To study the manufacturability and mechanical properties of a low alloy heat treatable medium carbon steel by additive manufacturing, 42CrMo4 (AISI4140) specimens were manufactured by laser powder bed fusion. Influences of processing parameters on relative density and crack density were investigated. Moreover, Charpy impact tests, microhardness, and tensile tests for specimens with and without preheating were also studied. Results showed that the primary defects in the microstructure of additively manufactured samples were the lack of fusion pores and microcracks. The variation of processing parameters influences the porosity and microcracks prominently. The impact toughness of specimens with preheating is around three times higher than specimens without preheating. However, post heat treatment for as-built specimens, such as tempering at different temperature ranges, did not improve the impact toughness further. The detailed fracture mechanism for the massive difference in the impact toughness was investigated
