Precipitates in microalloyed ultra-high strength weld metal studied by atom probe tomography

Abstract

Gas metal arc welding with metal-cored filler wires is frequently used to weld high strength steel constructions for lightweight and transportation applications. In the current study, microalloying is considered as strengthening concept for reaching the required mechanical properties by precipitation hardening. For this purpose, the typical microalloying elements Ti, Nb, V, and Al were added to the filler metal in a comparatively high amount (up to 0.5 m.%). All-weld metal samples with a yield strength of 1000 MPa and more were produced by gas metal arc welding. Laser-pulsed atom probe tomography was used to evaluate the potential of these elements to form clusters or precipitates and strengthen the weld metal. While Al and Nb did not form clusters, a strong tendency for clustering was found for V-and Ti-alloyed samples. The cluster size evolution and changes in chemical composition depending on the microalloying contents are discussed. Furthermore, the challenges arising from local alloying element enrichments and local differences in thermal history in the all-weld metal are addressed regarding sample preparation and data evaluation.

Influence of inhomogeneity on several length scales on the local mechanical properties in V-alloyed all-weld metal

“…Consequently, the last bead should have a higher strength and a lower toughness than the rest of the weld. However, the type of welding consumables investigated in this study is alloyed with V. This element is known for its potent strengthening effect in steels because of its tendency to form precipitates during reheating [4,20,21]. Therefore, hardness mappings and local Charpy V-notch tests were applied to study the local mechanical properties and the difference between the last weld bead and the underlying beads in this type of material.…”

“…The produced all-weld metal samples were martensitic and contained V(C,N) precipitates [4,13]. The purpose of this study is to demonstrate the existence of chemical and microstructural inhomogeneities in this type of martensitic multipass weld metal and to clarify their influence on the mechanical properties of the weld.…”

“…This was described by Keehan et al [2] for high-strength weld metals produced by shielded metal arc welding and by Powell and Herfurth [3] for gas metal arc welds. In another study, Haslberger et al pointed out that these segregations could also influence the formation of clusters and precipitates in V-alloyed weld metal [4].…”

Microstructure and mechanical properties of high-strength steel welding consumables with a minimum yield strength of 1100 MPa

“…However, in the investigated type of welded material an addition of min. 0.2 wt% vanadium is necessary to attain significant strengthening [5]. Due to the welding process, the material is reheated very shortly by subsequent welding passes.…”

“…This approach showed promising results, and the outcomes of further experiments with vanadium alloying are discussed elsewhere [5]. Based on these investigations, the strengthening effect of vanadium was incorporated in the alloying concept of this study.…”