Brittle intergranular fracture of a thread: The role of a carburizing treatment

Carburised mechanical components that have to sustain contact compressive loads or fatigue loads with limited tensile stresses, usually do not display brittle fracture in service. However, when high tensile stresses combined with high stress concentrations and a martensitic microstructure is present, such a damage mechanism may play an important role. In these cases, a careful control and optimization of the production processes is required. In the present investigation, the role of a carburizing treatment on the intergranular fracture behaviour of a pinion thread has been investigated. A simplified evaluation of the maximum tensile stress at the thread root after tightening is presented, and specific three-point bending and instrumented impact tests on Charpy-U carburised specimens were carried out, in order to highlight the role of hydrogen embrittlement and to provide guidelines for the optimization of the production process.