Papers by Author: Yutaka Kohno

Abstract: Electrochemical material characterization method was applied to the creep specimens of 12%Cr ferritic steel to evaluate the changes in precipitates during creep. Experimental results revealed that the peak current densities “Ip1” and “Ip2” appeared at specific potentials during anodic polarization curve measurement in 1M-KOH solution. These peak current densities corresponded to the selective dissolution of several kinds of precipitates, respectively. The Ip2, which reflected the amount of M23C6 and Laves phase, increased more siginificantly with creep than thermal aging. On the other hand, the effect of applied stress was not reflected on the change in Ip1, although it increased with thermal aging and creep as well as the Ip2.

Abstract: The recently developed small punch (SP) creep test was applied to four different heatresistant ferritic steels, namely, two kinds of conventional ferritic steels which had been actually used in the high-temperature components for long periods and two advanced high chromium ferritic steels for fusion reactor materials to investigate the applicability of the SP creep test. The ratio of the load of SP creep test to the stress of standard uniaxial creep test was calculated so that both the creep rupture curves (load/stress versus Larson-Miller parameter curves) were overlapped to convert the results of SP creep test into those of standard test. As a result, the ratio was determined to be 2.4, irrespective of the kind of ferritic steel. This result indicates that the creep rupture strength of heat-resistance ferritic steels can be estimated using a miniaturized plate-type specimen and this conversion coefficient 2.4 independent of the kind of ferritic steel.

Abstract: The thermally aged 10Cr-1Mo-1W-VNbN steels were charged with hydrogen by cathodic electrolysis and then were subjected to the thermal desorption spectroscopic analyses to examine an applicability of hydrogen as a tracer for evaluating the microstructure. The variation in hydrogen desorption characteristics with aging was discussed from the view points of microstructural changes. Experimental results revealed that the amount of hydrogen desorbed, CH, decreased monotonously as the aging proceeded and there was a relatively good correlation between the CH and Vickers hardness. The CH was expected to be a useful indicator for the material degradation. Additionally, the decomposition of the profile was attempted to understand the metallurgical meaning of hydrogen profile and apply the present method to the microstructural evaluation. As the result, it was suggested that the changes in two decomposed profiles with aging might reflect the variation of dislocation density and the precipitation process of M23C6 carbide and/or Laves phase, respectively.