Sulfur content plays a major role in improving the fatigue strength of ferrite-pearlite steels. Finely dispersed MnS plays an active role in providing transformation initiation sites for ferrite within austenite grains and interrupts the formation of large pearlite.
In the fatigue process of ferrite-pearlite steels, cracks initiate in weak ferrite crystal, where slips first form in ferrite and then accumulate to form microvoids and fatigue cracks. Therefore, ferrite strengthening is very important in improving the fatigue strength of steels with ferrite-pearlite microstructures.
To improve the fatigue strength of the F+P matrix, the ferrite crystal strength should be increased by grain refinement, and precipitation hardening is essential. Also, the strength ratio of ferrite to pearlite should be reduced. Refinement of ferrite crystals is possible by small additions of sulfur, which result in an increase of MnS distribution. The influence of sulfur contents is shown in the figure This metallurgical technology has made possible the use of ferrite-pearlite DHT steel for suspension components that need high toughness, such as I-beams for trucks.
For more information, click on the link below (subscription required). Then scroll to Figure 17.
H. Tahira, K. Funatani, Forge and Direct Heat Treatment Processes and Technologies, Heat Treating of Irons and Steels, Vol 4D, ASM Handbook, Edited By Jon L. Dossett, George E. Totten, ASM International, 2014, p 241–252, https://doi.org/10.31399/asm.hb.v04d.a0005994
