In recent decades, laser beam welding of high strength low alloy steel has replaced some other assembly processes used in the automotive, machinery, and agricultural equipment sectors. In this research, the effect of heat input on the microstructure and mechanical properties of the butt joint of high strength low alloy steel S500MC, which had been strengthened by thermomechanically controlled processing, was investigated. The joining process was performed in an autogenous mode at two different heat inputs (140 and 240 J/mm) using the laser beam welding process with a fiber source. Microstructural observations with an optical microscope and scanning electron microscope indicate that the microstructure of the fusion zone consists of acicular ferrite and scattered martensite packets. An increase in heat input reduced the contribution of martensite packets in the fusion zone. The presence of carbides in the coarse-grained heat affected zone created a fine martensitic packet moreover the packet size has increased with the higher heat input and the dissolution of carbides. In addition to the microstructural changes, the grain size in the weld zone and heat affected zone becomes larger than those observed in the base metal consequently, the properties resulting from the controlled thermomechanical treatment have been lost in both zone. This autogenous welding process produced a decrease in hardness in the fusion zone but an increase in local strength in the heat affected zone compared to the unaffected base metal. The tensile test confirmed that the fracture occurred within the mechanically weaker fusion zone. But at a high input of 240 J/mm due to the decrease volume fraction of martensite and coarse grain size, the strength of the fusion zone reaches 581 MPa, which shows a decrease of about 10% compared to the 140 J/mm heat input.
