Abstract

Fatigue crack initiation was studied on a typed API 51. X-65 steel in the conditions as received with and plasma ion nitrided. using optical microscopy with Ñomarski interferometry. The nitriding process gave origin to a 550-micrometer thick surface-modified layer, which contains a 5.1 micrometer thick white Layer composed of nitrides and a díffusion zone. The as-received steel presented surface deformation bands since the beginning of the fatigue test. Upon hydrogen charging various surface micro-cracks were noted to nucleated and then give rise to the main fatigue crack. Fatigue crack initiation on the nitrided steel occurred sub-superficially. The fatigue tests performed upon hydrogen charging did not induce any surface modifications as compared to as-received steel. The fatigue limits due to hydrogen decreased from 350 lo 300 MPa for the as received steel and from 350 to 300 MPa for the plasma ion nitrided steel.