Residual stresses can have a significant effect on the integrity of high-temperature structures. Consequently, the interaction between residual stresses and creep deformation is persistent source of uncertainty in the design and assessment of high-temperature pressure systems such as those used in nuclear power reactors. This study will investigate the role of combined primary and secondary creep during the creep relaxation of residual stresses. However, it has recently been shown that in some circumstances the majority of creep stress relaxation occurs during primary creep regime. In this study, we will remove the influence of primary creep by applying a creep pre-straining cycle to the specimens prior to the start of the experiment. In-situ creep relaxation tests will then be performed on pre-strained and non-pre-strained specimens, separated out the influence of these two regimes.