Analysis of creep in a welded ‘P91’ pressure vessel

The present work considers the uniaxial and multiaxial creep behaviour of the tempered martensite 9%Cr 1%Mo steel ‘P91’ and the creep behaviour of welds in a P91 pressure vessel. The microstructure of a base metal/weld metal transition in a thick section pipe was analysed by means of optical microscopy and hardness measurements. Special emphasis was given to three microstructural states: the base metal (BM), the weld metal (WM) and the intercritical heat affected zone material (IC-HAZ). A significant difference between these three microstructures was their subgrain size, which was measured in the transmission electron microscope and was found to be smallest for the weld metal and largest for the intercritical heat affected zone material. The uniaxial creep behaviour of the three material states was analysed and it was shown that the creep strength increased with decreasing subgrain size. The elastic modulus of P91 was measured and the uniaxial creep behaviour of the three material states was characterized and represented by (i) the Norton law and (ii) in terms of the Robinson model. A welded pressure vessel was creep tested and hoop and axial strains were measured for three welds in the vessel. A creep stress analysis of the welded pressure vessel was performed based on (i) Norton's law and (ii) the Robinson model concentrating on the accumulated hoop and axial strains in the welds. Measured and calculated axial and hoop strains were found to be in good agreement.