Damages of case hardened parts and samples are often related to hydrogen embrittlement due to the brittle fracture structure, showing torn up grain boundaries and pores and trenches on the grain boundary surface. On the other hand, these artefacts are also in line with a brittle fracture of high strength martensitic microstructure without hydrogen participation. To get a better understanding of the fracture mechanics of case hardened steels, hydrogen absorption during case hardening was investigated. Points of exploration were: process of case hardening, material, especially non metallic impurities; carburising parameters and tempering parameters. In addition, the influence of the above on mechanical properties were examined.
The current doctrine is that hydrogen embrittlement is caused by diffusible hydrogen only. The carburising process usually increases the total amount of hydrogen up to 4 ppm. Diffusible hydrogen leaves the material after quenching. Tempering accelerates this process.
To investigate the influence of hydrogen on embrittlement, two different steel grades with several degrees of non metallic inclusions were examined. Even though all those different steels were used, no influence on hydrogen content of the material was determined after carburising.
The influence of carburising on mechanical properties was examined by incremental step load tests. They showed that case hardened materials do not significantly tend to hydrogen embrittlement. If steel is electrochemically charged with diffusible hydrogen, it fails before the maximum load is reached. This indicates hydrogen embrittlement. However, no differences can be determined by means of fracture structures of these two states. Therefore, fracture structures of damages of case hardened components alone are not a sufficient characteristic for hydrogen embrittlement.