The effects of dislocation locking and strain ageing on the fatigue limit of low-carbon steel

Abstract

The fatigue behaviour of a low-carbon steel has been investigated with the object of clarifying the roles of initial dislocation locking and of strain ageing in the development of the fatigue limit. Comparisons of performances of two conditions of grain size (90 and 2500 grains/mm 2) were made in push-pull at 200 c/s. S-N curves were determined at room temperature in annealed, prestrained, quenched and quenchaged conditions; and at − 60°C in annealed and prestrained conditions. Observations were made of changes in damping, metallographic features and in tensile yield point behaviour during fatigue. In annealed, fine grained specimens the spread of plasticity appeared to be an important factor limiting the spread of fatigue damage. Striations were developed in a small proportion (<4 per cent) of surface grains by cyclic stresses just below the fatigue limit but, in general, they did not spread beyond grain boundaries and prologned cycling did not eliminate the sharp yield point. Stresses slightly above the limit were just sufficient to cause these plastic nuclei to grow. In contrast, with coarse grained specimens, stresses close to the limit caused rather widespread plastic deformation and eliminated the sharp yield point in the early stages of the test. The fatigue limit/U.T.S. ratio of annealed, fine grained specimens was superior to that of coarse grained specimens but prestraining removed this difference. In tests at − 60°C annealed, fine grained specimens appeared to retain a fatigue limit but none was observed with coarse grained specimens. It is concluded that initial dislocation locking makes an important contribution to the high fatigue limit of fine grained steels, but that it is unlikely to be important in a coarse grained steel of low yield strength/U.T.S. ratio. However, coarse grained and prestrained specimens did show a fatigue limit at room temperature. The relocking of dislocations by strain ageing may be important in these cases.