The Centro Ricerche Fiat (CRF) has the mission to develop and transfer innovative products, processes and methodologies to improve the competitiveness of the Stellantis Group. CRF conducts collaborative research initiatives at the national and international levels in partnership with all the key public and private stakeholders concerned with Sustainable Mobility, targeting specifically the industrial exploitation of research.
According to research, high strength steels with high amount of martensite such as the Usibor2000®, are susceptible to hydrogen diffusion leading to delayed cracking phenomena. Within FormPlanet, Usibor2000® was tested for automotive applications by stamping several A pillar reinforcements with this material, instead of the conventional Usibor1500®. The component could be produced in the same geometry without any thinning or wrinkles at FCA Cassino plant.
The hydrogen content was measured on the raw material blanks prior to the hot stamping process to evaluate this value before and after the process using the HELIOS 4 HOT PROBE equipment. After the hot forming process, the hydrogen content was measured again in different areas of each component to monitor the possible differences in terms of concentration area by area.
After assessing hydrogen content, chemical analyses, tensile tests and slow strain rate tests were carried out to fully characterise the material. In order to go deeper in the mechanical characterisation of Usibor2000®, tensile tests at different stress triaxiality were performed using Digital image Correlation.
The results proved that the Usibor2000® has low sensitivity to hydrogen absorption also in case of different production condition, a needed quality for the approval of the material for industrial applications. In addition, the raw material, before the heat treatment process, had negligible hydrogen content. Based on the results of this case study, CRF approved Usibor2000® for serial life automotive applications and can be used by the FCA Product Development department for the next car models.
After a proper preliminary calibration of HELIOS 4 HOT PROBE equipment, diffusible hydrogen content can be measured directly on sheet steel components by hot extraction technique. Test temperature is set ad hoc for the specific material from room temperature up to 180°C. Measurement is completely non-destructive and there is no need to remove coating.