Elizabeth Sackett

The adoption of advanced high-strength steels (AHSS) in the automotive industry has 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, D_eff, than FM AHSS at equivalent strength levels. At 800 MPa strength levels D_eff 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, D_eff 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.

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 D_eff 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, D_eff 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.

Conveyance tube manufacturing via a hot-finished, welded route is an energy-intensive process that promotes the rapid surface oxidation of curved surfaces. Previous studies have used computational and theoretical techniques to assess the oxidation of curved surfaces. However, experimental techniques for assessing the oxidation of curved surfaces, as well as for validating existing computational and analytical studies, have significant limitations that impact their ability to accurately recreate industrial processes. The challenges of thermogravimetric analysis (TGA) for in situ tests for the oxidation of cylindrical geometries were investigated, using an as-welded conveyance tube, and compared to an equivalent tube normalised in industry as well as computational predictions for the same geometry and thermal conditions. A core element of this work was the use of a refractory dummy sample to quantify thermal buoyancy and flow-induced vibration. There was a strong agreement between the oxide mass gain predicted by a computational model compared to that of the TGA sample, with only a 5% discrepancy. However, oxide thickness gain, measured using electron microscopy, showed poor agreement, particularly when comparing industrial and experimental results. This was attributed to the need for further work to account for transient heating, oxide porosity, atmospheric composition variation, and the effect of thermomechanical operations during conveyance tube manufacturing, e.g., hydraulic descaling.

Conveyance tube manufacturing via a hot-finished, welded route is an energy-intensive process which promotes rapid surface oxidation. During normalisation at approximately 950 °C to homogenise the post-weld microstructure, an oxide mill scale layer grows on tube outer surfaces. Following further thermomechanical processing, there is significant yield loss of up to 3% of total feedstock due to scale products, and surface degradation due to inconsistent scale delamination. Delaminated scale is also liable to contaminate and damage plant tooling. The computational thermochemistry software, Thermo-Calc 2023b, with its diffusion module, DICTRA, was explored for its potential to investigate oxidation kinetics on curved geometries representative of those in conveyance tube applications. A suitable model was developed using the Stefan problem, bespoke thermochemical databases, and a numerical solution to the diffusion equation. Oxide thickness predictions for representative curved surfaces revealed the significance of the radial term in the diffusion equation for tubes of less than a 200 mm inner radius. This critical value places the conveyance tubes’ dimensions well within the range where the effects of a cylindrical coordinate system on oxidation, owing to continuous surface area changes and superimposed diffusion pathways, cannot be neglected if oxidation on curved surfaces is to be fully understood.

In this investigation, 0.19–1.8 wt.% of Ge was introduced into a ternary Zn-Mg-Al alloy. The introduction of Ge had a significant impact on the microstructure, leading to the formation of Mg₂Ge. The area fraction of the eutectic phase diminished with increasing Ge additions. Small-scale test techniques were utilised to evaluate the mechanical properties due to the changes in microstructure. Zn-Mg-Al alloys were found to be inherently harder compared to standard hot-dip Zn-containing 0.2 wt.% Al. The hardness and strength of the Zn-Mg-Al alloys decreased with the increase in Ge additions.

Previous mechanistic models, proposed to explain the process of damage accumulation and stress redistribution between strong and weak regions inherent within the microstructure of α/β and near α titanium alloys, are validated through a matrix of experiments employing a non-standard variant of the alloy Ti 685. The grain size of the model material was deliberately processed to offer grains up to 20 mm in diameter, to facilitate constitutive measurements within individual grains. A range of experiments were performed under static and cyclic loading, with the fatigue cycle conducted under either strain or load control. Data will be reported to demonstrate significant variations in elastic and plastic properties between grains and emphasise the role of time dependent strain accumulation. Implications for the “dwell sensitive fatigue” or “cold creep” response of conventional titanium alloys will be discussed.