Inclusion chemistry and morphology in shielded metal arc (SMA) steel weld deposits

Abstract

The composition and morphology, and the size and distribution of the nonmetallic inclusions found within a series of shielded metal arc (SMA), C-Mn steel weld deposits, produced using AWS E7016 type electrodes, have been investigated using transmission electron microscopy (TEM) and associated x-ray microanalysis techniques. The majority of the inclusions were ≈ 0.1 μm to ≈ 1.0 μm in diameter, and microanalysis generally showed them to be rich in Mn, Si, and Ti, with lower levels of Al, Cu, and S. Most of the inclusions were heterogeneous, often containing Mn- or Cu-rich sulphide globules embedded in their surfaces. Inclusion composition could be related to that of the overall weld deposit, in a similar manner to that described previously for inclusion compositions and the flux type employed in submerged arc weld deposits. Specifically, the Ti content, Mn content, and the Mn/Si ratio of the inclusions could be related directly to that of the overall weld composition. Varying the basicity index (BI) of the electrode coating was found to have little effect on inclusion composition. No correlation was found between inclusion size and composition, although large inclusions (say ≥4 μm in diameter) were generally found to have atypical compositions, often indicating the presence of significant levels of elements such as Al, K, Ca, and Na. A further precipitate or inclusion type was found to be a feature of all the welds examined, namely a distribution of fine copper sulphide particles (≤100 nm in diameter). In addition, the observation of acicular ferrite nucleation events frequently showed them to be associated with the surface Mn- or Cu-rich inclusion globules of inclusions ≥0.2 μm in diameter.