The investigation was carried out with the carbon steel S 40 C on the superplasticity phenomena under constant load at a temperature within the ferrite+austenite region, and during the temperature cycling through A₁-A₃ transformation. The results obtained are as follows:
(1) At the test temperatures in the ferrite+austenite region, the creep strain in the case of heating the specimen directly to the test temperature is greater than that in the case of cooling the specimen to the test temperature after over-heating to 820°C. The larger creep strain observed at the temperature near 740°C is considered to be a kind of superplastic strain due to microduplex structure.
(2) Besides usual creep strain, the transformation strain occurs during the temperature cycling through A₁-A₃ transformation under constant load, and it increases with increase in the temperature cycling rate.
(3) The strain-rate sensitivity exponent m is 0.44 and 0.68 respectively in the creep test at 740°C and in the temperature cycling creep test between 700°C and 820°C at a cycling rate of 4 times per hour.