The background of the NIMS Creep Data Sheet Project, together with the preliminary study and facilities, material selection, and testing method, is summarized. The outcomes from the project are explained, focusing on the long-term creep strength of ferritic and austenitic heat-resistant steels. In some cases, the slope of the stress versus time-to-rupture curve in the long term differed from that in the short term in a manner that was markedly dependent on the type of material. Heat-to-heat variations in creep strength were recognized for ferritic and austenitic steels, even when the chemical compositions of the steels examined were within the range of specifications. The reasons for the heat-to-heat variations were differences in the chemical composition, in the amounts of minor elements, and in the grain size, among others. The existence of inherent creep strength was discovered in the very long term for ferritic heatresistant steels. The amounts of minor solute elements affect the inherent creep strength, independently of precipitation strengthening or the dislocation structure. An inflection point was observed in the tertiary creep stage for a low-alloy steel and for austenitic stainless steels when precipitation occurred during creep. A region-splitting analysis method was proposed for long-term creep strength evaluation for high-chromium ferritic steels. This method was used to review the allowable stress of high-chromium ferritic steels in Japan. A metallographic atlas, time-temperature-precipitation diagram, and fracture-mode map were proposed for ferritic and austenitic steels on the basis of creep-ruptured specimens.
The effect of minor alloying elements on the heat to heat variation in creep rupture time has been investigated for the nine heats of 18Cr 8Ni austenitic stainless steel (JIS SUS 304 HTB) at temperatures between 600 and 700°C for the periods from 30 to 1.8×10 5 h.The heat to heat variation in creep rupture time at first increases more than one order of magnitude then reduces and again increases with decreasing stress and increasing test duration. The first increase in heat to heat variation with increasing test duration, which is more pronounced at a lower temperature of 600°C, is caused by precipitation strengthening due to very fine Nb carbides having a size of 10 nm or less. The precipitation strengthening due to Nb carbides becomes disappeared by about 10 5 h at 650°C, because of their agglomeration during creep. This causes the reduction of heat to heat variation. The second increase in heat to heat variation at long times is more pronounced at higher temperature of 700°C and at long times above 10 4 h, but it does not appear at 600°C for the duration up to 10 5 h. The available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, clearly explains the second heat to heat variation. Accelerated void formation in a heat containing high Al also decreases the creep strength at long times. It is concluded that the present results improve the reliability of remaining life estimation for respective heats of SUS 304 HTB steel and hence reduce materials risk.
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