It is well known that in structural materials used for power and chemical plants, creep or fatigue damage accumulates during long-term use at high pressure and temperature due to the change in metallurgical structure. In the past, the life prediction of structural material used at high temperatures had been carried out through the destructive test data. Recently, however, there has been an attempt to utilize the data of various material properties measured by non-destructive testing methods.
The purpose of the present investigation was to study the relation between creep damage and material properties measured non-destructively, such as electric resistance, hardness and sigma phase area. The material used in this study was solution heat treated 18Cr-8Ni (Type 304) austenitic stainless steel.
The test specimens were subjected to the long-term heating and creep damage before testing. The preparatory conditions were: temperature of 750°C, loading period of 200-1500 hours and creep damage of 0.2-1.0. The creep damage was determined based on the time fraction rule, and the creep damage of 1.0 was defined as the normalized time when the test specimen was ruptured. The electric resistance, hardness and sigma phase area of the crept specimens were measured at room temperature.
The results obtained are summarized as follows.
(1) The ratio of electric resistiviy decreased with increasing creep damage in the range of small creep damage, and it became minimum at a certain value of creep damage. Then, the ratio of electric resistivity increased with increasing creep damage.
(2) Vickers hardness increased with increasing creep damage.
(3) The percentage of sigma phase area increased with increasing creep damage.
These results suggest that the creep damage during long-term service in austenitic stainless steel can be detected by means of a non-destructive method, such as the measurement of electric resistance, hardness or sigma phase area, and that it is possible to estimate material life by such non-destructive testing method.
