Abstract
In high Cr ferritic heat resistant steels, the creep strength of welded joint is closely associated with the strength of base metal itself. The creep property of base metal can be good indicator for ranking the strength level of welded joint or assessing its remaining-life, considering that it is not very difficult to scoop a small amount of material from the base metal part of pipings. In this study, the small punch (SP) testing technique was applied to five different heats of Gr.91 steel, which had been actually used for boiler pipings in 600 °C class USC power plants for long periods of time. The SP creep test was carried out at the temperature of 650 °C and under the load levels of 190, 230 and 300 N. Experimental results revealed that the rupture time was varied depending on the heat and there was maximum of one order difference even at same load condition. This difference in creep strength was qualitatively in good agreement with that determined by the conventional uniaxial creep test. Additionally, the correlation between the results of SP creep test and uniaxial one was also investigated using the load/stress conversion coefficients reported before.
