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A modified 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.92 using creep data in the NIMS Creep Data Sheets at 550 to 750°C. The tr versus minimum creep rate ε ˙ min plot, which is called the Monkman-Grant relation, deviates downward at low stresses and long times. Better correlation of the tr with the ε ˙ min is obtained by the replacement of tr with (tr /εr), where εr is the total or rupture strain. The (tr /εr) 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 creep life of Gr.92 can be predicted by evaluating the ε ˙ min, together with the εr evaluated from the stress and or ε ˙ min dependence. [ABSTRACT FROM AUTHOR]

The influence of boron nitride (BN) particles, W and Mo concentrations and creep test conditions of stress, temperature and test duration on the creep ductility parameter λ in UK R5 has been investigated for five heats of W-Mo-balanced 9Cr steel with 1.5% Mo equivalent at 600, 650 and 700 oC. With increasing time to rupture tr, the λ at first decreases only a little bit, then increases for up to a time showing a maximum peak and finally significantly decreases. The BN particles are responsible for the decreases in λ at long times by accelerating the formation of creep voids. The evaluation of creep rupture ductility based on the λ should be restricted to the long-time region at low stresses after reaching a maximum peak in λ, where the λ decreases with increasing tr mainly by a decrease in total elongation. [ABSTRACT FROM AUTHOR]

The creep deformation behaviour and its effect on the creep life and rupture ductility are investigated for W-Mo-balanced 9Cr steels at 600 °C to 700 °C. The creep life is described as tr = 1.5/[ ε ˙ min (dln ε ˙ /d ε)], where ε ˙ min and dln ε ˙ /d ε reflect the creep deformation behaviour in the transient and acceleration creep regions, respectively. The creep life is also correlated with the time to minimum creep rate tm as tr = gtm, where g slightly increases with decreasing stress. The logarithm of creep rate linearly increases with strain in the initial stage of acceleration creep region. This is followed by a decrease in dln ε ˙ d ε with strain at high stresses, resulting in ductile failure at large strains, while it is followed by an increase in dln ε ˙ /d ε with strain at low stresses, resulting in rupture at small strains. [ABSTRACT FROM AUTHOR]

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture tr exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in tr with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe2(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening. [ABSTRACT FROM AUTHOR]

The creep deformation behaviour and its effect on creep life have been investigated for Gr.91 by analyzing creep strain data in the NIMS Creep Data Sheet, 450 - 725°C and tr = 11.4-68,755 h. The creep and creep rate curves consist of transient region, where the creep rate decreases with time, and of accelerating region, where the creep rate increases with time after reaching a minimum creep rate. In the acceleration region, the logarithm of creep rate increases linearly with strain over a wide range. The creep life tr is inversely proportional to the minimum creep rate εmin times the increase in creep rate by strain d ln ε/dε in the acceleration region, as tr = c/[(εmin) (d lnε/dε)]. The constant c is given by 1.5, which corresponds to an inverse of the ratio of duration of acceleration region to the creep life. The creep deformation behaviour in the acceleration region is closely correlated with that in the transient region. The parameter d ln ε/dε is correlated with the time to minimum creep rate tm and the strain to minimum creep rate em, which characterize the creep deformation behaviour in the transient region. We obtain tr = 3.7 tm and tr = 2.4/(εmin/εm). This suggests that the creep life of Gr.91 is reasonably estimated by using the creep deformation parameters tm, εmin and εm, which can be evaluated by a short-time creep test for up to the end of transient region or for up to the beginning of acceleration region, corresponding to less than 30% of the creep life. It is concluded that the creep life of Gr.91 is reasonably estimated in short times by analyzing creep strain data without any stress extrapolation. [ABSTRACT FROM AUTHOR]