抄録
Rotating bending fatigue tests were conducted for a series of plasma-nitrided ultrafine ferrite-cementite steels 5 types of the ultrafine ferrite–cementite steels, i.e., 15C, 45C, 15C-P, 45C-P and low-Mn, were prepared with a double-melting method to improve their cleanliness. 15C and 45C contained 0.15 and 0.45 mass% of carbon, respectively. 15C-P and 45C-P were 0.1 mass% phosphorus-added versions of them. Low-Mn was a Mn-decreased version of 45C. All of the nitrided specimens had a hardened layer of 1 mm in depth, while the amount of hardening was small in low-Mn. In case of 15C and 15C-P, the hardness beneath the hardened layer largely fell due to grain growth during the nitriding, in contrast to 45C and 45C-P in which the grain growth was successfully suppressed. In the fatigue tests, due to the cleanliness improvement, inclusions never caused fish-eye fracture even in the nitrided specimens. Although the local stress amplitudes beneath the hardened layer exceeded the fatigue limit expected from the local hardness, the nitrided specimens showed surface fracture only. The fatigue strength improvements due to the nitriding were larger in 45C and 45C-P than 15C-P, and those improvements in 15C and low-Mn were negligible. These results meant that C-increase and P-addition improved the surface fracture properties. The results of low-Mn demonstrated that the hardened layers of the nitrided specimens were strengthened by Mn nitrides.
