Tube ballooning of axially confined tubes under increasing thermal loading is studied using a finite element model in an austenitic stainless steel, alloy D9. The model accounts for thermal, plastic and creep deformation of the tube. It is shown that (i) axial restrains can trigger the onset of plasticity, and (ii) activation of plasticity during the thermomechanical loading can significantly reduce the time for the development of large ballooning strains. © 2019 The Authors. Published by Elsevier B.V.