Abstract
In order to study the effects of surface pickling in acid solution and test atmosphere on the fatigue strength of an ultra-low carbon steel (Interstitial free steel), the reversed bending fatigue tests were carried out under a bending stress range of 140-340MPa in air and Ar atmospheres.
The results obtained were as follows.
(1) The fatigue life for the test in air strongly depended on the time of pickling in the acid solution. The life increased as the pickling time increased, however it did not depend on the pickling time for a time span of less than 10minute. The fatigue life for a10minute pickling time increased with decrease in bending stress and a fatigue threshold existed at around120MPa.
(2) The fatigue fracture surface showed that thecrack propagation process is dominated bx the formation of ductile fatigue striations and that the crack growth mechanism is a mixed mode of transgranular and intergranular.
(3) However, the crack growth process was mostly intergranular in surface region and gradually changed to transgranular in the interior region.
(4) The tendency of intergranular in the surface region was more distinguished in the longer pickling time and/or in air atmosphere.
(5) The micromechanism of intergranular was believed to be related to a combination of hydrogen atoms dissolved during the pickling and, oxygen atoms penetrating from the test atmosphere at the crack front in the grain boundary region.
