Strengthening mechanisms and mechanical properties of dual-stabilized ferritic stainless-steel joints processed by high-speed laser welding

Abstract

This study investigates the microstructural evolution, strengthening mechanisms, mechanical performance, and corrosion behaviour of dual-stabilized ferritic stainless steel (AISI 444) joints processed by high-speed laser welding at 9 m/min, followed by post-weld heat treatments (PWHT) at 400 and 500 °C for 24 h. The high-speed laser welding produced fully ferritic fusion zones (FZ) with epitaxial columnar α-ferrite, consistent with the Columnar-to-Equiaxed Transition (CET) framework and Thermo-Calc predictions that favour primary ferrite and suppress equilibrium intermetallic during rapid solidification. Scanning electron microscopy along with energy dispersive spectroscopy (SEM-EDS) revealed that PWHT promotes fine secondary precipitation; at 500 °C, Cr23C6 carbides appear in the FZ, in agreement with thermodynamic calculations. Tensile and instrumented indentation hardness (HIT) tests showed enhanced strength and hardness after PWHT (weld efficiency 104–107 %; highest HIT for PW500), while retaining ∼40–50 % ductility with fracture in the base metal. This was attributed to fine-scale precipitation, αFe and αCr′.

Electrochemical testing in 3.5 wt.% NaCl, combined with post-exposure surface imaging, demonstrated that PWHT degrades corrosion resistance relative to the as-welded condition, attributed to carbide precipitation and associated chromium depletion/sensitization; PW500 displayed intergranular features in the heat-affected zone (HAZ), whereas the as-welded condition showed pitting primarily in the BM adjacent to the FZ. Overall, high-speed laser welding combined with low-temperature PWHT yields joints with improved mechanical performance but compromised corrosion resistance, clarifying the processing–microstructure–property trade-offs for AISI 444 weldments.