Processes and Properties of Low-Temperature Pack Aluminized Coatings on Quenched-and-Tempered AISI 5140 Steel Assisted by Cold Pre-Torsion

Low-temperature pack aluminizing offers a promising approach to improving the surface properties of structural steels while limiting thermal damage. However, achieving sufficient coating growth at reduced temperatures remains challenging. Here, room-temperature pre-torsion was applied to quenched-and-tempered AISI 5140 (40Cr) steel with a tempered sorbite matrix prior to aluminizing at 700 °C for 3 h. Electron backscatter diffraction (EBSD) revealed a radial gradient in crystallographic misorientation, with the low-angle grain boundary (LAGB) fraction exceeding 62% near the surface. Pre-torsion increased the effective coating thickness from 236.4 to 298.5 μm (26.3%). Both coatings consisted predominantly of orthorhombic Fe2Al5, whereas monoclinic FeAl3 was also present throughout the coating in the pre-torsioned specimen, consistent with enhanced Al uptake and diffusion associated with the high deformation energy. Although pre-torsion reduced the maximum coating microhardness from 1010 to 920 HV0.2, potentially owing to localized Kirkendall microporosity and stress relaxation, it increased the hardened depth (>800 HV0.2) from 210–240 to 270–330 μm. In 3.5 wt.% NaCl solution, both coatings reduced the corrosion current density from 7.21 μA cm−2 for the bare substrate to 1.08 μA cm−2 for S-Al and 1.12 μA cm−2 for T-Al. These findings demonstrate that pre-torsion offers a promising strategy for modifying coating growth, increasing hardened depth, and maintaining corrosion protection during low-temperature pack aluminizing.