Application of SEM-EBSD for Measurement of Plastic Strain Fields associated with Weld Metal Hydrogen Assisted Cold Cracking

摘要

The requirement for more efficient use of materials for pipelines has lead to the application of high strength low alloy steels such as X70 and X80 in pipelines. As the strength of these alloys has increased so has the risk of hydrogen assisted cold cracking (HACC). In Australia to minimize construction time, the root runs of girth welds are produced by shielded metal arc welding using cellulosic electrodes without either pre or post heating. Well defined welding criteria have been developed and are incorporated into the weld procedures for the elimination of HACC in the heat affected zone but the risk of cracking to the weld metal is still of concern. It has been reported that plastic deformation occurs prior to the formation of hydrogen cracks in weld metal. Therefore the evaluation of plastic strains at the micro- and nano-scale and their relationship to the weld metal microstructure could be of great significance in assessing the susceptibility of welds to weld metal hydrogen assisted cold cracking (WMHACC). A method for analysing plastic strains on the micro- and nano-scales using electron backscattered diffraction (EBSD) has been developed. This technique is based on the degradation and rotation of diffraction patterns as a result of crystallographic lattice distortion resulting from plastic deformation. The analysis can be automated to produce an Image Quality (IQ) map in order to relate the spatial distribution of plastic deformation to microstructural features e.g. grains or cracks. The development and assessment of techniques using Scanning Electron Microscopy (SEM) and EBSD for the determination of local plastic strain distribution in E8010 weld metal used for the root pass of X70 pipeline girth welds is discussed.