Modified 9Cr-1Mo 강의 용접재에 대한 크리프 균열성장속도의 확률론적 해석

Abstract

Modified 9Cr-1Mo (ASME Grade 91, hereafter G91) steel has better mechanical properties at elevated temperatures than 2.25Cr-1Mo steel. Currently, the Modified 9Cr-1Mo steel is being considered as the structural components for a Gen-IV reactors, such as a very high temperature reactor (VHTR) and sodium-cooled fast reactor (SFR). Since these structures are designed for up to 60 years at elevated temperatures, they may cause creep crack growth (CCG) due to creep damage during the long service life. The understanding creep crack growth behavior is important for the reliability in elevated temperature structural components. Therefore, it is necessary to investigate the creep crack growth rate (CCGR) behavior for base metal (BM) of G91 steel, and especially, for its weldment. The G91 steel was used with a hot rolled G91 steel plate of a commercial grade. Heat treatment condition of the steel was normalized and tempered at 1050℃/1mim/mm and 770℃/3mim/mm, respectively. A groove shape for welding two plates was designed as single V-groove with 60 degree. Welded specimens were prepared by shielded metal arc welding (SMAW) method. To obtain the material properties data of the weldments, tensile test and creep test were performed at 600℃. Also, the CCG test were performed using 1/2 inch compact tension (CT) specimens for the applied load ranges of 3500N to 4500N at the identical temperature. After the CCG tests, fracture micrographs were observed by using scanning electron microscope (SEM) and optical microscope (OM). The C* creep fracture parameter was used to characterize the CCG rate of the welded G91 steel. In order to determine the material constants, B and q used in the creep crack growth rate equation, da/dt=B(C*)q, the CCGR data was analyzed by means of three methods; the least square fitting method (LSFM), a mean value method (MVM) and a probabilistic distribution method (PDM). The result showed that the MVM and PDM were better in predicting the CCGR than the LSFM. It was found to be two-parameter Weibull distribution for constants B and q. Using the Weibull distribution, a number of random variables were generated by the Monte Carlo simulation. The creep crack growth rates for 10% to 90% range could be predicted from the viewpoint of probability. The micrographs of crack propagations exhibited ductile dimple fractures, and the creep cracks were propagated by interconnection of the micro voids along grain boundaries.