The Incoloy 800H alloy heat transfer tube was tested for 20 000 h at 675 ℃ on simulation test bench in environment with 7 MPa high pressure steam inside the tube and micro positive pressure helium gas protection outside the tube. The microstructure evolution and mechanical property change were studied. The results showed that

the tensile properties of Incoloy 800H were excellent after test

which could meet the requirements of Technical Requirements of Incoloy 800H Tube for Steam Generator for new tubes. With the extension of test time

the tensile strength

specified plastic elongation strength and micro-Vickers hardness increased at first and then kept stable.After test for 20 000 h

the three parameters increased by 13.6%

7.9% and 17.7% respectively. The M23C6 carbides were mainly distributed along the grain boundaries and in the matrix

the second Ti(C

N) phases distributed in the matrix after test. The size of grain boundaries M23C6 carbides increased slightly with the extension of test time. The size of M23C6 carbides and secondary Ti(C

N) dispersed in the matrix increased with the extension of service time before test for 5 000 h. After test for 5000 h

the sizes of M23C6 carbides and secondary Ti(C

N) were 230～320 nm and 350 nm respectively

and then they were basically stable. No harmful phase like the σ phase and G phase was found. Incoloy 800H alloy has good resistance to steam oxidation and low risk of spalling.