Bainitic Structure Effect on Hydrogen Embrittlement

摘要

In the automotive industry, an important and critical process is the austempering of small details in the continuous line. The increase in use and advance in price of steels has been the reason to develop new alternative materials or material grades to have the same quality with lower cost. The article studies the steel C60E where minor changes in the chemical composition and structure affect the steel hardenability because of the slow cooling rate of quenching salt. Even the design of the detail remarkably influences the hardenability and therefore also the quality. Tensile strength and hardness (which are the main properties controlled in the heat treatment) are not the only parameters that affect the detail quality. After electroplating, the materials with lower hardenability are more susceptible to hydrogen embrittlement even with lower hardness. After electroplating the material ductility depends on the type of bainitic structure. The reason can be explained with the help of the TTT curves, which show the transformation of austenite into the bainite in various temperature ranges. Specimens were austempered at different process parameters (salt temperature and protective gas c-potential) and Ni-Cr electroplating with constant process parameters were used to load the hydrogen. The main structure characteristics were analyzed in the annealed and hardened condition, using various techniques of metallography, optical microscopy and scanning electron microscopy. Hardness and tensile tests were carried out on the hardened specimens. To study hydrogen embrittlement, the bending test was carried out. To analyze material hardenability, special tensile test specimens were prepared, following the original measures of safety belt tongue frames. The main goal of the study was to find the optimal properties of the steels with certain chemical composition and microstructure to produce safety belt tongues. The analysis of austempering process parameters showed the possibilities to avoid hydrogen embrittlement after Ni-Cr plating (hardness 45-51 HRC).