Kovacı, H.; Yetim, A.F.; Baran, Ö.; Çelik, A.

The effects of plasma nitriding on fatigue crack growth (FCG) properties of AISI 4140 low-alloy steel under variable amplitude loading (VAL) conditions were investigated. The compact tension specimens made of AISI 4140 were plasma nitrided at 400 °C, 500 °C and 600 °C for 1 and 4 h. CT specimens were subjected to fatigue under different VAL conditions including overload (OL), underload (UL), overload+underload (OL+UL) and underload+overload (UL+OL). The OL investigations revealed that the FCG life of untreated and plasma nitrided samples increased with respect to constant amplitude loading (CAL). Plasma nitrided samples exhibited less FCG life increase than untreated sample because of decreased crack tip plastic zone size and crack retardation period. In the case of UL, FCG lives of all samples decreased in comparison to CAL. The effect of tensile residual stresses formed after UL was suppressed by the compressive residual stresses produced by plasma nitriding. Also, it was seen that the increasing compound layer thickness and the presence of Braunite phase increased the brittleness of the material regardless of the loading type. The overall results showed that the loding type and mechanical/structural characteristics of nitride layers significantly affected the FCG behavior of the material.

Kovacı, H.; Yetim, A.F.; Baran, O.; Çelik, A.

The compact tension (CT) specimens manufactured from AISI 4140 steel were plasma nitrided at 400 °C, 500 °C and 600 °C for 1 and 4 h in a gas mixture of 50% N2 and 50% H2. CT specimens were subjected to fatigue crack growth under constant amplitude loading in order to analyze the effect of plasma nitriding on fatigue crack growth (FCG). The crack sizes of specimens were correctly detected by a digital image monitoring system. Also, the structural and mechanical properties of specimens were characterized by XRD, SEM, optical microscopy and 3D profilometer and microhardness tester. The structural and mechanical examinations showed that the surface hardness, layer thicknesses and surface residual stresses continuously increased with increasing plasma nitriding time and temperature. It was found that plasma nitriding improved the FCG life of the material due to the formation of compressive residual stresses and nitride layers in surface and near-surface regions. Additionally, the highest FCG increase rate was obtained from 500 °C to 4 h plasma nitrided specimen with respect to untreated specimen. The formation of compound layer and braunite layer constrained the continuous increase of FCG life because of increasing embrittlement and cracking tendency of nitride layers.