Effect of Nitrogen Ion Implantation on Fatigue Crack Initiation and Early Crack Growth Behavior of High Tensile Strength Steel

摘要

The aim of this work is to investigate the effects of nitrogen ion implantation on fatigue crack initiation, early crack growth behavior and fatigue life for high tensile strength steel (HT60). Three- point bending fatigue tests were carried out using an electro-hydraulic fatigue testing machine. The fatigue specimens used were edge notched specimen having a notch radius of 0.25 and 2 mm and smooth specimen. The fatigue crack initiation and the early crack growth were observed by using an optical and scanning electron microscopes adopting a plastic replica method. The main results obtained are summarized as follows. (l) The micro hardness of nitrogen ion implanted specimen was about 1.8 times greater than of non-implanted specimen. (2) The fatigue life was increased by nitrogen ion implantation when the maximum stress at the notch root was less than the static fracture stress of nitrogen ion implanted layer. (3) Comparing ion implanted specimen with non-implanted specimen, the fatigue crack initiation was delayed by nitrogen ion implantation. (4) The early crack growth behavior was sensitive to the microstructure so that a zig-zag pattern was observed at both sides of notched specimen on nitrogen ion implanted specimen.