CYCLIC STRENGTH OF NITRIDED CONSTRUCTIONAL STEEL.

The influence of gas nitriding on the cyclic strength of samples of normalized steel 45 is investigated in the present work. Using gas nitriding with program control of the composition of the saturable atmosphere allows the cyclic strength of smooth samples of constructional steel 45 to be increased by 114%. The increase in cyclic strength of steel 45 after gas nitriding is associated both with sharp increase in hardness of the surface layer (doubling of the microhardness) and with the creation of compressive residual stress in this layer. Using program control of the composition of the saturable atmosphere, the parameters of the zones of the nitrided layer may be changed in a goal-directed way, and thereby the level of cyclic strength of the material may be controlled. Fractographic investigations show that, after nitriding, fatigue failure of steel-45 samples develops under the strengthened surface layer in the form of a so-called fisheye. Fatigue cracks are nucleated from large nonmetallic inclusions.