Nilima Roy

Crack propagation studies and bond coat properties in thermal barrier coatings under bending

Ceramic based thermal barrier coatings (TBC) are currently considered as a candidate material for advanced stationary gas turbine components. Crack propagation studies under bending are described that were performed on plasma sprayed ZrO2, bonded by MCrAlY layer to Ni base superalloy. The crack propagation behaviour of the coatings at room temperature in as received and oxidized conditions revealed a linear growth of the cracks on the coating till the yield point of the super alloy was reached. High threshold load at the interface between the ceramic layer and the bond coat was required to propagate the crack further into the bond coat. Once the threshold load was surpassed the crack propagated into the brittle bond coat without an appreciable increase in the load. At temperatures of 800°C the crack propagated only in the TBC (ceramic layer), as the ductile bond coat offered an attractive sink for the stress relaxation. Effects of bond coat oxidation on crack propagation in the interface region have been examined and are discussed.

Simulation of bond coat properties in thermal barrier coatings during bending

Analytical models are presented for predicting bond coat properties of thermal barrier coatings (TBCs) during crack propagation studies induced by bending. Studies on crack propagation behaviour in TBCs were performed with plasma spray coated zirconia, bonded by a MCrAlY layer to Ni-base superalloys (Inconel 617 and CMSX-4). Such thermal barrier composites are currently considered as candidate materials for advanced gas turbine stationary components. Coating as a protective layer improves the surface properties of the substrate. At a temperature of 1073 K, the crack propagation was found to be confined to the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. The stress-strain behaviour is a function of the elastic modulus of coating, bond coat as well as that of substrate. Thus, from a knowledge of the elastic modulus of TBC and that of substrate, the elastic modulus of the bond coat needs to be evaluated which is a basic parameter for characterizing coating performance. In this paper, the elastic modulus of the bond coat has been determined by modifying the existing model for a two-layered composite beam to a three-layered composite beam.

Investigation and modeling of mechanical properties for thermal barrier coatings under bending

This paper is aimed at developing analytical models for predicting bond coat properties during crack propagation studies under bending on thermal barrier coatings (TBCs) and to reveal the stress distribution pattern along the directions of global axes and the equivalent stress for all types of specimens used in the present investigation. Crack propagation studies were performed with plasma spray coated zirconia, bonded by a MCrAlY layer to Ni-base superalloys (Inconel 617 and CMSX-4). Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. Coating as a protective layer improves the surface properties of the substrate. At a temperature of 800 degreesC, the crack propagation was found to be confined to the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. The stress-strain behaviour is a function of the elastic modulus of coating, bond coat as well as that of substrate. Thus, having known the elastic modulus of TBC and that of substrate, the elastic modulus of the bond coat needs to be evaluated which is a basic parameter for characterising coating performance. In this paper, the elastic modulus of the bond coat has been determined by modifying the existing model for two-layered composite beam specimen to a three layered composite beam specimen. FEM (finite element) analysis for all types of tested specimens was conducted with ANSYS software using a 2-D isoparametric solid element, defined by four nodal points having two degrees of freedom at each node. Simulations were carried out all specimens with selected load conditions in the elastic region of the stress-strain curves.. The outputs of the stresses along the direction of global axes i.e. sigma (x), sigma (y) (component stresses), the shear stress sigma (xy), the principal stresses sigma (1), sigma (2), sigma (3) and the equivalent stress, sigma (eq) have been presented and discussed.

Simulation and quantification of creep damage

This paper highlights assessment of creep damage of an 11-year service-exposed reformer tube made up of HP40 grade of steel of a petrochemical industry from replicated creep data. The existence of scatter in the void characterization, creep deformation and rupture data leads to substantial amount of uncertainty in the assessment of creep damage. A discontinuous Markov process, which takes into account the associated scatter observed in damage evolution, has been used for creep damage assessment. The damage parameter A* is compared with the damage prediction by Markov process, where A* is derived as the cumulative contribution from microstructurally determined cavities at grain boundaries (A1) as well as number of creep cavities or voids at the grain boundary triple points (A2). This damage parameter (creep cavitation /voids) is in close agreement with the damage prediction curves at low stress whereas at high stresses it could correlate well till the midregion of the predicted curves, above which cavity measurement was practically not feasible. Also the value of damage tolerance parameter λ (ratio of rupture strain to Monkman–Grant constant) infers that growth of cavities has been attributed to a purely diffusion controlled mechanism, grain boundary sliding mechanism or a combination of two.

High temperature mechanical properties of thermal barrier coated superalloy applied to combustor liner of aero engines

High temperature load controlled fatigue, hot tensile and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liner of aero engines are highlighted in this paper. Acoustic emission technique has been utilised to characterise the ductile-brittle transition temperature of the bond coat. Results revealed that the DBTT (ductile to brittle transition temperature) of this bond coat is around 923 K, which is in close proximity to the value reported for CoCrAlY type of bond coat. Finite element technique, used for analysing the equivalent stresses in the bond coat well within the elastic limit, revealed the highest order of equivalent stress at 1073 K as the bond coat is ductile above 923 K. The endurance limit in fatigue and the life of TBC coated composite under accelerated creep conditions are substantially higher than those of the substrate material. Fractographic features at high stresses under fatigue showed intergranular cleavage whereas those at low stresses were transgranular and ductile in nature. Delamination of the bond coat and spallation of the TBC at high stresses during fatigue was evident. Unlike in the case of fatigue, the mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular in creep.

Damage Analysis of Service Exposed Reformer Tubes in Petrochemical Industries

Abstract Accelerated creep or stress rupture data is used for remaining life assessment for life management studies of elevated temperature components, e.g. for reformer tubes where packed nickel catalysts are used for synthesis of hydrogen, ammonia etc. This research has become a regular task because of large range of time for failure (3 to 15 years) compared to designed life (11.4 years or 100,000 hours) and huge loss associated to damage, production and safety hazards. Utilization of appropriate inspection during plant shut down has been strategic short term life assessment. Tests have been typically done by high temperature mechanical properties, microstructure analysis and accelerated creep. Inspection of micro-cracks, hot spot formation, carburization/metal dusting for inner wall and oxidation, tube diameter increment for outer wall inspection have been traditional symptoms of expiry of tubes after service exposure. Aim of this review has been to study damage analysis of reformer tube in response to so wide time frame for failures and accidents involved, even after stipulation of designed time schedules.

Creep deformation and damage evaluation of service exposed reformer tube

Abstract This paper aims at studying the creep deformation behaviour and quantifying creep damage of ∼11 years service exposed primary hydrogen reformer tube made of HP40 grade of steel in a petrochemical industry, in terms of Kachanov’s continuum damage mechanics model (K model) and Bogdanoff model (B model) based on Markov process. Hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography were extensively carried out. The as received tubes did not possess any sign of degradation including voids or creep cavitations and decarburisation. There was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to aging and overheating. Accumulation of damage due to creep has been quantified through microstructural studies in terms of two damage parameters A and A*. Experimental scatter observed in creep deformation and creep strain rate curves of the material at 870°C and at various stress conditions, is probably due to scatter in creep cavitations/voids and also due to variation in the mode of fracture in top as well as bottom portion of the tube. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area, shifts with increase in true strain towards the higher population of void. The estimation of average time to reach a specific damage state from K model and B model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well.

Life Estimation and Creep Damage Quantification of Service Exposed Reformer Tube

Abstract This paper deals with evaluation of creep damage of ~11 years service exposed primary hydrogen reformer tube made of HP-40 grade of steel in a petrochemical industry, which has been carried out in terms of Kachanav’s continuum damage mechanics (CDM) model (K-model) and Bogdanoff model (B-model) based on Markov process. Residual life of the tubes was estimated based on hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography. Accumulation of damage due to creep has been quantified through microstructural studies. The as received tubes did not reveal any degradation in the material like creep cavitation or voids, but there was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to ageing and overheating. Scatter in creep deformation behaviour of the material is probably due to variation in mode of fracture and scatter in voids. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area shifts with increase in true strain towards the higher population of void. The estimation of mean time to reach a specific damage state from K- model and B-model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well. A remnant life of >10 years is estimated at the operating stress–temperature conditions of the top as well as bottom portion of the tube.

Performance and Life Assessment of Reformer Tubes in Petrochemical Industries

Abstract High temperature mechanical properties of service exposed reformer tubes are compared and revalidated with those of virgin material for the sake of health assessment studies. Technical interpretations of creep and stress rupture (accelerated creep) data usually are in close proximity in absence of defects. Results of extrapolation of accelerated creep data were optimized. This review article is aimed at studying life prediction methodology of reformer tubes operating at high temperature and aggressive chemical environment, in response to the designed time frame for preventing failures and accidents, even before stipulated design time schedules in some cases.

Damage Resistance of a Thermal Barrier Coated Superalloy Used in Aero Turbine Blade under Accelerated Creep Condition

This paper highlights the hot tensile and accelerated creep properties of a thermal barrier coated (TBC) AE 437A alloy used as a candidate blade material in aero engines. Acoustic emission techniqu ...