Corrosion fatigue behaviour of ion nitrided AISI 4140 steel

Machine components suffer from corrosion degradation of fatigue characteris tics and improvement can be attained by the application of a nitriding trea tment, particularly to low alloy steels. In the present study, the effect o f ion nitriding on corrosion fatigue performance of AISI 4140 steel has bee n investigated by conducting a series of rotary bending corrosion fatigue t ests at 95 Hz, in 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1, 3, 8 and 16 h prior to the tests. It was observed that distinct fatigue limit behaviour of ion ni trided steel in air completely disappeared in corrosive environment besides severe degradation in fatigue characteristics. An improvement reaching to 60% in corrosion fatigue strength can be attained by successive ion nitridi ng practice based on a fatigue life of 107 cycles. An attempt was made to e stablish an empirical relationship between corrosion fatigue strength and r elative case depth, which considers the size of the ion nitrided specimen. It was also determined that a power relationship holds between corrosion fa tigue strength and fatigue life of ion nitrided steel. The presence of whit e layer has resulted in additional improvement in corrosion fatigue resista nce, and it was observed that corrosion fatigue cracks were initiated domin antly under the white layer by pit formation mechanism. (C) 2000 Elsevier S cience S.A. All rights reserved.