Fatigue tests were conducted for a series of plasma-nitrided ultrafine ferrite-cementite steels under rotating bending. The plasma-nitriding was conducted at 500°C for 16 h. 4 types of the base ultrafine ferrite-cementite steels were prepared, i.e., 15C, 45C, 15C-P and 45C-P. 15C and 45C were contained 0.15 and 0.45 mass% of carbon, respectively. 15C-P and 45C-P were 0.1 mass% phosphorus-added versions of them. All of the nitrided specimens had a hardened area of 1 mm in depth and many of the nitrided specimens showed internal fracture in and beneath the hardened area in the rotating bending fatigue tests. In case of 45C and 45C-P of the nitrided specimens, the ferrite grains successfully remained ultrafine and the fatigue strengths were largely improved by the plasma-nitriding. On the other hand, the ferrite grains beneath the hardened area were enlarged in case of 15C and 15C-P during the plasma-nitriding and increase of fatigue strengths were small. Although the origins of the internal fractures were inclusions located in and beneath the hardened area, fatigue strength of the nitrided specimens appeared to be dependent on hardness at around the origin. In nitriding the ultrafine ferrite-cementite steels, therefore, preventing the ferrite grain growth beneath the hardened area was required to improve the fatigue strength. For this purpose, precipitation of particles, such as cementite, was found to be useful to pin the ferrite grain boundaries.
