Insights into weld metal hot cracking of austenitic stainless steels: Atomistic characterization of crack surfaces

Austenitic stainless steels (ASSs) are prone to hot cracking during welding. Solidification and liquation hot cracks are the two main hot-crack types. These cracks are known to occur in the presence of detrimental elements such as S, P, Nb and Ti. However, accurate quantification of these segregating elements is necessary in order to confirm the inherent mechanisms involved in hot-cracking phenomena. Until now there were no attempts to characterize the hot-crack surfaces at atomic-scale as this partly involves complex crack opening protocols and atomistic characterization methods. In the current scenario, using novel crack opening methods, we successfully opened hot-cracks in single and multi-pass welds of a low C high alloyed ASS. The crack surface investigations confirmed the nature of single-pass weld metal crack (WMC) to be solidification crack and the multi-pass WMC to be liquation crack. Atom probe tomography (APT) results showed predominantly ∼ 32 at. % S segregation at single-pass WMC. Whereas multi-pass WMC consisted of ∼ 28 at. % S, 14 at. % C, 0.4 at. % Ti and 0.4 at. % Nb. The segregating elements are concluded to be the main reason for WMCs occurrence in both cases, without stoichiometric phases formation at the crack surface. Additionally, Cr _eq and Ni _eq values calculated from weld metal composition indicate absence of primary ferrite during solidification which further increases the tendency to hot-crack formation.