Faculty Publications

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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 Ltd
Effect of shot peening coverage on surface nanostructuring of 316L stainless steel and its influence on low temperature plasma-nitriding
(ASTM International, 2017) Jayalakshmi, M.; Badekai Ramachandra, B.; Bhat, K.U.
Air-blast shot peening (ABSP) is a cost effective and industrially viable technique to produce nanostructured surface layer on metallic materials. In the present study, 316L stainless steel samples were subjected to shot peening at different peening coverage, from conventional to severe peening. Nanocrystalline structure was observed on the sample surface after peening and mechanical twins; intersection of multidirectional twins producing rhombic blocks were observed in the subsurface layer. Peening process led to the formation of strain induced martensite (?'), and its fraction was found to increase with the coverage. Depth of nanostructured layer and surface microhardness also increased with the increase in coverage, whereas surface roughness followed an opposite trend. Both peened and un-peened samples were subjected to plasma nitriding at 400°C for 4 h. Uniform and appreciably high case depth of about 45 ?m was observed in severely pre-peened (1000 % coverage) sample after nitriding treatment. No precipitation of CrN was observed. This highlights the marked influence of severe shot peening as a pre-treatment for low temperature plasma nitriding of austenitic stainless steels. © © 2017 by ASTM International.
Cumulative effect of FexN phases, roughness parameters, and asperity geometry on the anti-wear properties of low-temperature plasma nitrided Ti-Nb stabilized IF steel
(Elsevier Ltd, 2025) Sahoo, B.; Bhat, K.U.
Plasma-driven surface modification techniques like plasma nitriding (PN) are trending, especially for steel products. It is advantageous due to the higher order of process control and superior quality of property enhancement of the surface. This technique often employs a high processing temperature, which is one of its metallurgical and economical limitations. One of the renowned solutions is the implementation of lower processing temperatures. The current work is based on the low-temperature plasma nitriding of Ti-Nb stabilized interstitial-free steel at different processing temperatures ranging from 400 ºC to 500 ºC. The role of FexN phases, surface roughness parameters, and asperities geometry are thoroughly studied with respect to the anti-wear properties of the surface. The formation of ??-Fe4N and ?-Fe2–3N are detected in the XRD plot, whereas ??-Fe16N2 is confirmed in microscopy. The sample treated at 450 ºC presents the best anti-wear properties compared to other samples, primarily due to the presence of prominent ?-Fe2–3N phase and blunt surface asperities. A maximum reduction in wear volume of about 3 times the base value is recorded in the wear test. The microscopic and elemental analyses are conducted on the wear scars, wear debris, and counter-body worn-out surfaces to study the wear mechanism comprehensively. The work tries to illustrate the wear mechanisms schematically to understand the conceptual grounds associated with such theories. The spectrometric analysis in the depth direction is also performed, and it detects the trace of nitrogen up to about 7 µm depth for 500 ºC nitriding conditions. © 2025

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