In this study, the perforation behaviors of commercial Armox 500T and Armox 600T were investigated under various projectile velocities. The failure mechanisms were analyzed from the macroscopic to the nanoscale using X-ray computed tomography (X-ray CT), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Under complete perforation conditions, Armox 600T exhibited greater initial resistance owing to its higher hardness but suffered severe rear-side damage (plugging and spalling). By contrast, Armox 500T exhibited extensive ductile hole expansion, maintaining a residual volume of approximately 99%. Microstructural characterization demonstrated that adiabatic shear bands (ASBs) were prominent near the perforation surfaces and along the radial cracks in both steels. The occurrence of dynamic recrystallization and even localized melting inside the ASBs induced microcracks along the ASB/matrix interfaces and prior austenite grain (PAG) boundaries, eventually coalescing into the observed macroscopic radial cracks. These findings suggest that the design of high-performance armor steels should be based on an optimized balance between hardness and dynamic damage tolerance.

Die-casting molds undergo wear, erosion, and repeated thermal loading, which progressively degrade their surfaces and reduce their service life. Compared with conventional gas nitriding, sulfonitriding can improve wear resistance; however, its effects on other reliability-related properties, particularly erosion and thermal-shock fatigue, and the relationships between these properties and the compound-layer characteristics remain insufficiently understood. In this study, duplex surface treatments combining sulfonitriding with three microshot peening conditions, resulting in surface-roughness values of Rz 14, 18, and 22 μm, were applied to SKD61-based (FDAC) die-casting mold steel. The duplex-treated layers were evaluated based on their microstructure, hardness, wear resistance, erosion resistance, and thermal-shock-fatigue resistance, followed by a nondestructive assessment using Barkhausen noise. Sulfonitriding produced FeS-containing compound layers and reduced the wear of the counterbody and the tribo-pair compared with the results for gas nitriding, which is consistent with the solid-lubrication effect of FeS. The erosion resistance increased with the compound-layer thickness. The thermal-shock-fatigue tests indicated a preliminary trade-off between crack initiation and crack propagation as a function of the compound-layer thickness. These results demonstrate that sulfonitriding modifies the surface performance of SKD61 and that the compound-layer thickness is a key parameter governing mold reliability. Moreover, Barkhausen-noise analysis offers a nondestructive assessment of the compound-layer thickness after treatment.

Laser beam welding (LBW) offers low thermal distortion for thin austenitic stainless steel sheets but is sensitive to butt-joint fit-up and edge quality. This study compared machined and sheared joint edges under selected autogenous and ER308L filler-wire LBW conditions. Bead geometry, defects, microstructure, microhardness, tensile properties, and fracture behavior were characterized using conventional microscopy, EBSD-KAM, and three-dimensional digital image correlation (3D-DIC). The machined-edge autogenous condition (LBW-A-M) produced a stable bead with 0.04 mm top underfill and only a small number of internal discontinuities, while its tensile properties remained close to those of the base metal and fracture occurred in the base metal. In contrast, the sheared-edge autogenous condition (LBW-A-S) exhibited 0.24 mm top underfill, 0.18 mm misalignment, multiple pores, and localized strain near the weld, with tensile strength and elongation of 682.31 MPa and 44.04%, respectively. Under the selected filler-wire conditions, no measurable top underfill was observed, although pores remained. Because the autogenous and filler-wire modes used different process parameters and heat inputs, cross-mode differences are condition-specific rather than isolated filler-wire effects. Overall, joint-edge preparation and fit-up control remained important for thin-sheet LBW.