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Materials Frontier No.191

Title: Review of safety and advanced high-strength steels (AHSS) in auto service
Speaker: Prof. Andrej Atrens
Date/Time: 2017-7-6, 10:30am
Venue: Room 308, XU Zuyao Building
Inviter:Prof. YUAN Guanyin
An assessment is made of the consequences from hydrogen absorbed during auto service, as AHSS become more widely used in auto construction.
The influence of hydrogen on the mechanical properties of some AHSS was studied using the linearly increasing stress test (LIST) combined (i) with hydrogen absorbed during free corrosion, and (ii) with hydrogen from cathodic charging.
There is little hydrogen influence (i) for MS980, MS1180, 980DP and 1200 DP containing the highest possible hydrogen absorption during automobile service, and (ii) for a hydrogen-charged specimen of MS980-MS1500 eight minutes after cathodic charging, when the specimen contains 85% of the original hydrogen content.
In contrast, electrochemical-hydrogen charging causes an influence. The main manifestations are a decrease in yield strength, and the introduction of hydrogen assisted micro-fracture modes during specimen fracture at the ultimate tensile strength when the specimen has become mechanically unstable in the load controlled LIST. Hydrogen Enhanced Macroscopic Ductility (HEMP) decreases the yield strength. Hydrogen Assisted Micro-fracture (HAM) decreases ductility.
The relationship was established between the hydrogen charging over-potential and the equivalent hydrogen pressure during free corrosion, and during cathodic hydrogen charging, though the use of electrochemical permeation experiments and thermal desorption spectroscopy (TDS).
A two-site model explains the Y-intercept for Sieverts’ Law, as the density of filled hydrogen traps. The manner of filling of lower energy traps underpins extrapolation of Sieverts’ Law from the gas phase to electrochemical hydrogen charging. The steel hydrogen trapping behavior can be simplified as there being two types of hydrogen traps. These fill in turn. The deeper traps are filled by room temperature, and correspond to the Y-intercept for Sieverts’ law. The TDS indicates that the weaker traps are filled progressively by hydrogen gas charging and cathodic charging at increasing hydrogen fugacity. The TDS thus justifies the Sievert’s law extrapolation from the hydrogen gas pressures to the much higher fugacity for cathodic charging. Statistical mechanics provides the justification for Sieverts’ Law behavior in this range of hydrogen fugacity.

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