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ItemNew insights on recovery and early recrystallization of ferrite-pearlite banded cold rolled high strength steels by high speed nanoindentation mapping( 2021-03-15)This work unravels structure-property correlations at the micrometer length scale during recovery and early recrystallization of a cold rolled high strength steel comprised of ferrite and pearlite. High resolution nano-indention mapping is used to measure the hardness of ferrite, pearlite and the transition zone between them, which correlates well with the local microstructure obtained from scanning and transmission electron microscopy. A clustering algorithm is used to determine the properties of the constituent microstructural features. The variation in the hardness of the phases with annealing temperature agrees well with the dislocation activity observed using Transmission Electron Microscopy (TEM) and a simple mechanism to reconcile the observations is presented.
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ItemMicrostructure and mechanical property evolution of post weld heat treated bainitic steel welds( 2021-01-01)The achievement of excellent combination of high tensile strength and good ductility has been the key factor that facilitates the Advanced High Strength Steels (AHSS) to be the candidate materials for automotive applications. Carbide free bainitic steels are a class of AHSS that primarily have microstructure consisting blend of fine bainitic laths and interwoven retained austenite in the interlath boundaries of bainite. Solid state welding techniques are regarded to be the most sought after fabrication processes as they result in retention of basic microstructural constituents, especially nonequilibrium phases like bainite. Keeping this fact in view, friction stir welding was carried out on 3 mm thick carbide free bainitic steel plate with varying tool rotational speeds. The temperatures of stir zones were recorded. The microstructural changes and phase analysis from base metal (BM) to Thermomechancally Affected Zone (TMAZ) and Stir Zone (SZ) were studied. The microhardness profiling was done across the weld covering the BM, TMAZ and SZ. The abrupt increase in the hardness of stir zone and microstructure stability i.e., retention of bainitic microstructure along with the retained austenite were observed. The tool rotational speed of 150 RPM has been optimized. Post weld heat treatments were undertaken at different temperatures and increased soaking time on the welded samples of optimized rotational speed to facilitate bringing down the stir zone hardness to the base metal. Basing the phase analysis and EBSD data obtained, the post weld heat treatment cycle conducted at 500OC with a soaking time of 1 h has been optimized.
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ItemPOD of NDT Techniques Using High Temperature Oxidized Fatigue Cracks in an Aero Engine Alloy( 2021-06-01)Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT) techniques is essential for implementing damage tolerant (DT) methodology for aero-engines. This POD is usually established with the help of a large number of service expired aero-engine components containing several fatigue cracks. In the absence of such components, artificial defects such as electrical discharge machining (EDM) notches or starter cracks were explored. However, such artificial defects would not meet the key features such as tightness of the fatigue cracks and the possible oxidation in the crack opening and thus, limiting their usage. Therefore, in the current study, an innovative approach of generating fatigue cracks at 650 °C (~ typical aero-engine service temperatures) with key high temperature service degradation aspects of oxidation and fatigue cracking is demonstrated for the first time using Gleeble® test system. Further, POD is estimated by inspecting these laboratory generated fatigue cracks using fluorescent liquid penetrant technique (FLPT) and eddy current technique (ECT) under HIT (defect detected) vs. MISS (defect not detected) and â (signal response) vs. a (crack size) methodologies. The current study also discusses a statistical approach of random generation of crack sizes for use in NDT reliability analysis. In addition, an attempt has been made to understand the effect of a90/95 values on remnant life calculations. It is concluded that the eddy current response of oxidized fatigue cracks results in better (high sensitive) a90/95 values compared to the eddy current response obtained from non-oxidized fatigue cracks.
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ItemLaser Beam Welding of Advanced High-Strength Steels (Dual Phase Steels)( 2022-01-01)Presently, Dual Phase (DP) steels are highly accepted steels by the automotive manufacturers as structural materials. Microstructural features of these steels resulted special properties. Welding is an inevitable in automobile manufacturing. Nd:YAG fiber laser welding was used to produce Beads on plate on Dual Phase steel of 780 grade. Welding Beads were produced by varying the heat input from 17 to 24 J/mm2. Complete Depth of Penetration of weld beads was observed at all heat inputs. With increase in the heat input, weld zone dimensions were increased. Fusion zone was transformed by liquid to solid transformations, up on cooling. Heat Affected Zone was formed by solid–solid phase transformation, and the proportion of Martensite was decreased with increasing the distance from Fusion Boundary. The microhardness of the fusion zone was 370 ± 10 HV0.5 and that of the base materials was 270 ± 19 HV0.5. The increase in hardness in the fusion zone was due to the phase transformation in this zone.
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ItemStudies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies( 2011-12-01)Densification studies of 8 mol% yttria stabilized zirconia ceramics were carried out by employing the sintering techniques of conventional ramp and hold (CRH), spark plasma sintering (SPS), microwave sintering (MWS) and two-stage sintering (TSS). Sintering parameters were optimized for the above techniques to achieve a sintered density of > 99% TD. Microstructure evaluation and grain size analysis indicated substantial variation in grain sizes, ranging from 4.67 μm to 1.16 μm, based on the sintering methodologies employed. Further, sample was also sintered by SPS technique at 1425 °C and grains were intentionally grown to 8.8 μm in order to elucidate the effect of grain size on the ionic conductivity. Impedance spectroscopy was used to determine the grain and grain boundary conductivities of the above specimens in the temperature range of RT to 800 °C. Highest conductivity of 0.134 S/cm was exhibited by SPS sample having an average grain size of 1.16 μm and a decrease in conductivity to 0.104 S/cm was observed for SPS sample with a grain size of 8.8 μm. Ionic conductivity of all other samples sintered vide the techniques of TSS, CRH and MWS samples was found to be ∼0.09 S/cm. Highest conductivity irrespective of the grain size of SPS sintered samples, can be attributed to the low densification temperature of 1325 °C as compared to other sintering techniques which necessitated high temperatures of ∼1500 °C. The exposure to high temperatures while sintering with TSS, CRH and MWS resulted into yttria segregation leading to the depletion of yttria content in fully stabilized zirconia stoichiometry as evidenced by Energy Dispersive Spectroscopy (EDS) studies. © 2011 Elsevier Ltd and Techna Group S.r.l.