Electrochemical hydrogen charging to simulate hydrogen flaking in pressure vessel steels

The goal of this work is to propose a general mechanism for hydrogen induced crack initiation in steels based on a microstructural study of multiple steel grades. Four types of steels with strongly varying microstructures are studied for this purpose, i.e. ultra low carbon (ULC) steel, TRIP (transformation induced plasticity) steel, Fe-C-Ti generic alloy, and pressure vessel steel. A strong dependency of the initiation of hydrogen induced cracks on the microstructural features in the materials is observed. By use of SEM-EBSD characterization, initiation is found to always occur at the hard secondary phase particles in the materials.

https://biblio.ugent.be/publication/8656035/file/8656036.pdf

Initiation of hydrogen induced cracks at secondary phase particles

Serious blisters were found on the shell surface of the gas tank which operated in wet H2S environment during a regular inspection Gas sampling device was used to determine that the gas in the blister is hydrogen The causes of blisters were analyzed and investigated by means of chemical analysis, metallographic examines, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) The results revealed that the elongated MnS inclusions are the important reason for the hydrogen blisters The preventive measures of the hydrogen blisters are put forward based on the above study

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Mechanism Analysis of Hydrogen Blisters on the Surface of a Gas Tank and Research on Its Preventive Measures

This paper describes an approach to identify the influence of different stress type on hydrogen diffusion of the specimen which contains both tensile stress and pressure stress. The numerical method of simulating hydrogen diffusion by using software Abaqus is used to determine the influence. The simulated hydrogen diffusion is based on Ficks second law. By comparing the concentration of different nodes which locate in different stress type area, it can be concluded that the tensile stress will drive the hydrogen diffusion but the pressure stress will hinder the hydrogen diffusion. Introduction Hydrogen uptake in steel pipes can initiate or speed up various forms of degradation, such as external internal cracks, which will develop to be a blister [1]. Atomic hydrogen can be dissolved in the steel as a consequence of manufacturing process or environment. If there are corrosion phenomena on the steel surface, it will produce hydrogen atoms which can diffusion into the steel lattice [2]. There are two different diffusion mechanisms, first is vacancy diffusion, another is interstitial diffusion. The vacancy diffusion mechanism is atoms exchange with vacancies, it is applied to substitutional impurity atoms, its rate depends on the number of vacancies and activation energy to exchange. The interstitial diffusion is that the smaller atoms (H,C,O,N) can diffuse between atoms which is bigger than hydrogen atom and so on. It is more rapid than vacancy diffusion duo to more mobile atoms and more empty interstitial sites. Compared with the metallic atoms, the hydrogen atom is very small so that they can diffuse through the interstitial sites. Diffused hydrogen will be trapped at different micro structural features [3]. The diffused hydrogen atoms collected in trapping sites will recombine to form hydrogen gas which will generate high pressure [4]. With time and corrosion occurrence, the hydrogen pressure will become higher and can easily lead to the nucleation of microcracks, which finally coalesce leading to catastrophic failures, even at stresses which below the yield stress [5]. There is three different hydrogen induced damage: (i) Hydrogen Induced Cracking (HIC), (ii) Stress Oriented HIC (SOHIC) and (iii) hydrogen blistering. HIC is characterized by cracking which can be accompanied by through-thickness cracks. SOHIC is usually the results of the coalescence of HIC cracks. The blister is caused by hydrogen accumulation at inclusions. The recombination of atomic hydrogen to hydrogen gas generates high local stresses in the crack cavity, which can eventually exceed the yield stress of the material thereby leading to plastic defor1mation and surface blister [6]. When the crack cavity is full of local hydrogen gas pressure, stress gradient will be generated between the base metal and inclusions. The stress gradient may make the hydrogen atoms diffuse toward to the region which is full of high stress. The collected hydrogen atoms will increase over time, which will lead to the increase of the hydrogen pressure according to Sieverts law. The stress gradient also may make the hydrogen atoms diffuse away from the region which is full of high International Conference on Materials, Environmental and Biological Engineering (MEBE 2015) © 2015. The authors Published by Atlantis Press 573 stress. Diffusion of hydrogen will be simulated by using software Abaqus/Standard so that we can study which type of stress can promote the diffusion or impede the diffusion of hydrogen atom. Numerical simulations The specimen material was 20R. The finite element model is shown in Fig. 1. The elements of region of the crack tip are refinement. Before the analysis of diffusion hydrogen, the stress analysis should be conducted in order to obtain the stress filed distribution. The stress analysis model is shown in Fig. 2, a normal pressure is applied along the crack interface and the constraints are applied along the lateral side. The diffusion analysis model is the same as the stress analysis model except the boundary conditions. The inner wall of the diffusion model is charged at a constant hydrogen concentration (100ppm) which depends on the chemical composition of the sour environment. The concentration of the outer surface is assumed to be zero. The diffusion simulation adopt DC2D4 element. Fig. 1 Finite element model. Fig. 2 The loading model of stress analysis During the stress analysis, different groups of pressure which is 10MPa, 20Mpa, 30MPa, 40MPa, respectively, is applied. The pressure applied on the crack inner surface was used to simulate hydrogen pressure which produced by collected hydrogen gas inside the crack. The influence of different stress magnitude on hydrogen diffusion can be studied. Material model. The material model is based on the Ficks second law [7] as is shown in equation (1). During the process of hydrogen diffusion, the hydrogen atom diffusion can be affected by various factors, such as the stress, temperature and some other material self-factor like solubility and diffusivity. In a short, the hydrogen diffusion constitutive equation as is shown in equation (2) is adopted.

/pdf/influence-of-the-types-of-stress-on-hydrogen-induced-damage-nwezditk63.pdf

Hu Chen

Papers

Mechanism Analysis of Hydrogen Blisters on the Surface of a Gas Tank and Research on Its Preventive Measures

Influence of the Types of Stress on Hydrogen Induced Damage By Simulating Hydrogen Diffusion