Effect of Ferrite Grain Size on Cyclic Ratcheting Behavior of a Low-Carbon Niobium Bearing Microalloyed Steel

Abstract

Present investigation looks into the effect of ferrite grain size on ratcheting behavior of 0.14% carbon Nb-bearing microalloyed steel. Ferrite grain size has been varied by annealing treatment at 925, 1000, and 1075°C for one hour. Ratcheting behavior of the steels has been studied by asymmetrical cyclic stressing for 500 cycles at three different mean stresses, 5, 10, and 15% yield stress, but keeping the stress amplitude fixed at the yield stress level, corresponding to the respective ferrite grain size. It is observed that with increase in ferrite grain size final ratcheting strain decreases. Further, with increasing mean stress higher ratcheting strain develops in the specimens irrespective of grain size. It is also observed that while ratcheting strain with number of cycle follow different pattern depending upon ferrite grain size, the variation of maximum ratcheting strain with mean stress obeys a linear relationship for all the grain sizes. Effect of ratcheting on subsequent Low cycle fatigue (LCF) performance has been investigated by completely reversed strain controlled cyclic tests at 0.50% strain amplitude for the lowest ferrite grain size steel developed by annealing at 925°C. Comparing the post ratcheting LCF life with that of purely annealed material it is observed prior ratcheting affects subsequent LCF life. The post ratcheting LCF life follows exponential relationship with prior ratcheting strain.