Fatigue tests were conducted on two kinds of normalized S45C commercial steel bars that are of different purity. Amount and state of hydrogen are determined by thermal desorption spectroscopy (TDS) after cathodic charging. Effects of impurity elements and non-metallic inclusions have been discussed.
The fatigue life is less damaged in high purity type S45C (S45C/hpt) with same amount of hydrogen pre-charging, although the non-charged S45C steels exhibits almost same fatigue lives.
The TDS main peak around 335K is lower in S45C/hpt and hydrogen pick-up is smaller. This could be explained by the smaller amount of MnS. Though, it is only the subsidiary reason for the susceptibility.
The S45C/hpt being sufficiently de-oxidized, the Al2O3 inclusions are reduced in size and quantity. Furthermore, they are wrapped by MnS. The comparative S45C contains silicate inclusions. Both are elongated and cohesive to ferrite matrix; not resulting in the internal failure with hydrogen pre-charging.
Phosphorus would enhance hydrogen trapping along grain boundary and promote the fracture along it. Grain boundary facet being not found for both steels, it would not deal so much with the present hydrogen susceptibility.
With S45C/hpt, soluble nitrogen atoms are reduced by sufficient Al addition. This may lead to the late dislocation cell formation and responsible to the cyclic softening that lasts long at the beginning of strain cycling.
This cyclic softening acts to reduce the dislocation density in S45C/hpt. It would make the steel insensitive to hydrogen substantially through decreased hydrogen concentration.
