Influence of axial load on internal pressure creep of Mod.9Cr-1Mo circumferential welded pipe

Abstract

Mod.9Cr-1Mo steel(P91) is widely used for high temperature pipes in ultra-super-critical thermal power plants. However it was recently reported that the creep damage was detected at heat affected zone(HAZ) in welded joints of the pipes so-called the “Type IV” damage. So far, study on damage and life assessment methods for the welded joints of the P91 steel were concentrated on longitudinal welded pipes. On the other hand, circumferential welded joints are also susceptible to yield creep damage because of increase of axial stress by thermal stress superimposed with pipe weight itself. In this study, the effect of additional axial stress to the axial stress by the internal pressure on damage and rupture property is discussed based on internal pressure creep tests adding different levels of the axial stress using the P91 circumferential welded pipe. Rupture time of the circumferential welded pipe decreases with increasing additional axial stress level. The longitudinal cracking in the weld metal was observed in the specimen tested under lower additional stress level, and the “Type IV” cracking was observed in those under higher additional stress levels. The stress analysis results indicate that axial stress in the HAZ increases as increasing the additional stress level causing the “Type IV” cracking at a certain value of ratio of the total axial stress to the circumferential stress. Under the test condition where the “Type IV” cracking occurs is significantly shorter than predicted rupture time based on internal creep rupture data of the longitudinal welded pipes. The limited creep strain is introduced to predict rupture time of the circumferential welded pipe by considering the effect of stress multiaxiality. Eventually, times to rupture caused by the “Type IV” cracking of both the longitudinal and the circumferential welded pipes were accurately predicted by the limited creep strain concept.