Ashok K Ray

Analysis of failed reformer tubes

Reformer tubes from a fertilizer plant made of modified HK 40 steel which failed after 4 years service were investigated for failure mechanism and life evaluation. The investigation included hot tensile tests, hardness measurement, dimensional measurement, microscopy and a few accelerated creep tests. Analysis revealed that longitudinal cracks found in the tubes were caused by overheating because of inadequate feed flow caused by the choking of damaged catalyst. To avoid such choking, precautions should be taken while charging the catalyst that no broken piece of catalyst or any external material goes along. Nitrogen which is used for cooling down the tubes and which was retained inside during idle periods should be dry enough. Overheating during service is primarily responsible for significant degradation in mechanical properties and microstructures in the bottom portion of the reformer tubes.

Turbine blade failure in a thermal power plant

The failure of a LP (low pressure) turbine blade of a 220 MW thermal power plant is presented. The blade was made of martensitic stainless steel and the structure was tempered martensite. There was no evidence of degradation of blade material. The fracture took place at the aerofoil region, 113-mm from the root. Throughout the blade surface Si rich phases were detected. Several pits/grooves were found on the edges of the blades and chloride was detected in these pits. These were responsible for the crevice type corrosion. The probable carriers of Cl− were Ca and K, which were found on the blade. The failure mode was intergranular type. Possibly the ultimate failure was due to corrosion-fatigue.

Life of rolls in a cold rolling mill in a steel plant— operation versus manufacture

Failures of rolls occur due to improper manufacturing and operational parameters. Prematurely failed (spalled) roll samples collected from a reputed steel plant were examined for their chemistry, inclusion content, microstructures, carbide characteristics, hardness and retained austenite content. The residual stresses were also measured on the inner and outer surfaces of the spalled roll pieces. The higher content of retained austenite was primarily responsible for the spalling of indigenous rolls for which subzero treatment has been recommended. Several suggestions have also been made for smooth operation of the mill and consequently for the life extension of work rolls.

Failure of a super heater tube

The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of oxide layer was observed in the inner side of the failed tube. In the other tube the inner oxide layer was very small and the tube was thinned down due to fireside corrosion. The unexposed tube from the same lot revealed a ferritic bainitic microstructure. The failed tube also showed a ferrite bainite microstructure but most of the bainitic carbide had transformed to globular form. Considerable carbide precipitation both in grain boundary (GB) and inside grains were observed. A large number of cavities and micro-cracks were observed along the grain boundaries. A similar microstructure was observed on the same tube 10 cm away from the failed region. The micrograph at the fracture location showed a similar microstructure but the grains are very much elongated. The formation of globular oxide on the other tube was compared to that of the failed tube. Due to breakage of oxide scale of the failed tube, the metal surface of the failed tube was exposed to steam at a higher temperature and this accelerated the oxide growth. This led to creep damage of the material. Ultimately, failure occurred due to rapid overheating of the tube material.

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.

Thermal Barrier Coating System for Gas Turbine Application - A Review

Ceramic coatings are refractory metal compounds deposited on substrates to reduce thermal loss and to protect components from high temperature. Thermal barrier coatings (TBC) are composite overlay of bond coat and ceramic coat oil a superalloy substrate. Atomised deposition or splat deposition of fine semi-molten particle technique deposits thin coatings of brittle ceramic. Thermal and mechanical strains arising from service exposure require structural compliance tolerances. This is facilitated by brittle constituent deposition over a ductile substrate. Electron beam, physical vapour deposition and plasma spray technique lead to a tortuous intergranular network of coating Porous deposition technique is applied in all cases instead of cementation or continuous section thickness. Thermal barrier coating is inevitable in aerospace engine sections operating at limiting conditions of strains. Thermal barrier coatings help in protection of high temperature components for maximum utilisation of component lives, and maximum utilisation of energy by operating at optimum allowable temperature limits. Thermo mechanical behaviour of TBC is optimised by in-silu formation and transformation mechanisms of alumina from aluminium of substrate/bond coat and metastable tetragonal zirconia to stable tetragonal zirconia respectively at temperature of service. While the former produces a volumetric contraction, the latter produces volumetric expansion. In service the composite system provides auto-toughening effects in due course. An intergranular tortuous network of coating forms cracks on exposure of strain and the crack tip blunting forms cubic allotropy from metastable tetragonal phase, resulting in an increase in toughness due to elimination of c/a ratio. However, a prolonged exposure forms, localised spallation zones, which are initiated by volumetric expansion stresses associated with nickel enrichment of thermally grown oxides (TGO) at bond coat/ceramic coat interface, and auto-sintering. Bond coat is applied to produce mechanical adherence and stress relaxation effects. Generally M-CrAlY families of bond coating alloys are used for this purpose. Exposure to operating/test temperature produces thermally grown oxides (TGO) at interface. This occupies an intermediate zone in response to property interactions. TGO mainly consists of alumina being catalyzed by chromia and adhered by yttria. Active research is going oil to study the mechanisms of auto sintering and auto-toughening of TBC. Work is in progress to explore how to decrease thermal expansion mismatch stresses by application of composite. coatings made from functionally graded materials, microlaminated, and multilayered ceramic/ceramic or metallic/ceramic or metallic/metallic coatings. The application of laser scaling or remelting to reduce porosity of free surface and to increase glaze are other avenues to reduce diffusion of reactive gases and to increase internal heat, transfer respectively. The former increases life of bond coat/substrate, whereas the latter increases energy efficiency by maximum utilisation of heat. The main unsolved problem is spallation of ceramic coating, which is cohesively induced, in either side of interface and spread out to interfaces of adhesion. TBC increases. life more than two-fold for cases of aerospace engines. However localised spallation may rise by high temperature corrosion of bond coat/substrate, TGO stresses, gaseous/liquid contaminant diffusion/impregnation through tolerance networking of voids, and erosion.

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.

Investigation on the mechanical degradation of a steel line pipe due to hydrogen ingress during exposure to a simulated sour environment

The loss of ductility of a longitudinally welded steel pipe, brought about by hydrogen ingress during exposure to simulated sour environments, was investigated. The material was found to be very prone to hydrogen induced cracking (HIC). The presence of hydrogen-induced cracks were not necessarily detrimental to the ductility as demonstrated by ex-situ straining. It was concluded that mobile lattice hydrogen was responsible for ductile loss, while trapped hydrogen was responsible for HIC.

Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SiCw) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.