Mahesh Somani

• University of Oulu

In-depth analysis of phase transformations during continuous heating (2.4 °C/s) of 50 % cold-rolled Fe-6Mn-2Al-1Si-0.4C (wt.%) steel with 32 % pre-existing austenite was conducted using in-situ high-energy synchrotron X-ray diffraction and microstructural characterisation. Significant austenite decomposition was found to occur in two stages between...

An ultrafine bainitic steel was designed with segregation-induced martensitic/austenitic (MA) bands. The steel comprises almost 21% retained austenite (RA); the rest is mainly bainitic ferrite and a small fraction of martensite. The steel exhibits room temperature (RT) yield strength of ~ 914 MPa, ultimate tensile strength of ~ 1969 MPa, and total...

The AlMo 0.5 NbTa 0.5 TiZr refractory high-entropy alloy (RHEA) is one of the prominent systems that has recently attracted significant attention mainly due to its significant high-temperature strength. In this study, the effect of Rhenium (Re) addition on microstructure, chemical composition, mechanical properties, corrosion behavior, and high-tem...

A clear understanding of strain-induced martensitic transformation of filmy retained austenite (RA) near stable cracks is required. Literature shows RA transformation during fatigue crack growth (FCG) in steels containing blocky but not film-like RA, as the latter is known to resist strain-induced phase transformation. This work investigates the tr...

The purpose of this study is to understand in detail the micromechanical behavior of martensite and ferrite microphases in a medium‐Si 35CHGSA dual‐phase (DP) steel by varying the martensite volume fractions (MVF) and ferrite phase constituents. Light microscopy combined with microhardness measurements is supplemented by field‐emission scanning ele...

Laboratory rolling simulations, comprising of thermomechanically controlled rolling followed by ausforming and isothermal holding close to Ms temperature, were conducted on a medium-carbon (C) steel in order to understand the combined effect of prior straining and low temperature holding on the bainite transformation characteristics and resultant p...

The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1....

In this research, the influence of Re addition on the thermal stability of the AlMo0.5NbTa0.5TiZr refractory high-entropy alloy was investigated. The based alloy (Re-free) has a microstructure consisted of body-centered cubic (BCC) and hexagonal close-packed (HCP) phases. This dual-phase microstructure was thermally instable and decomposed into a m...

The influence of Re addition on the thermal stability of the AlMo0.5NbTa0.5TiZr refractory high-entropy alloy was investigated. While the base alloy (Re-free) which consisted of body-centered cubic (BCC) and hexagonal close-packed (HCP) phases, displayed thermal instability. Upon aging treatment in the temperature range of 1000-1200 °C, the Re-free...

The essences of the quenching and partitioning (Q&P) process are to stabilize the finely divided retained austenite (RA) via carbon (C) partitioning from supersaturated martensite during partitioning. The competitive reactions, i.e., transition carbide precipitation, C segregation and decomposition of austenite might take place concurrently during...

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 ferrit...

This paper reports the effect of heat treatment process on the ensuing microstructure evolution, and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). Heat treatments were carried out on the homogenized samples at 1000, 1100, and 1200 ˚C for 10, 24, and 48 hours. It was revealed that the phase dec...

To establish the characteristics and kinetics of dynamic softening in a Al0.3CoCrFeNi high–entropy alloy (HEA), isothermal compression tests were carried out in a suitable temperature range of 1273–1423 K at 10−2 and 10−1 s−1 in accord with our previous study. It was found that the discontinuous dynamic recrystallization (DRX) was the dominant micr...

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructur...

In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (∼19 kHz frequency) up to ∼1010 cycles at the load ratio (R) of −1 (fully reversed ten...

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 diffe...

A novel processing route is proposed to create a heterogeneous, multiphase structure in a medium Mn steel by incorporating partial quenching above the ambient, warm deformation, and partial recrystallization at high partitioning temperatures. The processing schedule was implemented in a Gleeble thermomechanical simulator and microstructures were ex...

To establish the characteristics and kinetics of dynamic softening in a Al 0.3 CoCrFeNi high–entropy alloy (HEA), isothermal compression tests were carried out in a suitable temperature range of 1273 – 1423 K at 10 -2 and 10 -1 s -1 in accord with our previous study. It was found that the discontinuous dynamic recrystallization (DRX) was the domina...

IDS is a thermodynamic-kinetic software combined with a microstructure tool developed to simulate the non-equilibrium solidification of steels. The present work introduces its main calculation module, SOL, and presents some essential results of that module, such as the formation of ferrite and austenite in different types of steels during their sol...

This study presents the characteristics of hot deformation behavior of a Al0.3CoCrFeNi high–entropy alloy in the temperature and strain rate ranges of 1023–1423 K and 10–3–10 s–1, respectively. The constitutive flow behavior was modeled based on the hyperbolic–sinusoidal Arrhenius–type equations and a mathematical relation was used to observe the i...

In order to establish the processing conditions to achieve the target of tensile strength of ∼1500 MPa with minimum retained austenite (RA) fraction of 10% in hot-rolled 3–4%Mn medium Mn steels, three hot-deformed steels with the base composition of Fe-0.3C–1Si-0.5Al (concentrations in wt.%) and variable contents of Mn (3 or 4 wt.%), Ni (1 or 2 wt....

The propensity of the retained austenite to transform to martensite under external strain is known to induce plasticity (TRIP) in steels. If the retained austenite has a high volume fraction, it generally has blocky morphology and increases the tensile ductility via TRIP effect but can potentially deteriorate the yield strength. The present work de...

Transmission electron microscopy (TEM) and 3D Atom probe tomography (APT) were used to investigate the elemental partitioning, carbon (C) redistribution and carbide precipitation associated with quenching and partitioning (Q&P) treatment of medium C steels (0.4 wt% C) alloyed with three levels of silicon (Si) (0.25, 0.75 and 1.5 wt%). Different typ...

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon content...

This work deals with the kinetic aspects of bainite formation during isothermal holding above and below the martensite start (Ms~275 °C) temperature using a low-alloy, high-silicon DIN 1.5025 steel in a range suitable for achieving ultrafine/nanostructured bainite. Dilatation measurements were conducted to study transformation behaviour and kinetic...

The characteristics of dynamic recrystallization (DRX) of a CoCrFeMnNi high–entropy alloy (HEA) was investigated via hot compression testing in the temperature range 950–1100 °C and at true strain rates of 10-2 and 10-1 s-1. The discontinuous DRX was found to be the dominant mechanism corroborating the microstructural evolution. The progress of the...

Phase transformation in low alloyed Fe-C steels during quenching and partitioning (Q&P), though authentically attributed to carbon diffusion, is scarcely quantified experimentally in terms of microstructural evolution and lack of fundamental backing at a quantum mechanics level. Herein, we report on a combined in-situ high energy synchrotron X-ray...

A new combination of factors enhancing the stabilization of austenite, including pre-existed austenite among quenched martensite, prior deformation, and partitioning at high temperatures is employed to create a multi-component refined microstructure in a medium Mn steel (Fe-4Mn-0.31C-2Ni-0.5Al-0.2Mo, wt.%). The microstructure evolution and phase fr...

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) were 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 were directl...

An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been...

The present paper aims to compare the microstructural and mechanical properties of CK45 carbon steel plates, joined by friction stir (FSW) and tungsten inert gas (TIG) welding methods. Besides visual inspection, the welded joints and the base material were subsequently evaluated in respect of optical microstructures, hardness and tensile properties...

The hot deformation characteristics of a FeCrMnN austenitic stainless steel containing 0.28 wt.% nitrogen (N) was investigated by hot compression tests using a Gleeble simulator in the temperature range of 800−1200 °C and at constant true strain rates of 0.01–10 s⁻¹ with all specimens deformed to ∼0.9 true strain. The influence of deformation condi...

A combination of experimental and numerical approaches was applied for constructing a dynamic model for thermomechanical processing, which was used for simulating laboratory rolling and cooling, and for designing a cooling path to enable phase transformation from austenite to ultrafine (∼50–100 nm) bainitic laths. Physical thermomechanical simulati...

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical...

The influence of boron as well as boron with niobium additions on the phase transformation behaviour, resultant microstructures, and mechanical properties of thermomechanically controlled hot-rolled and direct-quenched low-carbon bainitic steel plates was investigated. Also, the probable factors that could inhibit their specific merits on hardenabi...

The aim of this work was to derive new quality criteria based on steel composition and cooling pattern for continuous casting and for the subsequent cooling and reheating processes. The criteria were devised based on the outputs of multiphysics simulation tools for casting applications. The criteria were found to be good predictors of whether a ste...

New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble co...

A novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (DQ&P) was designed for developing tough, ductile, ultrahigh strength steels using 0.4 wt% carbon steels. A preliminary characterization of a laboratory-rolled, high-silicon DQ&P steel revealed an excellent combination of mechanical properties...

The unique concept of phase reversion involving severe deformation of parent austenite into martensite, followed by annealing for a short duration, whereby the strain-induced martensite reverts to austenite, was adopted to obtain nano-grained/ultrafine-grained (NG/UFG) structure in a Cu-bearing biomedical austenitic stainless steel resulting in hig...

Structural applications aspire to have an optimum combination of mechanical properties like strength, ductility, fracture toughness, fatigue life etc. With innovative material processing methods, it is possible to obtain excellent mechanical property combinations even with cost effective chemical compositions. This work aims to understand the influ...

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurrin...

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static...

Quenching and partitioning produces advanced high-strength steels that utilise transformation-induced plasticity for improved strength and deformability. Microstructures of these steels consist mainly of tempered martensite and carbon-enriched retained austenite. A novel processing route of direct-quenching and partitioning (DQP) facilitates carbon...

The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture...

Recent advances in transmission electron microscopy (TEM) in respect of structural characterization down to atomic scale have enabled confirmation of stabilization of long ignored hexagonal omega (ω) phase in steel. The presence of ω phase is suggested to increase the strength of steel, and one of the factors concerning its stabilization is enrichm...

Although quenched and partitioned (Q&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 c...

The effect of austenite deformation on carbon partitioning and transformation to athermal and isothermal martensite, and bainite during quenching and partitioning (QP) is described for three steel compositions: Fe-0.3C-0.6Si-1.1Al, Fe-0.3C-1.0Si and Fe-0.3C-0.5Si-0.5Al. Microstructures were characterized using SEM-EBSD, TEM and XRD. Austenite decom...

Recent advances in transmission electron microscopy (TEM) in respect of structural characterization down to atomic scale have enabled confirmation of stabilization of long ignored hexagonal omega (ω) phase in steel. The presence of ω phase is suggested to increase the strength of steel, and one of the factors concerning its stabilization is enrichm...

Quenching and partitioning produces advanced high-strength steels that utilise transformation-induced plasticity for improved strength and deformability. Microstructures of these steels consist mainly of tempered martensite and carbon-enriched retained austenite. A novel processing route of direct-quenching and partitioning (DQP) facilitates carbon...

The interaction of different alloying elements has a significant impact on the mechanical and microstructural properties of steel products due to the thermodynamic and kinetic effect. This article presents a statistically developed and validated model for austenite decomposition during cooling, based on a set of experimental continuous cooling tran...

Herein the effects of molybdenum and niobium on the microstructures and mechanical properties of laboratory‐rolled and direct‐quenched and direct‐quenched and tempered steels are revealed. The microstructures are martensitic with yield strength of 766–1119 MPa in direct‐quenched condition and 632–1011 MPa in direct‐quenched and tempered condition....

The processing of advanced multiphase high strength steels often includes isothermal treatments around the martensite start temperature (Ms) for achieving a refined microstructure comprising bainiteaustenite and/or bainite-martensite-austenite phase constituents. The objective of this research work was to investigate the structure-property relation...

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 solidi...

An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10 ⁻³ –10 s ⁻¹ , to a t...

In the current study, the hot deformation characteristics and workability of a CoCrFeMnNi high entropy alloy was characterized using processing maps developed on the basis of dynamic materials model in the temperature range 1023-1423 K and strain rate range 10 −3-10s −1. The processing map delineated various deterministic domains including those of...

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 comprisin...

This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling tran...

Deformation characteristics of a Ti + Nb stabilized IF steel have been investigated in the present study to clarify the associated strain hardening and/or dynamic restoration mechanisms. Single-hit hot compression experiments were performed in a Gleeble thermomechanical simulator in the temperature range 650–1100 °C at various constant true strain...

This study encompasses a comprehensive account of the abrasive wear properties of carbide-free, ultrahigh-strength bainitic steels processed through ausforming at three different temperatures well below the recrystallization stop temperature followed by bainitic transformation at temperatures close to the Ms temperature. Five medium-carbon, high-si...

The presence of a long‐abandoned hexagonal omega (ω) phase in steel samples is recently gaining momentum owing to the advances in transmission electron microscopy (TEM) measurements, even though it is already reported in other transition‐metal alloys. The stabilization of this metastable phase is mainly investigated in presence of C, even though th...

Multiphase advanced high strength low alloy steels having a refined microstructure of hard bainite and/or martensite laths, interspersed with finely divided retained austenite films, are the potential candidate materials for futuristic industrial applications, such as automotive industry, structural parts, crankshafts and shafts, and powertrain com...

In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low tempe...

Please cite this article in press as: Pashangeh S, et al. Microstructural evolution in a high-silicon medium carbon steel following quenching and isothermal holding above and below the M s temperature. J Mater Res Technol. 2020. https://doi. j m a t e r r e s t e c h n o l. 2 0 2 0;x x x(x x):xxx-xxx w w w. j m r t. c o m. b r Bainite Retained aust...

The microstructural mechanisms operating during the decomposition of austenite in a high-Si, medium‑carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) subjected to quenching and isothermal holding at temperatures above and below the martensite start (Ms) temperature for times up to 1 h have been investigated using a Gleeble 3800 thermomechanical simulato...

Models describing the constitutive flow behaviour of a metallic material are desired for appropriate process design and realization of defect-free components. In this study, constitutive equations based on the hyperbolic-sinusoidal Arrhenius-type model have been developed to define the hot deformation characteristics of a CoCrFeMnNi high entropy al...

During thermomechanically controlled processing (TMCP) of carbon steels, controlled rolling in no-recrystallization regime followed by directing quenching results in excellent mechanical properties, besides enabling cost-saving in respect of energy costs normally incurred during reheating and quenching procedure. The effects of three different comb...

The microstructural characterizations and partitioning of carbon element in the weld heat affected zones of a commercial API 5L X60 line pipe steel were studied by applying a high speed heating and cooling dilatometry technique in the present research work. The hollow cylindrical specimens were quickly heated to 1000°C, soaked for only 5 s followed...

New CCT equations have been developed and optimized to simulate the start temperatures of the austenite decomposition process in low-alloyed steels using experimental CCT data published in the literature. Exceptionally, this optimization does not apply the nominal compositions of the steels, but the corresponding soluble compositions of the grain b...

A model for static recrystallization by Zurob et al. [1] has been fitted to experimental stress relaxation [2] data obtained on a low-alloyed steel using a Gleeble thermomechanical simulator. The model has been implemented as an algorithm that calculates the stress relaxation as a function of time, including physical descriptions of the recovery an...

A finite element (FE) simulation model illustrating the stress relaxation test was established with the AbaqusTM software. The microstructural evolution of steel during relaxation includes the complex phenomena of recrystallization. While the compression introduces the planned deformation and stress into the test piece, subsequent softening relieve...

To develop an advanced high strength steel with reasonable ductility based on low alloying concept as well as micro-composite microstructure essentially consisting of bainite, martensite and retained austenite, a Si-bearing, low alloy medium carbon sheet steel (DIN1.5025 grade) was subjected to typical quenching and bainitic holding (Q&B) type isot...

The topic of durable coloration and passivation of metal surfaces using state-of-the-art techniques has gained enormous attention and devotion with unremitting efforts of researchers worldwide. Although femtosecond laser marking has been performed on many metals, the related coloration mechanisms are mainly referred to structural colors produced by...

The quenching and partitioning response following intercritical annealing was investigated for three lean TRIP-type high-Al steel compositions. Depending on the intercritical austenite fraction following annealing, the steels assumed either a ferrite/martensite/retained austenite microstructure or a multiphase structure with ferritic, bainitic and...

Hot forming combined with austempering and quenching and partitioning (QP) processes have been used to shape two cold rolled high silicon steel sheets into hat profiles. Thermal simulation on a Gleeble instrument was employed to optimize processing variables to achieve an optimum combination of strength and ductility in the final parts. Microstruct...

The direct quenching process is an energy- and resource-efficient process for making high-strength structural steels with good toughness, weldability, and bendability. This paper presents the results of an investigation into the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory rolled and direct-quenche...

In the direct quenching and partitioning (DQ&P) process, tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the...

A new experimental steel containing in weight percent 0.3C-2.0Mn-0.5Si-1.0Al-2.2Cr and 0.3C-1.9Mn-1.0Si-1.0Cr was hot rolled in a laboratory rolling mill and directly quenched within the martensite start and finish temperature range. It was then partitioned without reheating during slow furnace cooling to achieve tensile yield strengths over 1100 M...

Using a novel TMR-DQP processing route, two ultrahigh-strength steels have been developed with yield strengths up to 1100 MPa combined with good uniform and total elongations and low-temperature impact toughness. Processing involved thermomechanically controlled rolling including significant reductions below the recrystallization stop temperature (...

Hot deformation behavior of a Ni-free, Fe-17Cr-12Mn-0.28N-0.06C (wt.%) austenitic stainless steel, hereinafter coded as FeCrMnN, was investigated using hot compression tests conducted under different deformation conditions comprising temperature and strain rate ranges of 800-1200 °C and 0.01-10 s⁻¹, respectively. While the hot deformation at high s...