Effect of Nitrogen Content on the High Cycle Fatigue Properties of Titanium Microalloyed Steel

The high cycle fatigue properties of three industrial 0.12% Ti microalloyed steels with different nitrogen contents (56, 40, and 30 ppm in molten steel of tundish) are investigated. The results show that <20% of the fatigue crack initiation sites are oxide inclusions of size in 16.8 mu m approximate to 55.9 mu m, while the rest 80% are surface defects. No TiN inclusions cause fatigue failure and the fatigue limit strength slightly decreases with increasing N contents as the yield strength decreases with a coarser ferrite grain. Inclusions characterization in the section near the fracture shows that the average sizes of TiN inclusions in H56, M40, and L30 steels are 3.50, 3.22, and 2.89 mu m, and the maximum sizes are 6.92, 6.67, and 6.23 mu m, respectively. Calculating for a cooling rate of 0.2 K s(-1) using ChemAppPy precipitation model, the size of TiN inclusions will increase from 6.1 to 7.1 mu m, when increasing N content from 30 to 60 ppm. The relationships between nitrogen content, TiN inclusions, and fatigue failure quantified by experimental test and modeling show that the nitrogen content in steel can be relaxed up to 60 ppm when considering economical denitrogenization and fatigue safety.