Influence of high-ammonia-potential gas nitriding on the microstructure, mechanical properties, corrosion resistance, and wear behavior of AISI 4340 and AISI 1040 medium-carbon steels

This study investigates the effects of high-nitrogen-potential gas nitriding on nitride-layer formation, microstructure, hardness, corrosion resistance, and wear performance of unalloyed AISI 1040 (0.39 wt.% C) and alloyed AISI 4340 (0.35 wt.% C) steels. Gas nitriding was performed at 520 °C for 8 h using an atmosphere of 80% NH 3 and 20% H 2 . The nitrided specimens were characterized by X-ray diffraction (XRD), scanning electron microscopy, Vickers microhardness, electrochemical corrosion, and wear tests. Gas nitriding produced nitride layers with thicknesses of 6 µm for AISI 1040 and 13 µm for AISI 4340. XRD analysis identified Fe 3 N and Fe 4 N phases in both steels, whereas CrN was detected only in AISI 4340. The maximum hardness increased to 431 HV0.3 for AISI 1040 and 544 HV0.3 for AISI 4340. The corrosion resistance of nitrided AISI 4340 increased by a factor of 4.59, while the wear volume loss was reduced by approximately threefold compared with the untreated steel. These improvements were attributed to the increased nitride-layer thickness and the formation of hard nitride phases. No pores were observed within the compound layer, indicating the formation of a compact and continuous compound layer that is expected to improve both corrosion and wear resistance. Although AISI 1040 and AISI 4340 contain 0.39 and 0.35 wt.% C, respectively, the presence of Cr, Ni, and Mo in AISI 4340 promoted thicker nitride layers and CrN formation, resulting in superior surface properties. These findings highlight the critical role of alloying elements in governing nitrogen diffusion, nitride phase formation, and surface performance under identical nitriding conditions.