Hydrogen Embrittlement
Hydrogen Embrittlement is a common serious problem in steels applied in lightweight car construction, leading to delayed fractures such as cracks before its estimated life span
Hydrogen enters into the material via hydrogen adsorption through the surface of the component. These hydrogen atoms result from the dissociation of hydrogen containing molecules (present in atmosphere or solution). After dissociation and adsorption, a part of this hydrogen continues its path into the bulk material. Once absorbed into the microstructure, it is trapped in different microstructural sites.
Hydrogen trapped in interstitial sites is called “diffusible hydrogen” or “reversible hydrogen” and presents the lowest activation energy. When the material is exposed to external stresses or even residual stresses, diffusible hydrogen will diffuse to zones having a high hydrostatic stress, resulting in local increase of hydrogen concentration, and leading to the mentioned phenomenon of delayed cracking.
AHSS and PHS in the automotive sector
Advanced High Strength Steels (AHSS) and Press Hardening Steels (PHS) have become the dominant material choice for lightweight construction in automotive and transport sector. Steel is affordable, has good in-service performance, manufacturability, and recyclability.
AHSS and PHS by virtue of their strength allow downgauging and therefore weight saving, making them the preferred choice over conventional steels. However, their high ultimate tensile strength (UTS > 1000 MPa) makes them highly susceptible to delayed fracture by Hydrogen Embrittlement (HE).
Atomic hydrogen absorption in automotive components can take place in various phases of steel, part production and car life.
The constant increase in the application of AHSS and PHS demands accurate and reliable methods to assess HE susceptibility of new steels for lightweight construction. Many efforts have been devoted to defining test methods to characterise the HE susceptibility for AHSS sheets. However, no standardised tests have been issued yet and no unique nor well-defined methodology is available.
Hydrogen uptake during part manufacturing
