S B Kumar

Effect of Structural Heterogeneity on In Situ Deformation of Dissimilar Weld Between Ferritic and Austenitic Steel

Abstract Low-alloy steel and 304LN austenitic stainless steel were welded using two types of buttering material, namely 309L stainless steel and IN 182. Weld metals were 308L stainless steel and IN 182, respectively, for two different joints. Cross-sectional microstructure of welded assemblies was investigated. Microhardness profile was determined perpendicular to fusion boundary. In situ tensile test was performed in scanning electron microscope keeping low-alloy steel-buttering material interface at the center of gage length. Adjacent to fusion boundary, low-alloy steel exhibited carbon-depleted region and coarsening of matrix grains. Between coarse grain and base material structure, low-alloy steel contained fine grain ferrite-pearlite aggregate. Adjacent to fusion boundary, buttering material consisted of Type-I and Type-II boundaries. Within buttering material close to fusion boundary, thin cluster of martensite was formed. Fusion boundary between buttering material-weld metal and weld metal-304LN stainless steel revealed unmixed zone. All joints failed within buttering material during in situ tensile testing. The fracture location was different for various joints with respect to fusion boundary, depending on variation in local microstructure. Highest bond strength with adequate ductility was obtained for the joint produced with 309L stainless steel-buttering material. High strength of this weld might be attributed to better extent of solid solution strengthening by alloying elements, diffused from low-alloy steel to buttering material.

Effect of rate of addition of starter culture on textural characteristics of buffalo milk Feta type cheese during ripening.

The effect of rate of addition of starter culture on textural characteristics of buffalo milk Feta type cheese was investigated during ripening period up to two months. The textural characteristics of buffalo milk Feta type cheese in terms of hardness, cohesiveness, springiness, gumminess and chewiness were analyzed by using textural profile analyzer. The maximum hardness was found with cheese made using 1% culture, while the minimum was found with 2% culture. The cohesiveness and springiness decreased as the level of addition of starter culture increased. The chewiness of cheese also decreased, as the rate of addition of starter culture increased for cheese making. In addition to this, yield, moisture, fat, FDM, protein, salt and S/M of fresh buffalo milk Feta type cheese increased with the increase in rate of addition of starter culture; however, TS of experimental cheeses decreased.

Fatigue damage of a thermal barrier coated Ni-base superalloy

High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy were conducted in air. Results reveal that the endurance limits for the TBC and bond coated substrate were substantially higher than that of the base alloy, while the opposite was found for high stress, low cyclic life times. It appears that the increase in endurance limit for the TBC and bond coated superalloy is due,to load shifting to the bond coat, interdiffusion of A] from coating to substrate and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. The mode of fracture in the substrate at very high fatigue stress was intergranular whereas that at low stress was transgranular. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5400107 cycles during fatigue testing at 1073 K in air, due to a continuous alumina scale growth at the top coat (TBC) / bond coat interface.

Effect of coagulants on the quality of chhana and rasogolla obtained from admixture of buffalo milk and butter milk

A study was carried out to investigate the effect of different coagulant (lactic acid, citric acid and calcium lactate) on yield, sensory and textural characteristics of chhana and rasogolla made from admixture of buffalo milk and sweet cream butter milk (SCBM). The highest yield of chhana was observed with calcium lactate whereas the minimum yield was found with citric acid. There was no significant difference found with respect to flavour and colour and appearances scores, however, significant (p < 0.01) difference found in body and texture of chhana samples prepared with different coagulant. In addition to that, significant (p < 0.05) difference observed with respect to body and texture, flavour and porosity of rasogolla, but no significant difference was observed in colour and appearance as well as sweetness of rasogolla prepared with chhana obtained from varying coagulants. Among different coagulants, citric acid was found best suitable for chhana as well as rasogolla making.

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.

Three-Dimensional Thermomechanical Simulation and Experimental Validation on Failure of Dissimilar Material Welds

Dissimilar material weld joints, consisting of low-alloy steel and 304LN austenitic stainless steel (SS), have critical application in boiling water reactors in the nuclear industry. It was predicted that phase transformation adjacent to the fusion boundary and stress distribution across the transition joint play a key role in the structural degeneration of these welds. Quantitatively, to evaluate their contribution, two different joints were considered. One was fabricated with buttering material 309L SS (M/S Mishra Dhatu Nigam Limited, Hyderabad, India), and the other was produced with buttering material IN182 (M/S Mishra Dhatu Nigam Limited, Hyderabad, India). Base materials remained the same for both. Thermomechanical simulation on dissimilar material welds was performed using finite-element modeling to predict the thermal effect and stress prone area. Temperature-dependent thermal and structural properties were considered for simulation. Simulation results were compared with microstructural characteristics, and data were obtained from the in-situ tensile test. Simulation results exhibited that stress was at maximum in the buttering material and made the zone weaker with respect to adjacent areas. During the validation of results, it was observed that failure occurred through buttering material and endorsed the inference. The variation in mechanical properties of the two welds was explained considering the effect of thermal state and stress distribution.

Incorporation of Lactobacillus adjuncts culture to improve the quality of Feta-type cheese made using buffalo milk

Feta-type cheese was made from buffalo milk using commercial adjunct culture of Lactobacillus helveticus and Lactobacillus casei along with standard mesophillic cheese cultures. The sensory, biochemical and texture characteristics of the experimental cheeses were studied during ripening. Expert panellists observed, significant differences (P < 0.01) between the control and the experimental cheeses. The pH, titratable acidity, soluble protein and free fatty acid content of the experimental cheeses were found to be significantly (P < 0.01) higher than those of the control. The texture parameter values of the experimental cheeses were found to be significantly (P < 0.01) lower than values of the control.

Damage Resistance of a Thermal Barrier Coated Superalloy for Combustor Liners in Aero Turbines During Fatigue and Creep

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 found for high stress, low cyclic lifetimes. At high stress, the premature failure for these two materials is possibly due to high stress crack initiation/growth in the TBC/bond coat layers. Oxidation is the cause of the reduced life of the bare substrate as compared to the coated substrate while fatigue and creep experiments are carried out in an oxidizing environment. During 800 °C fatigue, the bare specimens behave differently from the coated specimens, but both the bond-coated only and bond coat + TBC specimens seem to exhibit very similar results that are within experimental scatter. Delamination of the bond coat, oxidation of the substrate and spallation of the ceramic layer were evident at very high fatigue and creep stresses. Lateral cracks that grew in the ceramic layer parallel to the stress axis were responsible for spallation of the top coat (TBC) at a very high fatigue stress, whereas, at low creep stress, spallation of the top coat was due to the growth of alumina scale (of thickness >3μm) at the top coat (TBC)/bond coat interface.

Influence of thermally grown oxide scale on fatigue resistance of a thermal barrier coated superalloy

Abstract The life of thermal barrier coating prior to spallation is dominated by micro-cracking in both the thermally grown oxide and the yttria stabilized zirconia top coat. The damage generated by this micro-cracking is expected to be a primary life limiting factor. High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted in air. It was observed that the coated materials had higher endurance limits than the bare superalloy and the premature failure for these two materials is possibly due to high stress crack nucleation and growth in the TBC/bond coat layers. Oxidation is also the cause of the reduced life of the bare substrate as compared to the coated substrate while fatigue testing is carried out in an oxidizing environment. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5,400,107 cycles during fatigue testing at 800 °C in air due to a continuous alumina scale growth (thickness >3 μm) at the bond coat/TBC (top coat) interface. La vie d’un revêtement thermique avant l’écaillage est dominée par la micro fissuration tant de la calamine développée thermiquement que de la couche supérieure de zircone stabilisée à l’oxyde d’yttrium. On s’attend à ce que le dommage engendré par cette micro fissuration soit un facteur primaire de limite de vie. On a effectué un essai de fatigue de force contrôlée, à haute température, à l’air, du superalliage Superni C263 revêtu d’une barrière thermique (TBC), avec seulement une couche d’ancrage et de l’alliage à nu. On a observé que les matériaux avec revêtement avaient des limites d’endurance plus élevées que le superalliage nu. La défaillance prématurée de ces deux matériaux est possiblement due à la nucléation et à la croissance élevées de fissures de contrainte dans les couches TBC/couche d’ancrage. L’oxydation est également la cause de la durée de vie réduite du substrat nu par rapport au substrat avec revêtement lorsque l’essai de fatigue est effectué dans un environnement oxydant. L’écaillage de la couche de céramique était évident à des contraintes très élevées de fatigue et également à de faibles contraintes de fatigue où l’échantillon composite TBC s’est détérioré après 5400107 cycles lors de l’essai de fatigue à 800 °C à l’air. La détérioration est attribuée à la croissance continue d’une écaille d’alumine (épaisseur > 3 μ m) à l’interface couche d’ancrage/TBC.

Experimental and Computational Investigation of Structural Integrity of Dissimilar Metal Weld Between Ferritic and Austenitic Steel

The structural integrity of dissimilar metal welded (DMW) joint consisting of low-alloy steel and 304LN austenitic stainless steel was examined by evaluating mechanical properties and metallurgical characteristics. INCONEL 82 and 182 were used as buttering and filler materials, respectively. Experimental findings were substantiated through thermomechanical simulation of the weld. During simulation, the effect of thermal state and stress distribution was pondered based on the real-time nuclear power plant environment. The simulation results were co-related with mechanical and microstructural characteristics. Material properties were varied significantly at different fusion boundaries across the weld line and associated with complex microstructure. During in-situ deformation testing in a scanning electron microscope, failure occurred through the buttering material. This indicated that microstructure and material properties synergistically contributed to altering the strength of DMW joints. Simulation results also depicted that the stress was maximum within the buttering material and made its weakest zone across the welded joint during service exposure. Various factors for the failure of dissimilar metal weld were analyzed. It was found that the use of IN 82 alloy as the buttering material provided a significant improvement in the joint strength and became a promising material for the fabrication of DMW joint.