Assessment of creep and rupture behavior of 2.25Cr-1Mo steel – A strain-based approach and its limitation

Increasing demand of 2.25Cr-1Mo steel, among different grades of low alloy Cr-Mo steels, in power plants and petrochemical industries is mainly because of its high creep resistance. Application of a strain based approach for a reliable assessment of creep and rupture behavior is thus of great importance. Creep data were generated under different microstructural conditions in the laboratory and collected from literature as well. The influence of thermal ageing and pre-strain on the shape of creep curves has been studied. Analysis of data revealed that irrespective of initial microstructures, thermal ageing increases the tendency to soften but not the pre-strain. Softening due to carbide coarsening thus appears to be the dominant mechanism of creep. Based on this, a strain based approach for creep life assessment was developed. A fairly accurate prediction of creep life up to 5% creep is achieved using eight materials constants extracted from the creep curves of steel having similar initial microstructure. The limitation and reliability of the approach used to assess creep behavior of service-exposed steels have been discussed

Probabilistic creep life assessment of austenitic stainless steel

Experimental investigations on the creep behaviour of engineering materials are frequently conducted for the purposes of design, development and service life prediction. However, tests on a single specimen yield a definite value for each material parameter. But when a number of specimens are tested, the parameter values randomly fluctuate from specimen to specimen. The scatter observed in creep deformation and failure data is of considerable technological importance because it greatly complicates the task of making accurate deformation and lifetime estimates for high temperature components. Repeat data on austenitic stainless steel provided a unique opportunity to identify stochastic creep properties and to use this information to build a probabilistic creep damage assessment for this alloy

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 > 10 years is estimated at 870 degree 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

Creep deformation behavior of 9Cr1MoVNb (ASME Grade 91) steel

Detailed investigations have been performed to examine the creep deformation behavior and microstructural evolution of modified 9Cr-1Mo steel which is widely used in high temperature power plant component. The creep data were analysed in terms of the temperature compensated power law and Monkman-Grant relation. The creep activation energy of Grade 91 steel was determined with 543 +/- 30 kJ/mol without threshold stress compensation, while after correcting the threshold stress; the activation energy is decreased to 303 +/- 15 kJ/mol. This value is close to the activation energy of creep in alpha-Fe. The calculated threshold stress showed a strong dependence on temperature. The creep behavior of the steel was described by the modified Bird-Mukherjee-Dorn relation. The rate controlling creep deformation mechanism was identified as the edge dislocation climb with stress exponent of n=5. Further, the value of creep damage tolerance factor (lambda) and stress exponent was used to identify the cause of creep damage, showed significant difference in the high and low stress regimes. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy to further elucidate the failure mechanisms. Whereas deformed microstructure was examined by transmission electron microscopy. The significant decrease in creep strength in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure

Study on the creep deformation behavior and characterization of 9Cr-1Mo-V-Nb steel at elevated temperatures

In this study, creep tests were conducted on a set of specimens made from the same tube of T91 steel at different temperatures (600 °C, 650 °C and 700 °C) with the applied stresses ranging from 30 MPa to 180 MPa and rupture times from 28.7 to 7957 h for understanding the creep mechanism. The stress dependence of minimum creep rates exhibits a power law with different exponents were found to be 14.3, 11 and 4.6 at 600 °C, 650 °C and 700 °C, respectively. Activation energy of creep was determined to be ~573 kJ/mol, which is ~53% higher than that of the lattice self-diffusion in α-iron. Based on the stress exponents, the modified Bird-Murkherjee-Dorn (BMD) equation was used to find the threshold stresses which has shown the strongly dependent on the test temperature. After incorporating the threshold stress, true activation energy of creep was found to be decreased ~271 kJ/mol, which is equivalent to that of lattice-self diffusion in α-iron. This subsequently led to evaluation of true stress exponent ~4. The results of creep data suggest that the rate-controlling mechanism of creep is dislocation climb for T91 steel. Microstructural observation of T91 steel is mainly consisting of lath martensite structure with fine MX carbonitrides and M23C6 carbides were characterized using the scanning and transmission electron microscopes. The stress dependence of creep rupture life has shown that the operating mechanism of creep is similar. Furthermore, creep data was analysed for validity of the Monkman-Grant relationship and creep damage tolerance parameter. The qualitative microstructure of the crept samples show that less number of MX carbonitrides within grains is responsible for creep cavity formation and decreasing the rupture life. Thus, microstructural degradation is attributed to M23C6 carbides coarsened and dislocation sub-structure boundaries are mainly responsible for creep damage. The fracture morphology of the samples has revealed with dimples is a typical transgranular ductile failure

Modelling of Elasto-Plastic Deformation and Crack Propagation Studies in Thermal Barrier Coated Superalloys

The present work highlights some of the results on deformation behaviour in elastic plastic condition under bending, for miniature test specimens of thermal barrier coatings (TBCs) on Ni base superalloys. The experimental data was simulated mathematically using beam elements in Finite Element Analysis. The analysis indicated concu-rrence of displacements, for each load case, with the experimental results in the elastic regime. Reasonable extent of concurrence was also obtained in the early stages of plastic deformation of elements. However, minor deviations occurred from experimental results in the case oflauge loads, i. e., large plastic deformation. A comp-uter code was developed considering elastic-plastic finite elements, for four point bend specimens with two degrees of f reedom at each node

Stress induced creep cavity

Studies monitoring cavity nucleation with strain after creep deformation has limited information with varying applied stresses. In the present communication, a large number of experiments were performed to investigate cavity nucleation at different stresses for a primary reformer tube. It has been found that with the increase in stress, cavity formation increases drastically, which is more prone to nucleate at grain boundary triple junctions rather than grain boundaries where the extent of initial inclusions/other second phase particles was kept unaltered. (C) 2014 Elsevier B.V. All rights reserve

Structural integrity of service exposed primary reformer tube in a petrochemical industry

This paper aims at quantifying creep damage, in terms of Kachanav's continuum damage mechanics (CDM) model and Bogdanoff model of service exposed primary hydrogen reformer tube. Quantification of creep damage was made from scatter in voids which was quantified from light optical microscopy (LOM), SEM (scanning electron microscope) equipped with image analyzing technique and EDX analyser, in terms of two damage parameters A and A*. Scatter in creep deformation behaviour of the material, is probably due to variation in mode of fracture and scatter in voids. Probability of rupture due to void area, shifts towards the higher population of void with increase in true strain. Experimental data for estimating creep damage agrees well with both the simulation based models. The value of damage tolerance parameter a infers that growth of cavities has been attributed to a purely diffusion controlled mechanism, grain boundary sliding mechanism, or a combination of both. (C) 2015 Elsevier Ltd. All rights reserved