Efforts from both industry and science must unite to transition the steel sector into a sustainable, low-carbon, circular economy in the near future. Decarbonizing iron and steel production is the primary focus, but other downstream processes such as multi-step heat treatments for quenched and tempered steels also produce substantial emissions. One emerging new steel class are the air-hardening ductile forging steels which are alloyed with 4% manganese and are therefore consider medium manganese steels. Through the omission of the energy intensive quench and tempering treatment significant amounts of CO2 emissions can be saved, leading to great interest from players along the process chain and from different industries. As the material was just recently standardized, not much knowledge exists about downstream the influence of downstream processes like plasma nitriding. In previous investigations, the AHD steels achieved high surface hardness of over 1000 HV. The generated nitriding layers showed an unusual appearance and phase composition. The goals of this work are to gain a better understanding of the causes of the high surface hardness and the unusual appearance and phase composition of the nitrided layers. Samples were nitrided together with reference materials with increasing temperatures from 400 to 600 °C for 12 hours each in an industrial plasma nitriding facility. Then its mechanical and chemical properties were characterized. The chemical analysis using electron probe microanalysis (EPMA) and X-ray diffraction (XRD) provided insides about the causes of the unusual appearance and phase composition of the nitrided layers. It is shown that the formation of aluminum nitrides is responsible for the high surface hardness and that the extensive decarburization of the nitrided layer is also closely related to the formation of fine aluminum nitrides. The unusual appearance of the nitrided layers and phenomenal phase formation is interpreted as a result of the extensive decarburization. It was found that under certain circumstances, diffusible carbon is involved in phase formation and causes the unusual appearance.