Thermal Stability Of High Chromium Ferritic Heat Resistant Steels

Microstructure evolution and thermal stability of two ferritic heat resistant steels with ultra low carbon content during long time aging at 650℃are investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction and stress rupture tests.After normalized at 1100℃for 1h, cooled in air and then tempered at 700℃for 1h and at 750℃for 1h, followed by air cooling, two ultra-low carbon test steels have tempered lath martensitic microstructure. M3B2 and MX phase form after tempering.The number of M3B2 particles in test steel with high tungsten and low molybdenum content is smaller than the test steel with low tungsten and high molybdenum content. M23C6-type carbide, do not precipitate in these two test steels after tempering due to low carbon content. During aging process the density of Laves phase in these two test steels reaches the highest value after 6000h, 3000h, respectively. Z phase has precipitated after aged for 3000 h. P92 steel is normalized at 1080℃for 1h, cooled in air and then tempered at 780℃for 2h, followed by air cooling, The lattice parameter of M23C6-type carbide is increased with the increase of aging time and reaches the highest value after aging for 7944h. The Z-phase is not detected in the P92 steel after aging at 650 for 10000h.The analysis of EBSD shows that residual austenite remain in the two test steels and steel P92. Besides, the most grain boundaries orientation differential of these three test steels are small-angle boundaries. After aging, the feature angle-60°of test steel with low tungsten and high molybdenum content almost turn into 0°, and the feature angle-60°of test steel with high tungsten and low molybdenum content decrease, which means the martensite lath substructure is instable. The feature angle-60°change of P92 steel is indistinctive after aging, which means martensite lath substructure keeps stable.High temperature strength degradation of ultra-low steel is more obvious than steel P92 due to formation of M3B2 and Z phase. High temperature strength degradation of steel with low tungsten and high molybdenum content is more obvious than that of steel with high tungsten and low molybdenum content.