Press-hardenable steels (PHS), such as 22MnB5 steel, present high tensile strength after hot stamping, approximately 1500 MPa. PHS sheets are usually coated with Al-Si to prevent surface decarburization and oxidation during the hot stamping process. However, the presence of an aluminized coating can affect the welding process and the final mechanical properties of the welded joints. Therefore, it is of great significance to understand the effects of heat treatment on the coating and its correlation with weldability. In the automotive industry, a usual method is projection welding which is based on the joining between a metallic stud, such as bolts and nuts, and a metallic sheet by resistance welding. This study is focused on the characterization of the Al-Si coating layer after heat treatment with different austenitization times and how the evolution of this layer impacted the joining performance between an M10 nut and the coated steel. Results showed that the coating morphology changed after applying different heating times, resulting in stratified layers due to the diffusion of elements, such as Al, Si, and Fe, between the steel and the coating. With longer times, in addition to thicker coating layers, the increase in Fe-rich phases and the decrease of Al-rich phases were observed. Even using the same projection welding parameters for all samples, the one with the longest austenitization time showed a decrease in the joint mechanical performance. Stereo and optical microscopy, visual inspection, and fractography were used to explain the weld quality deterioration and its correlation with the stratified layer.