Assessing the impact of carbonitriding on residual weld hydrogen in the manufacture of a gear component

In this paper, we report on the direct measurement of hydrogen flux from a machined car gear component immediately following carbonitriding treatment and prior to its welding to the gear train. Carbonitriding is a thermochemical process by which carbon and nitrogen are purposely introduced into the surface of steel components leading to an improvement of their wear and fatigue performance. Hydrogen bearing gases used in the treatment also cause hydrogen uptake in the steel. This hydrogen is substantially retained upon the quenching of the components that immediately follows carbonitriding, potentially leading to hydrogen cracking during subsequent manufacturing welding operations. The work was carried out to ensure a complete understanding of the conditions under which hydrogen, introduced during carbonitriding, is likely to present any risk of subsequent weld failure. Diffusion modelling of measured flux profiles indicated complete permeation of hydrogen through the gear piece at a concentration of 1.8 parts per million by weight (ppm) hydrogen upon the termination of carbonitriding., This compares with commonly specified residual hydrogen levels considered safe for welding of between 1 and 10 ppm hydrogen. However, the hydrogen cracking of welds is also very much informed by the welding thermal profile, geometry and metallurgy. As a result of the flux measurements and modelling, it was possible to adopt and monitor production procedures which assured avoidance of any risk of hydrogen damage during and after the weld process.