Department of Mechanical Engineering

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Abstract: In the present study, different finite element (FE) models were prepared to investigate weld induced residual stresses in thick multi-pass butt welded joint of SA516 Gr. 70 plates. Both 3D and 2D full geometry models and their axisymmetric half models were taken into consideration. The competence of these FE models on the accuracy of predicting residual stress distribution across the weld cross-section was investigated by comparing it with the experimental results. Blind hole drilling technique and deep hole drilling technique were employed to evaluate the surface and through-thickness residual stress distributions, respectively. In addition, the change in volume and yield strength of weld material due to austenitic phase transformation was also incorporated in the material modeling to observe the effect of solid-state phase transformation (SSPT) on the evaluation of residual stresses. Computed residual stresses obtained from different FE models indicate that the 3D FE models procured the best accuracy compared with the experimental results. On the other hand, 2D models can save a significant amount of computational time with reasonable accuracy. Incorporation of SSPT in the 3D FE full model exhibited a better agreement of predicted results with the experimental measurements. Graphic Abstract: [Figure not available: see fulltext.] © 2021, The Korean Institute of Metals and Materials.

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Metals and Materials International

Significance of Finite Element Models and Solid-State Phase Transformation on the Evaluation of Weld Induced Residual Stresses

In the present study, the Gas Tungsten Arc Weld (GTAW) joint of P91 steel was produced using the dissimilar IN617 grade filler wire. The welded joints have been subjected to different post-weld heat treatment (PWHT) to obtain the stable microstructure and mechanical properties for P91 heat-affected zone (HAZ). The PWHT was performed at 750 degrees C (less than Ac1) and 800 degrees C (close to Ac1). The welded joint was characterized for mechanical properties using tensile test, hardness test and Charpy impact toughness test. In order to understand the role of the dissimilar filler metal and PWHT on the Charpy impact toughness of the weld metal, Charpy impact toughness tests were conducted at room temperature. The Charpy impact toughness of the weld metal was measured 55 +/- 4 J for the as-welded condition. The PWHT has observed almost a negligible effect on Charpy impact toughness value. The tensile strength was measured 638 MPa for the as-welded joint and improved significantly after the PWHT. The role of the filler metal and PWHT on heterogeneity in microstructure and hardness produced along the HAZ of P91 steel were investigated in detail. The microstructure features of HAZ (grain size, particle size, fraction area of the precipitates and particle to particle distance) were measured experimentally using the ImageJ software as a possible contributor to the hardening mechanism. The HAZ (CGHAZ) devoided from the carbide precipitates showed the peak hardness of about 395 HV in as-welded condition and the minimum of 189 HV was measured for inter-critical HAZ (ICHAZ). The PWHT at 800 degrees C produced the homogenous microstructure and a negligible hardness variation along the HAZ. The precipitate free delta-ferrite region was identified in the CGHAZ region near to fusion line for as-welded condition and remain unaffected during the PWHT.

INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING

Effect of post-weld heat treatment and dissimilar filler metal composition on the microstructural developments, and mechanical properties of gas tungsten arc welded joint of P91 steel

The paper presents the detailed microstructure study and mechanical behavior of P91 welded joint produced by gas tungsten arc welding process (GTAW) using the dissimilar nickel grade superalloy Inconel 718 filler (ERNiFeCr2). The microstructural changes in the weld fusion zone (WFZ) and heat-affected zone (HAZ) were also studied for varying heat treatment conditions. The effect of the Inconel 718 filler on the tensile properties, hardness and Charpy impact toughness (CIT) of the welded joint was also studied for as-welded and heat-treated conditions. The post-weld heat treatment (PWHT) was performed in a temperature range of 650–1060 °C. The microstructure study showed the formation of an unmixed zone at the interface of the HAZ and weld metal and remain unaffected during the PWHT. The region of weld metal near the boat fusion line showed the cellular and columnar dendrites. The center region of the bulk weld metal was solidified as the columnar dendrites and cellular dendrites along with segregation of the elements at inter-dendritic boundaries. The dendrite core showed the segregation of the Ni, and Cr; however, at dendrite boundaries, major segregation was observed for the elements Nb, Mo and Ti. The heat treatment up to a temperature of 810 °C has observed a negligible effect on the WFZ microstructure; however, a significant change in microstructure was observed for the P91 HAZ region. A significant change in the microstructure of the WFZ was measured after the normalizing at 1060 °C, followed by tempering at 760 °C for 2 h (complex heat treatment). The tensile strength of the welded joint was measured 817 MPa with a joint efficiency of 146 %for as-welded condition and a maximum of 901 MPa with a joint efficiency of 161% for complex heat treatment. The welded joint CIT was measured a maximum of 48 J in as-welded condition and get reduced after the PWHT and measured a minimum of 35 J after complex heat treatment. A hardness variation was observed along the welded joint for as-welded and PWHT condition. In the as-welded condition, the hardness of the WFZ was measured about 417 ± 21 HV and showed a minute change with an increase in tempering temperature. The minimum hardness was measured in the inter-critical HAZ region for the as-welded joint and produce a significant level of heterogeneity in hardness variation along the P91 HAZ. PWHT has observed a considerable reduction in the hardness value of the P91 HAZs except for ICHAZ. A negligible level of hardness variation along P91 HAZ was measured after PW 4. © 2020 Elsevier Ltd

International Journal of Pressure Vessels and Piping

Microstructure and mechanical behavior of P91 steel dissimilar welded joints made with IN718 filler

Existing Reynolds Averaged Navier–Stokes-based transition models do not accurately predict separation induced transition for low pressure turbines. Therefore, in this paper, a novel framework based on computational fluids dynamics (CFD) driven machine learning coupled with multi-expression and multi-objective optimization is explored to develop models which can improve the transition prediction for the T106A low pressure turbine at an isentropic exit Reynolds number of Re2is = 100, 000. Model formulations are proposed for the transfer and laminar eddy viscosity terms of the laminar kinetic energy transition model using seven non-dimensional pi groups. The multi-objective optimization approach makes use of cost functions based on the suction-side wall-shear stress and the pressure coefficient. A family of solutions is thus developed, whose performance is assessed using Pareto analysis and in terms of physical characteristics of separated-flow transition. Two models are found which bring the wall-shear stress profile in the separated region at least two times closer to the reference high-fidelity data than the baseline transition model. As these models are able to accurately predict the flow coming off the blade trailing edge, they are also able to significantly enhance the wake-mixing prediction over the baseline model. This is the first known study which makes use of ‘CFD-driven’ machine learning to enhance the transition prediction for a non-canonical flow. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Energies

Transition Modeling for Low Pressure Turbines Using Computational Fluid Dynamics Driven Machine Learning

This paper presents an assessment of machine-learned turbulence closures, trained for improving wake-mixing prediction, in the context of LPT flows. To this end, a three-dimensional cascade of industrial relevance, representative of modern LPT bladings, was analyzed, using a state-of-the-art RANS approach, over a wide range of Reynolds numbers. To ensure that the wake originates from correctly reproduced blade boundary-layers, preliminary analyses were carried out to check for the impact of transition closures, and the best-performing numerical setup was identified. Two different machine-learned closures were considered. They were applied in a prescribed region downstream of the blade trailing edge, excluding the endwall boundary layers. A sensitivity analysis to the distance from the trailing edge at which they are activated is presented in order to assess their applicability to the whole wake affected portion of the computational domain and outside the training region. It is shown how the best-performing closure can provide results in very good agreement with the experimental data in terms of wake loss profiles, with substantial improvements relative to traditional turbulence models. The discussed analysis also provides guidelines for defining an automated zonal application of turbulence closures trained for wake-mixing predictions. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Assessment of Machine-Learned Turbulence Models Trained for Improved Wake-Mixing in Low-Pressure Turbine Flows

This investigation attempts to explore the weld characteristics of a laser welded dissimilar joint of ferritic/martensitic 9Cr-1Mo-V-Nb (P91) steel and Incoloy 800HT austenitic nickel alloy. This dissimilar joint is essential in power generating nuclear and thermal plants operating at 600–650 °C. In such critical operating conditions, it is essential for a dissimilar joint to preserve its characteristics and be free from any kind of defect. The difference between the physical properties of P91 and Incoloy 800HT makes their weldability challenging. Thus, the need for detailed characterization of this dissimilar weld arises. The present work intends to explore the usage of an unconventional welding process (i.e., laser beam welding) and its effect on the joint’s characteristics. The single-pass laser welding technique was employed to obtain maximum penetration through the keyhole mode. The welded joint morphology and mechanical properties were studied in as-welded (AW) and post-weld heat treatment (PWHT) conditions. The macro-optical examination shows the complete pen-etrations with no inclusion and porosities in the weld. The microstructural study was done in order to observe the precipitation and segregation of elements in dendritic and interface regions. Solidification cracks were observed in the weld fusion zone, confirming the susceptibility of Incoloy 800HT to such cracks due to a mismatch between the melting point and thermal conductivity of the base metals. Failure from base metal was observed in tensile test results of standard AW specimen with a yield stress of 265 MPa, and after PWHT, the value increased to 297 MPa. The peak hardness of 391 HV was observed in the P91 coarse grain heat-affected zone (CGHAZ), and PWHT confirmed the reduction in hardness. The impact toughness results that were obtained were inadequate, as the maximum value of impact toughness was obtained for AW P91 heat-affected zone (HAZ) 108 J and the minimum for PWHT Incoloy 800HT HAZ 45 J. Thus, difficulty in obtaining a dissimilar joint with Incoloy 800HT using the laser beam welding technique was observed due to its susceptibility to solidification cracking. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Fulltext

Materials

Study on microstructure and mechanical properties of laser welded dissimilar joint of p91 steel and incoloy 800ht nickel alloy

Robotic pick and assembly using deep learning and hybrid vision/force control

Vision based method for improving position accuracy of four-cable-driven platform

In this work, we report unique hybrid composite film fabricated with the amalgamation of metal, semiconductor and polymers for hydrogen sensing application at room temperature. Fabrication of a novel nanocomposite film based on tin oxide (SnO2) nanosheets with polyaniline (PANI) doped with palladium (Pd) is performed using the hydrothermal synthesis technique. Functional aspects of the fabricated films are investigated with XRD, Raman spectra, FESEM, and FTIR spectral analysis. Interactions of the H-2 gas molecules with SnO2, SnO2-Pd, PANI, PANI-SnO2, PANI-SnO2-Pd nanocomposite are also theoretically studied. Using first-principles density functional theory, the effects of gas adsorption on the electronic and transport properties of the sensor are examined. The computations show that the sensitivity of the SnO2 to the H-2 gas molecules is considerably improved after hybridisation with Pd and, the sensitivity of the PANI to the H-2 gas molecules is considerably improved after hybridisation with SnO2. Gas sensing characteristics of fabricated films of SnO2, PANI and composite of SnO2/PANI/Pd are also experimentally investigated at mom temperature with varying concentration level ranging from 50 to 400 ppm. The highest sensitivity among all the films at room temperature has been observed as -540\% for the SnO2/Pd film at 0.4\% of the target gas and performance factor (the ratio of response percentage to total cycle time) is evaluated highest in Pd doped PANI-SnO2 film. Our results reveal the promising future of SnO2, PANI and Pd associated hybrid films in the development of ultra-high sensitive gas sensors.

MICRO AND NANO ENGINEERING

Room temperature hydrogen sensing with polyaniline/SnO2/Pd nanocomposites

The aim of the present work is to investigate the role of Ni-based filler on Charpy impact toughness (CIT) of the P91 welds joint. The Gas Tungsten Arc Weld (GTAW) joint of creep strength enhanced ferritic/martensitic P91 steel was produced by using the dissimilar Ni-based IN617 and IN718 filler. The welds joint subjected to varying post-weld heat treatment (PWHT) in the temperature range of 650?810 ?C. Post weld normalizing based tempering (PW 4) was also carried out at a normalizing temperature of 1040 ?C.The weld metal shows the cellular dendrites and equiaxed dendrites for IN617 filler along with the segregation of the secondary phase particles at inter-dendritic areas and boundaries. For IN718 filler, columnar and cellular dendrites with well-defined primary and secondary arm were observed along with segregation of the particles at inter-dendritic areas and boundaries. PW 4 provides the uniform distribution of the particles inside the austenitic matrix of weld metal for both filler condition. The CIT test was conducted at room temperature for each condition of the filler and PWHT.The CIT value of the WFZ was measured maximum for as-welded condition and showed a minute change after the PWHT. However, a significant decrease in CIT was measured after the PW 4 for both the filler. ? 2021 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials, Processing \&amp; Characterization.