Diffusible hydrogen behavior and delayed fracture of cold rolled martensitic steel in consideration of automotive manufacturing process and vehicle service environment

This study aims to elucidate the behavior of diffusible hydrogen and delayed fracture in martensitic steel with 1500 MPa strength during automotive painting process and under vehicle service conditions. A sequential process of automotive pretreatment line and vehicle service environment is simulated to evaluate the hydrogen pick up in each process. In case of the automotive painting line, the absorption of hydrogen is within the common range in the process of phosphating treatment and electrodeposition. The baking process plays an effective role for desorbing the diffusible hydrogen absorbed during the automotive pre-treatment such as zinc-phosphating, and electrodeposition process. In case of the corrosion environment under the automotive driving conditions, hydrogen induced delayed fracture is accelerated as the exposure time increases. Further, it is clarified that severe plastic deformation are the significant factors for hydrogen induced delayed fracture under with low pH value and present of chloride ion in a chemical solution parameter. In summary, hydrogen is transported constantly during electrodeposition sequential line process of automobile manufacturing below the hydrogen content of 0.5 ppm, which is not critical value for leading to hydrogen delayed fracture based on results of slow strain rate tensile tests. However, exposure to extreme conditions under service environment of vehicle, such as acidic solution and chloride chemistry solution that result in high level of hydrogen absorption, severe plastic deformation in the sheared edge, and constantly applied internal or external stresses, can cause the hydrogen induced delayed fracture in the fully martensitic steels.