Ultrasonic fatigue tests were performed up to 10 9 cycles. Two different plasma-nitriding treatments were performed on a quenched and tempered 42CrMo4 cast steel. Thus, crack initiation of cyclically loaded materials occurs predominantly at internal nonmetallic inclusions (NMIs). Surface treatments such as shot peening, deep rolling, or nitriding are known to be very effective for the protection of a surface against fatigue crack initiation, due to surface hardening and residual compressive stresses introduced below the surface.
