Hydrogen Embrittlement in Advanced High Strength Steels and Ultra High Strength Steels: A New Investigation Approach

In order to reduce CO₂ emissions and fuel consumption, and to respect current environmental norms, the reduction of vehicles weight is a primary target of the automotive industry. Advanced High Strength Steels (AHSS) and Ultra High Strength Steel (UHSS), which present excellent mechanical properties, are consequently increasingly used in vehicle manufacturing. The increased strength to mass ratio compensates the higher cost per kg, and AHSS and UHSS are proving to be cost-effective solutions for the body-in-white of mass market products.
In particular, aluminized boron steel can be formed in complex shapes with press hardening processes, acquiring high strength without distortion, and increasing protection from crashes. On the other hand, its characteristic martensitic microstructure is sensitive to hydrogen delayed fracture phenomena and, at the same time, the dew point in the furnace can produce hydrogen consequently to the high temperature reaction between water and aluminum. The high temperature also promotes hydrogen diffusion through the metal lattice under the aluminum-silicon coating, thus increasing the diffusible hydrogen content. However, after cooling, the coating acts as a strong barrier preventing the hydrogen from going out of the microstructure. This increases the probability of delayed fracture. As this failure brings to the rejection of the component during production, or, even worse, to the failure in its operation, diffusible hydrogen absorbed in the component needs to be monitored during the production process.
For fast and simple measurements of the response to diffusible hydrogen of aluminized boron steel, one of the HELIOS innovative instruments was used, HELIOS II. Unlike the Devanathan cell that is based on a double electrochemical cell, HELIOS II is based on a single cell coupled with a solid-state sensor. The instrument is able to give an immediate measure of diffusible hydrogen content in sheet steels, semi-products or products, avoiding time-consuming specimen palladium coating with a guided procedure that requires virtually zero training.
Two examples of diffusible hydrogen analyses are given for Usibor®1500-AS, one before hot stamping/ quenching, and one after hot stamping, suggesting that the increase in the number of dislocations during hot stamping could be the main responsible for the lower apparent diffusivity of hydrogen.