The Effect Of Tempering Microstructure On The Fatigue Fracture Of Cold-Rolled Low Carbon Steel

This paper focuses on the effect of microstructure features after tempering in low carbon steel on the propagation rate of fatigue crack.Four different types of microstructure are obtained by tempering at different temperatures in low carbon steel after cold rolling with deformation rate (DR) of 60% and 80%. Tension-tension tests are conducted on standard fatigue samples. Hardness is measured and microstructure features after different tempering are observed by optical microscopy (OM) and transmission electron microscopy (TEM). a-N curve and da/dN-ΔK curve are determined from the date obtained by the fatigue test. Morphology of fracture surface is observed by scanning electron microscopy (SEM). Comprehensive analysis is obtained from experiment results.Results show the hardness of low carbon steel after quenching and cold rolling decreases as tempering temperature increasing, Recrystallization happens at 500℃and ferrite become the dominant microstructure at 575℃in sample with 60% DR, as for the case of sample with 80% DR, the temperature is 475℃and 560℃. When the tempering temperature reaches 600℃, equiaxed ferrite grain and chain-like carbides are typical features of both sample with 60% and 80% DR.The a-N curve and da/dN-ΔK curve show that crack propagation in sample with 60% DR after tempering at 575℃have the lowest rate, and the highest crack propagation rate for sample after 80% DR is researched when the tempering temperature is 515℃. Crack propagation rate of sample with 60% DR after tempering at 515℃and that of sample with 80% DR after tempering at 560℃are comparable. In a word, different crack propagation rates are induced by different microstructure features in sample after difference of DR and tempering temperature.SEM observation shows that steady propagation area with little amount of secondary crack on plain surface in sample with 60% DR after tempering at 575 ℃is of typical features of fatigue fracture; in fast spreading area, mixture features of different type of fracture surface, the appearance of plastic envelope zone, the disappearance of secondary crack and the formation of shear ligament area between fatigue planes are observed. In sample with 80% DR after tempering at 560℃, more secondary cracks are observed on plain steady propagation area, fine narrow ligaments distributes between fatigue planes in fast spreading area; in fast spearing area, fluctuation increased and dimple is formed without secondary crack. In sample with 80% DR after tempering at 500℃, step pattern without secondary crack is observed in steady propagation area; no plastic envelope zone is detected in fast spreading area.The microstructure of specimen with 60% DR after tempering at 575℃is consisted of recrystallized ferrite, carbides in grain interior and unrecrystallized deformed lath. The mechanisms of low crack propagation rate are concluded as follows: delamination crack is initiated at weak interface in lath during the propagation of fatigue crack; fine ferrite grain in crack tip forms plastic envelope area which leads to the close of the crack; the carbides induce the deviation of propagating orientation or bridge the front section of crack. The comprehensive effect of these three mechanisms decreases the crack propagation rate greatly.