Proceedings of the Third International Conference on Metals & Hydrogen P42

Impact of mechanical destabilization of retained austenite on hydrogen trapping of TRIP-aided advanced high strength steels

Sebastian Cobo (1)1 , Thomas Dieudonné (1)1 , Laura Moli Sanchez (2)2 , Thierry Sturel (1)1

  • (1) 1

    Automotive Product Research Center, ArcelorMittal Maizieres Global R&D, Voie Romaine 57283 Maizières-Lès-Metz, France

  • (2) 2

    ArcelorMittal R&D Gent, Zelzate, Belgium


TRIP-aided advanced high strength steels are increasingly being developed for light weighting applications in automotive aiming at an optimum compromise between strength and ductility. Carbide free bainite (CFB) and Quenching & Partitioning (Q&P) designs containing between 10 and 20% retained austenite achieve enhanced ductility by the transformation induced plasticity (TRIP) effect induced by the mechanical destabilization of the retained austenite during the forming processes.  Strain applied by forming could make high strength steels susceptible to delayed cracking when hydrogen content in the microstructure exceeds a critical level.  Mobile hydrogen and the interaction with dislocations and microstructural features can induce brittleness following HEDE (Hydrogen induced Decohesion) and/or HELP (Hydrogen induced local plasticity) mechanisms. The nature of hydrogen trapping is therefore vital; as hydrogen trapped on reversible sites remains mobile and therefore harmful, hydrogen on irreversible sites is not mobile and does no play any relevant embrittlement role.  In contrast to conventional ferritic steels the presence of retained austenite in modern TRIP-aided steels significantly modifies the distribution of hydrogen trapping sites. More important, this distribution is expected to be further modified by the forming processes due to the mechanical destabilization of retained austenite. In this work the role of retained austenite in the trapping of hydrogen is investigated on CFB and Q&P steel samples using deuterium charging techniques. Thermal desorption spectroscopy (TDS) was used to describe the nature of hydrogen trapping after charging and its evolution with applied plastic deformation by cold rolling. Changes induced by retained austenite decomposition on the TDS patterns are quantified establishing the relationship between the evolving microstructure and the changing nature of the hydrogen trapping.  The consequences on the  definition of suitable delayed fracture risk assessment tests are discussed as hydrogen induced damage is compared in constant load punched hole tensile tests and slow strain rate test (SSRT) in which the trip effect mainly takes place before and during the test respectively.