Structural carbon steel (CS) and weathering steel (WS) were exposed to various atmospheric climates of Thailand for one year. After the exposure, the samples were cleaned and corrosion losses were ...
The growth rate of fatigue cracks is accelerated under high pressure hydrogen because of hydrogen embrittlement. Hydrogen diffusion is enhanced by a hydrostatic stress field, and an accumulation of hydrogen in the fracture point is necessary to cause hydrogen embrittlement. In this paper, fatigue crack growth behavior under different hydrogen pressure was first investigated. Hydrogen diffusion behavior was analyzed by a finite element analysis based on the Fick's diffusion theory with a modification in the stress gradient term. Fatigue crack growth rate was accelerated by higher pressure because of increased hydrogen concentration around the crack tip region. Clear relationship was seen between the crack growth acceleration rate and the rate of hydrogen accumulation in the crack tip region.
It is important to predict the stress driven hydrogen induced cracking at the weld joint on the basis of computational mechanics from the view point of engineering problem. One of authors has been proposed α multiplication method which magnifies the hydrogen driving term in the diffusion equation to realize correctly hydrogen concentration behaviors. In this study, on the basis of proposed numerical analysis, behaviors of hydrogen diffusion and concentration during cooling process of y-grooved weld joint were analyzed and the mechanism of hydrogen induced cracking was investigated. The behaviors of hydrogen diffusion and concentration for the model of y-grooved weld joint was analyzed by combining α multiplication method with the coupled analyses of heat transfer – thermal stress – hydrogen diffusion. As a result, hydrogen was found to diffuse from weld metal to base metal through HAZ (Heat Affected Zone), and concentrate at the position of blunt angle side of weld groove bottom. It was found that hydrogen concentrates at the position of the local maximum value of hydrostatic stress gradient. This analytical result was found to well predict the actual hydrogen induced cracking of the y-grooved weld joint. Therefore, it was considered that prediction of hydrogen induced cracking becomes possible using this method of analysis.
Abstract Current hydrogen pipeline code ASME B31.12 requires that pipe materials shall be qualified for adequate resistance to fracture in hydrogen gas based on Article KD-10 of ASME BPVC, Sec. VIII, Division 3. In order to assess the integrity of a hypothetical hydrogen pipeline, fracture toughness and fatigue crack growth tests under gaseous hydrogen at up to 21MPa were first conducted using a recent Grade X65 linepipe with fine grained bainitic microstructure. Fatigue crack growth in the pressurized linepipe with semi-elliptical surface flaw was calculated by the procedures described in the Article KD-10 using the da/dN data obtained from the X65 linepipe and the fatigue crack growth equation specified in ASME B31.12. Pressure cycles were applied to the pipe with a surface flaw to investigate the effects of pressure range and design factor. The critical crack size was analyzed using the failure assessment diagram (FAD) concept which is also specified in Article KD-10. Significant fatigue crack growth was not observed under the lower design factor such as fD = 0.5 with small pressure range, while fatigue crack growth was drastically accelerated under the higher design factor and large pressure fluctuation. Integrity assessment by FAD analysis for longitudinal semi-elliptical crack and girth weld flaw clarified how the toughness value affects the critical condition.
It is important to predict the stress driven hydrogen induced cracking at the weld joint on the basis of computational mechanics from the view point of engineering problem. In this study, On the basis of proposed numerical analysis, behaviors of hydrogen diffusion and concentration during cooling process of y-grooved weld joint were analyzed and the mechanism of hydrogen induced cracking was investigated. One of authors has been proposed a multiplication method which magnifies the hydrogen driving term in the diffusion equation to realize correctly hydrogen concentration behaviors. In this study, the behaviors of hydrogen diffusion and concentration for the model of y-grooved weld joint was analyzed by combining a multiplication method with the coupled analyses of heat transfer – thermal stress – hydrogen diffusion. As a result, hydrogen was found to diffuse from weld metal to base metal through HAZ (Heat Affected Zone), and concentrate at the position of blunt angle side of weld groove bottom. It was found that hydrogen concentrates at the position of the local maximum value of hydrostatic stress gradient. This analytical result was found to well predict the actual hydrogen induced cracking of the y-grooved weld joint. Using this method of analysis, prediction of hydrogen induced cracking becomes possible.
The severity of sour environments has been determined in accordance with the European Federation of Corrosion 16 and NACE MR0175/ISO 15156-2:2015 standards for carbon and low-alloy steels, based on the experimental results of sulfide stress cracking (SSC). However, the severity map obtained from SSC test results cannot be applicable to the hydrogen-induced cracking (HIC) susceptibility. In this study, the hydrogen permeability and crack area ratio of HIC under various pH and H2S partial pressures (pH2S) were measured to establish the link between the sour environmental severity and HIC susceptibility using grades X65 to X80 steels for linepipes. In addition, the hydrogen concentration at the location of the HIC was calculated by the finite element analysis. The results showed that the sour environmental severity map obtained from hydrogen permeation tests changes with time because the hydrogen permeability reached maximum values in the early stage and steady-state values in the later stage. Then, the HIC susceptibility did not correspond to the maximum permeability, but to the steady-state hydrogen permeability. In addition, the hydrogen content at the location of the HIC did not correspond to the maximum hydrogen permeability but corresponded to the steady-state hydrogen permeability, because HIC occurred in the center segregation part and the hydrogen atoms required a certain time to diffuse from the metal surface to the mid-thickness. These results suggest that the HIC susceptibility is dominated by the severity map obtained from the steady-state hydrogen permeability.
Abstract The fracture toughness test under 100% hydrogen with testing pressure of 21 MPa was conducted on a recently produced Grade X80 linepipe and its seam and girth welds. For comparison, two X70 linepipes and one X65 linepipe and their welds were tested as well. For the X80 linepipe, the seam weld heat affected zone (HAZ) exhibited the same level of fracture toughness as the base metal. The toughness is higher than the minimum requirement in ASME B31.12 when it is measured by the crack initiation toughness not by the conventional 0.2mm offset value in a J-R curve. The girth weld HAZ of the X80 linepipe showed higher fracture toughness than the seam weld HAZ. Using the fracture toughness values obtained from the weld fracture toughness tests in 21 MPa hydrogen, the critical depth of planar axial flaw was determined for different measured values of toughness and design factor using assessment procedures in API 579. The critical flaw depth strongly depends on the residual stress assumption used in the assessment. The allowable planar flaw dimensions for girth welds were determined using the assessment procedure in Annex A of API 1104. It was shown that the girth weld of the X80 linepipe would have workable allowable flaw dimensions due to the high toughness measured in the tests. However, girth welds with the required minimum toughness of ASME B31.12 would have allowable flaw dimensions much smaller than the workmanship criteria in API 1104 or equivalent standards. There can be multiple causes for this outcome which should be investigated further.
