Local hydrogen accumulation after cold forming and heat treatment in punched advanced high strength steel sheets

• Published in: Journal of alloys and compounds 2021-03, Vol.856, p.158226, Article 158226
• Main Authors: Drexler, A., Bergmann, C., Manke, G., Kokotin, V., Mraczek, K., Leitner, S., Pohl, M., Ecker, W.
• Format: Article
• Language: English
• Subjects: Accumulation; Advanced high strength steels; Automobile industry; Cold punching; Cold working; Diffusion simulation; Electron backscatter diffraction; Environmentally assisted cracking; Heat treatment; High strength steel; High strength steels; Hydrogen; Hydrogen embrittlement; Local Hydrogen concentration; Metal sheets; Microstructure; Plastic deformation; Punched edge; Retained austenite; Shear; Stress distribution; Thermal desorption spectroscopy; Ultimate tensile strength
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2020.158226

Hydrogen embrittlement is one of the most crucial problems in the application of advanced high strength steel (AHSS) sheets for the automotive industry. Especially, the severe plastic deformation in punched edges makes the components susceptible to hydrogen assisted cracking (HAC). While small amounts of hydrogen are measured in the bulk material, hydrogen concentration increases in the micrometer-sized shear affected zone many times, along with severe plastic deformation. To contribute to the understanding of local microstructure and local stress states on the hydrogen accumulation in the shear affected zone, two industrial AHSS were investigated. Both steels had the same ultimate tensile strength of 1200 MPa, but different uniform elongations. High-pressure torsion (HPT) deformed samples were used to represent the material state in the shear affected zone. Thermal desorption spectroscopy (TDS), X-ray diffraction (XRD), magnetic retained austenite measurements and electron backscattering diffraction (EBSD) in a high resolution secondary electron microscopy (SEM) were applied among other techniques to gain more insights with microstructural resolution. The hydrogen analysis results of the HPT samples were correlated with the local hydrogen trapping capacity in punched edges. Finally, a simplified “two-zone” model considering the bulk and punched edge as separate zones was developed in order to estimate the local hydrogen concentration of punched AHSS sheets as a function of hydrogen bulk concentration. In a concluding remark it is shown, that the effect of the hydrostatic stress field as trap site is negligibly small compared to other traps. •Role of microstructure and plastic deformation on hydrogen trapping is investigated.•A heat treatment at 150 °C decreases hydrogen uptake from electrochemical sources.•Hydrogen analysis- and model-based evaluation provides hydrogen localization.•Punched edges exhibit a by a factor of 5–10 increased local hydrogen concentration.•Volume residual stresses have a negligible effect on hydrogen accumulation.