Abstract Lattice expansion in austenitic stainless steel after nitrogen insertion in the temperature range between 320 and 380 °C is a well established phenomena whereas conflicting data on the existence of expanded phases in martensitic stainless steels are published. However, inferring the structure from X-ray diffraction data alone can be misleading, as the distinction between a martensite structure with a very low nitrogen content obtained after rapid quenching and a ferrite/cementite mixture after additional annealing is not straightforward. In this presentation the results of nitrogen plasma immersion ion implantation (PIII) into differently preheated steels (martensite and ferrite structure of 1.4034 (X46Cr13), 1.4057 (X17CrNi16.2), 1.4021 (X20Cr13) and 1.4104 (X14CrMoS17)) are reported. X-ray diffraction (XRD) data and N to Fe ratios obtained from time-of-flight secondary ion mass spectroscopy (SIMS) are presented together with hardness values. The influence of the treatment parameters was investigated. A wear reduction of a factor of 10–100 was found for all implanted samples, whereas initially brittle martensites showing largest decrease.