The present work deals with the effects of nonmetallic inclusions on hydrogen assisted cracking (HAC) and its fracture morphology. Three point bending test was carried out, using hydrogen charged and uncharged specimens whose sulphur contents were changed in the range 10 to 510ppm. The hydrogen diffusion coefficients of these steels were measured to investigate the hydorgen trapping effect of inclusions. The follownig results were obtained in this work.
The critical stress intensity (Kc) values for the hydrogen uncharged specimens decreased with increasing inclusions. Therefore, inclusions had a detrimental effect on ductile facture of steels. In this test, deep voids induced by MnS inclusions were observed on the fracture surfaces of the specimens with higher sulphur contents. For the hydrogenated specimens, the threshold stress intensity (KTH) values increased but the crack propagation rates decreased with increasing inclusions. Therefore, it was concluded that inclusions suppressed both the initiation and propagation of HAC. Though the dominant fracture mode was quasi-cleavage in this test, secondary cracks induced by MnS inclusions were observed on the fracture surfaces of high sulphur steels at high K values. However, such secondary cracks diminished with decreasing K levels. At KTH levels, all of the steels exhibited the same fracture appearances independent of sulphur contents. The above mentioned effects of inclusions on HAC were due to the reduction of hydrogen concentration at the crack tip and the delay of hydrogen diffusion to the crack tip, which were caused by the hydrogen trapping of sulphides.
