High nitrogen containing stainless steel was prepared by mechanical alloying (MA) method from SUS 447 powder. The SUS 447 powder was ball-milled at N2 atmosphere and then HIP treatment was carried out. The obtained stainless steels were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). With the increase in the time of MA treatment, dissolved nitrogen content of the stainless steel was increased, and ferrite phase changed into austenite phase. The evaluation of corrosion behavior was carried out by the electrochemical corrosion test. For the stainless steel with MA treatment without nitrogen, it was found that the pitting potential in sulfuric acid solution including the chloride was very noble. And clear pitting corrosion was not observed for the stainless steel with MA treatment with nitrogen. The results of electrochemical corrosion test indicated that localized corrosion was suppressed by the MA treatment.
There is a possibility that alternating current will cause corrosion in a defect of the coating on buried steel pipeline. In this research, we developed an evaluating method with which we could evaluate alternating current corrosion of the steel in a short time compared with conventional evaluating method by mesuring corrosion weight loss. The results of this research showed that alternating current corrosion of buried steel under cathodic protection could be estimated by measuring the IR-drop-free potential, especially peak (maximum) potential of the steel using IR-drop-free probe. It was also found that corrosion protection of the steel could be achieved when peak potential of the steel was over 100mV less noble than free corrosion potential of the steel.
Localized corrosion damages from pitting or crevice corrosion attacks frequently appear on stainless steel structure, notably on the equipment for chemical plant. It is known the initiation process of localized corrosion produces current noise. This behavior is generated by the breakdown and repassivation of passive film on an electrode, the analysis of the current noise would be important signals for monitoring the initiation of localized corrosion. We have been developing an analysis for current noise using Random Pulse method.1) This paper presents the experimental results that show the analysis we have development has a capability to identify the corrosion phenomena of the electrode. Furthermore, we found the waveform of the current changes responding to the applied potential pulse gives the data characteristic of corrosion on an electrode.
Recently, the monitoring method is required in order to estimate the corrosion rate strictly for timely repairing and renewal of underground steel structures. In this study, the monitoring method for microbially influenced corrosion with Sulfate-reducing Bacteria (SRB) of steel material immersed in underground water was investigated. It was found that the polarization resistance method with constant direct current, to impress larger current and longer time than the conventional method, could be used in such environment. It was also found that the coefficient to convert the reciprocal of polarization resistance to the corrosion rate was approximately 2mV, which was about 10% of the conventional coefficient.
On the catholic protection for marine steel structures, total weight demand of aluminum alloy anode is remarkably influenced by the life of anode set up at site, therefore an accurate expected anode life is indispensable factor for the catholic protection design. The analysis of the site survey data that had been executed in the past was tried to verify the relation to which the potential and the current density decayed according to time in the catholic protection of steel in seawater. By this analysis, in the protective current density condition below the dissolved oxygen diffusion limiting current density of the examination sea area, it is confirmed that the relationship (slope parameter) of potential vs. current density is certainly linear with the corresponding to the cathode potential. In view of a linear relation between the potential and the current, the anode life can be evaluated by the cathode potential because the anode generation current is decided certainly by not the ohmic resistance calculated from anode shape but the cathode potential. That is, it is necessary to review the method of calculating the anode life because the anode generation current is in the relation controlled automatically by not the ohmic resistance of the protection circuit but the cathode potential. This paper describes the outline of the anode design based on slope parameter obtained by the site survey data analysis.
To evaluate the remaining lifetime of high-temperature parts of fossil-fuel power plants precisely, the influence of the solidified direction and the crack-surface oxidized zone on the crack growth rate of Ni-based directionally solidified super alloy was investigated by using the results of long-term fatigue tests. Three types of specimens, 0°, 45° and 90° specimens, in which the load direction was parallel, diagonal and normal to the solidified direction, were prepared to evaluate the effect of solidified direction on the crack growth rate. The crack initiation lives were influenced by the solidified direction and the crack initiation life of the 0° specimen was longer than the 45° and 90° specimens. Also the crack growth rates were influenced by the solidified direction at the same cycle number. Under 600°C, the fatigue crack growth rates of 45° and 90° specimens were almost the same. The fatigue crack growth rate of 0° specimen was slower than the crack growth rates of 45° and 90° specimens during the stress intensity factor range ΔK was less than 30MPa√m. And under 860°C, the fatigue crack growth rates of 0°, 45° and 90° specimens were almost the same. The long-term fatigue-crack growth rate under 860°C once decreased with the growth of the crack. And this behavior seemed to arise from the residual-stress field produced by the volumetric expansion owing to the growth of the oxidized layer, the growth of which was evaluated by the experimentally obtained parabolic oxidation law.
The quantitative estimation of the failure stress of a lithium tantalate (LT) single crystal due to thermal stress was investigated. Circular plate test specimens were heated in a silicone oil bath, then subjected to large thermal stress by room temperature silicone oil. Cracking occurred during cooling. The transient heat conduction analysis was performed to obtain temperature distribution in the test specimen at the time of cracking, using the surface temperatures measured in the test. Then the thermal stress was calculated using the temperature profile of the test specimen obtained from the heat conduction analysis. It is found from the results of the thermal stress analysis and the observation of the cracking in the test specimens that the cracking induced by thermal stress occurs mainly in cleavage planes due to the stress component normal to the plane. As for a size effect of failure stress, large-sized circular plate test specimens show lower failure stress than small-sized ones. Four-point bending tests were also performed to examine the relation between the critical stress for the cracking induced by thermal stress and the four-point bending strength. Several discussions were made for size effect of failure stress from the viewpoint of the Weibull's weakest link model.
WC-12Co thermally sprayed coating, formed by high velocity oxygen-fuel spraying, is widely used in industry for its high wear resistance properties. Unfortunately, WC-12Co coatings are plagued by low corrosion resistance in aqueous solutions. Previous studies have shown that adding Cr to the coating improves corrosion resistance, but the effect on corrosion fatigue fractures remains unknown. In this study, first, two kinds of coatings (WC-12Co & WC-10Co-4Cr) were deposited on SS400 steel by high-velocity flame spraying. Second, fracture behavior was investigated through corrosion fatigue testing in an Na2SO4 aqueous solution (R=0, f=14Hz). The cause of the corrosion fatigue fractures was found to be corrosion pits formed at the interface between the coating and the substrate. The corrosion fatigue life for WC-10Co-4Cr was longer than that for WC-12Co in low cyclic stress areas (stress amplitude σa<75MPa). This extended corrosion fatigue life is assumed to be due to decreased corrosion response, because the passivation film on the surface of the WC-10Co-4Cr coated materials was quickly restored, even after it had been mechanically broken.
It is investigated that the effect of anodizing times and dipping times of the aluminum in a 4 mass% oxalic solution bath at 313 K and that of the hydrothermal temperature (373K, 423K, 473K) on the alkali resistance time (ART) and film structure of treated films. As the temperature of the hydrothermal treatment increased, the ART also increased considerably, The SEM observation of the cross sections of hydrothermally sealed anodized films revealed that, they were, in general, composed of an outer layer consisting of cube-shaped hydrate crystals (outer hydrate layer), an middle layer having no distinct morphological features (inner hydrate layer), and an inner oxide layer containing the original anodized oxide film (oxide layer). The actual microstructures of sealed films could be classified into three groups corresponding to three regions of anodizing times. In the case of short anodizing times (region I), the oxide layer was absent. In the case of long anodizing times (region III), the thickness of the oxide layer was constant. In region II, intermediate region between region I and region III, the thickness of the oxide layer increased as the anodizing time increased. The thickness of outer hydrate layer was about 0.5μm within all region. In addition, if the sum of the anodizing time and dipping time was the same, the thickness of the inner hydrate layer was also the same. And, the ART of the hydrothermally sealed anodized films was affected by the inner hydrate layer thickness and it's characteristics.
This study is concerned with the split Hopkinson bar method for tension test of strip specimens. The split Hopkinson bar method (SHB) has been used for impact tests such as compression and shearing, and is currently modified for tension test. It is demanded that the impact tensile behaviour of high-tensile strength steel plates is made clear in the design of light-weight and high stiffness automotive parts for weight reduction in materials. However, the method of impact tension test using the SHB has not been established because the testing conditions such as specimen geometry and fastening are quite different from them in compression test. In this paper, the characteristics of the SHB method for tension test are examined using by some experiments and numerical simulations. Particularly the appearance of initial peak in stress-strain curves like an increase of yield point is discussed from the viewpoint of the specimen geometry and the attachment of a specimen and stress bars with screws. It is found that the fastening method affects strongly the responses of stress waves measured on an output bar and then causes the virtual peak in the initial portions of stress-strain diagram.
In vehicle manufacturers, it is demanded to develop the design of light-weight and high stiffness automotive body for reduction of CO2 emission. Recently, in order to satisfy the two conflicting demands, the tailored blanks technique using high tensile strength steels attracts attention. Then for contributing the fundamental data it may be important to examine not only the static and fatigue properties but also the dynamic characteristics of impact deformation strength of welded structural members of high strength steel sheets. In the present study the measurement of impact tensile strength of welded structural members for vehicles is performed by means of the modified split Hopkinson bar method for tensile test.