Effect of Strain Ageing on Fatigue Limit of Low Carbon Steel

Abstract

Rimmed, silicon-killed and aluminum-killed steels (0.1%C) were fatigued in rotating bending. The solute content was modified by annealing, low-temperature quenching and slow cooling. The strain ageing potentials which were measured in terms of the change in static yield strength were in the range of 4.3∼17.5%, when specimens were prestrained. by 2.5∼6% and then held at 100°C for 40min. The fatigue limits were observed to be in the range of 14∼24kg/mm².
Although both the fatigue limit and the strain ageing potential increased by quenching, generally, no close relationship between them was apparent. A much better correlation was found between the fatigue limit and static yield strength. However, a knee in σ-N curve, at which stage-I cracks became non-propagative, shifted towards a small number of cycles with the increase in strain ageing potential. The fatigue limit of quenched specimens, in which stage-I cracks had been introduced by cyclic stressing above the limit, Was enhanced by statical ageing at 100°C for 40min. before fatigue. Contrary to this, with the slowly cooled steel, no strengthening effect due to ageing was observed.
For the fatigue limit of quenched specimens, the same strengthening effect as statical ageing is to be expected by strain ageing during stressing for a large number of cycles. The increase in the limit by quenching, statical ageing and compressive residual stress was estimated to be about 9, 1∼2 and 2.5∼5kg/mm², respectively.
It is concluded that strain ageing during fatigue can make a contribution to the arrest of propagating stage-I cracks, but the effective contribution to the increase in fatigue limit due to quenching is made by substitute hardening and compressive residual stress rather than strain ageing during fatigue.