Aging effects on the fatigue performance of deep rolled bar steels

The effects of solute nitrogen on the strain aging response after deep rolling and on the effects of deep rolling at a temperature selected to maximize the difference in the dynamic strain aging response on the fatigue behavior of two medium carbon bar steels was evaluated in reverse bending at room temperature. The nominally 0.38 wt pct carbon hot rolled bar steels included a plain carbon steel alloy specifically designed to have an elevated free nitrogen content and a steel alloyed with 0.09 wt pct vanadium, 0.010 wt pct titanium, and 0.020 wt pct aluminum in order to reduce solute nitrogen through precipitation. The static strain aging response of the two alloys was evaluated at room temperature on samples prestrained to 2.5% and aged at temperatures between 100 °C and 260 °C. The microalloyed steel exhibited a peak strain aging index at 220 °C, while the plain carbon steel exhibited a constant strain aging index at all temperatures tested nearly the same as the peak value of the microalloyed steel. Stress reversal tests at temperatures between 100 °C and 300 °C showed that the microalloyed steel exhibited limited dynamic strain aging (DSA) over the temperature range tested while at 150 °C the plain carbon steel exhibited a maximum in DSA. Samples were deep rolled at room temperature and at 150 °C, and aged after room temperature deep rolling at 100 °C, a temperature selected to maximize the difference in strain aging index between the two alloys. The effect of deep rolling at room temperature, is an increase in the endurance limit of 55% over the as received condition. For the microalloyed steel, the room temperature deep rolled endurance limit was equivalent to the samples aged at 100 °C and those rolled at 150 °C. In contrast the plain carbon steel with an elevated free nitrogen content exhibited 9.2% increase in endurance limit when aged and an 18.2% increase after deep rolling at 150 °C. The enhanced response of the high nitrogen plain carbon steel was attributed to the development of more stable dislocation structures and correspondingly residual stress fields due to the effects of solute pinning on dislocations.