Investigation of the effects of nitriding process on the dimensional and geometric tolerances of AISI 4340 steels

The AISI 4340 quenched and tempered steel is widely used in various industries, such as aerospace, automotive, and defense, due to its high strength, toughness, and wear resistance properties. Nitriding surface hardening processes plays a significant role in increasing the surface hardness of this material, enhancing its wear resistance, and improving its fatigue strength. However, dimensional and geometric changes that occur during the nitriding process can adversely affect systems operating with precise tolerances. In this study, the effects of the nitriding process applied to AISI 4340 steel on part dimensions, geometric tolerances, surface roughness, surface hardness, diffusion depth, and white layer thickness were examined. A total of 18 samples with three different geometric forms were used in the research. Six hours of nitriding was applied to three samples from each geometry, while 12 hours of nitriding was applied to the other three samples. Measurements conducted on the parts revealed dimensional changes ranging from 3.5 to 6 µm for the 6-hour nitriding process and from 15 to 19 µm for the 12-hour nitriding process. It was found that these changes caused most tolerance grades in the IT5 to IT9 range to fall out of tolerance or reduced tolerance grades to values below 10 µm, making manufacturing more challenging. While the nitriding process negatively affected symmetry tolerance, it did not affect parallelism or concentricity tolerances. As the nitriding duration increased, surface hardness decreased; however, the increase in nitriding duration also led to an increase in diffusion depth and white layer thickness.