In this study, conventional and advanced methods with instrumented spherical indentation techniques were applied, respectively, to estimate mechanical properties of steels used in gas pipelines. The advanced method has recently been proposed by authors. The instrumented spherical indentation tests were performed three times for each steel to confirm the reproducibility. Meanwhile, the tensile tests were performed for axial and circumferential directions, respectively. Then, the estimated results of the true stress-true strain curve, yield stress and tensile strength were compared with those obtained by tensile test to validate the estimation accuracy in these methods. The results showed that the indentation load-depth curves were reproducibly obtained in each steel. And then, the tensile test results were in good agreement with those estimated by the advanced method rather than the conventional method. Thus, the utility of the advanced method was successfully demonstrated. From the results, it can be expected that the advanced method with the instrumented spherical indentation technique validated in this study become useful as a maintenance technique for gas pipelines and other steel structures.

View full abstract

Lath martensitic steels were pre-strained before hydrogen charging, and subjected to thermal desorption analysis (TDA) . The measured TDA curves was analyzed by peak separation with Gaussian distribution functions to know the state of hydrogen. Reasons for the effects of pre-straining were explored, together with the effects of pre-straining on the hydrogen embrittlement susceptibility. Pre-straining made hydrogen pick-up increased:It was trapped by dislocation and grain boundary, in combination with further formation of vacancy and vacancy cluster as hydrogen traps. They are originated by the cooperation of the charged hydrogen and internal stress introduced by pre-straining. These changes can be brought about through the link of the following four processes: (1) hydrogen trapping at dislocation, (2) vacancy production through dislocation interaction, followed by pairing with hydrogen, (3) agglomeration of vacancy-hydrogen pair to vacancy cluster-hydrogen complex, (4) transport of vacancy-hydrogen pair toward grain boundary to increase hydrogen along it. Processes (3) and(4) are in competition to each other. When the steel is aged at 200°C before hydrogen charging, the state of hydrogen was hardly affected by pre-straining. This is because the dislocation, which was newly formed by pre-straining, loses the ability of hydrogen trapping through strain aging with carbon atom, concerning to the process (1) in the above: The processes (2) to (4) are consequently lacked. When the strength of the steel is higher, pre-straining facilitates to increase the amount of hydrogen trapped by grain boundary, through localized dislocation density close to grain boundary. Thus the steels with higher strength become more susceptible to hydrogen embrittlement with grain boundary facet.

View full abstract

It is needed to obtain the fracture toughness of butt-welded joints of heavy thick steel plates accurately to ensure strength and reliability of upper structure of very large container ships using heavy thick steel plate with 50 to 100mm thickness.
In our previous study, the tensile test by using the center notched small size tension test specimen had been proposed to obtain the fracture toughness K_IC for unstable brittle fracture initiated at the tip of a fatigue crack which generates on the surface of butt-welded joint of heavy thick high-tensile strength steel plates then propagates from there.
In this study, the fracture toughness K_IC values of butt-welded joints of heavy thick high-tensile strength steel plates (YP36, YP40, YP47) for ship building by the center notched small size tension tests were evaluated. As the results, by using the center notched small size tension test specimen, valid fracture toughness K_IC can be obtained and by adding the effect of the weld residual stress the fracture toughness K_IC values of the welded joints by the center notched small size tension tests show good agreement to them by the center notched wide plate tension tests.

View full abstract

Based on the recent results of material testing in high pressure hydrogen environment, mechanisms of the hydrogen induced cracking are discussed as follows.
(1) The hydrogen induced cracking shows the morphology of multiple-site cracks on the metal surface and grows continuously into the inner of metal. The terminology of hydrogen embrittlement is not correct use in this case.
(2) The origin is the plastic deformation by loading. An active slip band results in localized rupture of the oxide film on the metal surface and creates freshly slip steps. Therefore, there is no effect of hydrogen during the elastic deformation by loading.
(3) The dissociation of hydrogen molecule into atomic hydrogen takes place easily on the freshly created slip step. At the same time, rupture of the oxide film generates the dislocation source and the slip is localized by the dislocation emission.
(4) The dislocation emission operates as a hydrogen intake mechanism and hydrogen operates in parallel the activity of mobile dislocations. Therefor, hydrogen can enter the metal by the plastic deformation and hydrogen entry can activate the plastic deformation.
(5) Two models of hydrogen induced cracking are shown;one is a mechanics model and the other a crystallographic model. Both models constitute a pair of intersecting two slip planes and a crack plane divided by the two slip planes. It is also shown that the difference of body centered cubic metal from face centered cubic metal in the crystallographic model.

View full abstract

A new low-nickel steel plate has been developed for liquefied natural gas (LNG) tanks, which has excellent performances equivalent to those of the conventional 9%Ni steel under ultralow temperature. In this study, properties of the developed steel plates were evaluated to assess its applicability to LNG storage tanks. The brittle crack initiation properties of the developed steel plates and its welded joints were evaluated by CTOD tests and cross weld notched wide tests. Duplex ESSO tests were conducted to demonstrate brittle crack arrestability of the base plate. Fatigue properties and physical properties were also examined. Additionally, safety evaluations for applicability of the developed steel plate to LNG tanks were discussed based on fracture mechanics.

View full abstract