Study On Phase Transformation Of Modified High Cr Ferritic Heat-Resistant Steels

A new high Cr ferritic heat-resistant steel was developed based on the 9-12%Cr ferritic heat resistant steels by adjusting its opimal element,including reduce C content increase Cr content and add Co Mo W elements, then smelting the elements into the high vacuum melting furnace and casting moulding. The tested heat resistant steels are studied on the microstructure and mechanical properties influenced by cooling rate, cooling rate and tempering temperature, by using differential thermal analysis(DTA), scanning electron microscope(SEM), X-ray diffraction(XRD) and electronic tensile creep. The conclusion can be drawn as follows:The results show that the steel has magnitic transformation, austenite formation, M23C6-type carbide dissolution and transformation from austenite to δ-ferrite, successively, between 600-1100 ℃. The starting temperature of austenite formation is 853 ℃. The martensitic transformation will occur during air cooling or cooling at 40 ℃/min. The starting temperature of the martensitic transformation is 382 ℃. Nb-containing MC-type carbide is very stable and cannot dissolve into austenite at the testing temperature, completely.The microstructure of the tested steel is typical lath martensite after heat treatment at 1050 for 1h. The globular carbides are distributed along martensite lath boundaries an℃ d prior austenite boundaries. The influence of cooling rate on the microstructure and precipitates were also investigated, with the decrease of cooling rate, the martensite lath will get coarsen, the carbide will gradually increase toward the crystal boundary and grow up. The XRD results showed that the type of carbides is M23C6. With the increase of cooling rate the hardness become higher gradually.After heat treatment at 1100℃ for 1h, cooled in air, the microstructure is typical lath mart ensite, a small amount of δ-ferrite and some undissolved carbide with NbC type in the matrix. After heat treatment at 1100℃ for 1h, and tempering at 780℃ for 2h, the microstructure is typi cal temped martensite, the main precipitated phase in the matrix is M23C6. The carbide is cras si and continuous distributed over the martensite and δ-ferrite boundaries, the phase precipitate from martensite boundaries is fine.The creep strength is good in the present steels after heat treatment at 1100 for 1h and ℃then tempered at 780 ℃for 2h, cooled in air. Under the condition of 650℃/200 MPa and 650℃/170 MPa,the rupture life is 411.4h and 1499.1h respectively, it has greater improved compared with T/P92 and P/T122 heat resistant steel published by NIMS.