In many modern engineering material modelling approaches irreversible, viscous behaviour is limited to post-yield (plastic) deformation. Irreversibility in the so called elastic domain of deformation can however be readily demonstrated experimentally. Understanding “elastic” viscous effects is important as commonly deformation of components in operation will induce sub-yield stress states. Consider, for example, components in sectors like the power generation industry that are exposed to ‘two-shifting’ operating strategies which involve frequent start-up and shut-down cycles, imposing significant stresses on the components. Rate-dependent viscous behaviour at stress states below yield is encountered in such components and is more pronounced due to elevated operating temperatures. Understanding the behaviour of such components is crucial in the development of predictive material models that can inform their design and safe operation. In the present work viscoelastic irreversibilities, (i.e., stress relaxation below yield) are demonstrated in a 316 stainless steel material at elevated temperatures of 400⁰C, 600⁰C and 700⁰C. Through experimental observation of stress relaxation, cyclic hardening and rate-dependency associated with viscoplasticity, a thermodynamically-based constitutive model is calibrated using various novel waveforms at a strain rate of 0.03%s-1 and at various strain holds (0.1%, 0.15%, 0.3%, 0.35% and 0.5%). The model is validated against experimental results of a realistic waveform experienced by an in-service steam header. The calibrated material model can be used to correct otherwise static stress distributions for in-service components at elevated temperatures. By incorporating a viscoelastic term, damage can be characterised for this region of deformation.