One of the steels commonly used in the industry in mechanical devices, tools, and structural steels is SAE 1045. A possible replacement for this alloy is SAE 15B30 with similar mechanical properties and has been used in the automotive and construction industry. To select an alloy, care is needed, for example, weldability. Making a comparative study of these alloys, can boron steel replace SAE 1045? Knowing that boron steels have high hardenability but may have improved weldability due to the low carbon equivalent. Therefore, it is extremely important to know the factors that can interfere with the quality of the final product after the welding processes, as the mechanical properties and microstructures in the heat-affected zone (HAZ) may change. Knowing that in the automotive and civil sector we have several welding processes, it is extremely important to know the results of a boron steel after a welding process; thus, this work intends to compare the weldability of SAE 15B30 and SAE 1045 welded by shielded metal arc welding (SMAW) and also to analyze the microstructural changes after welding by optical microscopy, scanning electron microscopy, and its relationship with hardness using the test of Vickers microhardness. To perform the welds, both in SAE 1045 steel and in SAE 15B30 steel, the butt joint with a V-shaped chamfer was chosen. The results showed that the boron steel, with high hardenability, obtained a microstructure with bainite in the coarse-grained heat-affected zone (CGHAZ), unlike SAE 1045, which presented pearlitic microstructure. In addition, the hardness of boron steel increased by approximately 90% over the base metal (BM), and the hardness of SAE 1045 increased close to 50% over the BM.