Michael J. Morgan

Predicting Tritium and Decay Helium Effects on Burst Properties of Pressure Vessels

Burst testing is used to assess the performance of stainless steel pressure vessels designed to contain tritium, a radioactive isotope of hydrogen. Burst ductility of tritium-exposed vessels is reduced in time as a result of the combined embrittlement effects from tritium that has diffused into the microstructure and its radioactive decay product, helium-3. A materials system model and finite element procedure were developed to predict burst pressure and the vessel volume change (ductility) during burst testing. The model is used to predict changes in burst pressure and ductility from the tritium service history, known values of tritium diffusivity, and published data on the effects of tritium and helium on the tensile properties of stainless steel. Good agreement has been achieved with actual burst test data for unexposed vessels. It is shown that the service history could be used to derive values of tritium concentration in the metal and the depth of penetration in the vessel sidewall. These values could be used in the finite element model to predict values of burst pressure and burst ductility for tritium-exposed vessels.Copyright

Comparison of Decoupled and Coupled Analyses for Hydrogen Transport in Fracture Specimens

Hydrogen embrittlement is an important issue in many industries. Fracture resistance of metals is often weakened by the presence of hydrogen. In this paper, two diffusion models are compared for hydrogen transport analysis. One is the coupled model where the concentration of hydrogen in the lattice is integrated with mechanical properties. The other is the decoupled model in which the hydrogen diffusion is independent of the mechanical properties; but depends on the stress state. Finite element analyses are performed for a boundary layer specimen with a blunting crack and a four-point bend specimen with rounded notch. Hydrogen concentration profiles around the blunt crack (or notch) are compared under different boundary conditions and material properties. It is observed that, in spite of the difference in constitutive models, there is a similarity between hydrogen concentration in normal interstitial sites by the two models. In case that large plastic strain is present (such as those in low to moderate strength steels) there is a substantial difference in hydrogen concentration between the two models.Copyright

HYDROGEN EFFECTS ON THE FRACTURE TOUGHNESS PROPERTIES OF FORGED STAINLESS STEELS

The effect of hydrogen on the fracture toughness properties of Types 304L, 316L and 21-6-9 forged stainless steels was investigated. Fracture toughness samples were fabricated from forward-extruded forgings. Samples were uniformly saturated with hydrogen after exposure to hydrogen gas at 34 MPa or 69 and 623 K prior to testing. The fracture toughness properties were characterized by measuring the J-R behavior at ambient temperature in air. The results show that the hydrogen-charged steels have fracture toughness values that were about 50-60% of the values measured for the unexposed steels. The reduction in fracture toughness was accompanied by a change in fracture appearance. Both uncharged and hydrogen-charged samples failed by microvoid nucleation and coalescence, but the fracture surfaces of the hydrogen-charged steels had smaller microvoids. Type 316L stainless steel had the highest fracture toughness properties and the greatest resistance to hydrogen degradation.

HYDROGEN EFFECTS ON FRACTURE TOUGHNESS OF TYPE 316L STAINLESS STEEL FROM 175 K TO 425 K

The effects of hydrogen on the fracture-toughness properties of Type 316L stainless steel from 175 K to 425 K were measured. Fracture-toughness samples were fabricated from Type 316L stainless steel forgings and hydrogen-charged with hydrogen at 34 MPa and 623 K for two weeks prior to testing. The effect of hydrogen on the J-Integral vs. crack extension behavior was measured at various temperatures by fracturing non-charged and hydrogen-charged samples in an environmental chamber. Hydrogen-charged steels had lower toughness values than non-charged ones, but still retained good toughness properties. The fracture-toughness values of hydrogen-charged samples tested near ambient temperature were about 70% of non-charged values. For hydrogen-charged samples tested at 225 K and 425 K, the fracture-toughness values were 50% of the non-charged values. In all cases, fracture occurred by microvoid nucleation and coalescence, although the hydrogen-charged samples had smaller and more closely spaced microvoids. The results suggest that hydrogen effects on toughness are greater at 225 K than they are at ambient temperature because of strain-induced martensite formation. At 425 K, the hydrogen effects on toughness are greater than they are at ambient temperature because of the higher mobility of hydrogen.

Hydrogen Effects on the Burst Properties of Type 304L Stainless Steel Flawed Vessels

The effects of hydrogen and burst media on the burst properties of Type 304L stainless steel vessels were investigated. The purpose of the study was to compare the burst properties of hydrogen-charged stainless steel vessels burst with different media: water, helium gas, and deuterium gas. A second purpose was to provide data to improve an existing finite-element model for predicting burst behavior. Burst tests were conducted on hydrogen-charged and uncharged axially-flawed cylindrical vessels. The results indicate that samples burst pneumatically had lower volume ductility than those tested hydraulically. For pneumatic burst tests, samples burst with deuterium gas had slightly lower ductility than helium gas tests. For uncharged samples, burst pressure was not affected by burst media. For samples pre-charged with hydrogen, deuterium burst pressures were about 80% of the hydraulic or helium burst pressures. Hydrogen-charged samples had lower volume ductility and slightly higher burst pressures than uncharged samples. The results of the tests were used to verify and improve a previously developed predictive finite-element model. The existing finite-element model can qualitatively predict the expected changes in burst properties with hydrogen or tritium service, but a better material property database is required for quantitative predictions.Copyright

Investigation of Hydrogen Transport Related Fracture in Stainless Steels

Stress-assisted hydrogen diffusion analysis was performed on arc-shaped tension specimen (C-specimen) fabricated from Type 21-6-9 stainless steel. Two-dimensional finite element method was applied for the determination of elastic-plastic crack front stress fields, which was later coupled with hydrogen diffusion analyses. The extension of Ficks rule was used as the governing equation for the diffusion analysis. The distributions of hydrogen concentration were compared under various Internal Hydrogen Embrittlement (IHE) and Hydrogen Environment Embrittlement (HEE) conditions. Without any external hydrogen pressure in IHE conditions, the increment of hydrogen concentration driven by the gradient of hydrostatic stress was offset by the out-gasing driven by gradient of hydrogen concentration at the crack front region. As the hydrogen pressure increases, the hydrogen concentration at the crack front region becomes dominant and show peak value around the crack tip. Compared with the results of IHE cases, the hydrogen concentrations in HEE conditions could reach the level of steady state in a relatively short time which is attributed to the high solubility and diffusivity of the material at high temperature.Copyright