Role of MnS inclusions in hydrogen assisted cracking of steel exposed to H 2 S saturated salt solution

The role of MnS inclusions in the crack growth behaviour of quenched and tempered medium carbon steel exposed to H₂S saturated salt solution has been studied. In this environment, crack extension in the short transverse orientations occurs in a discontinuous manner which is associated with the development of inclusion nucleated subsidiary cracks ahead of the main crack. The high hydrogen activity of the H₂S saturated salt solution causes both matrix embrittlement and the accumulation of hydrogen at the inclusion/matrix interface in a molecular form. In the case of specimens containing pre-existing cracks, the formation of secondary cracks in the short transverse plane ahead of the main crack is caused by pressurization effects at colonies of closely spaced large inclusions. The discontinuous growth observed in these specimens is attributed to the operation of the pressurization mechanism.