Delayed cracking caused by hydrogen embrittlement results in decreased toughness, ductility, and load-bearing capacity of a material. It is a concern with some Advanced High-Strength steel (AHSS) grades.
Three triggers must be present for there to be an embrittlement risk:
- Use of a susceptible material, like steel with tensile strength above approximately 1000 MPa and/or the presence of retained austenite in the microstructure,
- An environment or processing step where hydrogen is available to diffuse into the steel, and
- Presence of a high stress condition, either from forming or during product usage and life cycle.
The interaction between hydrogen and the steel crystallographic structure in the presence of these conditions limits the ability of the steel to deform under load. The steel part will fracture at a lower stress than is anticipated under other conditions. Of particular concern is that fracture may not occur at the time of stamping, since hydrogen can diffuse out of the lattice over time.
High strain areas on parts formed from AHSS are at increased risk for delayed cracking, since the distorted crystallographic lattice allows for an increased hydrogen solubility. Cut edges are especially troublesome, since the shearing operation work hardens the steel (increasing strength) and damages the microstructure (increasing strain).
AHSS grades that contain retained austenite are also at increased risk for delayed cracking from hydrogen embrittlement. Stamping strains provide the driving force for the TRIP effect, where austenite coverts to martensite. The austenite crystallographic lattice allows for increased hydrogen solubility, so the TRIP effect creates a hydrogen-rich high strength martensite – conditions that promote a greater risk of embrittlement.
Sources of hydrogen include:
- Breakdown of mill or press applied lubricants,
- Annealing or heat treating atmospheres, and
- Electrochemical operations in steel mills or vehicle production such as pickling, electrogalvanizing, welding, phosphate coating, and paint pretreatment like e-coating.
Actions to minimize the risk of hydrogen embrittlement includes not over-pickling the surface prior to coating (since pickling uses an acid), heat treating in dew-point controlled atmospheres (below -10 °C or even lower for higher strength grades), and using the e-coating operation as a means to degas the steel.
The procedures to characterize delayed fracture resistance typically involve using a machined sample, pre-strained and placed in an acidic solution for a defined time. SAE J3215S-94 is one such procedure.
Note that cracking in simulative tests does not necessarily indicate that parts made from that material would crack in an automotive environment due to the specific conditions that need to be present for hydrogen embrittlement to occur. When selecting any AHSS, results from simulative tests should be considered in conjunction with the forming and processing history of the steel, the strain state of the formed and processed part, and the operating environment of the part.