On the microstructure of multipass high-strength weld metals and their heat-affected zones

Fusion welding of high-strength steels is seen as the primary bottleneck in obtaining the desired mechanical properties in corresponding steel constructions. During this welding process, the molten base metal solidifies together with any added filler metal to a weld metal whose microstructure is fixed upon solidification and offers limited potential for subsequent modification. Furthermore, the base metal adjacent to the molten material is heat treated with locally varying thermal cycles, resulting in the so-called heat-affected zone. Multipass welding further complicates this aspect, as certain regions of both the weld metal and the base metal are exposed to the heat of several welding passes. Consequently, understanding these locally varying microstructures is crucial for enabling advanced lightweight steel constructions in the future. To address this, the aim of the present doctoral thesis is to investigate the microstructure-property relationships of multipass high-strength weld metals and their heat-affected zones, with a particular focus on a fundamental evaluation of the present microstructural constituents. For this purpose, high-resolution electron microscopy and atom probe tomography were performed. Furthermore, high-energy X-ray diffraction was employed for phase quantification and in situ observation of the microstructural behavior during thermal and mechanical loading. Mechanical properties were examined using standardized tensile and Charpy V-notch tests, as well as hardness measurements. The microstructure of a multipass high-strength all-weld metal with a yield strength exceeding 1100 MPa was found to consist primarily of martensite along with various types of bainite and retained austenite. Physical simulation of the coarse-grained heat-affected zone of this all-weld metal enabled the construction of one of the first welding continuous cooling transformation diagrams for weld metals at corresponding strength levels. Examination of the entire all-weld metal revealed an unexpectedly high fraction of retained austenite, reaching up to 7 %. Elevated concentrations of this metastable phase were localized in regions reheated into the intercritical temperature regime between A1 and A3 by the subsequent welding beads. Retained austenite in certain regions of this all-weld metal was observed as stand-alone phase or within martensite-austenite constituents. In the last bead, it remained largely mechanically stable during tensile strain until the onset of plastic deformation. Retained austenite was also examined in the heat-affected zone of a high-strength steel with a yield strength of 700 MPa. It was found that reheating to temperatures near the liquids line (coarse-grained heat-affected zone) resulted in the formation of a similar fraction of retained austenite as observed during subsequent reheating to the intercritical regime (intercritically reheated coarse-grained heat-affected zone). Ultimately, severe embrittlement was observed in a multipass high-strength all-weld metal with a yield strength of 690 MPa during stress-relief post weld heat treatment. This embrittlement was attributed to the formation of coarse, Mn-rich cementite precipitates.