CREEP DAMAGE EVALUATION FOR BOILER TUBES BY INTERNALLY PRESSURIZED CREEP TESTS

Creep damage behavior under multiaxial creep conditions has been investigated for the typical boiler tubes of T22(2.25Cr-1Mo) and TP321H(18Cr-10Ni-Ti) steels by means of long-term internally pressurized creep tests. Creep strain under pressurized creep for both steels was smaller than that under uniaxial creep. The strain ratios to the uniaxial creep for both steels were almost constant up to about 0.7 in the life fraction and the constant values of the strain ratios were about 0.13 for the T22 steel tube and 0.28 for the TP321H steel tube. For the ductile T22 steel tube, the extensive grain deformation was observed and voids were formed with difficulty even in the final stage of creep. For the less ductile TP321H steel tube, microcracks were observed on grain boundaries in the early stage of creep. However, they existed only in the outer surface side within 1 mm depth, even in the final stage of 0.78 in the life fraction. The replication method was applicable for the evaluation of the grain deformation for the T22 steel and of the microcracks for the TP321H steel. © 1990, The Iron and Steel Institute of Japan. All rights reserved.