Microstructure characterization and its relationship with impact toughness of C–Mn and high strength low alloy steel weld metals – a review

Abstract

Prediction of impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the innumerous parameters involved. The common practice that associates this property with the microstructure of the last bead of multipass weldments has been proven to be unsatisfactory, because the amount of acicular ferrite, the most desirable constituent, may not always be the main contributor to toughness. Parameters such as the recrystallized fraction, the presence of micro-phases and inclusions may also have a relevant role. Thus, to consider the influence of all these parameters, the method proposed by the International Institute of Welding (IIW) is not sufficiently comprehensive and so complementary techniques are necessary. This situation is more relevant for high strength steel weld metals, where very refined microstructures may not be adequately resolved, resulting in misidentification of the microstructure. The use of scanning electron microscopy as an auxiliary technique to optical microscopy has been successfully used for many decades to study C–Mn and low alloy weld metals, mainly when refined microstructures are to be assessed. Recently, in addition to the previously mentioned techniques, Electron Back Scattering Diffraction (EBSD) has also been used to enable a more effective analytical procedure. This technique, which provides valuable information about grain boundaries, is advantageous for refined microstructures to confirm constituents such as acicular ferrite, bainite, and martensite. This work proposes a contribution to the microstructural characterization of C–Mn and high strength steel weld metals based on the analysis carried out by optical microscopy, scanning electron microscopy and EBSD techniques, involving the influence of recrystallization in multipass welds, microstructural constituents, microphases, and inclusions. The microstructure/toughness relationship analysis of some experimental results obtained over the last decades for weld metals with ultimate tensile strength varying from 400 to 1000 MPa was done using the methodology proposed in this work to check its effectiveness and to explain the impact toughness behavior.