Cyclic deformation behavior and low cycle fatigue life of normalized medium carbon steel with hydrogen charging

Abstract

The effects of hydrogen on low cycle fatigue properties have been fundamentally examined using normalized JIS S45C steel. With the aid of hydrogen thermal desorption analysis, the influence of the increase in carbon content has been discussed. The S45C steel easily pick-up hydrogen. But it is relatively insensitive to hydrogen absorption. This increased amount of hydrogen pick-up is firstly caused by large amount of pearlite structure. Further sulfur takes an important role for hydrogen pick up during cathodic charging. Hydrogen plays two kinds of roles during fatigue tests. One is hydrogen in dislocation core and makes plastic deformation easier in the first monotonic tension stage. The other is hydrogen/vacancy pair, and suppresses dislocation motion and increases stress amplitudes. The latter should be the fundamental issue for fatigue life. The amount of absorbed hydrogen itself is not only important, but the difference of hydrogen concentration between surface and inside is more important. When the fatigue crack initiates inside, the fatigue life is severely damaged to less than 10% of non-charged one. Taking the lower limit value of hydrogen to form internal fish-eye type cracking, S45C is less sensitive to hydrogen in cyclic straining. This comes from the pearlite structure that resists crack propagation.