At higher magnification, the cracking appears to be thermal fatigue. At any point in time, a sudden temperature gradient over 200°F between the hot steel and blowing media induces significant stresses. Those thermal stresses would be closer to the yield strength of the steel. These repeated thermal cycles initiate thermal fatigue cracks both at the ID and OD surfaces, dependent upon stress levels. The appearance of the cracks suggests that these may have followed the inter-dendrites due to lack of ductility. Higher carbon in the steel may have diminished fatigue strength due to reduced ductility. Higher carbon content likely played a role in the failure to propagate the cracks at a faster rate.