The published results of thermal fatigue studies, which also included metallographic observations during the tests, are analyzed. All tests were performed on flat corset samples, in which, as in products, the plastic deformation (εpl) in the cycle can vary widely. In addition, they allow you to vary the εpl by changing the parameters of the sample shape. A method and device for determining the tensile stresses in the cooling half-cycle in tests on corset samples are proposed and tested. They are an important parameter, as they control the formation and growth of cracks in the material of products during the development of thermal fatigue.