The hydrogen induced blistering and internal cracking behavior of a TRIP-assisted (Transformation Induced Plasticity) high strength steel is investigated. TRIP-assisted steels are multiphase steels with a microstructure consisting out of ferrite, bainite, retained austenite, and some martensite. Each microstructural constituent demonstrates a different behavior in the presence of hydrogen. Additionally, such steels exhibit an austenite to martensite transformation upon deformation, which influences the hydrogen induced crack formation of the material. In order to assess this influence and to better understand the interaction of such a multiphase material with hydrogen, the hydrogen induced cracking effect is evaluated for a TRIP-assisted steel in the undeformed and cold deformed, i.e. 10% by tensile deformation, condition. The austenite/martensite phase ratio differs for both conditions, allowing one to evaluate the impact of the TRIP effect on the hydrogen sensitivity of the material. The materials are cathodically charged with hydrogen at various charging conditions, i.e. charging time and current density, in order to assess their sensitivity to hydrogen induced cracking. Subsequently, crack initiation and propagation is investigated by microstructural analysis using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Hydrogen induced crack initiation occurs preferably at martensitic islands, while crack propagation takes place along segregation lines, which also consist of martensite networks. Increased deformation leads to less internal crack initiation, but to a more outspoken crack propagation. On the one hand, deformation leads to strain hardening of the material, which leads to an increase of the critical hydrogen pressure necessary for internal crack initiation. On the other hand, deformation of TRIP-assisted steel results in the formation of martensite and provides a more favorable crack path for continuous crack propagation. Moreover, the synergetic effect of hydrogen induced and assisted cracking mechanisms is illustrated.
- Hydrogen induced cracking
- TRIP steel