Thermal Fatigue Behaviour or 18-8 Stainless Steel under Tensile or Compressive Stress at Higher Temperature

Abstract

Thermal fatigue tests of 18-8 stainless steel were carried out under two different conditions, using a testing equipment that has a property to produce the strain and temperature cycling either independently or under a certain predetermined co-relation.One condition was such that the specimens were subjected to tensile stress at higher temperature, and in the other, compressive stress was applied to the specimen at higher temperature.After the tests, the microstructures of the fractured specimens were examined carefully, and the test results were discussed in connection with the various features of the thermal fatigue phenomenon in detail.Conclusion are as follows:(1) Under 700-200°C temperature cycling, thermal fatigue life is shorter when the specimens are subjected to tensile stress at higher temperature, than when the specimens are subjected to compressive stress at the same temperature. But under 500-200°C temperature cycling, no marked difference can be seen.(2) The specimens subjected to compressive stress at higher temperature show a large bulging at the center of the testing portion, as the number of cycling increases, but the specimen subjected to tensile stress at the same temperature did not show much deformation. These phenomena seem to be due to the difference of the strain and temperature cycling combined with the different temperature distribution along the axis during the heating and cooling period, and also with the constraint of the straining at the fillet parts.(3) From the study of microstructure, the precipitation and corrosion were recognized especially in the specimens heated up to 700°C. It is assumed that corrosion has a large effect on the thermal fatigue life of the 18-8 stainless steel.