In recent years, the growing attention to the problem of pollution linked to the growing number of cars has prompted the automotive industry to invest in the construction of low-emission cars. Reducing the weight of the car plays a major role in this respect.
In fact, a reduction in weight means a reduction in fuel consumption and therefore a reduction in carbon dioxide emissions. Among the various components of a car, structural components have a significant impact on the overall weight of the car. A redesign of the body, chassis, bodywork, with a view to lightening, leads to a significant reduction in overall weight.
Figure 1: Different contribution to lightweight. Courtesy of FCA.
The use of metallic materials in the automotive industry
The choice of material is of fundamental importance for this purpose. The use of lighter materials is not immediately applicable, as materials used in the automotive industry have to meet various criteria. Some of these criteria are the result of safety and pollution regulations, others are related to customer requirements. The sheet materials must guarantee the safety of passengers by absorbing energy in the event of a collision.
They must be also easy to process, recyclable and ensure the recovery of the vehicle at the end of its life. Finally, one of the most important selection criteria is the low cost they must guarantee. This is the result of the sum of the cost of the raw material, testing and production of the component.
Materials for the automotive industry have to meet multiple requirements, including safety, pollution, recyclability, as well as be cheap to manufacture into a component
Some of the alternative materials that can be used for lightweighting solutions in the automotive industry are metal polymers and composites. Polymeric and composite materials have been used for applications with low production volumes due to their production flexibility. They have a low weight and are corrosion resistant, anisotropic, and have good mechanical properties.
Although these advantages are well known to the industry, the use of composites has been hampered by high material costs, slow production rates and poor recyclability. Metallic materials are today the most widely used materials in car manufacturing. The use of aluminium and magnesium alloys has increased significantly compared to previous years. Due to their low density they have the potential to reduce the weight of the vehicle bodywork. They are more expensive than steel, so their use is limited to those components where improved functionality justifies the higher cost.
Steel has undergone major technological developments in the past decades allowing the production of metallic parts with high strength and good ductility properties.
Steel has undergone considerable technological development compared to the past. This has led to the redesign of various components by all car manufacturers. The established technology of steel has allowed the development of different alloys with different strength and ductility properties. In conventional steels, traditional hardening methods such as solid solutions and grain refining are used, while in Advanced High Strength Steels (AHSS) less conventional methods based on phase transformations are applied.
Figure 2: Advanced steel application. Courtesy of FCA.
The increased strength properties of the steels permit the use of thinner thicknesses, and thereby, the reduction in component weight. New technologies have allowed the development of steels with high strength and, at the same time, good ductility properties, allowing a good formability and energy absorption capacity during an impact. These properties combined with low cost, weldability and recyclability make steel the first choice for the material in car design.
Advanced steels have become the first choice when selecting materials in car design because of their properties, low cost and recyclability.
What is Hydrogen Embrittlement
In general, increasing the mechanical strength of a steel also increases its susceptibility to Hydrogen Embrittlement (HE). HE is manifested by degradation of the mechanical properties of the material due to the presence of hydrogen.
Hydrogen may be absorbed by the steel during the manufacturing process of the component or during its service. For example, hydrogen may be absorbed during the electrochemical cataphoretic process or for hot stamping steels during stamping due to the presence of humidity in the furnace. A brackish environment or the use of salt in the streets can promote corrosion reactions with the formation of atomic hydrogen.
High-strength steels are more susceptible to Hydrogen Embrittlement, a degradation of the material’s mechanical properties due to the presence of hydrogen within the steel
Once absorbed, hydrogen diffuses and concentrates into defects such as grain boundaries, dislocations and phase boundaries. Tensile stresses dilate the crystalline lattice of steel promoting hydrogen diffusion. Since many body components are produced by cold forming techniques that can induce high residual stresses, a small amount of absorbed hydrogen can lead to a significant accumulation of hydrogen. If the hydrogen content reaches a critical value inside the steel, it can lead to a significant reduction in mechanical properties, strength and ductility.
Absorbed hydrogen can lead to cracks, grain boundaries or dislocations, as well as a reduction in strength and ductility
Figure 3: Crack due to HE
In AHSS steels where the strength increase is based on phase transformations, the phenomenon of HE still requires more investigations. Especially when an austenitic phase is present, differences in the transport and solubility properties of hydrogen in austenite and ferritic or martensitic matrix, as well as austenitic tendency to transform into martensite during plastic deformation are aspects that still need to be studied.
Knowledge of the phenomenon and its modelling are of fundamental importance at the design stage to prevent the phenomenon and not to compromise the integrity of the component and the safety of the passenger.
Mechanical characterisation of susceptibility to Hydrogen Embrittlement
The phenomenon of HE occurs through different phases: absorption, diffusion, accumulation and interaction of hydrogen with metal.
HE is studied through the development of standardised and non-standardised laboratory tests. To evaluate the effect of hydrogen, it must be introduced into steel. For this purpose, electrochemical charging is often used. During electrochemical loading the sample is immersed in a solution and works as a cathode so that the hydrogen reduction reaction of atomic hydrogen occurs on its surface. The adsorbed atomic hydrogen diffuses into the metal until the sample is completely saturated.
Acting on the charging parameters, such as type of solution and current density, it is possible to obtain the concentrations of hydrogen of interest for the alloy studied. The phenomenon of hydrogen diffusion is governed by Fick’s law. The physical parameter that regulates the phenomenon is the diffusion coefficient, which is determined through electrochemical permeation tests.
Slow strain rate tests and four-point bending tests can be used to study the effect of hydrogen in metallic parts
The quantification of the reduction of mechanical properties due to hydrogen is carried out through slow strain rate tests on specimens charged with hydrogen. The low strain rate allows hydrogen to migrate to the stressed areas promoting fracture. The mechanical properties are then related to the hydrogen content in the test specimen. It is thus possible to determine the hydrogen content for which the mechanical property of interest decreases below a fixed threshold. This content is called critical hydrogen content.
Figure 4: Variation of tensile strength as a function of hydrogen content.
Another mechanical test to evaluate the decay of mechanical properties in the presence of hydrogen is the four-point bending test technique. A specimen charged with hydrogen is bended in the elastic regime at different maximum tension values through the incremental step loading technique.
As a result of the diffusion and subsequent accumulation of hydrogen, the specimen may break due to delayed fracture by HE. New mechanical characterisation tests have been proposed in order to make the evaluation of the effects of hydrogen on steel faster and more economical. Indentation tests have been identified as an effective method to characterise the elastic-plastic behaviour of metallic materials.
Since hydrogen interacts with dislocations affecting the ductility of the material, these tests are good candidates to evaluate the effect of hydrogen on metallic materials. Research is continuing the quantification of the phenomenon of HE using indentation tests and the extension of these techniques during the production of a component.
Hydrogen Embrittlement in high-strength automotive steels
Hydrogen Embrittlement is a phenomenon that affects high-strength automotive steels. The parameters that drive the hydrogen during the different phases in which it enters the material, diffuses and damages can be quantified through the execution of a mechanical characterisation campaign. FormPlanet aims to offer Hydrogen Embrittlement tests to determine the parameter that regulates the hydrogen diffusion; to quantify the degradation of mechanical properties and identify the critical hydrogen content.
Knowledge of this information can be used during the design phase of automotive components made of high strength steel in order to avoid fractures because of HE. In this way, it is possible to allow a wider use of high strength steels resulting in a greater reduction in vehicle weight.
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.
