Effects of strengthening mechanisms on fatigue properties of ultrafine ferrite-cementite steels

Fatigue tests were conducted using a series of ultrafine ferrite-Cementite steels with the ferrite grain size below 1 mu m in order to investigate the effects of precipitation, solid solution and dislocation strengthening on the fatigue properties. Some steels contained 0.15% of carbon and others 0.45% to investigate the effects of precipitation strengthening since the carbon addition increased the number of cementite particle in case of the ultrafine ferrite-cementite steels. 0.1% of phosphorus was added to some steels to discuss the effects of solid solution strengthening. The effects of dislocation strengthening were investigated by comparing the as-rolled steels with the annealed versions. As the result, tensile strength of the prepared ultrafine ferrite-cementite steels ranged from 721 to 1048 MPa. All of the prepared ultrafine ferrite-cementite steels showed high fatigue strength and the fatigue limit ratios (fatigue limit/tensile strength) exceeded 0.5. The fatigue strength of the ultrafine ferrite-cementite steels was higher than that of ferrite-pearlite steels, and as high as that of tempered martensite steels. In conclusion, all of the above strengthening mechanisms effectively improved the fatigue strength of the ultrafine ferrite-cementite steels. The reasons why the ultrafine ferrite-cementite steels stably showed high fatigue strength would be because the microstructures were not only ultrafine but also uniform.