Mahesh Somani - Academia.edu (original) (raw)

Papers by Mahesh Somani

Materials Science Forum, Jan 5, 2021

Although quenched and partitioned (Q&amp;amp;P) steels are traditionally alloyed with Si,... more Although quenched and partitioned (Q&amp;amp;P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&amp;amp;P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the Ms -Mf range, retaining about 20% austenite at the quench-stop temperature (TQ), and held for 1000 seconds above TQ in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&amp;amp;P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&amp;amp;P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Feb 1, 2020

The minerals, metals & materials series, 2018

The TMR-DQP* processing route comprising thermomechanical rolling followed by direct quenching an... more The TMR-DQP* processing route comprising thermomechanical rolling followed by direct quenching and partitioning, has shown huge potential for the development of tough, ductile ultra-high-strength steels, both for structural and wear-resistant applications. The approach comprised designing suitable chemical compositions, establishing appropriate DQP processing conditions with the aid of physical simulation, and finally testing laboratory rolled DQP material with the emphasis on cost-effective process development, amenable for industrial hot strip production. Evaluation of DQP processed samples cooled slowly following DQP processing, thus simulating coiling, confirmed achieving the desired martensite-austenite microstructures and targeted mechanical properties. Ausforming in no-recrystallization regime (Tnr) resulted in extensive refining and randomization of the martensite packets/laths besides fine division of interlath austenite, thus resulting in an all-round improvement of mechanical properties. Preliminary investigations on alloys designed with 0.2 C have shown promising properties not only for structural applications, but also wear-resistance purposes.

Journal of The Mechanical Behavior of Biomedical Materials, Dec 1, 2018

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016

Abstract The concept of phase reversion involving severe cold deformation of metastable austenite... more Abstract The concept of phase reversion involving severe cold deformation of metastable austenite to generate strain-induced martensite, followed by a specific temperature–time annealing sequence, was used to obtain grain size varying typically from nanograined/ultrafine-grained (NG/UFG) to coarse-grained (CG) regime with the objective to elucidate the interplay between strain hardening behavior and grain structure. The study underscores that irrespective of the grain structure and mechanisms operating during deformation (twinning vs. strain-induced martensite), the generic nature of strain hardening is unaltered. However, there were subtle differences in the three-stage strain hardening ability that were governed by noticeably different deformation mechanisms. There was transition in deformation mechanism from essentially nanoscale twinning in NG/UFG and sub-micron grained (SMG) structures to strain-induced martensite transformation in the fine-grained (FG) and CG structures, a behavior related to increase in the stability of austenite with decrease in grain size.

Proceedings of the International Conference on Martensitic Transformations: Chicago, 2018

The effects of microalloying (Nb, V) and aluminum on the constitutive flow behavior and static re... more The effects of microalloying (Nb, V) and aluminum on the constitutive flow behavior and static recrystallization (SRX) characteristics of microalloyed TWIP steels (Fe-20Mn-0.6C-Al-(Nb,V)) have been investigated using hot compression testing. The effects of temperature, strain and strain rate were determined to estimate the activation energy of SRX as well as the powers of strain and strain rate. While microalloying with V up to 0.3% seems to have little effect on the SRX kinetics, 0.026%Nb significantly slowed down the SRX rate. Addition of 4.9%Al not only increased the flow stress and deformation activation energy, but also retarded the SRX kinetics in comparison to that of the steels with low Al (1.5%), with or without V.

The key objectives of this project were: 1. To investigate, by laboratory test simulation and hot... more The key objectives of this project were: 1. To investigate, by laboratory test simulation and hot rolling trials, the effect on the microstructural evolution of the loading path taken to achieve a given overall cumulative strain. 2. To identify the mechanisms responsible for differences in constitutive behaviour. 3. To formulate constitutive models which help provide an understanding of this behaviour. Instances of constitutive models are obtained for a range of steel qualities, including austenitic, ferritic and duplex structures, using experimental data obtained from the laboratory tests, are validated using results from such tests and hot rolling operations to which the models are applied. To achieve these objectives the partners have cooperated in supplying materials, in using their experimental testing and rolling equipment to implement the required loading paths, in providing metallurgical services for material characterisation, and in sharing their facilities for development and validation of the constitutive models. It has been found, in this work, that comparable mechanical loading paths in the experimental laboratory test simulation trials have given rise to significant differences in rheological and structural behaviour: 1. An abrupt decrease in strain-rate, in tension tests can give rise to a lag in the flow stress behaviour behind that for a mechanical equation of state, and an even greater lag in the rate of static recrystallisation, and, in the case of ferrite, cells larger than those obtained without the drop in strain rate. 2. Strain reversal in both tension/compression and torsion tests produces a noticeable Bauschinger effect and a plateau in the flow stress curve. For interpass times greater than 10 s, there is also a change in the static recrystallisation rate depending on the magnitude of the strains applied. For ferrite, the grains are coarser, the cells larger and low angle boundaries more numerous. 3. A strain reversal coupled with an abrupt decrease in strain rate gives rise, for both torsion and tension/compression, to a Bauschinger effect and plateau more noticeable than without the drop in strain rate. Conversely, if the change in strain rate is an increase, results for torsion exhibit opposite i.e. less pronounced results. 4. A double hit in the same direction can cause a lag in the recrystallisation curve behind that for the first compression but, if the reductions are sufficiently high, the curve will ultimately be higher than that for a single reduction of the same total absolute strain. Equivalent triple hits with individually lower reductions may, however inhibit recrystallisation completely. If the two compressions are applied in perpendicular directions, recrystallisation may take longer to complete, although in the short term, the rate may be faster. For comparable thermal loading paths in an austenitic steel, differences are insignificant. In the laboratory rolling trials, differences have been observed between loads for the high and moderate speed rod rolling schedules and between loads for the monotonic and forward-reverse plate rolling schedules. In the latter application, differences in deformation have also been found. However, unlike experimental tests, effects of loading path on structural evolution, if existent, appear at best to be secondary. Difficulties in detecting any differences have arisen because of the presence of phases such as bainite and wide variations in grain size associated partly with heterogeneity in the initial structure. Equally importantly, the angle between corresponding strain paths is not large, overall, for the simple rolling processes considered. To predict the effect of loading path on material behaviour, constitutive models have been formulated by all the partners.

Transactions of the Indian Ceramic Society, 1988

This paper reports the preliminary results of fabricating Al2O3-ZrO2composites by Hot Isostatic P... more This paper reports the preliminary results of fabricating Al2O3-ZrO2composites by Hot Isostatic Pressing (HIPing). The idea was to produce toughened (by addition of ZrO2) and completely dense Al2O3—ZrO2 composites. 15 mol % of ultrafine unstabilized ZrO2 was milled with Al2O3. The as-mixed powder was cold isostatically pressed (CIPed), sintered at 1400°C for 1 hr and HIPed at 1450°C for 0.5 hr. The samples reached their theoretical density values. The results were compared to HIP diagrams described in the literature. It could be seen that in spite of some simplifying assumptions, theoretical predictions matched with the experimental results. Boundary diffusion was interpreted to be the dominant mechanism.

Metals

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studie... more In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

$Research paper thumbnail of Effect of Heat Treatment Regime on Microstructure and Phase Evolution of Almo0.5nbta0.5tizr Refractory High Entropy Alloy$

Journal of Materials Engineering and Performance

A combination of physical simulation and laboratory rolling experiments, including thermomechanic... more A combination of physical simulation and laboratory rolling experiments, including thermomechanical rolling and low-temperature ausforming, was conducted for designing a suitable processing route to enable phase transformation from austenite to ultrafine bainite in a medium-carbon steel. Following low-temperature ausforming at 500-550 °C, two different cooling and holding paths were tried in the study: (1) water cooling close to martensite start temperature (300 °C), followed by isothermal holding (route A), and (2) air cooling to 350 °C followed by isothermal holding (route B). For reference, a third sample was directly water-cooled to 300 °C after hot rolling without ausforming treatment, followed by isothermal holding (route C). Field emission scanning electron microscopy and electron backscatter diffraction, as well as x-ray diffraction, were employed for microstructural analysis and correlations with the mechanical properties evaluated in respect of hardness and tensile propert...

SSRN Electronic Journal, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

FEMS - (Federation of European Materials Societies)Excellent combinations of strength and toughne... more FEMS - (Federation of European Materials Societies)Excellent combinations of strength and toughness can be obtained from high-carbon nanobainite, but this requires heat-treating highly alloyed steels for long periods of time. In this project we aim to develop very fine bainitic microstructures in medium carbon steels (0.3-0.5 wt.%) in a more cost–effective way, using shorter processing times via thermomechanical ausforming. Tensile strengths above 1600 MPa are aimed at to give hot rolled steels with enhanced wear resistance combined with good toughness. Suitable compositions and processing parameters have been developed using modelling and physical simulation. We will present the main achievements so far obtained under the auspices of a Research Fund for Coal and Steel (RFCS) project, TIANOBAIN

A novel concept of direct quenching and partitioning (DQ&P) processing route has been exp lored w... more A novel concept of direct quenching and partitioning (DQ&P) processing route has been exp lored with the specific aim of producing ultra -high strength structural steels with yield strengths ≈1100 M Pa co mb ined with good uniform and total elongations and impact toughness. The approach used was to design suitable compositions based on high silicon and/or aluminiu m content, establish the DQ&P parameters with the aid of physical simulation on a Gleeble simu lator and finally, design the DQ&P p rocessing route for trials on a laboratory ro lling mill. Dilatation experiments were made on a Gleeb le simulator to determine the appropriate cooling rates and quench stop temperatures for obtaining martensite fractions in the range 60-90%. Two types of dilatation tests were conducted starting with either strained or unstrained austenite prior to quenching to roughly simulate industrial rolling with low and high fin ish rolling temperatures, respectively. Laboratory rolled samples were direc...

steel research international, 2021

The residual compressive stresses and dimensional changes related to the lattice strains of retai... more The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high‐Si, medium‐carbon steel (Fe‐0.53C‐1.67Si‐0.72Mn‐0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE‐SEM) combined with electron backscatter diffraction (EBSD) analyses, and X‐ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high‐carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the ina...

Metallurgical and Materials Transactions B, 2020

In this study, a set of thermodynamic, kinetic, and microstructure data is presented to simulate ... more In this study, a set of thermodynamic, kinetic, and microstructure data is presented to simulate the non-equilibrium solidification of Fe-Al-Mn-Si-C alloys. The data were further validated with the experimental measurements and then used in a thermodynamic–kinetic software, IDS, to establish the effect of the alloying and cooling rate on the solidification behavior of high-AlMnSi (Al ≥ 0.5 wt pct, Mn ≥ 2 wt pct, Si ≥ 1 wt pct) steels. The modeling results were additionally validated by conducting electron probe microanalysis (EPMA) measurements. The results reveal that (1) solidification in high-AlMnSi steels occurs at much lower temperatures than in carbon steels; (2) increasing the cooling rate marginally lowers the solidus; (3) the microsegregation of Mn in austenite is much stronger than that of Si and Al due to the tendency of Al and Si to deplete from the liquid phase; (4) the residual delta ferrite content may be influenced by a proper heat treatment but not to the extent tha...

Materials Science and Engineering: A, 2020

As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial prop... more As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial property, but their mechanical strength is low. In order to improve the yield strength of a 304Cu (17%Cr-7%Ni-3%Cu) alloy through substantial grain refinement, a research investigation has been taken up to conduct the reversion annealing treatment comprising a heavy (71%) cold rolling reduction followed by annealing at various temperatures (650-950 °C) and durations (1-5400 s). The microstructure evolution was examined by electron backscatter diffraction and further characterized by magnetic measurements, and mechanical properties were determined by tensile and hardness testing. The precipitation of Cu was confirmed by transmission electron microscopy. It was found that the reversion of deformation-induced martensite to austenite took place by the shear mechanism, followed by subgrain formation and continuous recrystallization resulting in quite non-uniform grain size distribution. The finest reversed grains were around 0.6 µm in size, but also much larger austenite grains and a small fraction of unreversed martensite existed in the final structure despite annealing at least up to 800 °C. Coherent Cu particles were observed after aging for 1.5 h at 700 and 650 °C, while the yield strength could be improved to 507 and 791 MPa, 2 respectively, i.e. by ~2-3 times that of the annealed steel. The ductility of the steel remains still high, the fracture elongation being 36%.

Journal of the Mechanical Behavior of Biomedical Materials, 2020

The significance of phase reversion-induced nanograined/ultrafine-grained structure on the load-c... more The significance of phase reversion-induced nanograined/ultrafine-grained structure on the load-controlled deformation response and related mechanism in copper-bearing austenitic stainless steel,