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A modified theta projection model is proposed for the description of creep behaviour. The model shows very good flexibility in fitting the entire test data of the creep-resistant steel 12Cr1MoV over the three creep stages. The capability of describing the primary to secondary stage makes it easier to precisely evaluate the steady state or minimum creep rate using the model. The model is derived and modified from the original theta projection method. It also keeps the four indeterminate coefficients and can meet the convergence condition easily during regression by the least squares method. In this paper, the minimum creep rates of 12Cr1MoV are also obtained and discussed. [ABSTRACT FROM AUTHOR]

A modified version of the Monkman-Grant equation, which can provide a more accurate means of predicting creep rupture life than the standard Monkman-Grant formula, has been investigated for Gr.91 using creep data in the NIMS Creep Data Sheets at 450–725 °C. The maximum time to rupture t r was 1.2 × 105 h. The t r versus minimum creep rate ε ˙ min plot, which is called the Monkman-Grant relation, deviates downward at low stresses and long times. The difference between the maximum and minimum t r at a same ε ˙ min becomes more significant with decreasing stress and increasing test duration. Better correlation of the t r with the ε ˙ min is not obtained by the replacement of t r with (t r / ε r), where ε r is the total or rupture strain. The magnitude of data scattering is larger in the (t r / ε r) versus ε ˙ min plot than in the t r versus ε ˙ min plot. The (t r / ε m), where ε m is the strain to minimum creep rate, is inversely proportional to the ε ˙ min over a wide range of stress, temperature and test duration and the magnitude of data scattering is only a little bit even at low stresses and long times. The (t r / ε m) versus ε ˙ min plot gives us more reliable relation for Gr.91 than the t r versus ε ˙ min and (t r / ε r) versus ε ˙ min plots. • Monkman-Grant eq. with "strain to minimum creep rate" accurately predicts creep life. [ABSTRACT FROM AUTHOR]

Abstract: Uniaxial isothermal stress relaxation tests (SRT) were performed on a tungsten-alloyed 10% Cr cast steel (G-X12Cr Mo W V Nb N 10 1 1) at temperatures of 580, 600 and 620 °C and initial strain levels of 0.2, 0.5 and 0.8%. Inelastic strain rates for different stresses were estimated from the stress versus time data generated from the tests. Conventional creep tests were also conducted on the same material at 580, 600 and 620 °C and at different stress levels. The strain rate data estimated from SRT were compared with minimum creep rates derived from the creep tests; the strain rates estimated from SRT with 0.8% initial strain level are in better agreement than those estimated from SRT with 0.2 and 0.5% initial strain levels. In order to ascertain the technique, stress relaxation behaviour was estimated from creep test data and compared with the stress relaxation curves obtained from SRT at corresponding temperatures. The stress relaxation curves obtained from SRT with 0.8% initial strain level are in good agreement with the stress relaxation curves estimated from the creep tests. It is concluded that the stress relaxation test with initial strain level of 0.8% could be considered as an appropriate short-term test technique for estimation of creep strength of newly developed materials. [Copyright &y& Elsevier]

Abstract: Long-term creep and rupture data of NIMS Creep Data Sheets show that the time to rupture at a minimum creep rate of 10−5%/h is longer than 105 h for most ferritic steels, while it is shorter than 105 h for most austenitic steels, Ni base and Co base superalloys. This is correlated with the larger total elongation in ferritic steels than in austenitic steels and in Ni base and Co base superalloys at a minimum creep rate of 10−5%/h. However, 67% of the average stress to cause rupture at the end of 105 h is lower than the stress to produce a minimum creep rate of 10−5%/h for all the materials examined. This indicates that the ASME allowable stress in a high-temperature creep region is determined by creep rupture data but not by creep strain data. The creep and creep rate curves are compared between 2.25Cr–1Mo steel and 18Cr–8Ni austenitic steel at approximately the same minimum creep rate. 18Cr–8Ni austenitic steel reaches a minimum creep rate in shorter time and spends shorter time in the acceleration creep before creep rupture than 2.25Cr–1Mo steel at low stress and long time conditions. This is correlated with different microstructure evolution during creep. [Copyright &y& Elsevier]

Abstract: In order to improve creep strength of tempered martensitic 9Cr steel at 650°C, alloy design for a dispersion of fine and thermally stable precipitate particles has been investigated for 9Cr–3W–3Co–VNb steel and 9Cr–2W–VNbTi steel. Creep tests were carried out at 650°C for up to about 10000h. By reducing carbon concentration below 0.02%, a dispersion of nano-size MX nitrides of vanadium and niobium along boundaries as well as in the matrix is achieved in 9Cr–3W–3Co–VNb steel with 0.05% nitrogen. This gives rise to excellent creep strength at 650°C, as shown by much higher creep rupture strength than conventional 9Cr steels P92 and T91. Excess addition of nitrogen, 0.07 and 0.1%, promotes the coarsening of MX nitrides and the formation of Z-phase during creep. A dispersion of nano-size TiC carbides is also achieved in 9Cr–2W–VNb steel containing 0.05% Ti by high-temperature normalizing at 1300°C followed by tempering. This effectively decreases minimum creep rate and improves creep life. [Copyright &y& Elsevier]