Effects of B Doping and Quenching Rate on Charpy Impact Properties in AISI 4130 Steels

It is well known that the element boron (B) improves the hardenability of low- to medium-carbon steels, reducing the need for expensive alloying elements. This study focuses on investigating the effect of quenching rate on B segregation at prior austenite grain boundaries (PAGBs) and Charpy impact energy in medium-carbon AISI 4130 steel with a tempered-martensitic microstructure. NanoSIMS data after quenching from austenite at different rates, including water quenching (WQ), He-gas quenching (HeQ), and Ar-gas program quenching (ArQ), were used to analyze B-doped 4130 steel specimens. The results revealed that the concentration of B along the PAGBs, and the extent of B segregation and boro-carbide precipitation increased with decreasing quenching rate. This phenomenon could be explained by the critical time required for non-equilibrium segregation of B during quenching. Charpy impact test data showed that the B-doped 41B30 specimens and slowly quenched specimens exhibited higher absorbed energy than the non-B-doped 4130 specimens and rapidly quenched specimens, respectively, particularly in the impact transition temperature region. This improvement in impact energy can be attributed to several factors. First, B doping enhances the resistance to brittle quasi-cleavage fracture by strengthening the grain boundaries through cohesive bonding. Second, B facilitates slip transfer across the grain boundaries, promoting plastic deformation and enhancing toughness. Finally, B doping may suppress the formation of transition carbides during tempering, further contributing to improved impact properties.