Papers by Author: Shinichi Komazaki

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: As advanced ferritic/martensitic heat-resistant steels generally have a complex structure consisting of several microstructural units (lath, block, packet, and prior austenite grain), it is very hard to separate the contribution of each microstructural unit (or its each boundary) to the strengthening mechanism in such steels. Here we explore the role of each microstructural unit in strengthening of advanced high Cr steel through nanoindentation experiments performed at different load levels. Nanoindentation results are analyzed by comparing with microstructural observations and discussed in terms of prevailing descriptions of strengthening mechanism.

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.

Abstract: Both the change in DBTT and the degradation of creep properties of the aged CrMoV cast steel which had been actually used as a main steam valve were evaluated from the conventional SP test and the SP creep (SP-C) test using by SP test specimens. The CVN DBTTs of the virgin and aged steels were estimated to be 280 and 369 K by the conventional SP test, respectively. This increase in DBTT after the long-term service exposure was considered to be caused by carbide-induced embrittlement due to coarse carbides. The SP-C test results revealed that the creep rupture strength of the aged steel was inferior to that of the virgin steel. The ratio, F / s , between the load of the SP-C test (F) and the stress of the uniaxial test ( s) was determined to be 2.4 for the same value of the Larson-Miller parameter in the creep rupture data. This linear relationship enables us to convert the results of the SP-C test into those corresponding to a conventional uniaxial creep test. Consequently, the SP testing method is expected to be appropriate tool for the determination of material deterioration and, in turn, the assessment of remaining-life of high-temperature components.

Abstract: We are developing a Small-Punch SCC testing apparatus with an in-situ observation system applied to high temperature water and high pressure. This new testing apparatus is used to evaluate the Environmentally Assisted Cracking effect. A constant loading rate was carried out with different values of speed, of water flow rate and different polishing of the sample. In BWR (Boiling Water Reactor) conditions (9MPa and 288°C) it was found that the loading rate, water flow rate and surface preparation of the sample have some influence if the test is performed during a short time (maximum 4 days). After the test, by SEM observation, studying the secondary microcracks, it is possible to find all the sequences of EAC process. The in-situ observation system permits to study the strain of the sample during the test.

Abstract: The susceptibility to environmental embrittlement (EE) of automobile spring steels was investigated using six different steels. A SSRT test and TDS analysis were applied to specimens subjected to wet-dry cyclic corrosion tests in a NaCl solution. Experimental results revealed that the reduction in ductility after the corrosion tests was pronounced with increasing strength level. This degradation was closely associated with the resistance to pitting corrosion. Consequently, the hydrogen absorbed in steel and the corrosion pit as a geometric damage were responsible for the EE of spring steels. The hydrogen in rust layer had no significant influence on the EE.