Evaluation of Internal Pressure Creep Rupture Life of Boiler Tube Using Miniature Tensile Creep Test

Abstract

Many studies have been made for evaluating the creep rupture life of a boiler tube. The working stress of a tube under internal pressure is calculated mostly by the mean-diameter formula adopted in design for pressurized components. However, this stress is not always representative of a complex stress state under internal pressure. So, focusing on the representative stress, internal pressure and uniaxial creep tests were conducted using unused and 90,000-h used boiler tubes of Type 321 stainless steel in this study. A miniature specimen (1.5 mm in width, 1.5 mm in thickness, and 25 mm in length) for the uniaxial test was taken from the middle of the tube wall in the direction of the maximum principal stress. As a result, the mean-diameter formula did not present good agreement between internal pressure and uniaxial creep rupture lives. Hence, as an alternative stress, von Mises equivalent stress, which presents good correlation between uniaxial and multiaxial creep rupture properties, was chosen. The equivalent stress, σ*, at the middle of a tube wall was calculated using Rimrott’s solution to internal pressure creep of thick-walled tubes considering large deformation. Because σ* increases with creep strain under internal pressure, σ* at εθo = εθof / 2 (where, εθo and εθ0f are tangential and tangential rupture strains on the outer diameter, respectively) was employed as the representative stress. Consequently, this stress resulted in good agreement between both creep rupture lives for the unused tube. However, the internal pressure creep rupture life of the used tube was inconsistent with the uniaxial life, which was attributed to the existence of an outside layer due to exposure to combustion gas. Using a reduction rate of the rupture ductility of the layer, the internal pressure creep rupture life of the used tube was evaluated well with the unused tube.