A New Method of Creep Ruptures Data Extrapolation Based on the Work
For structural and machine components in service at high temperature and pressure, such as those used in power generating and petrochemical plants, as well as aero-engines and metallurgical machineries, creep life assessment is a major concern because of safety and economic reasons. In this paper, a new model for creep life prediction is proposed based on energy processes. The work of making the material failure of applied loads is used to describe the creep processes respectively. By mathematical technique, the expression of the model is, where is Larson-Miller constant, and is also material constant at certain condition. This function Moreover, owing to its simple expression, it is very easy to use this model to predict the creep life. To check the validity of the new model proposed in this paper, the 250000h creep rupture strength estimates for 2.25Cr1.0Mo steel under stress control at 743~793K have been performed. The results are in a good agreement with the experimental data.
Creep life predictions in nickel-based superalloys
Abstract Creep life prediction techniques such as the Larson-Miller parameter, life trend diagrams, the Monkman-Grant relationship and the Dobes-Milicka relationship are critically reviewed. Problems associated with the scatter in creep life data and the inability of these relationships to accommodate for service-induced material degeneration are discussed. These effects are rationalized in terms of the erratic tertiary creep behaviour of complex engineering alloys. A new creep design relationship based on the sum of the primary creep life tp plus the secondary creep life ts, normalized by the sum of the primary creep strain ϵp plus the secondary creep strain ϵs, varying as a function of secondary creep rate ϵ has been proposed. The relationship is described by t p +t s ϵ p +ϵ s ϵ M =K where M and K are constants. The equation is accurate over a wide range of stresses (350–700 MPa) and temperatures (760–890°C) in IN-738LC. The method systematically reveals the creep degeneration effects with increasing service life. The relationship appears to be independent of any changes in the predominant deformation mechanism.
Apparent and effective creep parameters in single crystals of a nickel base superalloy—II. Secondary creep
Abstract At intermediate and high temperatures (700–1000°C) primary creep of nickel base superalloy single crystals appears as an incubation period (part I) with a very low creep rate ( ge −5 / h ). Following a transition period where ge increases rapidly, secondary creep sets in with a constant creep rate of the order ge ∼- 10 −4 / h . Secondary creep is analized in terms of effective stress σ e = σ a − σ i ( σ a , T ) and effective creep parameters Q e and n e are estimated Experimental determinations of the apparent creep parameters yield n a = 11 and Q a ∼- 135 kcal/mole in this stage. For creep rates of the order of 10 −4 /h, internal stress measurements, as estimated from stress dip tests, point to values of σ i in the vicinity of 0.85−0.95 σ a . When performed under various temperature and applied stress conditions, they lead to the determination of σ i su∗ = ( ∂σ i ∂T) σ a equal to −0.5 MPa/°C and of σ' i = ( ∂σ i ∂σ a ) T equal to 0.2. An effective activation energy of about 60 kcal/mole and an effective stress exponent of the order of 1 are derived from these results. Direct observation of dislocation and precipitate structures in relation with effective creep parameters during secondary creep suggests a high temperature creep model for nickel base superalloys in the range of temperature and applied stress where shearing of the γ' phase does not control the straining process. During secondary creep, strain is mainly the result of climb of dense edge dislocation networks in the γ-γ' interfaces: the dislocation density (− 10 9 /cm 2 ) is kept constant by the glide of screw dislocations between γ' precipitates. Effective creep parameters are typical of a diffusion controlled process: edge dislocations are climbing in the γ-γ' interfaces. This mechanism is responsible for the mass transport leading to the oriented coarsening of the hardening phase taking place during high temperature deformation and also for the typically unstable plastic flow usually observed in this type of high performance material.