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Laboratory experiments on press hardened steels in different delivered states exposed to hydrogen
(2015)
The legal and economical demands in the automotive industry lead to increasing efforts reducing the CO2 emissions. A way to achieve that goal is to reduce the total weight of the car and therefore the fuel consumption. With the application of high strength steels in car body manufacturing it is possible to decrease the sheet thickness of the used materials and therefore the weight of the automobile. Components made of high strength steels can be produced by cold forming as well as by hot forming. In high strength steel the hydrogen content plays a crucial role for the component behavior. It is known that the hydrogen diffusivity and solubility are based on several factors, for example increasing dislocation density by cold working processes, vacancies, chemical composition and grain boundaries. The understanding and the control of the interaction between hydrogen and the high strength press hardened steels is an important factor for the application of the materials. The scope of this work is the determination of hydrogen diffusion and permeability kinetics in press hardened steel samples. An electrochemical hydrogen permeation method was applied on boron-manganese steel grades with different rolling reduction in initial ferrite/pearlite matrix as well as for the as-quenched martensite microstructure. The diffusible hydrogen contents of the specimens were measured using the carrier gas hot extraction technique (CGHE).
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by Evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).