ toughness, as measured by CTOD and CVN tests.