Value Of Larson-Miller Parameter Research Articles

Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube

In this work, a Cr35Ni45Nb alloy tube for use in a reformer furnace is subjected to high temperature (871 °C and 927 °C) creep regimes, and the microstructural changes (e.g. carbide precipitation and phase transformation) and consequent creep damage are investigated. The results of creep tests under closely similar applied stresses give different values of the Larson–Miller parameter (LMP) at different temperatures. As a result, the calculated creep rupture time at 871 °C based on the LMP value measured at 927 °C is approximately twice that which was actually measured at 871 °C. The microstructure of the as-cast tube is found to consist of an austenitic matrix with networks of MC (NbC) and eutectic (M23C6) carbides located both at grain boundaries and between dendrites. It is noted that the M23C6 carbide is not a primary eutectic carbide that can be observed in the as-cast Cr35Ni45Nb alloys. It can be argued that the primary eutectic M7C3 carbides were transformed into M23C6 due to the heat of spiral welding during the tube manufacturing process. After creep, the NbC carbides at both locations were mostly transformed to the G-phase (Nb3Ni2Si) and all the precipitates formed inside the austenitic matrix were composed of the M23C6 and G-phase. Creep cavities were initiated around the G-phase and grew cracks along the grain boundaries due to the formation of a Cr-depleted zone and the G-phase.

High temperature creep properties of a linear friction welded newly developed wrought Ni-based superalloy

AD730™ Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ′ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750MPa stress levels, and 850°C under 100 and 200MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730™ Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5 × 103 and 24.5 × 103. It was found that in the investigated temperature range, the PWHTed AD730™ has similar creep characteristics as Udimet™720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.

Life assessment of die-cast Mg–5Al–1.7Ca alloys under creep service conditions

The creep rupture behavior of a Mg–5Al–1.7Ca alloy produced via die casting was investigated at temperatures between 423 and 498K, and the creep rupture life of the alloy was evaluated using the Larson–Miller parameter (LMP). It was demonstrated that the minimum creep rate and creep rupture life followed the phenomenological Monkman–Grant relationship. The value of the LMP was uniquely described by the logarithm of the applied stress when the Larson–Miller constant was set at 20. In addition, the applied stress and the LMP value followed a linear relationship below the yield stress. These results can be utilized to predict the long-term creep rupture life of the alloy under a given temperature condition and applied stress.

Uncertainty in Damage Assessment and Remaining Life Prediction of Engineering Materials Used In Petrochemical Industry

In this paper creep damage assessment of about 11 years’ service exposed HP-40 grade of steel used in hydrogen reformer of a petrochemical industry has been carried out in terms of a discontinuous Markov process. Experimentally determined conventional creep data under identical testing condition were used in the present investigation. Scatter and damage accumulation due to creep deformation were evaluated through microstructural assessment using light optical microscope and scanning electron microscope. Quantification of creep damage was made from replicated creep data in terms of two damage parameters A and A*. Statistical analysis of void area fraction has been carried out extensively for the both top and bottom portions of the reformer tube at 870 o C in the stress range of 52-68 MPa. In addition, the proposed probabilistic model has been compared with the Kachanav’s Continuum Damage Mechanics (CDM) model. Both the approaches displayed quantitative experimental support. A residual life of A¢ÂŸÂ© 10 years is estimated at 870 o C/ operating stress. For 55 years’ service exposed Catalytic Cold Cracking (CCU) reactor vessel and Feed Processing Unit (FPU) distillation column materials of a petrochemical industry remnant life assessment studies were estimated by incorporating the uncertainty involved in calculation of LMP (Larson Miller Parameter) values and from extrapolation of stress vs. LMP plot. Variability of normalized creep damage for reactor and column materials is well approximated with the aid of Weibull distribution. As expected, it is observed that the distributions shift towards the higher range of damage with increase in service exposure time.

Damage assessment of service exposed reactor and distillation column materials of a petrochemical industry

This work highlights quantification of normalized creep damage incorporating uncertainty with respect to Larson–Miller parameter (LMP) at various intervals (with service exposure) of a 55years service exposed reactor and distillation column materials of a petrochemical industry. Usually, remaining life is predicted from extrapolation of stress vs. LMP plot without incorporating the uncertainty involved in calculation of LMP values. Uncertainty involved in calculating LMP value has been given prime importance in this work because it is a function of rupture time. Variability of normalized creep damage for reactor and column materials is well approximated with the aid of Weibull distribution. As expected, it is observed that the distributions shift towards the higher range of damage with increase in service exposure time.

Analysis of creep rupture life of Mg–Al–Mn alloy produced by die-casting

Creep rupture tests were performed for a die-cast Mg–Al–Mn alloy, AM50, at temperatures between 423 and 498 K under stresses between 30 and 120 MPa, and the creep rupture lives were evaluated by using the Larson-Miller parameter (LMP). The value of LMP is uniquely described by the logarithm of the applied stress for the alloy, when the Larson-Miller constant is chosen as fifteen rather than twenty. The Larson-Miller constant depends predominantly on the morphology of intermetallic phases in microstructure for magnesium alloys.

Assessment of creep rupture life of heat resistant Mg–Al–Ca alloys

Creep rupture tests were performed for three kinds of Mg–Al–Ca die-cast alloys at temperatures between 423 and 498 K, and the creep rupture lives were evaluated by using the Larson–Miller parameter (LMP). The value of LMP is uniquely described by the logarithm of the applied stress for the alloys, when the Larson–Miller constant is chosen as 20. The applied stress and the LMP value follow a linear relationship below the yield stress, which can be utilized to predict the long-term creep rupture life for the alloys under a given condition of temperature and applied stress.

Changes in Electrochemical Properties of Directionally Solidified Ni-base Superalloy Due to Creep and Damage Evaluation.

Changes in electrochemical properties of directionally solidified Ni-base superalloy CM 247 LC due to creep have been investigated. The electrochemical polarization technique has been applied to creep-ruptured and creep-interrupted specimens under various creep condition in order to examine a detectability of creep damage. Experimental results on electrochemical polarization measurements revealed that the peak current density and which appeared at a specific potential during potentiodynamic polarization reactivation measurements in dilute glyceregia solution increased with increased thermal history and showed a good correlation with Larson-Miller Parameter value. Ip and Ipr value reflected both a thermal aging effect and a stress effect, and increased with a creep life fraction. The increases in Ip and Ipr were mainly corresponded to increases in preferential dissolution rate of γ' phase at γ/γ' interface. This preferential dissolution was considered as a result from degradation of corrosion resistance of γ'phase at γ/γ'interface which may be attributable to creep damage accumulation. As a consequence, the creep damage in CM 247 LC can be quantitatively evaluated by the electrochemical technique.