Department of Mechanical Engineering

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The bimetallic welds are frequently utilized for pipeline transport system of the nuclear power plants. The occurrences of welding defects generally depend on the filler electrode as well as the electrode coatings during shielded metal arc welding process. This study involves the design of austenitic stainless steel welding electrodes for SS304L–SA516 bimetallic welds. The objective of research work includes the novel design of Al2O3–TiO2–CaO–SiO2 coatings by combining two ternary phase systems using extreme vertices mixture design methodology to analyze the effect of key coating constituents on the weld metal chemistry and mechanical properties of the welds. The significant effect of electrode coating constituent CaO on weld metal manganese content is observed which further improves the toughness of bimetallic weld joints. Various regression models have been developed for the weld responses and multi objective optimisation approach using composite desirability function has been adopted for identifying the optimized set of electrode coating compositions. The role of delta ferrite content in promoting the favourable solidification mode has been studied through microstructural examination. © IMechE 2021.

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Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture

Combining CaO–SiO2–TiO2 and CaO–SiO2–Al2O3 ternary phase systems for design of bimetallic welds

This work investigates the reformed austenite in the super duplex stainless /pipeline steel dissimilar weld, fabricated using gas tungsten arc welding process (GTAW) employing super duplex filler (ER 2594). Multi-pass weld was fabricated at a constant heat input of 0.51 kJ/mm. Super duplex filler 2594 solidifies in ferritic mode and has low cracking susceptibility due to a low level of (P + S) impurities. Upon the weld metal cooling below the ferrite solvus, the austenite precipitation occurs, starting from the grain boundaries. Investigation reveals precipitation of grain boundary (GBA), Widmanstatten (WA), and Intergranular (IGA) austenite along with the minor presence of partially transformed austenite (PTA) in sequential order. The temperature ranges and morphology of each precipitated austenite has been discussed. The reformed austenite influences the mechanical properties of weld significantly. Based on the characteristics of different austenite, their influence on properties can be established. High chromium concentration of weld establishes its superior pitting corrosion resistance. © 2020 Elsevier B.V.

Materials Letters

Investigations on reformed austenite in the microstructure of dissimilar super duplex/pipeline steel weld

This research article investigated the rotor-casing contact phenomena of a high-speed rotor-bearing system under mass unbalance situation. Here, large deflection rotor-bearing model with a flexible shaft, a rigid disk loaded with an unbalance mass and contact phenomenon between a disk and casing of the rotating system has been used. A nonlinear mathematical model of a rotating system has been developed to study the steady-state stability by elucidating the state of bifurcation and route to chaos phenomena. The steady-state bifurcations and plausible route to chaos phenomena have been explored by numerically solving the governing motion upon gradually varying the design parameters i.e., flexible co-efficient, friction, shaft speed and unbalance mass. The rub-impact onto this large deflection model along with mass unbalance advocates the changes in dynamic state and ill-fated instability where the system losses its structural stability due to the presence of critical bifurcation and sensitivity toward chaotic responses. The present system enables a complex dynamic evolving with periodic, multi-period periodic solutions, sequences of period-doubling to chaotic motion on a gradual change in design variables. The results obtained will contribute to predict and identify the parametric conditions at which the system is safe from any contact between casing and rotor–disk. © 2021 Elsevier Ltd

International Journal of Non-Linear Mechanics

Large deflection model for rub-impact analysis in high-speed rotor-bearing system with mass unbalance

Abstract: Thermal barrier coatings (TBCs) have seen considerable advancement since the initial testing and development of thermal spray coating. Thermal barrier coatings are currently been utilized in various engineering areas which include internal combustion engines, gas turbine blades of jet engines, pyrochemical reprocessing units and many more. The development of new materials, deposition techniques is targeted at improving the life of the underlying substrate. Hence, the performance of the coating plays a vital role in improving the life of substrate. The scope for advancement in thermal barrier coatings is very high and continuous efforts are being made to produce improved and durable coatings. Thermal barrier coatings have the potential to address long term and short-term problems in gas turbine, internal combustion and power generation industry. The study of thermal barrier coating material, performance and life estimation is a critical factor that should be understood to introduce any advancement. The present review gives an overview of the thermal spraying techniques and current advancements in materials, mechanical properties, understanding the high temperature performance, residual stress in the coating, understanding the failure mechanisms and life prediction models for coatings. Graphic Abstract: [Figure not available: see fulltext.] © 2020, The Korean Institute of Metals and Materials.

Metals and Materials International

Thermal Barrier Coatings—A State of the Art Review

The present study aims to investigate the process of vacuum generation in a second throat vacuum ejector system employed for the high altitude testing of rocket motors. The study shows that the vacuum generation progresses with four distinct stages during the initial start-up process. The initial stage consists of a gradual evacuation process (first stage) which is followed by a transition region (second stage) in which the pressure starts to drop fastly and eventually leading to a rapid evacuation stage (third stage). This is followed with the reappearance of a gradual evacuation stage (fourth stage) which terminates with the onset of the started mode operation. It is found that the various stages of evacuation are closely linked to the internal shock wave movement in the supersonic nozzle. The gradual evacuation is found to be caused by the relatively large nozzle exit pressure caused by the multiple shock cells in the nozzle. The rapid evacuation occurs due to the sudden movement of the first shock cell to the nozzle exit and the reappearance of the gradual evacuation happens during the transformation of the nozzle jet from an overexpanded to underexpanded state. The parametric studies show that with increase in diffuser convergent cone angle, the contribution of vacuum generation from the fourth stage reduces due to a reduction in the maximum possible jet expansion. A same trend in vacuum generation is observed when nozzle distance from the second throat section of the ejector diffuser reduces. © 2021 Elsevier Masson SAS

Aerospace Science and Technology

Starting transients in second throat vacuum ejectors for high altitude testing facilities

The present research work investigates the effect of the re-austenitizing based post-welded tempering (PWNT) treatment on microstructure evolution and fracture characteristics of the P91 welds joint under creep exposure condition. The major problem associated with the P91 welded joint is the failure from the soft inter-critical heat affected zone (ICHAZ) during elevated creep service conditions. The failure is associated with the altering of the microstructure of the P91 steel as a result of the welding heat, which leads to the formation of ICHAZ. To investigate the creep performance of the welded joint, creep tests have been conducted at 620 degrees C for applied stress of 200 MPa (PWNT 1) and 150 MPa (PWNT 2). The crept specimen was carried out in detail by using the scanning electron microscope (SEM). The PWNT have observed a significant effect on fracture characteristic, and it shifted from the soft ICHAZ to overtempered base zone. The inter-metallic Laves phase was observed in each region of the weldments for specimen exposed at 620 degrees C for applied stress of 150 MPa (PWNT 2). (C) 2020 The Authors. Published by Elsevier Ltd.

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MATERIALS TODAY-PROCEEDINGS

Role of Mo in the nucleation of intermetallic laves phase

In the present work, the thermal contact resistance in a double fin-and-tube heat exchanger is investigated using multiphysics numerical approach. A three-dimensional (3D) Computational Fluid Dynamic (CFD) model is constructed in commercial software COMSOL Multiphysics® by coupling steady-state conjugate heat transfer and turbulent fluid flow. The impact of contact resistance at the fin and tube interface is analyzed by quantitatively evaluating the thermal-hydraulic characteristics. It is found that contact resistance of 3.3 × 106 Km2/W can reduce the overall performance by approximately 6% when compared without thermal resistance at the contact interface. The developed model can predict the effectiveness of the fin-and-tube heat exchanger design with or without thermal contact resistances. Furthermore, the model can be employed to improve the overall heat exchanger performance used in various applications. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.

Lecture Notes in Mechanical Engineering

Impact of Thermal Contact Resistance on Thermal-Hydraulic Characteristics of Double Fin-and-Tube Heat Exchanger

Soft materials exhibiting large deformation under external stimuli have gained an increasing attention in the recent past because of their potential applications in soft transducers aimed at achieving biomimetic actuation. This paper theoretically analyzes the effect of internal properties, including entanglements, crosslinks and finite extensibility of polymer chains along with inherent viscoelastic properties of polymers on the performance of Dielectric Elastomer actuator (DEA) in dynamic modes of actuation. A physics based nonaffine material model proposed by Davidson and Goulbourne is used to model the polymer chains entanglements, crosslinks and finite extensibility. To incorprate the viscoelastic properties, a rheological material model based on the additive decomposition of the isotropic strain energy density into equilibrium and viscous parts is implemented. A computationally efficient method, which relies on the principle of least action is used for extracting the governing equation representing the dynamic motion of the DE actuator. The results demonstrate that DEAs with strong entanglements and crosslinks along with small finite extensibility of polymer chains exhibits lower deformation level in DC dynamic modes of actuation. It is inferred that the strong entanglements and crosslinks in polymer chains enhances the resonant frequency, but debilitate the intensity of vibration of viscoelastic DEAs. Further, the periodicity and stability of the nonlinear oscillations exhibited by the viscoelastic DEAs are assessed by employing the Poincare maps and phase portraits. The results of the present investigation can be applied effectively in bridging the mechanism between the microcosmic polymer chains and macroscopic dynamic behavior of viscoelastic DE actuators. © 2021, The Author(s), under exclusive licence to Springer Nature B.V.

Nonlinear Dynamics

Effect of polymer chains entanglements, crosslinks and finite extensibility on the nonlinear dynamic oscillations of dielectric viscoelastomer actuators

Gas Tungsten Arc Welded dissimilar joint of modified 9Cr–1Mo (P91) and SS304H steel plate is prepared using Ni-based ERNiCrCoMo-1 super alloy filler and are post-weld heat treated (PWHT) at 760 °C for 2 h. In particular, joints are investigated for metallurgical and mechanical properties in different heat-affected zones. Furthermore, an attempt has been made to understand the following: (i) Is there any effect of mixing of Ni-based superalloy filler material on the joint? (ii) How does the PWHT process change the metallurgical and mechanical properties of joints and different sections? (iii) Does the PWHT process beneficial as compared to as-welded (AW) joint? Experimental results indicate that improper mixing of filler materials led to the formation of an unmixed zone near the weld metal and HAZ interface. The hardness, residual stress, and impact energy results confirmed the variation in mechanical properties along the weldments. The tensile strength varied from 539 MPa in as-joint to 516 MPa in PWHT, and impact energy varies from 165 J in as-joint to 40 J in PWHT condition. The maximum hardness of 375 H V is observed in P91 CGHAZ. The maximum longitudinal residual stress is estimated as 200 MPa at the top surface of the welded plate, i.e., about 2 mm depth, while the maximum magnitude of the transverse residual stresses is observed as 109 MPa at the same depth of 2 mm. In P91 HAZ, the longitudinal and transverse residual stress magnitudes are found to be in a range of 0 to ˗70 MPa. The SS304H HAZ shows the maximum value of longitudinal and transverse residual stress level at a depth of 3 mm, and it was 144 MPa and 92 MPa, respectively. The PWHT showed a drastic reduction in residual stress value in a different zone of the weldments. © 2021 Elsevier Ltd

International Journal of Pressure Vessels and Piping

Study on evaluation of through-thickness residual stresses and microstructure-mechanical property relation for dissimilar welded joint of modified 9Cr–1Mo and SS304H steel

This study aims to find out the effect of welding heat inputs on microstructure and mechanical properties relationship of naval high strength steel. The physical weld thermal cycles with four different heat inputs between 10 to 200 kJ/cm were experimented using the thermomechanical simulator Gleeble®3800. The weld coarse-grained heat-affected zone (CGHAZ) microstructure was characterized in detail by using optical microscopy, field emission scanning electron microscope (FESEM), and transmission scanning electron microscope(TEM) facilities and mechanical properties such as impact toughness, hardness and tensile strength were obtained using Charpy impact, microhardness, and tensile testing facilities. It was often noticed that the impact toughness decreased with increasing heat inputs attributed to the grain and bainite lath coarsening and the formation of new phases with different morphologies in CGHAZ regions. At lower heat input, the microstructure consisted of a fully martensite structure, while at higher heat inputs consisted of granular bainite and bainitic ferrite; however, in medium-range heat inputs, it consisted of bainite and martensite. The different phases were confirmed through TEM images. The mechanical properties were found to be decreased with an increase in heat input. It was due to the formation of coarse bainite lath, granular bainite, and hard M/A constituents. The weld structure properties were established in detail. © 2021 Elsevier Ltd