Abstract
Body-in-white components are commonly manufactured through hot stamping of aluminized high strength steel (AlSi-HS-HSS). The hot stamping involves heating of the coated steel blank at 900–950 °C for certain time to attain fully alloyed coating and fully austenitic microstructure. The blank is then formed and quenched in water-cooled dies to obtain a high strength martensitic steel. It is well known that the martensitic microstructure is sensitive to hydrogen-induced cracking (HIC). The hydrogen is absorbed into the steel during the austenitization step of the hot stamping if the process is not controlled. The source of hydrogen is the moisture in the furnace. The hydrogen absorption is dependent on the dew point in the furnace and heating dwell time and its content may serve as indication for HIC sensitivity. It is commonly accepted that diffusible hydrogen may cause a severe degradation of the mechanical properties of AlSi-HS-HSS if is above critical level and therefore its accurate measurement is critical and important. The hydrogen content can be measured effectively using thermal desorption spectroscopy with mass spectrometry (TDMS). However, the hydrogen desorption and the obtained TDMS spectrum might be influenced whether the analysis
is performed using as-received specimens with the AlSi coating remained or specimens for which the AlSi coating was removed.
The aim of this work was to reveal whether removing the AlSi coating is necessary prior TDMS analysis and how this is affecting the accuracy of evaluating the hydrogen content in AlSi-HS-HSS. Therefore, the diffusible and deeply trapped hydrogen content were analysed in AlSi-HS-HSS in as-received condition and after mechanical removing of the coating using grinding. The hot stamping was carried out at three different dew points and using one heating time. After hot stamping the specimens were stored in liquid nitrogen to preserve desorption of the diffusible hydrogen.
The results showed significant influence of specimen preparation prior TDMS analysis on the shape of the TDMS spectrum and on the evaluated diffusible and deeply trapped hydrogen values. The TDMS spectra obtained from the ground specimens exhibited almost perfect gaussian shape. Thus, the hydrogen evaluation was less challenging and more straightforward to perform as compared to the TDMS spectra of the AlSi coated specimens. This had also strong impact on the evaluated hydrogen contents for the diffusible hydrogen. Notably higher hydrogen contents were measured for the diffusible hydrogen using the specimens on which the AlSi coating was removed.