Papers by Keyword: Hydrogen Trapping

Abstract: Modern advanced high strength steels (AHSS) for the automotive sector often contain retained austenite which promotes remarkable combinations of strength and ductility. These high strength steels may however be subject to a risk of hydrogen embrittlement. For the current contribution, hydrogen trapping and embrittlement behaviour were investigated in AHSS compositions having different levels of retained austenite. Hydrogen permeation tests revealed that hydrogen diffusion was slower for increased levels of retained austenite, being controlled most likely by reversible trapping at austenite-matrix interfaces. External hydrogen embrittlement tests via step loading also revealed that resistance to hydrogen was lower for increased levels of retained austenite. It was suggested that during step loading the hydrogen accumulated at austenite-matrix interfaces, leading to cracking when the applied stress was high enough.

Abstract: Effect of hydrogen trapping and poisons on diffusion behavior of hydrogen in commercial cold-rolled low carbon steel was investigated by means of electrochemical hydrogen permeation techniques. The experimental results reveal that diffusion rate and diffusion flux of hydrogen in the materials gradually increase with increasing the number of hydrogen charging and outgassing, and lag time significantly shortens with them, therefore, hydrogen trapping impede diffusion behavior of hydrogen in the materials. Different poisons in the hydrogen charging solution have also resulted in a certain influence on the assessment of hydrogen diffusion behavior.

Abstract: Martensitic sheet steel is increasingly being used in advanced car body construction, especially in areas where high crash loads are expected. Using such steels appropriately the weight of individual components can be reduced by up to 20 percent. Martensitic steel sheet is commercially available in the strength range of 1200 to 1900 MPa, either as cold forming or hot stamping grade. Whereas the strength of such martensitic steels is practically only a function of the carbon content, other properties such as ductility, toughness, bendability and delayed cracking resistance are severely influenced by other alloying elements and the particular thermal processing route. The paper discusses the influence of various key-alloying elements such as Nb, Mo and B on these properties and suggests routes to optimize the steel’s behavior with respect to the manufacturing and application related aspects.Keywords Martensite, prior austenite grain size, delayed cracking, grain boundary segregation, hydrogen trapping, niobium, molybdenum

Abstract: The present work aims to study the hydrogen embrittlement process in API 5L X60 and API 5L X80 steels. The tests were performed using two kinds of hydrogen sources to work with two conditions of hydrogen damage: environmental hydrogen embrittlement and internal hydrogen embrittlement. The mechanical behavior of API 5L X60 and API 5L X80 steels in tensile tests, with and without hydrogen, were studied. Under environmental hydrogen embrittlement conditions, the API 5L X60 steel presented a softening process observed by the decrease in yield strength and increase in its deformation. The API 5L X80 steel was more susceptible to the phenomenon due the deformation decrease of hydrogenated samples. In notched samples, both steels were susceptible to embrittlement as shown by the decrease in elongation. Under internal hydrogen embrittlement conditions, in both steels the changes in deformation were significant and can be attributed to changes in the hydrogen trapping due to the hydrogenation process used, the chemical composition and microstructure. It was observed that the fracture surface morphology of hydrogenated samples of both steels was ductile by microvoids coalescence, and that the distribution of dimples per unit area was higher in the API 5L X60 steel. It can be concluded, as reported in the literature, that the reversible hydrogen trapping observable in environmental hydrogen embrittlement is more damaging than irreversible hydrogen trapping, observable in internal hydrogen embrittlement.

Abstract: Microstructure influence on hydrogen trapping in a Cr-Mo type steels −2.25Cr 1Mo and 2.25Cr 1Mo 0.25V− was studied by means of electrochemical permeation test, thermal desorption spectrometry, scanning and transmission electron microscopy analysis. Both steels, used in hydrogenation reactors, in as received and artificial aged conditions exhibit a bainitic microstructure with CrxMoy and CrxMoyVz carbides finely dispersed. The hydrogen diffusivity for the 2.25Cr-1Mo-0.25V is lower than 2.25Cr-1Mo due to its higher carbide precipitation. At aged conditions TDS on samples cathodically charged with hydrogen showed an increase on the hydrogen trapping capacity for 2.25Cr-1Mo and a reduction for the vanadium modified steel, compared with the as-received state.

Abstract: Deuterium retention in single crystal and polycrystalline tungsten and molybdenum exposed to low-energy (38200 eV/D), high ion flux (10211022 D/m2s) deuterium plasmas at various temperatures were examined with the D(3He,p)4He nuclear reaction at a 3He energy varied from 0.69 to 4.0 MeV, and with thermal desorption spectroscopy. The surface morphology was examined by scanning electron microscope. Blisters formed on the Mo surfaces under plasma exposure are significantly larger in size than those for W. The D retention in the W and Mo samples increases with the exposure temperature, reaching its maximum at about 500 and 530 K (for ion fluxes of 1021 and 1022 D/m2/s), respectively, and then decreases as the temperature grows further. For polycrystalline W and Mo exposed at temperatures above 400 K, the D retention in the bulk (far beyond the ion implanted zone) is dominant. Plastic deformation caused by deuterium super-saturation within the near-surface layer is suggested as a mechanism for blister formation and creation of defects responsible for deuterium trapping at depths up to several micrometers.

Abstract: The presence of hydrogen in steel decreases its toughness and formability leading to hydrogen embrittlement. To understand the failure mechanisms of steel due to the presence of hydrogen, a profound insight in the hydrogen household of the steel is needed. This includes a study of the solubility, the diffusion and the trapping of hydrogen. Next, the absorption and desorption behavior during and after electrolytic charging must be well determined. This was investigated in this research for steels with various types of traps, e.g. dislocations, microcracks, grain boundaries and precipitates such as TiC and Ti4C2S2. The samples were cathodically charged at three different current densities: 0.8mA/cm2; 8.3mA/cm2 and 62.5mA/cm2. It was noticed that the cathodic current density used for hydrogen loading had a great influence on the results. Observation of the samples by scanning electron microscopy (SEM) showed that at the highest current density major damage of the surface had occurred. Hence it was decided to study more systematically the influence of charging current density on the resulting surface aspect and on hydrogen absorption and desorption. The hydrogen charging kinetics, maximum hydrogen solubility and hydrogen desorption behavior have also been evaluated for the different current densities during charging.

Abstract: Hydrogen absorption of incoherent TiC particles that were once reported to be strong hydrogen traps in iron at room temperature was investigated by means of thermal desorption spectrometry (TDS). The results indicated that incoherent TiC particles in iron do not trap hydrogen at all at room temperature even they are cathodically charged for a long time. Only at high temperatures and in atmosphere containing hydrogen source, incoherent TiC particles can trap hydrogen. The origin of hydrogen trapped by incoherent TiC particles was justified to be water vapor in the atmosphere during heat treatment.

Abstract: A new method has been developed to determine the activation energy for hydrogen desorption from steels by means of thermal desorption spectrometry (TDS). This method directly fits the Kissinger’s reaction kinetic formula dX/dt=A(1-X)exp(-Ed/RT) to experimentally measured thermal desorption spectrum and best fit yields the activation energy (Ed) and the value of constant A. It has been proven that this new method is applicable to precise measurement of the activation energy for hydrogen desorption from incoherent TiC particle, coherent TiC precipitate, grain boundary and dislocation in 0.05C-0.20Ti-2.0Ni and 0.42C-0.30Ti steels.