Microstructural analysis of weld metal microstructures in a low carbon high strength microalloyed steel

Microstructural observations and analysis of weld metal in a 800 MPa grade low carbon high strength microalloyed steel are studied by means of potical microscopy,SEM-EDS(Scanning Electron Microscopy-Electron Dispersed Spectrum) and EBBSD(Electron Backscattering Diffraction).The weld metal consists of bainite and acicular ferrite.Acicular ferrite laths or plates are nucleated on inclusions which consisted of an Al₂O₃ core and an outer layer of titanium oxide.Although multi-variant ferrite plates emanate radially from an inclusion,there are only some of them could enlengthen rapidly.It indicates growth direction of acicular ferrite preferred orientation.Results of EBSD analysis also indicate that the acicular ferrite grains in weld metal are not crystallographically oriented randomly,probably a certain orientation relationship being kept.Those acicular ferrite laths or plates nucleated on inclusions grown in opposite direction have the same orientation relationship,presumably because they have a Kurdjumov-Sachs(K-S) orientation relationship with the prior austenite grains.