Effect of pre-strain on hydrogen embrittlement of high strength steels

Abstract

To mitigate global warming, society must reduce greenhouse gas emissions. The transportation sector is one of the largest energy consumers and greenhouse gas emitters. One way to mitigate the emissions of this sector is to reduce the weight of vehicles using Advanced High Strength Steels (AHSS), particularly dual phase steels (DP). However, these steels are subject to hydrogen embrittlement which hinders their full potential and application inside the automotive industry.
The effect of pre-strain level and in-situ pre-straining on hydrogen embrittlement was investigated for DP1000. Therefore, in-situ tensile test was performed at 0.5% and 2% plastic pre-strain to study hydrogen embrittlement by measuring strain at fracture. This test was done under ex-situ and in-situ pre-strain conditions. In addition, sample charging time was varied from 15 to 60 minutes for 2 % plastic pre-strain. Thermal Desorption spectroscopy was performed to measure hydrogen uptake. Fracture surface was studied by scanning electron microscopy. Results showed that pre-strain plays an important role in hydrogen embrittlement, namely 0.5% and 2 % plastic pre-strain reduced the final strain to fracture by almost 50 % for both in-situ and ex-situ charging cases. However, no significant increase in hydrogen embrittlement occurs for the increment from 0.5% to 2% pre-straining, except for the 0.1 wppm increase in hydrogen content. Differences in results for in-situ and ex-situ testing were only observed for 2% pre-strain at 60 minutes charging. Increase in charging time resulted in increased hydrogen uptake, however, no severe effects were observed in hydrogen embrittlement. For future work it is recommended to study different extra pre-strain levels, further increase the charging time and investigate the effect of various DP1000 microstructures, such as ferrite, martensite and retained austenite ratios.