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- Hydrogen diffusion (5)
- Hydrogen embrittlement (5)
- Hydrogen (4)
- Diffusion (3)
- Neutron radiography (3)
- Blister (2)
- Energy dispersive diffraction (2)
- In situ (2)
- Iron (2)
- Neutron tomography (2)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metallic crystal lattice. For the investigations reported in this contribution, hydrogen thermal desorption analysis has been carried out to observe the influence of hydrogen desorption on the lattice of super martensitic stainless steel during continuous heating. The lattice expansion parameter and the phase transformations have been monitored during the thermal desorption process, and the influence of hydrogen on such characteristics has been evaluated. It was found that hydrogen has a significant influence on both the lattice parameter and on the thermal expansion. However, hydrogen has no influence on phase transformation during thermal desorption. The hydrogen's desorption behavior in this process was also observed and it turned out that hydrogen desorbs in two stages, i.e., firstly diffusible hydrogen and trapped hydrogen afterward.
Energy dispersive synchrotron diffraction for in-situ analyses of hydrogen behavior in steels
(2013)
The extent of hydrogen embrittlement phenomenon is dependent on the interaction of hydrogen with the material i.e. diffusion behavior, lattice distortions, phase transformations and residual stresses developments. For the better understanding of the hydrogen interaction with the metal, possibilities of performing in-situ observations of the mentioned factors has to be accomplished. The EDDI (Energy Dispersive Diffraction) beamline at the BESSY II facility in Berlin provides this possibility and was used in order to investigate the aspects of this phenomenon in in-situ scale.
The energy dispersive method entails a synchrotron X-ray white beam with a wide energy range (20–120 keV) which allows a fixed position of the sample and the detector, and combined with the high photons flux it allows a very short measurement time for obtaining a full scale X-Ray Diffraction patterns.
In this contribution, the successful usage of high energy X-rays, applied for the first time for hydrogen behavior observations, provided the ability for evaluating in-situ the hydrogen diffusion behavior and the hydrogen interaction with stresses fields in the material.
Steel samples were charged with hydrogen and examined by Synchrotron radiation.
Two different methods were used in this research; the first is diffusible hydrogen concentration evaluation extracted from the lattice distortions, i.e. changes of the lattice parameter. The second is calculation of the residual stresses present in the sample and the interaction of hydrogen with them, using the known sin2ψ method.
In situ analysis of hydrogen behaviour in stainless steels by high energy synchrotron radiation
(2011)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metal matrix. The present contribution provides a report of first in situ investigations of hydrogen effects on the lattice of the austenite and the martensite phase in a supermartensitic stainless steel using hard synchrotron radiation for respective energy dispersion diffraction measurements. Lattice distortions, such as the planar spacing expansion, have been monitored during the complete hydrogen effusion process over 24 h and are directly correlated to the dissolved hydrogen in the metal matrix. Among other results, it turned out that hydrogen at the introduced concentration level causes a reversible lattice expansion and after hydrogen effusion the lattice nearly regained its original dimensions. Hydrogen much more significantly affects the austenite phase with respect to lattice expansion than the martensite phase, but no phase transformations occur during the hydrogen desorption process.
The present contribution highlights recent trends in investigating hydrogen diffusion and cracking in steel welds. For such studies, supermartensitic stainless steels (SMSS) have exemplarily been selected. These materials have been used for offshore and marine constructions for about two decades now. They present a versatility of improved properties such as high strength to thickness ratio, good weldability and good corrosion resistance. However, as shown by respective failure cases, SMSS welds might become prone to hydrogen assisted cracking and the degradation phenomena are more easily to investigate due to the nearly homogeneous martensitic microstructure than in other materials. Generally, it has to be distinguished between cracking that occurs during or shortly after fabrication welding, or during subsequent operation of SMSS components. In order to achieve crack avoidance during fabrication and service, conclusive test sequences have to be applied, ranging from field tests at real components and full scale tests investigating the materials behavior under real service conditions to basic and small scale tests, such as tensile and corrosion tests, oriented more towards a materials ranking. Considerable testing of SMSS welds has been carried out and the present paper particularly summarizes spotlights on 1:1 scale component testing of welded tubulars, slow strain rate testing and basic tests oriented to elucidate the hydrogen behavior and degradation in SMSS weld microstructures. Also, permeation tests, hydrogen dependent degradation of mechanical properties and thermal desorption spectroscopy are adressed. As a specific item, first results of lately conducted investigations for tracking hydrogen movement in such weld microstructures by using high energy synchrotron radiation are elucidated.
TRIP-steels offer a good combination between strength and ductility. Therefore TRIP-steels are widely used in the automobile industries. The aim of this work is to study the stability of involved phases during heating and to identify the kinetics of the occuring phase transformations. For that purpose, in-situ diffraction measurements, using high energy synchrotron radiation were conducted. The analysis revealed the decomposition of the metastable austenitic phase into carbide and ferrite along the heating process and the regeneration of the austenite by further heating of the sample.
Hydrogen interaction with residual stresses in steel studied by synchrotron X-ray diffraction
(2014)
The residual stress state in a material has an important role in the mechanism of cracking, induced or assisted by hydrogen. In this contribution, the beamline EDDI in BESSY II instrument in Berlin was used in order to investigate the influence of hydrogen upon the residual stresses state existing in a Supermartensitic stainless steel sample. The method used for investigating the residual stresses is the sinus square ψ method. This method involves the usage of high energy X-ray diffraction in order to measure the residual stress state and magnitude. It was found that hydrogen presence has a significant influence upon the magnitude of the residual stresses, as its value decreases with high hydrogen content. This effect is reversible, as hydrogen desorbs from the sample the residual stress magnitude gains its initial value before hydrogen charging.
Modern methods like carrier gas hot extraction enable the quantification of dissolved hydrogen as well as the determination of the hydrogen trapping and diffusion behavior. This method was applied in order to compare for the first time the hydrogen diffusion and trapping behavior in electrochemically charged and welded duplex stainless steel (1.4462). Characteristic extraction temperatures (400, 650, and 900 °C) were used to quantify the amounts of diffusible hydrogen and trapped hydrogen for the base material and the weld metal, and in order to calculate the effective diffusion coefficients corresponding to the specific temperature. The comparison of the charging methods showed that electrochemically charged samples have a higher content of diffusible hydrogen than the welded samples. In addition, the effusion times increase in welded samples, which indicate a higher amount of trapped hydrogen. In electrochemically charged weld samples, a significant lower concentration of hydrogen was determined than in the base material. In addition, the effective diffusion coefficients were calculated for every microstructure and charging method. It was found that the base material has a higher effective hydrogen diffusion coefficient than that of the weld metal. This effect is due to the tortuous path of hydrogen diffusion in the weld metal.
Hydrogen embrittlement (HE) is a widely known phenomenon and under investigation already for more than a century. This phenomenon, though thoroughly studied, is not yet completely understood, and so far, there are several suggested mechanisms that try to explain the occurrence of HE. One important factor of understanding the HE phenomenon and predicting hydrogen-assisted failure is the descent knowledge about the hydrogen transport behaviour in the material. Neutron radiography is a proven method for tracking hydrogen diffusion and it was applied successfully in various research studies. In the presented study, we examined the hydrogen effusion behaviour in duplex stainless steel by means of neutron radiography and calculated the effective diffusion coefficient from the obtained transmission images.
We investigated the hydrogen distribution spatially and temporally in technical iron at room temperature. Samples were charged electrochemically and subsequently analysed by means of neutron radiography and tomography. The radiographic images allowed for a time-resolved analysis of hydrogen fluxes. The three-dimensional distribution of hydrogen measured by neutron tomography delivered valuable information for the damage analysis of hydrogen-induced cracks. For the first time hydrogen concentration gradients inside the material could be detect directly together with the cracks. The neutron radiography and tomography results were gained at the Research Reactor BER II of the HZB in Berlin.