Carbon diffusion behavior at 850℃ in 45# steel under 1 keV high-flux Ar+ ion irradiation was investigated. The steel samples had been pre-decarburized at 850℃ in H2 +H2O for 2 h to form a columnar ferrite surface layer of 200 μm. Cross-section structure of the samples indicates that

at the dual phase zone

carbon diffuses to the surface

especially in grain boundaries

leading to the movement of phase boundary between the ferrite and austenite phases towards the surface. Under the ion irradiation

however

the carbon diffusion is restrained

as can be seen by comparing the irradiated samples to the non-irradiated. This effect cannot be explained by any established mechanism of radiation-influenced diffusion. An explanation based on nonlinear effect is proposed.