Faculty Publications
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Item Formation of microstructural features in hot-dip aluminized AISI 321 stainless steel(University of Science and Technology Beijing, 2018) Huilgol, P.; Rajendra Udupa, K.; Udaya Bhat, K.Hot-dip aluminizing (HDA) is a proven surface coating technique for improving the oxidation and corrosion resistance of ferrous substrates. Although extensive studies on the HDA of plain carbon steels have been reported, studies on the HDA of stainless steels are limited. Because of the technological importance of stainless steels in high-temperature applications, studies of their microstructural development during HDA are needed. In the present investigation, the HDA of AISI 321 stainless steel was carried out in a pure Al bath. The microstructural features of the coating were studied using scanning electron microscopy and transmission electron microscopy. These studies revealed that the coating consists of two regions: an Al top coat and an aluminide layer at the interface between the steel and Al. The Al top coat was found to consist of intermetallic phases such as Al7Cr and Al3Fe dispersed in an Al matrix. Twinning was observed in both the Al7Cr and the Al3Fe phases. Furthermore, the aluminide layer comprised a mixture of nanocrystalline Fe2Al5, Al7Cr, and Al. Details of the microstructural features are presented, and their formation mechanisms are discussed. © 2018, University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature.Item Insights into formation of gradient nanostructured (GNS) layer and deformation induced martensite in AISI 316 stainless steel subjected to severe shot peening(Elsevier B.V., 2018) Jayalakshmi, M.; Huilgol, P.; Badekai Ramachandra, B.R.; Udaya Bhat, K.Severe peening is a well-accepted top-down approach to engender surface nanocrystallization in austenitic stainless steels. In the present study, AISI 316 grade austenitic stainless steel is subjected to severe peening through air blast shot peening technique. Study is aimed at analyzing the microstructural features of the peened layer and deformation induced martensite through transmission electron microscopy technique. Gradient nanostructured (GNS) layer formed as a result of high strain rate, multi-directional deformation during severe peening found to extend to about 500 ?m from the surface. Nucleation of deformation induced martensite is not limited to shear band intersections as affirmed by the published literature related to severe peening. It is observed to nucleate at multiple locations in the austenite matrix. Martensite units thus formed, coalesce with each other to form continuous layer of lath martensite layer at about 15–20 ?m from the surface. Upon further deformation, lath morphology transforms to dislocation cell-type; resulting in fine martensite crystallites at the topmost layer of the peened surface. © 2018 Elsevier B.V.Item Effect of acid pickling treatment of stainless steel substrate on adhesion strength of electrodeposited copper coatings using non-cyanide electrolyte(Elsevier Ltd, 2023) Bharadishettar, N.; Udaya Bhat, K.In recent years, copper-based antimicrobial coatings have gained popularity in healthcare and public recreation facilities. The morphology, topography, and adhesion strength are decisive properties for copper coatings to have long-term antimicrobial effectiveness in hospital environments. This work explores the effect of multistage acid pickling treatment of AISI 304 stainless steel substrate on the adhesion strength of the copper coating. The copper coating was obtained by electrodeposition using an alkaline non-cyanide electrolyte. After the fourth stage of acid pickling, the copper coating had an excellent adhesion strength, up to 9 MPa. Glow discharge optical emission spectroscopy (GDOES) examination revealed no oxide scales or other contaminants on the SS surface after the fourth (final) stage of acid pickling. Using a non-contact optical profilometer, it was observed that the roughness of the substrate increased with each stage of the pickling treatment. The surface topography analysis confirms the increased density of the interlocking sites, which favors the adhesion of the coating. On the other hand, the microstructure of the copper coating showed a cauliflower-like morphology with an average nodule size of 28 nm. Transmission electron microscopy confirmed that the coatings have nano-scaled crystallites with internal twins inside the grains of copper coatings. © 2023 Elsevier LtdItem Microstructural Features Intimated in Automotive Grade IF Steel Subjected to Conventional and Severe Shot Peening(Springer, 2024) Sahoo, B.; Udaya Bhat, K.A significant amount of interstitial-free or IF steel is used to manufacture automotive body parts due to its high ductility, high formability, and low yield strength. But, the major drawback of this steel is the lower surface hardness. The current investigation intended to enhance the surface hardness by employing shot peening at different coverages. The work also studied the microstructural features intimated after the treatment and its effect on the surface hardness. The optical and transmission electron microscopy (TEM) results showed a prominent grain refinement and dislocation hardening, which improved the micro-hardness to 2.5 times. Tri-junctions, sub-grains, twins, nanocrystalline regions, and several dislocation-induced microstructural features, like dislocation bands, dislocation forests, dislocation walls, dislocation cell structure, etc., were detected in the samples after peening. These features bear a beneficial impact on the surface hardness of the substrate. A spatial filter (Sobel filter) was used to refine the image and detect the presence of NbC precipitates near the grain boundary. Using Gatan DigitalMicrograph software, the thermal imaging technique effectively identified thinner grain boundaries near the segregation zone. © ASM International 2024.Item Tailoring the surface characteristics and mechanical behavior of Ti-Nb stabilized IF steel through controlled shot peening coverage(Elsevier Ltd, 2025) Sahoo, B.; Udaya Bhat, K.; Kumar, D.S.The rising demand for a qualitative surface opens a new window of research in the domain of mechanical surface treatment, known as severe shot peening, especially in the automotive industry. The effectiveness of this method is usually affiliated with various process parameters, of which peening coverage is the most sought-after. It is anticipated to elevate the surface characteristics by proficiently optimizing the peening coverage. On this ground, the current investigation tries to gather the beneficial effect of peening coverage on the surface properties of Ti-Nb stabilized interstitial-free steel subjected to severe shot peening by considering four different coverages (100 %, 500 %, 1000 %, and 2000 %). The work attempts to interpret the impact of peening coverage on grain refinement and dislocation-induced microstructures at different depths of the as-treated sample. The crossectional microscopy unveiled a prominent grain refinement hardening and dislocation hardening in 2000 % peening coverage up to a depth of 90–120 µm, firmly agreeing with the microhardness depth profile. The optical microscopy identified four zones of deformation (severe deformation, deformation, transition, and undeformed zone) in the sample treated with the highest coverage. The transmission electron microscopy demonstrated the dominance of certain dislocation-derived features like dislocation forest, dislocation cells, tangled dislocations, dislocation bands, nanocrystalline region, stress concentration region, etc., at the deformed zone of the treated samples. Interestingly, the trace of these features was detected at a greater depth for the highest-peened sample than the lowest-peened sample, affirming the beneficial aspect of higher peening coverage. The stored energy and thermal stability assessment in the as-received and as-treated sample was done in the differential scanning calorimeter, revealing the favorable impact of severe peening on the substrate. The surface topographical study in a 3D profilometer also unveils the variation in the surface roughness and functional volume parameters. The present investigation also analyzed the maximum depth and mean density of furrows to verify the severe plastic deformation in the as-treated sample. © 2024Item Synergistic investigation on Fe[sbnd]N phases and gradient microstructure supported anti-indentation and anti-scratch performance of low-temperature plasma ion nitrided Ti[sbnd]Nb stabilized IF steel(Elsevier B.V., 2025) Sahoo, B.; Udaya Bhat, K.Plasma ion nitriding (PIN) is a remarkable surface modification process that utilizes a thermochemical environment to treat the substrate by diffusion-induced phenomena, allowing it to modify complex-shaped objects, especially steel components. However, the foremost shortcoming of PIN is the higher processing cost, which can be minimized by reducing the nitriding temperature. This leads to a prominent variation in the sample's microstructure, often resulting in a gradient microstructure in the depth direction, which can be beneficial for specific industrial applications like scratch resistance, abrasion resistance, etc. The current investigation performs an extensive study to extrapolate the gradient microstructure-induced indentation and scratch resistance of low-temperature PINed (400 °C to 500 °C) interstitial-free steel through microscopy, diffractometry, spectroscopy, microhardness test, indentation test, and scratch test (constant and progressive loading). The transmission electron microscopy and scanning electron microscopy findings suggested a clear trace of gradient microstructure containing various Iron nitride phases (?-Fe2-3N, ?'-Fe4N, and ??-Fe16N2), size and distribution of which affect the scratch resistance. The sample treated at 450 °C shows the best result, with an overall improvement in scratch hardness of 3.2 times the base value. The coefficient of friction, track depth, traction force variation, etc., are also studied and correlated with spectroscopy and microscopy findings. © 2025 Elsevier B.V.