Faculty Publications
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Item Hot-dip Aluminizing of Low Carbon Steel in Al & Al-5wt % Cr Baths(Elsevier Ltd, 2018) Huilgol, P.; Bhat, K.U.; Udupa, K.R.Hot dip aluminizing of low carbon steel is carried out in pure aluminium bath and Al-5wt% Cr bath. The coating is characterized by scanning electron microscopy and chemical composition of the coating is analysed by EDS (energy dispersive spectroscopy) attached to SEM. The coating consists of three regions, viz., outer aluminium topcoat, intermediate Fe-Al intermetallics layer and the base alloy. The intermetallics layer consists of FeAl3 and Fe2Al5 phases. Fe2Al5 is the major phase in the intermetallics layer. The growth kinetics of intermetallics layer is parabolic in nature implying that it is diffusion controlled. Addition of chromium forms Al7Cr dispersed intermetallics phases in the aluminium topcoat. Addition of chromium has no influence on the morphology of the intermetallics layer. Scratch resistance of the coating is carried out to evaluate the scratch hardness of the coating. Chromium addition improves scratch resistance of the coating. © 2018 Elsevier Ltd.Item Microstructural characterization of low temperature plasma-nitrided 316L stainless steel surface with prior severe shot peening(Elsevier Ltd, 2016) Jayalakshmi, M.; Huilgol, P.; Badekai Ramachandra, B.R.; Bhat, K.U.Surface nanocrystallization by severe deformation has proven beneficial as pre-treatment to plasma nitriding. It aids in achieving thicker nitride layers at lower temperatures thus making the process more economical. In austenitic stainless steels, severe deformation leads to formation of strain induced martensite on the surface while plasma nitriding alone forms expanded austenite. However, structural characteristics of surface layer of pre-deformed steel after plasma nitriding is still a matter of debate. In present study, 316L stainless steel was subjected to severe shot peening: followed by plasma nitriding at 400 °C for 4 h. Characteristics of sample surface before and after treatment were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that, this duplex treatment leads to formation of about 45 ?m thick nitride layer; without CrN precipitation. This is significantly high compared to reported data considering the temperature and duration of nitriding treatment employed. Selected area electron diffraction pattern from topmost surface confirmed the co-existence of austenite and martensite while subsurface layer was predominantly consisting of lath martensite. This indicates that major phase in the nitrided layer is martensitic in nature and nitrogen supersaturation leads to transformation of small fraction of martensite to expanded austenite. © 2016 Elsevier LtdItem 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 Metastable microstructures at the interface between AISI 321 steel and molten aluminum during hot-dip aluminizing(Elsevier B.V., 2018) Huilgol, P.; Udupa, K.R.; Bhat, K.U.The microstructure at the interface between AISI 321 stainless steel and molten aluminum was investigated which occurs during the process of hot-dip aluminizing. Microstructural characterization was carried out by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The study revealed the formation of metastable FeAlm and multiple twinned Al13Fe4 phases at the interface between steel and aluminum. Multiple twinned Al13Fe4 phase exhibits pseudo tenfold electron diffraction pattern. Another metastable phase Al3(NiFe) with an orthorhombic structure was formed as one of the eutectic phase mixture in the solidified aluminum topcoat. The Al3(NiFe) phase in the eutectic shares crystallographic orientation relationship with the Al matrix. Metastable intermetallic phases are being reported for the first time during hot-dip aluminizing. © 2018 Elsevier B.V.Item Microstructural investigations on the hot-dip aluminized AISI 321 stainless steel after diffusion treatment(Elsevier B.V., 2019) Huilgol, P.; Udupa, K.R.; Bhat, K.U.The microstructure of the hot-dip aluminized AISI 321 stainless steel was studied after diffusion treatment at 900 °C for 3 h. The microstructural characterization was carried out by scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. The microstructure of the as aluminized steel consisted of two regions, viz.; aluminum topcoat and aluminide layer. During the diffusion treatment, the coating transformed into a layered structure consisting of four layers. The Fe2Al5 phase was formed in the outermost layer and the presence of Al13Fe4 quasicrystalline approximant phase was observed. The innermost layer adjacent to the base metal transformed to ferrite phase with NiAl precipitates. Next, to this layer, a disordered FeAl phase was observed. The lattice parameter of the disordered FeAl phase was found to be larger than that of the ordered B2 FeAl phase. The layer between outer Fe2Al5 phase and disordered FeAl phase consists of a mixture of three phases, namely Fe2Al5, disordered FeAl and a new phase with the simple cubic structure. The phase with simple cubic structure shares cube on cube crystallographic orientation relationship with the disordered FeAl phase. © 2019 Elsevier B.V.