Creep Rupture on the Tubular Specimens under Internal Pressure at High Temperature

Abstract

In the tube design of boilers and other high temperature and high pressure installations, most of the tubes under internal pressure are designed on the basis of creep or creep rupture strength of simple tension bar specimens. In practice, these are used under multiaxial stresses. It is required therefore that the most representative formula will be determined experimentally which is to be applied to the designing of tubes subject to internal pressure at high temperature.In the present study, the creep tests on tubular specimens of 18-8 Mo steel (thin-walled tube), 21/4 Cr-1 Mo steel (thick-walled tube) and a low carbon steel (electric resistance welded tube) under internal pressure were carried out. The data of the stress rupture tests on tubular specimens were compared with those of simple tension bar specimens cut from the same tubes. The results are summarized as follows:(1) A reasonable estimation of creep rupture life of the tube subject to internal pressure may be made from the uniaxial creep rupture data by using the Mean diameter formula. This agreement is independent of the steels, the size of the tubes, and of the temperature and time. The reason for this agreement is regared as the redistribution and the levelling of circumferential stress that occurred in the creep stage over the tube wall.(2) The creep rupture test on the welded tube of a low carbon steel under internal pressure was carried out. The tube was the stretch reduced at 800°C after welding. In this test, the welded tube showed almost equivalent creep rupture strength as compared with the seamless tube.(3) From this result, the welded tube can be used in high temperature service by giving proper treatment to eliminate the weld structure.