Creep Resistance of Ferrite–Martensite Steel for Nuclear Reactors

High-temperature ferrite–martensite steels with 12% Cr are characterized by high strength and creep resistance at 600–750°C. In the high-safety BREST reactor, 16Kh12MVSFBR (EP-823) steel is used to manufacture fuel-rod casings. Literature data regarding its mechanical characteristics in tensile tests at 20–750°C are available, as well as limited information regarding the long-term strength, but nothing has been reported regarding its creep rate. In the present work, the creep of EP-823 steel is investigated for 3-kg samples from three melts. Cylindrical samples (~5–6 mm) undergo compressive tests in air at 600–760°C, under a stress of 70–310 MPa. In the compressive tests, the duration is no more than 11 h. The structure after quenching and tempering consists of tempered martensite and some (6–12 vol %) δ ferrite, with grains no larger than 20 μm. The experimental data and the approximation for the creep are in better agreement when plotted in logarithmic coordinates (log σ–log έ) than in semilogarithmic coordinates (σ–log έ). Analysis of the results in terms of the Hollomon equation P_S =(T/1000)[C_S – log(έ)] and the Larson–Miller equation P_E = (T/1000)[C_E – log(σ)] yields formulas for the creep rate at specified stress (100–220 MPa) in the form log(έ) = –19.355 + 9.17(T/1000)log (σ); and for the creep limits with specified tolerances on the creep rate (0.1–1%/h) in the form log(σ) = 4.304 – 0.109(T/1000)(20 – log(έ)). Similar results are obtained in calculating the creep limits and creep rates by the pair models and the Hollomon (Larson–Miller) models, but the Hollomon models are preferable because they take account of all three variables. The results of control tests in identical tensile conditions are presented. It is shown that, in determining the strength characteristics, there is a linear relation between the results of the compressive and tensile tests: σ_0.2co = 1.3 σ_0.2te. Analysis of the creep indicates that the creep rate for steel of the same type (09Kh12V3BR) and the same melt, with different loading (tension or compression) are similar, whereas the creep rate for steel of the same type (EP-823 steel) and different melts may differ by an order of magnitude even in the same loading conditions (compression).