Junichi Kusumoto

Creep strengthening by lath boundaries in 9Cr ferritic heat-resistant steel

Abstract The interactions between dislocations and lath boundaries in Grade 91 steel were observed by an in situ transmission electron microscopy tensile test at 973 K. Dislocations glided slowly and bowed out in a martensite lath interior. The ends of the dislocation were connected to the lath boundaries. In a tempered specimen, the pinning stress caused by the lath boundary was estimated to be >70 MPa with a lath width of 0.4 μm. In crept specimens, lath coarsening reduced the pinning effect.

Residual Life Assessment of 2.25Cr-1Mo Boiler Pipe by Small Punch Creep Test

The non-destructive techniques such as the microstructural assessment are usually performed for residual creep life assessment of plant components. Since these techniques have limited accuracy, destructive assessment techniques such as uniaxial creep tests are required to improve the accuracy of the assessment. However, if this type of destructive assessment technique are applied, the sampling and the weld repair damages the material, and also the assessment will become expensive. On the other hand, small punch creep (SPC) test [1,2], which uses miniature-sized specimens, does not cause any serious sampling damages, and its assessment accuracy is high since it is a mechanical assessment technique. However, in applying the SPC test to the residual creep life assessment of the boiler in service, there are some issues to be studied [1,3,4]. In order to apply SPC test to the residual creep life assessment of the 2.25Cr-1Mo steel boiler pipe, the relationship between uniaxial creep stress and the SPC test load has been studied. And the Omega method [5] by SPC test was also studied.Copyright

Ultrasonic Guided Wave Testing System for Piping

In the operation of electric power plant, it is necessary to understand the condition of the pipe quantitatively. The purpose is to prevent the accident caused of the wall thinning portion due to erosion or corrosion. The thickness of the pipe is partially measured with the ultrasonic thickness gage now. However, it is difficult to investigate the wall thinning portion in the pipe quantitatively without confirming it beforehand. In this study, for the purpose to identify the location of thinning, we have developed the ultrasonic testing system using the guided wave that was able to investigate partially the wall thinning portion of all aspects of pipe to be inspected at a time. This system propagates guided wave by using an electro-magnetic acoustic transducer. We understand the extensive condition of the pipe by using this system. Therefore, the efficient investigation of the pipe is available. For that reason, it is expected to contribute to the reliability improvement of facilities and the reduction of maintenance and repairs.Copyright

Evaluation of Creep Deformation Mechanism of Heat Resistant Steel by Stress Change Test

In order to evaluate creep deformation mechanism of heat resistant steels, stress change tests were conducted during creep tests. In this study, it was confirmed that the dislocation behavior during the creep tests was in viscous manner, because no instantaneous plastic strain was observed at stress increments. Transient behavior was observed after stress changes for all kinds of steel in this work. Mobility of dislocation was evaluated by the observed backward creep behavior after stress reduction. Internal stress was evaluated by the change of creep rate in stress increment, and mobile dislocation density was evaluated with the estimated mobility of dislocation and the change of creep rate in stress increment. It was found that the variation of mobile dislocation density during creep deformation showed the same tendency as the variation of creep rate. Therefore mobile dislocation density is the dominant factor that influences the creep rate variation in creep deformation of heat resistant steels investigated in this work. The mobility of dislocation showed a good correlation with 1/T and it is related with the amount of solute Mo that is a solution strengthening element. Microstructure of crept specimens was observed by TEM to discuss the validation of these results.© 2007 ASME

Development of Compact Small Punch Creep Testing Equipment

Japan has many aged thermal power plant facilities and some boilers in such facilities have been in operation for more than 100,000 hours, so the importance of creep residual life evaluation of components exposed to severe conditions has been increasing. Although creep residual life of such components can be evaluated by destructive methods relatively accurately, they significantly affect the component to be sampled, take a long time to carry out and are high-cost. To solve these problems, Kobe Material Testing Laboratory Co., Ltd. and Kyushu Electric Power Co., Inc. have jointly developed compact-sized and less expensive small punch creep testing equipment for accurate creep residual life evaluation. Outer dimensions of developed equipment are 660W × 335D × 807H (mm); weight is 130kg. Small punch creep test can be conducted by this testing equipment, in order to identify creep property using 8mm diameter and 0.5mm thickness specimens taken from tubes or pipes of thermal power plant facilities, and evaluate remaining life. The testing equipment is expected to improve efficiency and reduce costs to evaluate material characteristics and creep residual life for facilities in power plants and various industries.Copyright

A Study on Estimation of Internal Pressure Creep Life Using Small Punch Creep (SPC) Tests for Boiler Pipes

The small punch creep (SPC) test is possible to predict residual creep life at a high accuracy. But, the results of SPC tests cannot be compared with uniaxial creep or internal pressure creep results directly. In this report, the relationship between SPC test results and uniaxial creep test results in ASME A335 P11 (1.25Cr-0.5Mo Steel) was studied. The obtained relationship between SPC load and equivalent uniaxial creep stress formed a simple linear equation under the wide range of test temperature and test period. Then, the SPC results can be compared with uniaxial results by converting SPC loads to the equivalent uniaxial creep stresses. The relationship between SPC test results and internal pressure creep tests results was also studied. The internal creep life of as-received P11 pipe was almost same as SPC result when the hoop stress was converted to the SPC load. The creep lives of internal pressure creep influenced materials also showed good correspondence with SPC results. Therefore SPC can estimate the residual life of internal pressure creep influenced materials.Copyright