Brittle fracture in hot-stamped AHSS due to hydrogen embrittlement.
Nowadays the increasing complexity of automotive components together with the need to reduce fuel consumption led to the use of materials which have a good combination of strength and ductility parameters. This has led to the development of Advanced High Strength Steels (AHSS), which have high strength and formability.
AHSS are materials whose excellent mechanical properties are obtained through the realisation of multiphase microstructures and different hardening methods. A hardening process involves stamping the sheet when it still has a soft microstructure, therefore, producing the heat treatment when it is still in the press in order to harden the component. This process is known as hot stamping.
22MnB5 steel is the most used grade for hot stamping. The use of hot-stamped AHSS has increased significantly in recent years in the automotive industry. It allows obtaining components with complex shapes with low springback and residual stresses. However, the hot-stamping process can cause the absorption of hydrogen into the material leading to the phenomenon of hydrogen embrittlement.
Hydrogen embrittlement in hot-stamped steels
Hydrogen can penetrate the material when the sheet is heated in the furnace. The water vapor present in the furnace dissociates at high temperatures by forming atomic hydrogen with consequent absorption in the steel bulk. During cooling, the material hydrogen solubility decreases, thus causing hydrogen trapping. The hydrogen present in the material can diffuse into the component during service and cause unexpected failures even after long time periods. This phenomenon is known as hydrogen delayed fracture.
Hot-stamped AHSS are particularly susceptible to hydrogen delayed fracture due to their final microstructure which is predominantly martensitic. In fact, martensite is formed by highly distorted crystal lattices due to the presence of interstitial carbon. It leads to residual stresses in the lattice. Hydrogen interacts with these intrinsic stresses in martensite making it highly sensitive to hydrogen embrittlement.
A methodology to assess hydrogen delayed fracture in advanced steels
FormPlanet offers different methodologies to assess the susceptibility of steels to hydrogen delayed fracture. The four-point bending test is one of them. In this test, the hydrogen absorption is obtained by quenching specimens in a water vapor rich environment. The quantity of hydrogen absorbed by the specimen is controlled at the laboratory level by acting on parameters such as the dew point of the furnace atmosphere and the dwell time in the furnace.
The hydrogen charged specimens are then bent under a four-point bending load configuration to generate a stress gradient promoting hydrogen diffusion and accumulation. A crack may appear after a certain time period to indicate a material failure. A complete fracture map of the material is created through the choice of different hydrogen charging parameters and bending levels. The information obtained from the test is used in the design phase of the component to identify the safe regions of the material and avoid undesirable fractures.
Francesco Aiello
Master’s degree in mechanical engineering. He is a PhD student in Industrial Engineering at the Department of Civil and Industrial Engineering of the University of Pisa. His research interest is the study of the phenomena of hydrogen embrittlement in high-strength steels.
