Creep Rupture Tests on Thin-Walled Tubular Specimens of a Low-Carbon Steel and Cr-Mo Steels under Internal Pressure

Abstract

In the design of boilers and other high temperature and high pressure installations, most of the tubes under internal pressure are still designed on the basis of the creep or creep rupture data of simple tension bar specimens. In practice, these tubes are loaded under multiaxial stresses. It is, therefore, a very interesting and important problem to elucidate the correlations between the creep or creep rupture strengths under simple tension and the multiaxial stress state.In the present study, the creep rupture tests on thin-walled tubular specimens of a low carbon steel and two sorts of low alloy steels, 1.25 Cr-0.5 Mo and 2.25 Cr-1 Mo steels, under internal pressure at constant temperature were carried out. The tensile creep rupture tests on bar specimens cut from the same tubular materials were also made, and their correlation stated above has been studied. The results obtained are summarized as follows.(1) Generally speaking, the creep rupture data of tubular specimens of Cr-Mo steels agreed most closely with the simple tension data on bar specimens of the materials when the equivalent stress on the wall of the tube was calculated by the average diameter formula, though according to the ASME formula, this agreement was obtained with somewhat less accuracy.(2) The visible cracks in the ruptured area were few in number and were very faint and small in size when the rupture occurred in the short duration of test. On the other hand, the visible cracks were larger and distributed widely over a bulging area when it took longer time to rupture.(3) The values of hoop strains and the micro-structures of ruptured tubular specimens were similar to those of simple tension tests.(4) It seems that the rupture is governed by the maximum shear stress criterion, since the ruptured surfaces in all the tests were the planes having the angle of about 45°with the tangential planes of the tubes.(5) The axial elongation in the ruptured tubular specimens were seldom observed.