Hydrogen Embrittlement Susceptibility of New Generation Advanced High-Strength Steels for Automotive Applications

The adoption of advanced high-strength steels (AHSS) in the automotive industry have significantly increased in recent years driven by weight reduction and enhanced crashworthiness. Hot dip galvanised sacrificial coatings are regularly applied to these steels for corrosion protection. In this investigation, the scanning vibrating electrode technique (SVET) demonstrated that hydrogen evolution on the steel substrate is taking place when these sacrificial coatings are damaged during service, increasing the risk of hydrogen embrittlement. The hydrogen embrittlement susceptibility of a new generation of nano-precipitate ferritic, FNP, AHSS have been studied and compared against conventional dual phase ferritic-martensitic, FM, AHSS at equivalent strength levels. Hydrogen permeation tests have shown that FNP AHSS have lower effective diffusion coefficients, Deff, than FM AHSS at equivalent strength levels. At 800 MPa strength level Deff were 1.68×10-7 cm2/s and 1.87×10-7 cm2/s for FNP800 and FM800 respectively. At higher strength levels, 1000 MPa, Deff were 7.45×10-8 cm2/s and 1.45×10-7 cm2/s for the FNP1000 and FM1000, respectively. Slow strain rate tests (SSRT) showed that FNP AHSS displayed over 35% higher resistance to hydrogen embrittlement than conventional FM AHSS. Quantitative fractographic analyses confirmed that the new ferritic nano-precipitate microstructure retains much more ductile behaviour than conventional martensitic-ferritic even under the most severe hydrogen charging conditions tested.