In the oil sand production field in Canada, steel pipes are used for injecting steam into the sands. Pipes for steam distribution are subjected to high operating temperature such as 350 degrees C for a long time. In this study, in order to insure the reliability of high strength pipe for steam distribution, long-term heating tests were conducted for X80 grade UOE pipe. To simulate the long-term operation at 350 degrees C for 40 years, accelerated tests were conducted at 400 degrees C for up to 4400 hours. The effect of initial bainite microstructure on tensile properties after long-term heat treatment was investigated by using two kinds of steel pipes with different bainite microstructure. One is accelerated cooling (ACC) type, and the other is heating on-line process (HOP) type just after accelerated cooling in plate manufacturing process.

It was revealed that strength decrease in ACC type steels produced by only accelerated cooling was mainly governed by decomposition of hard phase MA (Martensite-Austenite constituent) and recovery of dislocations. On the other hand, HOP type steels had small amount of MA and nanometer-sized carbides by applying HOP after accelerated cooling. The HOP type steels had almost the same initial strength in spite of small amount of MA. Fine niobium carbides gave enough precipitation strengthening. Formation of fine niobium carbides was promoted by the addition of niobium. The precipitates were stable after long-term heat treatment at 400 degrees C. It is confirmed that the fine niobium carbides in HOP type steels remained even after long-term heat treatment. The suppression of initial MA also lead to minimize the strength decrease by MA decomposition. Therefore, HOP type steels tend to have higher resistance to the strength decrease after long-term heat treatment.

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