Creep is the main degradation mechanism in steel components operating at high temperatures in the... more Creep is the main degradation mechanism in steel components operating at high temperatures in thermal and nuclear units. Therefore, acquiring creep data during equipment operation is vital for a realistic calculation of the remaining useful life of these units. However, there are severe limitations to obtain sufficient material for standard size specimen fabrication. Therefore, replacing conventional specimens with small-size specimens, which can be fabricated from a limited amount of material, can facilitate the assessment of power-plant components by creep tests without structural damage. This study aims to compare the tensile and creep test results of CrMo ferritic steel using conventional and small-size specimens to assess the effectiveness of using miniaturized specimens for analysis. The results of this study confirm the applicability of miniaturized-specimen creep testing for the reliable estimation of the materials remaining life.
The present study shows feasibility of synthesising steel-aluminium based metal intermetallic lam... more The present study shows feasibility of synthesising steel-aluminium based metal intermetallic laminate (MIL) composites by an industrially viable and repetitive roll bonding process and subsequent annealing. Roll bonding process has been utilised to achieve solid state bonding of interstitial free (IF) steel and aluminium with simultaneous reduction in sheet thickness. Further annealing of roll bonded laminate sheets gives rise to IF steel-aluminides/aluminium multilayer composites. Formation of Al 5 Fe 2 phase at the interface is confirmed by X-ray diffraction (XRD) analysis.
In recent years research on the processing, structure and mechanical behavior of nanocrystalline ... more In recent years research on the processing, structure and mechanical behavior of nanocrystalline (d < 100 nm) and ultra-fine grained (100<d<1 lm) materials has thrived[1]. Both the promising structural properties, such as elevated strength, good wear resistance and high tough-ness, as well as the potential for superplasticity at low temperatures and high strain rates [2,3], constitute the driving force for this flourshing field. Processing homog-eneous bulk nano and ultra-fine grained (ufg) structures is complex endeavor.
A 210MW turbine blade failed prematurely. It is a free standing blade. The blade failed at 250mm ... more A 210MW turbine blade failed prematurely. It is a free standing blade. The blade failed at 250mm from the tip. The failed blade along with a deformed portion were sent to NML to find out the cause of the failure. The material of the blade is hardened and tempered martensitic microstructure; it shows no degradation. The mechanical property of the blade material is nearly similar to that of the standard alloy of similar nature. Fractographic analysis of the blade shows no intergranular mode of deformation. The fracture surface consists of embedded silica particles as well as blisters associated with chlorine. It has been concluded that the blade failed due to initiation of cracks by chloride assisted corrosion with silica particle impingement. Later these cracks grow under fatigue loading and lead to failure of the blade.
The major objective of the project is to develop major facilities to carry out (i)R&D (ii) Co... more The major objective of the project is to develop major facilities to carry out (i)R&D (ii) Consultancy and servi-ces and (iii) Training programme related to nondestructive evaluation of materials.
Study of creep behavior of base metal (without weld) and welded specimens P91B steel over a range... more Study of creep behavior of base metal (without weld) and welded specimens P91B steel over a range of temperature (600-650 °C) and stress (50-180 MPa) showed similar values of minimum creep-rates for both specimens at higher stress regime (> 100 MPa) whilst, significantly higher creep rates of welded specimens at lower stress regime Considering that welded specimen comprised of two distinct structural regimes, i.e. weld affected zone and base metal, a method was proposed for estimating the material parameters describing creep behavior of those regimes. Stress-strain distribution across welded specimen predicted from finite element analysis based on material parameters revealed preferential accumulation of stress and creep strain at the interface between weld zone and base metal. This is in-line with the experimental finding that creep rupture preferentially occurs at inter-critical heat affected zone in welded specimens owing to ferrite-martensite structure with coarse Cr 23 C 6 particles.
Experimental investigations on the creep behaviour of engineering materials are frequently conduc... more 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. Several studies, employing theory of probability concepts have been developed to predict and characterize the variations in creep damage parameter. So far the deterministic approach has been used which is based on only two limiting states of the material i.e., ω= 0, no damage and ω = 1, completely damaged. This imposes severe limitations on the validity and practical applications of this model. For instance, it is not possible to trace the damage accumulation during service using the NDT techniques.
The paper presents creep test data on standard P91B steel specimens made from two distinct region... more The paper presents creep test data on standard P91B steel specimens made from two distinct regions of a welded plate over a range of stresses (50-190 MPa) and temperatures (600-650°C). The analysis of test data revealed that the samples having a weld zone within the gage length (cross-weld samples) have lower long term rupture strength than the samples made of the base metal. Estimated weld strength factors (WSF) of this steel were found to be higher than those reported for P91 steel. The study also showed that the effect of welding on loss of rupture ductility is much more prominent than its effect on the reduction in rupture strength. In presence of welded zone the extent of local deformation in ruptured samples was not as prominent as in the samples without weld. Creep damage tolerance factors (λ) were estimated from the creep strain versus time plots. This also showed that the magnitude of λ is significantly reduced in the presence of welding. Examination of microstructure and measurement of density revealed that this difference is primarily due to the formation of cavities in the heat affected zones of welded specimens. In the lower stress regime a few test specimens without any welded region did not fail even after very long creep exposure. Diameters of these specimens were found to have increased in spite of measureable increase in length due to creep. This unusual effect has been attributed to oxide scale growth. It shows up when the increase in diameter due to the growth of oxide scale becomes greater than the decrease in diameter due to the accumulation of creep strain.
ABSTRACT Multimaterial fabrication such as joining of steel and aluminum is currently prominent i... more ABSTRACT Multimaterial fabrication such as joining of steel and aluminum is currently prominent in a variety of industries. Friction stir welding is a novel solid-state welding process that causes good joint strength between steel and aluminum. However, the phenomenon contributing significant strength at the interface is not yet clear. In the present study, the interface of the friction stir lap-welded aluminum and coated steel sheet having joint strength maximum (71.4 pct of steel base metal) and minimum, respectively, under two parameter combinations, i.e., 1000 rpm 50 mm min−1 and 500 rpm 100 mm min−1, was exclusively characterized by X-ray diffraction, transmission electron microscopy (TEM), concentration profile, and elemental mapping by electron-probe microanalysis. A TEM-assisted EDS study identifies the morphologies of large size Al13Fe4 and small size Fe3Al-type intermetallic compounds at the interface. The diffusion-induced intermetallic growth (thickness) measured from a backscattered image and concentration profile agreed well with the numerically calculated one. The growth of these two phases at 1000 rpm 50 mm min−1 is attributed to the slower cooling rate (~3.5 K/s) with higher diffusion time (44 seconds) along the interface in comparison to the same for 500 rpm 100 mm min−1 with faster cooling rate (~10 K/s) and less diffusion time (13.6 seconds). The formation of thermodynamically stable and hard intermetallic phase Al13Fe4 at 1000 rpm and travel speed 50 mm min−1 in amounts higher than 500 rpm and a travel speed of 100 mm min−1 results in better joint strength, i.e., 71.4 pct, of the steel base metal.
High-temperature, high-cycle fatigue and accelerated creep properties of thermal barrier coated (... more High-temperature, high-cycle fatigue and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liners of aero engines are highlighted in this paper. An acoustic emission technique has been utilised to detect the ductile-brittle transition temperature of the bond coat. Results revealed that the ductile-to-brittle transition temperature (DBTT) of this bond coat is around 650 • C, which is in close proximity to the value reported for CoCrAlY type of bond coat. 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 was 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. It is recommended that these TBCs should not be put into use above 800 • C for more than 1000 h of continuous operation as the strength of the TBC superalloy drops to negligible levels.
Mechanical property evaluation is a powerful tool for estimating the residual life of in-service ... more Mechanical property evaluation is a powerful tool for estimating the residual life of in-service components and health monitoring. In the present investigation such an assessment was carried out to estimate the remaining life of inservice plain carbon steels that had been in use for 45 years in a feed processing unit (FPU) of a petrochemical plant. Health assessment of both base and weld joints of the material has been carried out through accelerated stress rupture tests and other mechanical property evaluation. At the operating conditions, it is noted that the materials still have about 10 years of residual life. However, surprisingly weld joints of the column of this unit exhibited poor notch toughness (5-10 J). It could not be proved beyond doubt, if the weld developed poor notch toughness during fabrication or degraded during service. Annealing of weld joints at 530 C for 1.5 h improved the notch toughness significantly, hence is recommended accordingly.
Abstract: This paper deals with an evaluation of the lifetime of a thermal barrier coated (TBC) C... more Abstract: This paper deals with an evaluation of the lifetime of a thermal barrier coated (TBC) C263 superalloy under fatigue and creep loading. Results revealed that both TBC and bond-coated substrate had higher endurance limits than the base alloy, while the opposite was ...
A generalised constitutive description of deformation of engineering alloys that exhibit a domina... more A generalised constitutive description of deformation of engineering alloys that exhibit a dominant tertiary creep is presented that extends a previous treatment of creep deformation. The model parameters of the constitutive equations that are determined by analysis of creep curves are supplemented by the Young's modulus to provide a database from which illustrative calculations are made. Simulations of constant strain rate, stress relaxation and high strain low cycle fatigue (both strain and stress control) are made for the superalloy IN738LC; the predictions are compared with limited available experimental data. Rfmtmf-On prfsente une description gfnfralisfe de la dfformation d'alliages industriels qui subissent un fluage tertiaire dominant; cette description dfveloppe un traitement antfrior de la dfformation par fluage. Ides paramftres du modfle des 6quations constitutives qui sont dfterminfes par l'analyse des courbes de fluage, sont complftfs par le module de Young pour fournir une base de donnfes fi partir de laquelle sont effectufs les calculs. Des simulations de vitesse de dfformation constante, de relaxation de la contrainte et de fatigue fi faible nombre de cycles ~i dfformation 61evfe (contrfle de la deformation et de la contrainte) sont effeetufes pour le superalliage IN738LC; les prfvisions sont comparfes avec les quelques donnfes expfrimentales disponibles. Zusammenfassung-Es wird eine verallgemeinerte Beschreibung der Verformung yon Legierungen, bei denen terti/ires Kriechen dominiert, vorgelegt; diese Beschreibung erweitert eine friihere Behandlung der Kriechverformung. Die Modellparameter der Grundgleichungen werden aus der Analyse yon Krieehkuryen bestimmt und ergfinzt um den Elastizi/itsmodul, um eine Grundlage fiir illustrierende Berechnungen zu schaffen. Fiir die Legierung IN738LC werden Simulationen fiir Verformung bei konstanter Dehnungstrate, f/ir Spannungsrelaxation und f/ir Ermiidung bei hohen Dehnungsamplituden und niedrigen Zyklenzahlen (sowohl dehnungswie auch spannungskontrolliert) durchgefiihrt. Die Ergebnisse werden mit den wenigen vorhandenenen experimentellen Beochtungen verglichen.
The boiler tubes and pipes in the present day coal fired power plants are designed against damage... more The boiler tubes and pipes in the present day coal fired power plants are designed against damage arising from the interaction of creep and fatigue. The present investigation reports the microstructural evolution in P92 grade martensitic steel during pure fatigue and hold time fatigue tests conducted at 600 o C. Fatigue life significantly dropped for hold time fatigue tests in comparison with pure fatigue tests. The drop in fatigue life was more for hold time fatigue tests conducted with compressive hold. Grain boundary oxidation and cracking was identified as the major cause for the decrease in fatigue life under compressive hold. Annihilation of dislocations and pinning of dislocations by MX precipitates were observed to be the microstructural changes during cyclic deformation.
One of the promising ways for mitigation of Type IV cracking-a failure by cracking at the intercr... more One of the promising ways for mitigation of Type IV cracking-a failure by cracking at the intercritical /fine grained heat affected zone, a life limiting problem in advanced 9-12 Cr ferritic steel weld like that of P91 is through modification of alloy composition by addition of boron. Addition of boron was observed to improve the microstructure at the weld zone and hence the creep strength. In the present work, boron (100 ppm with controlled nitrogen) added P91 steel after normalizing at 1050C and 1150C and tempered at 760C were studied for the creep behavior in the base metal and welded condition in the temperature range of 600-650C. Creep strength was characterized in terms of stress and temperature dependence of creep rate and rupture time. Weld creep life was reduced compared to the base metal with rupture occurring at the ICHAZ (Type IV crack). However at longer time (at lower stress levels) exposure creep crack moves from weld metal to HAZ (Type II crack). Rupture life was found to superior for the base and weld in the boron containing steel when higher normalizing temperature is used. Estimation of 10 5 h was attempted based on short term rupture data available and weld strength factors were calculated. Observed values are better for P91BH condition than the values for P91BLcondition as well as those available for P91 in open literature
Acoustic Emission technique has advance from laboratory use to application such as integrity of p... more Acoustic Emission technique has advance from laboratory use to application such as integrity of pressure vessels and many other structures. In this in-house project, we have looked into the condition monitoring of prototype horton sphere by acoustic emission technique during hydro-test. The damage mechanism occurred during hydrotest are characterized based on AE parameters. The predicted damage mechanisms are simulated on the similar materials. The results show that the predicted damage mechanisms include oxide scale peeling, breaking, chipping etc. The source location study identifies the damage prone zone as verified by ultrasonic NDE. The hydrotest of the Horton sphere, which was left in atmosphere for a decade, indicates that major material degradation occurs near the bottom flange. The degradat-ion was identified as loose and adherent oxide scales.
Creep is the main degradation mechanism in steel components operating at high temperatures in the... more Creep is the main degradation mechanism in steel components operating at high temperatures in thermal and nuclear units. Therefore, acquiring creep data during equipment operation is vital for a realistic calculation of the remaining useful life of these units. However, there are severe limitations to obtain sufficient material for standard size specimen fabrication. Therefore, replacing conventional specimens with small-size specimens, which can be fabricated from a limited amount of material, can facilitate the assessment of power-plant components by creep tests without structural damage. This study aims to compare the tensile and creep test results of CrMo ferritic steel using conventional and small-size specimens to assess the effectiveness of using miniaturized specimens for analysis. The results of this study confirm the applicability of miniaturized-specimen creep testing for the reliable estimation of the materials remaining life.
The present study shows feasibility of synthesising steel-aluminium based metal intermetallic lam... more The present study shows feasibility of synthesising steel-aluminium based metal intermetallic laminate (MIL) composites by an industrially viable and repetitive roll bonding process and subsequent annealing. Roll bonding process has been utilised to achieve solid state bonding of interstitial free (IF) steel and aluminium with simultaneous reduction in sheet thickness. Further annealing of roll bonded laminate sheets gives rise to IF steel-aluminides/aluminium multilayer composites. Formation of Al 5 Fe 2 phase at the interface is confirmed by X-ray diffraction (XRD) analysis.
In recent years research on the processing, structure and mechanical behavior of nanocrystalline ... more In recent years research on the processing, structure and mechanical behavior of nanocrystalline (d < 100 nm) and ultra-fine grained (100<d<1 lm) materials has thrived[1]. Both the promising structural properties, such as elevated strength, good wear resistance and high tough-ness, as well as the potential for superplasticity at low temperatures and high strain rates [2,3], constitute the driving force for this flourshing field. Processing homog-eneous bulk nano and ultra-fine grained (ufg) structures is complex endeavor.
A 210MW turbine blade failed prematurely. It is a free standing blade. The blade failed at 250mm ... more A 210MW turbine blade failed prematurely. It is a free standing blade. The blade failed at 250mm from the tip. The failed blade along with a deformed portion were sent to NML to find out the cause of the failure. The material of the blade is hardened and tempered martensitic microstructure; it shows no degradation. The mechanical property of the blade material is nearly similar to that of the standard alloy of similar nature. Fractographic analysis of the blade shows no intergranular mode of deformation. The fracture surface consists of embedded silica particles as well as blisters associated with chlorine. It has been concluded that the blade failed due to initiation of cracks by chloride assisted corrosion with silica particle impingement. Later these cracks grow under fatigue loading and lead to failure of the blade.
The major objective of the project is to develop major facilities to carry out (i)R&D (ii) Co... more The major objective of the project is to develop major facilities to carry out (i)R&D (ii) Consultancy and servi-ces and (iii) Training programme related to nondestructive evaluation of materials.
Study of creep behavior of base metal (without weld) and welded specimens P91B steel over a range... more Study of creep behavior of base metal (without weld) and welded specimens P91B steel over a range of temperature (600-650 °C) and stress (50-180 MPa) showed similar values of minimum creep-rates for both specimens at higher stress regime (> 100 MPa) whilst, significantly higher creep rates of welded specimens at lower stress regime Considering that welded specimen comprised of two distinct structural regimes, i.e. weld affected zone and base metal, a method was proposed for estimating the material parameters describing creep behavior of those regimes. Stress-strain distribution across welded specimen predicted from finite element analysis based on material parameters revealed preferential accumulation of stress and creep strain at the interface between weld zone and base metal. This is in-line with the experimental finding that creep rupture preferentially occurs at inter-critical heat affected zone in welded specimens owing to ferrite-martensite structure with coarse Cr 23 C 6 particles.
Experimental investigations on the creep behaviour of engineering materials are frequently conduc... more 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. Several studies, employing theory of probability concepts have been developed to predict and characterize the variations in creep damage parameter. So far the deterministic approach has been used which is based on only two limiting states of the material i.e., ω= 0, no damage and ω = 1, completely damaged. This imposes severe limitations on the validity and practical applications of this model. For instance, it is not possible to trace the damage accumulation during service using the NDT techniques.
The paper presents creep test data on standard P91B steel specimens made from two distinct region... more The paper presents creep test data on standard P91B steel specimens made from two distinct regions of a welded plate over a range of stresses (50-190 MPa) and temperatures (600-650°C). The analysis of test data revealed that the samples having a weld zone within the gage length (cross-weld samples) have lower long term rupture strength than the samples made of the base metal. Estimated weld strength factors (WSF) of this steel were found to be higher than those reported for P91 steel. The study also showed that the effect of welding on loss of rupture ductility is much more prominent than its effect on the reduction in rupture strength. In presence of welded zone the extent of local deformation in ruptured samples was not as prominent as in the samples without weld. Creep damage tolerance factors (λ) were estimated from the creep strain versus time plots. This also showed that the magnitude of λ is significantly reduced in the presence of welding. Examination of microstructure and measurement of density revealed that this difference is primarily due to the formation of cavities in the heat affected zones of welded specimens. In the lower stress regime a few test specimens without any welded region did not fail even after very long creep exposure. Diameters of these specimens were found to have increased in spite of measureable increase in length due to creep. This unusual effect has been attributed to oxide scale growth. It shows up when the increase in diameter due to the growth of oxide scale becomes greater than the decrease in diameter due to the accumulation of creep strain.
ABSTRACT Multimaterial fabrication such as joining of steel and aluminum is currently prominent i... more ABSTRACT Multimaterial fabrication such as joining of steel and aluminum is currently prominent in a variety of industries. Friction stir welding is a novel solid-state welding process that causes good joint strength between steel and aluminum. However, the phenomenon contributing significant strength at the interface is not yet clear. In the present study, the interface of the friction stir lap-welded aluminum and coated steel sheet having joint strength maximum (71.4 pct of steel base metal) and minimum, respectively, under two parameter combinations, i.e., 1000 rpm 50 mm min−1 and 500 rpm 100 mm min−1, was exclusively characterized by X-ray diffraction, transmission electron microscopy (TEM), concentration profile, and elemental mapping by electron-probe microanalysis. A TEM-assisted EDS study identifies the morphologies of large size Al13Fe4 and small size Fe3Al-type intermetallic compounds at the interface. The diffusion-induced intermetallic growth (thickness) measured from a backscattered image and concentration profile agreed well with the numerically calculated one. The growth of these two phases at 1000 rpm 50 mm min−1 is attributed to the slower cooling rate (~3.5 K/s) with higher diffusion time (44 seconds) along the interface in comparison to the same for 500 rpm 100 mm min−1 with faster cooling rate (~10 K/s) and less diffusion time (13.6 seconds). The formation of thermodynamically stable and hard intermetallic phase Al13Fe4 at 1000 rpm and travel speed 50 mm min−1 in amounts higher than 500 rpm and a travel speed of 100 mm min−1 results in better joint strength, i.e., 71.4 pct, of the steel base metal.
High-temperature, high-cycle fatigue and accelerated creep properties of thermal barrier coated (... more High-temperature, high-cycle fatigue and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liners of aero engines are highlighted in this paper. An acoustic emission technique has been utilised to detect the ductile-brittle transition temperature of the bond coat. Results revealed that the ductile-to-brittle transition temperature (DBTT) of this bond coat is around 650 • C, which is in close proximity to the value reported for CoCrAlY type of bond coat. 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 was 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. It is recommended that these TBCs should not be put into use above 800 • C for more than 1000 h of continuous operation as the strength of the TBC superalloy drops to negligible levels.
Mechanical property evaluation is a powerful tool for estimating the residual life of in-service ... more Mechanical property evaluation is a powerful tool for estimating the residual life of in-service components and health monitoring. In the present investigation such an assessment was carried out to estimate the remaining life of inservice plain carbon steels that had been in use for 45 years in a feed processing unit (FPU) of a petrochemical plant. Health assessment of both base and weld joints of the material has been carried out through accelerated stress rupture tests and other mechanical property evaluation. At the operating conditions, it is noted that the materials still have about 10 years of residual life. However, surprisingly weld joints of the column of this unit exhibited poor notch toughness (5-10 J). It could not be proved beyond doubt, if the weld developed poor notch toughness during fabrication or degraded during service. Annealing of weld joints at 530 C for 1.5 h improved the notch toughness significantly, hence is recommended accordingly.
Abstract: This paper deals with an evaluation of the lifetime of a thermal barrier coated (TBC) C... more Abstract: This paper deals with an evaluation of the lifetime of a thermal barrier coated (TBC) C263 superalloy under fatigue and creep loading. Results revealed that both TBC and bond-coated substrate had higher endurance limits than the base alloy, while the opposite was ...
A generalised constitutive description of deformation of engineering alloys that exhibit a domina... more A generalised constitutive description of deformation of engineering alloys that exhibit a dominant tertiary creep is presented that extends a previous treatment of creep deformation. The model parameters of the constitutive equations that are determined by analysis of creep curves are supplemented by the Young's modulus to provide a database from which illustrative calculations are made. Simulations of constant strain rate, stress relaxation and high strain low cycle fatigue (both strain and stress control) are made for the superalloy IN738LC; the predictions are compared with limited available experimental data. Rfmtmf-On prfsente une description gfnfralisfe de la dfformation d'alliages industriels qui subissent un fluage tertiaire dominant; cette description dfveloppe un traitement antfrior de la dfformation par fluage. Ides paramftres du modfle des 6quations constitutives qui sont dfterminfes par l'analyse des courbes de fluage, sont complftfs par le module de Young pour fournir une base de donnfes fi partir de laquelle sont effectufs les calculs. Des simulations de vitesse de dfformation constante, de relaxation de la contrainte et de fatigue fi faible nombre de cycles ~i dfformation 61evfe (contrfle de la deformation et de la contrainte) sont effeetufes pour le superalliage IN738LC; les prfvisions sont comparfes avec les quelques donnfes expfrimentales disponibles. Zusammenfassung-Es wird eine verallgemeinerte Beschreibung der Verformung yon Legierungen, bei denen terti/ires Kriechen dominiert, vorgelegt; diese Beschreibung erweitert eine friihere Behandlung der Kriechverformung. Die Modellparameter der Grundgleichungen werden aus der Analyse yon Krieehkuryen bestimmt und ergfinzt um den Elastizi/itsmodul, um eine Grundlage fiir illustrierende Berechnungen zu schaffen. Fiir die Legierung IN738LC werden Simulationen fiir Verformung bei konstanter Dehnungstrate, f/ir Spannungsrelaxation und f/ir Ermiidung bei hohen Dehnungsamplituden und niedrigen Zyklenzahlen (sowohl dehnungswie auch spannungskontrolliert) durchgefiihrt. Die Ergebnisse werden mit den wenigen vorhandenenen experimentellen Beochtungen verglichen.
The boiler tubes and pipes in the present day coal fired power plants are designed against damage... more The boiler tubes and pipes in the present day coal fired power plants are designed against damage arising from the interaction of creep and fatigue. The present investigation reports the microstructural evolution in P92 grade martensitic steel during pure fatigue and hold time fatigue tests conducted at 600 o C. Fatigue life significantly dropped for hold time fatigue tests in comparison with pure fatigue tests. The drop in fatigue life was more for hold time fatigue tests conducted with compressive hold. Grain boundary oxidation and cracking was identified as the major cause for the decrease in fatigue life under compressive hold. Annihilation of dislocations and pinning of dislocations by MX precipitates were observed to be the microstructural changes during cyclic deformation.
One of the promising ways for mitigation of Type IV cracking-a failure by cracking at the intercr... more One of the promising ways for mitigation of Type IV cracking-a failure by cracking at the intercritical /fine grained heat affected zone, a life limiting problem in advanced 9-12 Cr ferritic steel weld like that of P91 is through modification of alloy composition by addition of boron. Addition of boron was observed to improve the microstructure at the weld zone and hence the creep strength. In the present work, boron (100 ppm with controlled nitrogen) added P91 steel after normalizing at 1050C and 1150C and tempered at 760C were studied for the creep behavior in the base metal and welded condition in the temperature range of 600-650C. Creep strength was characterized in terms of stress and temperature dependence of creep rate and rupture time. Weld creep life was reduced compared to the base metal with rupture occurring at the ICHAZ (Type IV crack). However at longer time (at lower stress levels) exposure creep crack moves from weld metal to HAZ (Type II crack). Rupture life was found to superior for the base and weld in the boron containing steel when higher normalizing temperature is used. Estimation of 10 5 h was attempted based on short term rupture data available and weld strength factors were calculated. Observed values are better for P91BH condition than the values for P91BLcondition as well as those available for P91 in open literature
Acoustic Emission technique has advance from laboratory use to application such as integrity of p... more Acoustic Emission technique has advance from laboratory use to application such as integrity of pressure vessels and many other structures. In this in-house project, we have looked into the condition monitoring of prototype horton sphere by acoustic emission technique during hydro-test. The damage mechanism occurred during hydrotest are characterized based on AE parameters. The predicted damage mechanisms are simulated on the similar materials. The results show that the predicted damage mechanisms include oxide scale peeling, breaking, chipping etc. The source location study identifies the damage prone zone as verified by ultrasonic NDE. The hydrotest of the Horton sphere, which was left in atmosphere for a decade, indicates that major material degradation occurs near the bottom flange. The degradat-ion was identified as loose and adherent oxide scales.
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Papers by Dr R N Ghosh