Quantitative analysis of fatigue behavior, fatigue damage and fatigue fracture surfaces of low carbon bainitic steel (SAE 15B13)

Quenched and tempered medium carbon steels (SAE 1038) are widely used in the automotive industry due to good combination of high strength and toughness. However, the quenching and tempering treatments account for a significant fraction of manufacturing cost. Therefore, there is a need to develop alternative steels that eliminate the need for heat treatments at the component level. One attractive option is to consider the replacement (Q+T) steel by low carbon bainitic steel (SAE (15B13) that can be cold worked in high hardness condition, and therefore do not require hardening and tempering treatments at the component level. Since automotive parts undergo cyclic loading under dimensional constraint, a comparison of strain controlled low cycle fatigue behavior of (Q+T) steel and the alternate bainitic steel is of interest.; It is observed that 40% cold worked and low temperature aging process (T = 200{dollar}spcirc{dollar}C) improves the cyclic mechanical properties of bainitic steel. However, both the bainitic steel and quenched and tempered steel undergo cyclic softening. The softening is due to the re-arrangement of the dislocations in the formation of cell sub-structure. Cold working of the bainitic steel increases the fatigue strength on the expense of the fatigue ductility. At the same hardness level, 40% cold worked and aged bainitic steel has higher fatigue life than quenched and tempered steel, at low strain amplitudes. The differences in the strain-life behavior are attributed to the differences in the fatigue mechanisms arising from different microstructure.; In bainitic steel the persistent slip bands (PSBs) form continuously up to the specimen failure at approximately constant rate, whereas the rate of PSBs formation decreases significantly with the number of cycles in the quenched and tempered steel. Similarly, the micro-crack initiation rate is lower in the quenched and tempered steel. On the other hand, the crack propagation resistance is more in bainitic steel. The crack propagation is predominantly intergranular in the bainitic steel, whereas it is transgranular in the quenched and tempered steel. It is concluded that for fasteners and other automotive engine part applications, 40% cold worked and aged bainitic steel can replace the quenched and tempered steel.