A review of the latest developments regarding the heat treatment effects on hardness, impact toughness, and microstructure in AISI 1040, 4140, and 4340 medium carbon structural steels

This review compiles and compares recent research on the influence of various heat treatments on the hardness, impact toughness, and microstructural evolution of AISI 1040, 4140, and 4340 medium carbon structural steels. These steels vary in alloy composition - AISI 1040 is a plain carbon steel, AISI 4140 is Cr-Mo steel, and AISI 4340 is Ni-Cr-Mo alloy steel. For consistent comparison, hardness and impact toughness values reported in the literature were standardized to Rockwell C (HRC) and Charpy impact energy (J). It was observed that due to formation of a ferrite – pearlite structure, annealing produced the softest steels (~ 20 HRC, > 120 J). Normalizing refined pearlite grains and balanced the properties (26–37 HRC, ~ 140 J). Quenching, because of martensite formation, yielded highest hardness (59–70 HRC) but lowest toughness (~ 35 J), while tempering improved toughness (80–150 J) while maintaining a moderate hardness. Heat treatments such as austempering, up-quench austempering (UQA), ultra-fast heat treatment (UFH), laser heat treatment (LHT), deep cryogenic treatment (DCT) and shallow cryogenic treatment (SCT), improved performance by altering mechanical properties through bainitic transformation, carbide precipitation, and surface hardening. Overall, the findings highlight that the hardness-to-toughness relationship varies significantly with the selected heat treatment, underscoring the critical role of tailored heat treatment processes in optimizing the mechanical performance of these steels.