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 behavi our of welds in a P91 pressure vessel. The microstructure of a base me tal/weld metal transition in a thick section pipe was analysed by mean s of optical microscopy and hardness measurements. Special emphasis wa s 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 the ir subgrain size, which was measured in the transmission electron micr oscope and was found to be smallest for the weld metal and largest for the intercritical heat affected zone material. The uniaxial creep beh aviour of the three material states was analysed and it was shown that the creep strength increased with decreasing subgrain size. The elast ic modulus of P91 was measured and the uniaxial creep behaviour of the three material states was characterized and represented by (i) the No rton law and (ii) in terms of the Robinson model. A welded pressure ve ssel was creep tested and hoop and axial strains were measured for thr ee 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 w elds. Measured and calculated axial and hoop strains were found to be in good agreement.