The small punch (SP) creep testing technique was applied to five heats of Gr.91 steel, which had been actually used for boiler pipings in the different USC power plants for long periods of time. As a result, it was found that the difference in creep strength between the heats, that is, the heat-to-heat variation, was well reflected in the SP creep property. In addition, the conventional SP test was carried out at room temperature and 650°C to obtain the load/stress conversion coefficient (F/σ) which is necessary for the conversion of SP creep rupture data to the uniaxial ones. The F/σ for each heat was estimated by the displacement to maximum load (um) using the correlation of F/σ = um h0/0.320. The experimental results revealed that the F/σ showed no significant change and it was 2.4-2.5 irrespective of heats. It was also found that the SP creep rupture data were relatively well converted to the uniaxial one with those coefficients.
