A simplified evaluation of the maximum tensile stress at the thread root after tightening showed that the critical brittle
fracture stress has to be reduced by about 200 MPa in order to induce the intergranular fracture after tightening and literature data show that such an embrittlement is surely feasible, since it can be obtained if 1 ppm of hydrogen is introduced in
the microstructure during carburizing. The presence of several secondary cracks at the fracture surfaces further support the
hypothesis of hydrogen embrittlement