Development of fatigue design curves for thick-section 9Cr-1Mo ferritic steel forgings

Abstract

Strain-controlled low-cycle fatigue (LCF) data have been generated on 9Cr-1Mo thick-section (300 mm) tube plate forgings under simulated postweld heat treatment conditions at 723, 773 and 793 K employing a constant strain rate of 1 × 10^-3 s^-1. LCF tests were performed over strain amplitudes in the range ±0.25 to 1.00%. The fatigue life was found to decrease with increasing temperature. The effect of temperature was more pronounced at low strain amplitudes. Within the range of strain amplitudes examined, the experimental data on fatigue life at elevated temperatures were found to match well with the life predicted by the Tomkins crack growth model. Tentative fatigue design curves for elevated temperatures have been formulated for 9Cr-1Mo based on information generated in the laboratory using the philosophy outlined in the ASME code case N-47. A fatigue design curve applicable for the temperature range 300-644 K was computed based on room temperature tensile properties, incorporating the concept of fatigue limit and mean stress.