Kazum, Oluwole (2018) Mechanisms of hydrogen-induced cracking in ultrahigh-strength steels. PhD thesis, James Cook University.

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Abstract

There has been a significant increase in the application of high-strength steels in the engineering and offshore industry. Nanostructured bainitic steels (BS-200 and BS-350) and austenitic twinning-induced plasticity (TWIP) steel have emerged as two promising materials for engineering and offshore applications, due to their extraordinary mechanical properties. Generally, steel corrosion is a major problem, especially in the offshore industry. Hence, several corrosion protection techniques such as protective coating, alloying and cathodic protection have widely been employed. The cathodic protection system is preferred by corrosion experts in the offshore industry as it provides a more effective method of protecting steels from general corrosion. However, high-strength metallic materials, in general, are prone to a localized form of corrosion known as hydrogen-induced cracking (HIC).

The diffusion of hydrogen can occur during the cathodic protection potentials and cause catastrophic failure of the material under tensile loading. HIC undermines the integrity of structural components and leads to huge financial losses, as well as potential environmental pollution. The degree of HIC susceptibility is influenced by the microstructure of the material. Generally, steels containing a predominantly ferrite phase are more susceptible to HIC than those containing an austenite phase. Ultrahigh-strength steels generally contain refined microstructures with different phases, grain sizes and other features such as dislocations, twinning and precipitates. These features can trap or enhance the mobility of hydrogen and thus affect the steel's susceptibility to HIC. Therefore, it is critical to study the HIC susceptibility of ultrahigh-strength nanostructured bainitic steels and TWIP steels to determine their potential applications for steel pipeline and structural components in the offshore oil industry. This thesis aims to determine the HIC susceptibility of these ultrahigh-strength steels by experimentally calculating the hydrogen diffusivity in these materials, and by mechanical property testing in conducive environments.

Nanostructured bainitic steels comprised of refined microstructures including ferrite with a large component of dislocation density and austenite phases. The microstructural features contribute to the steel's superior tensile strength (˃ 800 MPa) and ductility (≥ 30 pct). To understand the HIC susceptibility of the nanostructured bainitic steels, electrochemical hydrogen permeation tests and micro-hardness tests were carried out on the steel, and the results were compared to those from a nominal mild steel. Electrochemical hydrogen permeation results showed that the nanostructured bainitic steel containing 79 pct of ferrite phase (BS-200) exhibited lower effective hydrogen diffusivity compared to steel containing 47 pct ferrite phase (BS-350) and mild steel by about two orders of magnitude. The effective hydrogen diffusivity for each steel was found to increase as the cathodic charging current density was increased. However, the increase was not significant in BS-200 compared to the other steels. This was attributed to the trapping effect of the refined microstructural constituents: bainitic ferrite laths, retained austenite films and higher dislocation density in the bainitic ferrite of the BS- 200 steel.

In order to understand how hydrogen diffusion and hydrogen concentration in the steel affect the mechanical properties, micro-hardness tests were performed on charged samples. The results showed softening in nanostructured bainitic steel and hardening in mild steel. The BS-200 and BS-350 nanostructured bainitic steels softened by ~ 5% and 12%, respectively. The softening was higher in BS-350 nanostructured bainitic steel. This was attributed to the interaction of hydrogen with dislocations, which enhanced dislocation mobility and hence softening. For the mild steel, the hardness was attributed to supersaturation of dissolved hydrogen that gave rise to the formation of voids and cracks. This generated additional stress and new dislocation that enhanced hardening. The effect of hydrogen diffusion on hardness was, however, found to be limited to the subsurface region of BS-200 compared to BS-350 nanostructured bainitic steel and mild steel.

In TWIP steel, the microstructure is comprised of a single austenite phase stabilized by a high amount of manganese. TWIP steel has an excellent combination of strength and elongation which is achieved through work hardening, deformation twinning and small grains size ~ 5 μm. The hydrogen permeation rate in TWIP steel was about three orders of magnitude lower than the permeation rate in mild steel. Interestingly, the effective hydrogen diffusivity in TWIP steel was about two times higher compared to mild steel. The higher hydrogen diffusivity in TWIP steel compared to mild steel can be attributed to diffusion been primarily through the low energy grain boundaries of TWIP steel, as austenite grains have very low effective hydrogen diffusivity. Tensile tests were carried out in pre-charged and in-situ hydrogen charged conditions to evaluate the HIC susceptibility of the steel. The resistance to HIC based on the susceptibility indices (IHIC) for elongation (Ɛf) and reduction in area (RA) showed that TWIP steel was about 60% more resistant in its pre-charged condition than mild steel. The fractured surface analysis of the pre-charged sample revealed ductile failure in TWIP steel characterized by a uniform distribution of fine dimples and micro-voids. In mild steel, the surface showed brittle fractures. However, in the in-situ condition, TWIP steel exhibited brittle failure with a mix of dimples and large cracks attributable to more cathodically evolved hydrogen available for diffusion along the grain boundaries for crack propagation. The findings from this study have been disseminated through the following publications.

Item ID:	56201
Item Type:	Thesis (PhD)
Keywords:	nanostructured bainitic steel, cathodic-charging current, hydrogen permeation, hydrogen diffusivity, twinning-induced plasticity (TWIP) steel
Related URLs:	https://researchonline.jcu.edu.au/48525/
Copyright Information:	Copyright © 2018 Oluwole Kazum
Additional Information:	Publications arising from this thesis are available from the Related URLs field. The publications are: Chapter 4: Kazum, Oluwole, Beladi, Hossein, Timokhina, Ilana B., He, Yinghe, and Kannan, M. Bobby (2016) Hydrogen permeation in nanostructured bainitic steel. Metallurgical and Materials Transactions A, 47A (10). pp. 4896-4903.
Date Deposited:	26 Nov 2018 03:54
FoR Codes:	09 ENGINEERING > 0912 Materials Engineering > 091207 Metals and Alloy Materials @ 100%
SEO Codes:	86 MANUFACTURING > 8611 Basic Metal Products (incl. Smelting, Rolling, Drawing and Extruding) > 861103 Basic Iron and Steel Products @ 100%
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