Abstract
We discussed the susceptibility for sulfide corrosion cracking of pre-strained mild steel and high-strength steels.
The tested materials were four kinds of steels (SB42B, HT50, HT60 and HT80), and the test pieces, which were circular rods, were strained by tension test at room temperature before being submitted to sulfide corrosion test. Sulfide corrosion test was carried out under constant tensile load in the solution' of 0.5% acetic acid saturated with hydrogen sulfide at the temperature of 35°C, and the time to failure was measured
It became clear that the effect of pre-strain on the time to failure was closely associated with the structure o each steel. In the material like HT80 of which the structure was tempered martensite, the time to failure shortened with the increase of pre-strain until the test piece began local shrinkage, but conversely the time to failure lengthened with the increase of pre-strain beyond the uniform elongated zone. In the material like HT60, the time to failure shortened with the increase of pre-strain all over the zone of pre-strain. In the materials like SB42B and HT50 of which the structure was ferrite plus pearlite, the time to failure lengthened with the increase of pre-strain until the test piece began local shrinkage, but beyond this region of pre-strain the time to failure already did not practically change.
The volume of the diffusible hydrogen absorbed in steel increased with the increase of pre-strain, and this relation was observed with all kinds of steels.
Though it is believed that sulfide corrosion carcking is introduced by the diffusible hydrogen absorbed in steel, there is not any relation established between sulfide corrosion susceptibility and the volume of absorbed hydrogen.
As far as the local shrinkaged HT80 is concerned, the failure occurred at the part separated from the center of local shrinkage.
Moreover, small longitudinal cracks were observed at the core of local shrinkage for all kinds of steels. It is thought that these cracks occurred only though the diffusible hydrogen in steel.
