KONDRAT'EV Sergey Saint Petersburg State Polytechnical University

Spoluautoři ANASTASIADI Grigoriy P., RUDSKOY Andrey I.

Iron-nickel and nickel-based superalloys, intended to operate in dynamic conditions of load at temperatures 800-1000 ° C, i.e. at T = (0.65-0.80) Tsol (where T and Tsol - working temperature and the solidus of used alloys) have an unstable structure, prone to continuous change during the operation [1 - 2] . This process cannot be stabilized by the thermal treatment on the following mode: heating to 1050-1200 °C, followed by cooling and long aging. The brittleness of alloys increases during continuous operation due to the evolution in the structure of the intermediate phases - σ, χ, μ, G, Z, which have different chemical composition, significantly different from the composition of the alloy and compositions of the matrix γ- phase and main strengthen γ΄- phase [ 1-3]. Iron-chromium-nickel-based alloys with a high content of carbon - to 0,30-0,70 % (weight), stabilized with niobium, titanium, molybdenum, tungsten and other elements, are primarily used for manufacturing equipment used in the processing of oil and gas, working at temperatures of 1100-1200 °C. Operating temperature of these alloys reaches T = (0.85-0.90) Tsol. Oversized blanks are produced mainly by casting. Problems of structural instability in these alloys are enhanced due to cast condition, characterized by a large initial structure and chemical heterogeneity, as well as the impact of higher operating temperatures and significantly longer duration of operation, reaching 140 thousand hours. Price P. and Grant N. were the first (Trans. AIME, 1959, v. 215, No. 4, p. 635-637) who experimentally constructed phase diagram of alloys Fe-Ni-Cr to a temperature of 1300 °C. It is probably still the only one because to achieve the necessary balance for lower temperatures exposures to 10,000 hours are needed. Despite the founded structure instability, the experience of industrial application of such alloys indicates their satisfactory performance. Analysis of the results of various studies suggests that the main factors, that provide high performance of Fe-Cr-Ni based superalloys, are: - low level static load of 5 MPa for a working temperature of 1150 °C up to 70 MPa - for 800 °C; - presence of a large number (up to 30 % by vol.) coarse eutectic of carbide base (М7С3) and γ- phase in the initial molded structure, along with the matrix solid solution (γ); - additional hardening of the cast structure by relatively stable niobium carbide (NbC); - continuous transformation of the initial phases in the structure during prolonged high exposures: chromium carbides (М7С3) change the crystal structure and change to others (with a smaller ratio of C/M ), niobium carbides dissolve with separation of niobium in matrix solid solution and the formation of intermetallic phases; - continuous release and dissolution of various intermediate intermetallic phases in the alloy structure of complex chemical composition in the presence of a stable matrix - γ- solid solution containing % (at.): 25Cr, 33Ni, 37Fe. МmCn carbide is formed in the matrix "on place" due to the diffusion of excess carbon to it, formed during the modification of the eutectic chromium carbide by the reaction 23М7С3 → 7М23С6 + 27С. Intermetallics FeCr, Cr5NiFe, Fe7Cr4Ni8Si, Cr7Ni5Si3N3FeNb are formed at the boundary carbide phases and matrix. During long exposure under load (800 °C, 70 MPa, 6000 h) the growth kinetics of phases varies, so the chromium carbides may be formed at the boundaries of intragranular dislocation sites, as well as nitrides and intermetallic compounds. References: 1. Dewar M.P., Gerlich A.P. Correlation between experimental and calculated phase fractions in aged 20Cr32Ni1Nb austenitic stainless steel containing nitrogen // Metallurgical and Materials Transactions A. – 2012. – V. 13. – № 4, April. Preprint submitted. – 19 p. 2. Kenik E.A., Maziasz P.J., Swindeman R.W., Cervenka J., May D. Structure and phase stability in cast modified-HP austenite after long-term ageing // Scripta materialia. – 2003. –V. 49. – P. 117-122. 3. Rudskoy A.I., Oryshchenko A.S., Kondrat'ev S.Yu., Anastasiadi G.P., Fuks M.D., Petrov S.N. Special Features of Structure and Long-Term Strength of Cast Refractory Alloy 45Kh26N33S2B2 // Metal Science and Heat Treatment. – July 2013. – V. 55. – № 3-4. – P. 209-215.