Hydrogen Embrittlement Susceptibility of Ferritic High Strength Steels

Abstract

Advanced High Strength Steels (AHSS) are essential to reduce weight and consequently CO2 emissions in the automotive industry. However, they are vulnerable to Hydrogen Embrittlement (HE), because many strengtheningmechanisms present in their microstructures are also a cause of HE. The contributions of individual microstructural features to HE is not yet completely understood. Several challenges occur when studying HE in these steels. Firstly, since all strengthening mechanisms interact with hydrogen, isolating the effect of singular features is complicated. Moreover, HE is a time-sensitive phenomenon which requires experimental setups that minimise time between hydrogen charging, testing and measuring in order to avoid desorption between steps. Two types of iso-parametric microstructures were created for this project, in order to isolate the effect of individual features. Firstly, ferritic steels containing either Vanadium Carbide (VC) or Titanium Carbide (TiC) nano-precipitates of different size distributions since the addition of nano-precipitates is a promising candidate to provide both strength and
ductility. Secondly, Dual Phase (DP) steels with varying amounts ofmartensite in a ferrite content, since these steels are the most widely used in the automotive sector.

This research begins by subjecting nano-precipitate strengthened steels to annealing treatments designed to achieve two distinct precipitate size distributions. A 2 hour heat treatment leads to precipitates of around 10 nm in size, that grow to more than 10 nm after 20 hours. Heat treatments are performed in either N2 or H2 gas in order to provide both a reference steel as well as one that charges the precipitates with hydrogen. After hydrogen contents were measured using Thermal Desorption Spectroscopy (TDS), the hydrogen gas is found to predominantly charge large incoherent precipitates with hydrogen.
These precipitates of sizes larger than 100 nm are present in themicrostructure from the steelmaking process. Smaller (semi-)coherent carbides with sizes on the order of 10 nmare not observed to contain hydrogen after the treatment. Hydrogen trapped in incoherent precipitates is trapped irreversibly in carbon vacancies inside the precipitate bulk, meaning that it does not diffuse throughout the steel. The activation energies could only be determined for TiC precipitates, which range from69 kJ/mol to 115 kJ/mol. This type of trapping inhibits accumulation of hydrogen at critical areas such as crack tips, which means that no HE is observed in these specimens. Secondary Ion Mass Spectrometry was performed to visualise hydrogen trapped in the incoherent precipitates. Hydrogen in the TiC precipitates is primarily stored at the interface with the matrix, whereas in VC
precipitates it is distributed throughout the entire bulk. This is explained as an effect of a higher C-vacancy concentration in VC.....