Thermal fatigue tests under in-phase condition were conducted on two typical types of steels, 0.16%C carbon steel (S15C) and austenitic steel (SUS304). Effects of in-phase and out-of-phase on fracture mode were examined. Characteristic features of microcracks under in-phase condition were found to be similar for both steels. The results obtained are summarized as follows:
(1) Life reduction with in-phase thermal cycling (tension at high temperatures, compression at low temperatures) is remarkable when the maximum cycle temperature is high enough to produce creep strain and the strain range is fairy large.
(2) Under in-phase condition, many grain boundary cracks initiate. They seem to reduce the ductility of materials more effectively than surface cracks. Only surface cracks are found under out- of-phase condition.