Creep deformation and stress relaxation of a martensitic P92 steel at 650 °C

Abstract

This paper develops methods to predict creep stress relaxation in the presence of combined boundary conditions and explores the influence of primary-secondary stress dependent creep properties on predictions for a martensitic P92 steel at temperature of 650 °C. A series of forward creep and elastic follow-up experiments have been conducted. A summary is provided of empirical creep equations for forward creep and creep stress relaxation (elastic follow-up) tests, including the link to the experimental procedure. The creep stress relaxation tests have been performed with two rigs to give elastic follow-up factors of 1.17 and 1.7. Both time hardening and strain hardening approaches have been considered where the strain hardening model provided more accurate predictions compared to time hardening; except at relatively low stress levels. The difference between stress relaxations predicted using strain hardening and time hardening approaches are considerable. The model predicts the creep stress relaxation accurately in the early stage of relaxation, indicating that the majority of stress relaxation occurs where primary creep needs to be taken into account. This study highlights the importance of stress dependent creep model to predict stress relaxation, especially with high level of initial residual stresses

Influence of thermal ageing on the creep behaviour of a P92 martensitic steel

“…Other works [2]- [8] have proposed that thermal ageing, under laboratory conditions, is sufficient to induce changes in the microstructure and creep deformation. In practice service components operating at high temperature are subject to a wide range of stress states, especially in the presence of geometrically non-linearities, such as a crack [1], [17] [18]. Certainly, the creep deformation and fracture mechanisms for 9% Cr 1% Mo steels when subjected to low levels of stress, responded differently compared to when exposed to high stresses [16], [17] [18].…”

“…In practice service components operating at high temperature are subject to a wide range of stress states, especially in the presence of geometrically non-linearities, such as a crack [1], [17] [18]. Certainly, the creep deformation and fracture mechanisms for 9% Cr 1% Mo steels when subjected to low levels of stress, responded differently compared to when exposed to high stresses [16], [17] [18]. It is common to consider conventional creep models presuming one mechanism operates across all the stress levels [10], [12], [15], [19]- [22].…”

“…To evaluate the effect of thermal ageing on creep behaviour it is important to consider the stages of both primary creep and secondary creep, especially since the P92 steels exhibit significant primary creep strain [1], [18]. To overcome this difficulty, a primary-secondary stress dependent creep model has been developed by Khayatzadeh et al [1], [18] which takes into account creep behaviour for an un-aged steels over for a wide range of applied stresses from 40MPa to 170MPa at 650℃ [18]. This model is now employed to investigate the effect of thermal ageing on creep deformation over a wide range of stresses.…”

“…Uniaxial creep stress relaxation tests with an elastic follow-up factor of Z=1.17 were conducted on un-aged and aged P92 steels at 650℃ to examine the effect of thermal ageing [18]. The initial residual stress, in this test rig, was introduced by applying a misfit to a level of 123MPa.…”

“…Table 4 provides the details of the uniaxial creep stress relaxation tests conducted on both the un-aged and aged specimens at 650℃. Full details about the procedure for performing the tests can be found in [1] [18], [29], [30]. The purpose of this research is to study creep stress relaxation at a very low elastic follow-up condition.…”