Abstract

Advanced high strength steels (HSSs), such as dual phase steels, are widely used in the automotive industry due to their excellent combination of strength and ductility. In certain applications, they might be exposed to hydrogen (H) which is known to be detrimental for the deformation. H embrittlement (HE) is still not fully understood. It might drastically reduce the energy absorbed in a crash event and limits the use of HSSs in car bodies. Although H diffusion is a highly time dependent phenomenon, so far, the combined effect of dynamic strain rates and electrochemical H pre-charging has not been studied. Therefore, a reproducible methodology has been developed. Tensile specimens were electrochemically H pre-charged and immediately tested in a split Hopkinson tensile bar setup. To distinguish between the effect of strain rate and HE, static tests have been conducted using the same procedure. Results show that the HE resistance decreased due to higher H amounts in the sample for all strain rates. The HE increased when slower strain rates were applied due to higher probability of H to diffuse to regions of stress concentration ahead of a crack tip and as such accelerating failure. At the highest strain rate considered (900 s-1), the material still lost about 10% of its ductility.

Highlights

  • In the automotive industry, advanced high strength steels (AHSs), such as dual phase (DP) steels, are widely used due to their outstanding combination of both formability and strength

  • The effect of the H diffusivity during the tensile test on the hydrogen embrittlement (HE) susceptibility was estimated by increasing the strain rate from static (1.67*10-2 and 1.67 s-1) to dynamic (450 and 900 s-1) conditions

  • The degree of HE was increased when slower strain rates were applied. This was linked to higher probability of H to diffuse to regions of stress concentration ahead of a crack tip at slower strain rate and, as such, accelerating failure

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Summary

Introduction

In the automotive industry, advanced high strength steels (AHSs), such as dual phase (DP) steels, are widely used due to their outstanding combination of both formability and strength. The use of these AHSs has been stimulated since these materials can both guarantee an increased safety together with the weight reduction required to meet the rigorous CO2 emissions regulations. DP, transformation induced plasticity (TRIP) and high strength low alloyed (HSLA) steels are widely used grades in the automotive industry and were already subject of numerous H related research [2,3,4,5,6,7]

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