Multiphase Steels Research Articles

Effect of amorphous carbon film on the phosphate formation in a multi-phase steel

The role of the amorphous carbon layer on phosphate formation is investigated in multi-phase steel. To investigate site-specific characterization for the remained organic contaminants which possess C-C/C-H species, the amorphous hydrogenated carbon (a-C:H) which possesses both C-C bond and C-H bond is deposited on the steel surface using the electron beam-induced deposition method. As the amorphous carbon film introduces, the main factor determining the dissolution rate in the multi-phase steel is changed from carbon contents on the surface to external amorphous carbon film when no external oxides exist. Despite the high electrical transportation of sp2-bonded carbon, the carbon layer acts as an effective physical barrier due to the amorphous structure, resulting in the phosphate-uncovered surface. The detailed microstructure analysis clarifies the inhibition mechanism of the phosphate formation on the carbon-deposited steel surface.

Hierarchically patterned multiphase steels created by localised laser treatments

The realisation of sophisticated hierarchically patterned multiphase steels has the potential to enable unprecedented properties in engineering components. The present work explores the controlled creation of patterned multiphase steels in which the patterns are defined by two different crystal structures: face centre cubic or fcc (austenite) and body centre cubic or bcc (martensite). These austenite/martensite mesostructures are generated by solid–solid phase transformations during the application of localised laser heat treatments in a Fe-Ni-C alloy. In particular, four patterned configurations are analysed in this work consisting of one or two horizontal austenite line structures imprinted in a base of as-quenched or tempered martensite. Digital image correlation analysis during tensile testing of the developed materials showed that both the strength of the base martensite and the mesostructure at the gauge have a strong effect on the resulting properties. Clear differences were observed among the configurations in strain partitioning, hardening of the different constituents and failure. The uniform elongation and tensile strength are increased with respect to that of the reference martensite and austenite, respectively. Concepts explored in this work can be extended to more complex patterns and other base microstructures, opening novel strategies to engineer properties in steel and other alloys.

Quantitative prediction of yield strength of highly alloyed complex steel using high energy synchrotron X-ray diffractometry

The overwhelming impact of complex-phase microstructures to mechanical response in multiphase steels requires accurate constitutive properties of the individual phases. However, precise prediction of individual phase properties to their mechanical response is critical and sophisticated, and commonly requires multiscale characterizations and numerous approximations. In this work, by employing full phase information and semi-empirical analytical models, we accurately predict the yield strength of deformed variants of Ce-modified SAF2507 super duplex stainless steel (SDSS). High energy synchrotron X-ray diffraction (HE-SXRD) reveals the phase fractions of major phases along with secondary phases of Cr2N and eutectic CexFey. Average lattice strain/crystallite size of the austenite and ferrite/martensite phases from the measured volume is estimated through the Williamson-Hall method. A unique composite strengthening type analytical model is used to estimate yield strength by taking individual strengthening contributions from all phases, their grain sizes, stored dislocation densities, solid solution, and precipitates. Close agreement between reconstructed and experimental yield strength is observed for several cold and cryogenic rolled SDSS. A combination of HE-SXRD and analytical model offers a time-effective virtual design pathway to engineer high-strength steel.

Microstructure and mechanical properties of a novel cast multi-phase stainless steel

Cast stainless steels with high strength and toughness are very important for industrial production, but the strengthening methods are very limited. In this study, a novel cast multi-phase stainless steel (CMPSS) containing ferrite, martensite and austenite was prepared and its strengthening mechanism was investigated. After solid solution and tempering at 520 °C for 4 h, the yield strength of the sample reaches over 800 MPa, and the total elongation exceeds 13%. This tempering strengthening is mainly achieved by precipitation of the nanoscale Laves phase pinning dislocations in ferrite. However, when prolonging the tempering time or increasing the tempering temperature, the Laves-phase particles precipitate and coarsen along the phase boundary, which seriously damages the strength and toughness of the sample. In addition, high-temperature tempering also leads to a decrease in retained austenite (RA) content by reducing the amount of C and Mn elements in the lath martensite and promotes the precipitation of the R phase (molybdenum-rich intermetallic compound) in ferrite, which is also detrimental to the mechanical properties of CMPSS.

The Microstructure Prediction during the Welding Process of Fe–C–Si Steel Using Physical and Numerical Simulation: A Comparative Study

Overall, medium‐carbon, multiphase steels subjected to welding processes can be described as difficult and require dedicated process conditions. In these investigations, an attempt is made to characterize the welding process of commercial 55Si7 steel with a high carbon equivalent (CE = 0.73). An experimental tungsten inert gas (TIG) welding process is performed by the use of preheating. Then, to understand and explain the processes during welding, a numerical (finite element method [FEM]) and physical (dilatometry) simulation is carried out. Measurement points located in the heat‐affected zone (HAZ) are selected based on the obtained data of thermal cycles. Significant differences in the results obtained with the three approaches are assessed and indicated. The cause of the discrepancies in the test results is also identified. The essence of the implementation of material data, which precisely defines the kinetics of phase transformation, is confirmed. A detailed analysis of the contribution of the phases is performed, together with their morphological assessment, as well as the hardness distributions are conscientiously compared. Moreover, it seems that the segregation of the chemical composition of austenite and the initial microstructure plays a crucial role in terms of the quality of the prediction.

Effect of Post-austempering Cold Work on Thermal Stability of Retained Austenite and Mechanical Properties of Medium Carbon High Silicon Multiphase Steels

Effect of Post-austempering Cold Work on Thermal Stability of Retained Austenite and Mechanical Properties of Medium Carbon High Silicon Multiphase Steels

Characterization and Parametric Optimization of EN-10149-2 Steel Welded Joints Made by MIG Welding

Advanced high strength steels (AHSS) are multiphase steels that contain various concentrations of ferrite, bainite, martensite, and retained austenite phases. The unique physical characteristics of AHSS present some challenges to welding and bonding processes. AHSS have tendency to change microstructure and mechanical properties by any welding process like other steels. Based on the significant applications in various industries, the experimental works are carried out on one of AHSS materials, EN-10149-2 S700MC, with thickness of 2 mm steel sheet metal by MIG welding process parameters such as current, voltage, and welding speed. For all combinations of process parameters, the butt joints are fabricated. The butt joints are inspected through different nondestructive testing for defects detection. All the defect-free joints are characterized through microstructure (at different zones of joint), hardness, and tensile tests. From the obtained mechanical test results, the process parameters are optimized through design of experiment techniques (DoE). The analysis from DoE method is used to identify which parameter is the most significant among all parameters. Confirmatory test is also done by taking the optimal parameters and the result shows that the improvement of the response variables is acceptable and also shows that the method that is used for optimization was valid.

Effect of various phase fractions of bainite, retained austenite, intercritical ferrite and pearlite on the wear behaviour of multiphase steels

In this study, the dry sliding wear behaviour of multiphase steels (MPS) was investigated with the help of a pin/ball-on-disc wear setup. The newly developed high strength and highly ductile MPS specimens consisting of various fractions of bainite, retained austenite, intercritical ferrite and pearlite, were obtained by a combination of continuous cooling for different times (0 s, 10 s, 20 s and 40 s) after austenitization followed by isothermal heat treatment at various temperatures (300 °C, 350 °C and 400 °C) for 10 min from a moderately high C (∼0.67 wt%) - high Si (∼1.71 wt%) steel. The worn-out surfaces and wear debris revealed that the primary material removal mechanism was of abrasive types. Wear scars were seen like a ploughed field-like structure. The depth of wear scars and amount of material collected at the periphery were affected by various phases and their fractions in the MPS specimens. The wear behaviour of the multiphase steels was affected by the distribution of plastic deformation due to continuous sliding of wear ball and application of loads on the surface of MPS specimens which was analysed by hardness variation of the sub-surfaces beneath the wear scars. At a particular austempering temperature, the wear volume loss and specific wear rate of the MPS specimens increased with the increase of continuous cooling time irrespective of the applied wear loads, and it was attributed to the formation of softer intercritical ferrite as well as pearlite formed due to longer continuous cooling. However, the bainitic structure formed by austempering without continuous cooling led to the highest wear resistance irrespective of the isothermal temperatures.

Corrosion Behavior of Multiphase Bainitic Rail Steels

Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained from bainitic-austempering (BAT) and bainitic-quenching and -partitioning (BQ&P) processes were investigated via immersion and electrochemical tests in 3.5 wt.% NaCl solution. The corroded surface and rust after immersion and electrochemical tests were analyzed via electron microscopy, Fourier transform infrared spectra, and x-ray diffraction. The multiphase bainite + martensite/retained austenite island showed higher corrosion resistance than that of the pearlitic one. The acicular bainite obtained from the BQ&P process showed slightly higher corrosion resistance than the granular bainite + martensite structure obtained from the BAT process.

Multimodal assessment of mechanically induced transformation in metastable multi‐phase steel using X‐ray nano‐tomography and pencil‐beam diffraction tomography

A combination of X-ray nano-tomography and pencil-beam diffraction tomography was utilized for multimodal assessment of the mechanically induced transformation of individual retained austenite grains during tensile deformation in a 0.1C-5Mn-1Si multi-phase steel. In the present study, a newly developed high energy (20 - 30 keV) and high resolution (spatial resolution of 0.16 µm in this study) X-ray nano-tomography technique was applied for the first time to the in-situ observation of a steel under external loading. The gradual transformation, plastic deformation, and rotation behaviour of the individual austenite grains were clearly observed in 3D. It was revealed that the early stage of the transformation was dominated by the stress-assisted transformation that can be associated with measured mechanical driving force, whilst the overall transformation was dominated by the strain-induced transformation that is interrelated with measured dislocation multiplication. The transformation behaviour of individual grains was classified according to their initial crystallographic orientation and size. Noteworthy was the high stability of coarse austenite grains (i.e., 2.5 μm or larger in diameter), contrary to past reports in the literature. Characteristic rotation behaviour and wide data dispersion were also observed in the case of the individual austenite grains. It was conclusively demonstrated that such characteristic behaviour partly originated from interactions with surrounding soft and hard phases. The origins of these characteristics are discussed by combining the image-based and diffraction-based information.

Inverse Problem in Stochastic Approach to Modelling of Microstructural Parameters in Metallic Materials during Processing

The need for a reliable prediction of the distribution of microstructural parameters in metallic materials during processing was the motivation for this work. The model describing the evolution of dislocation populations, which considers the stochastic aspects of occurring phenomena, was formulated. The validation of the presented model requires the application of proper parameters corresponding to the considered materials. These parameters have to be identified through the inverse analysis, which, on the other hand, uses optimization methods and requires the formulation of the appropriate objective function. In our case, where the model involves the stochastic parameters, it is a crucial task. Therefore, a specific form of the objective function for the inverse analysis was developed using a measure based on histograms. The elaborated original stochastic approach to modeling the phenomena occurring during the thermomechanical treatment of metals was validated on commercially pure copper and selected multiphase steel.

Phase evolution within multiphase stainless steels during simulated hot isostatic pressing cycles

Stainless steel hardfacing alloys are being developed for wear and corrosion resistant applications in pressurised water reactor environments. Two examples of this, the austenitic Tristelle 5183 and triplex RR2450 were produced by gas-atomisation before undergoing consolidation using hot isostatic pressing. The phase evolution of these alloys during simulated hot isostatic pressing cycles was observed in-situ, using synchrotron X-ray diffraction. During these cycles, the metastability of the gas-atomised powders is revealed, which influences the rate of high-temperature γ→δ transformation within the RR2450 alloy. Additionally, a high-strength silicide phase, named π-ferrosilicide, forms within these alloys. It decomposes by a eutectoid π→δ+M7C3 transformation, demonstrating a high carbon solubility within this phase. The observations of this study demonstrate the need to carefully consider the process parameters during hot isostatic pressing for such complex alloys, since alloy phase transformation rates are heavily influenced by the starting condition of the gas-atomised powder.

Edge crack test methods for AHSS steel grades: A review and comparisons

Due to the economic efficiency of shear cutting, mechanical cutting is used for almost every sheet metal component processed in large numbers. The process parameters recommended both in the literature and in factory regulations generally apply to low- strength, single-phase steel alloys with low ECS. This does not necessarily apply to current types of high-strength alloys, such as those used in body and chassis components for passenger cars or in frame structures of commercial vehicles. As a result, metal working industries and teaching institutes have developed a range of non-standard testing methods to identify the ECS of metals. The ability to prevent edge cracking by the choice of appropriate cutting parameters eliminate the need for costly and time-consuming testing, which in turn reduces machine, tool, material and labor costs. This paper investigates various edge crack testing methods based on current single and multi-phase steel alloys and objectively evaluates them based on multiple criteria. The findings were then linked to the state of knowledge. A particular aspect of this document as compared to a conventional review paper is that the materials used can be regarded as constant across all test methods. This enables an objective comparison and evaluation of the methods used. A simple discussion or comparison of the recorded parameters of the state of knowledge is not expedient due to the large number of metallic alloy systems per material group and the fact that the alloy composition is generally not explicitly documented.

Numerical Simulation and Experimental Analysis of Thermal Cycles and Phase Transformation Behavior of Laser-Welded Advanced Multiphase Steel

This work presents the results of the comparison between simulations and experiments of the laser welding of advanced multiphase steel. The numerical simulations of welding with different parameters were carried out using the Sysweld software. The geometry of the weld and structural constituents as well as stress distributions were analyzed. The simulated thermal cycles were incorporated in the JMatPro software to determine the phase transformation kinetics during cooling. The experimental tests were performed to compare the simulations with the real results. According to them, the shape of the weld and its width were symmetrical according to the weld axis. The simulated values were higher compared to the real one at heat inputs higher than 0.048 kJ/mm. The microstructure investigations conducted with scanning electron microscopy showed a good agreement in the fusion zone. Some differences in the microstructure of heat-affected zone and transition zone were identified. According to the phase transformation simulations, the weld should be composed of only martensite. The real microstructure was composed of martensite and some fraction of bainite in the heat affected zone. The more complex microstructure consisting of ferrite, bainite and retained austenite was present in the transition zone. The results of measured and simulated hardness indicated the good agreement with the difference of 17 HV0.1.

Revealing the interdependence of microstructure evolution, micromechanics and macroscopic mechanical behavior of multi-phase medium Mn steels

The mechanical response, on a microscopic and macroscopic level, and the deformation-induced martensitic transformation (DIMT) were investigated in multi-phase medium Mn steels (MMnS) with 6, 8 and 9 wt% Mn using in situ high-energy synchrotron x-ray diffraction during tensile loading. Prior to the in-situ analysis, a similar heat treatment finishing with an intercritical annealing was imposed on all MMnS. The initial microstructure prior to tensile loading was investigated by electron backscatter diffraction analysis. The volume fraction of austenite (γ) after the heat treatment decreases from 60.2% to 50.7%, and 23.6% with increasing Mn content from 6 to 8 and 9 wt% Mn, respectively. This is mainly due to the difference in the formation of athermal α′-martensite. Athermal ε-martensite also formed in the MMnS with 8 and 9 wt% Mn, whereas no athermal ε-martensite formed in the MMnS with 6 wt% Mn. The alloys have quite different deformation behavior due to the different microstructures, and the majority of the load is carried by the phase that forms a continuous network throughout the steel, which in turn influences the DIMT. These results reveal the importance of assessing both phase-specific strain/stress and the inherent mechanical stability of the austenite in order to predict the macroscopic mechanical properties of the steel. As an example, this is witnessed by the comparison of MMnS9 and MMnS8. Austenite in MMnS9 bears about half the load as compared to austenite in MMnS8 during early deformation due to a continuous network of athermal α′-martensite resulting in significant load partitioning from austenite to α′-martensite. Thus, the mechanical driving force for DIMT in MMnS9 is reduced and therefore causes lower DIMT kinetics in MMnS9 than in MMnS8, even though MMnS9 has lower inherent austenite stability.

Determination of retained austenite in multiphase steels by magnetic force microscopy

Abstract The use of atomic force microscopy (AFM) in the magnetic force mode (MFM) for the determination of austenite (nonmagnetic phase) in duplex and TRIP (transformation-induced plasticity) steels is described. The influence of the sample preparation for the MFM measurements is investigated, showing that a final electropolishing stage is necessary in the case of TRIP-steels. The results of the determination of austenite in the range of 6 – 15% with AFM were compared with those obtained with electron backscattered diffraction (EBSD) and light optical microscopy. For TRIP-steels an identification of retained austenite with MFM was found to show a better resolution than classical light optical microscopy.

Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite.

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

Structure and properties of reinforcing rolled coils by v-alloyed dual-and multi-phase C-Mn-Si-steel

The aim of the work was to study the influence of vanadium doping on two- and multi-phase steels. The schemes of thermomechanically controlled rolling on the wire line of the sectional rolling mill 400/200 for the production of reinforcing wire with a diameter of 6.0 mm in rolls were studied. For the production of reinforcing wire, vanadium alloyed C – Mn – Si steels, which have two-phase (DP – ferritic-martensitic (bainite)) and multi-phase (MP – ferritic-martensitic (bainite) - pearlitic) microstructures, were used. During thermomechanical controlled rolling, TMCR charts were used, including TLH stacking head temperatures of 1024°C to 1063°C. It was established that the applied rolling modes ensure the formation of the MP microstructure. In this 6.0 mm wire, high tensile strength and ductility indicators were achieved in the bends(YS0.2 = 530-550 MPa; TS = 785-885 MPa; El5 = 15.0 - 29.0 %), which fully corresponded specification requirements of national standards such as ASTM A 615 (USA), JIS G 3112 (Japan) and KSD 3504 (Republic of Korea). Keywords: thermomechanical controlled rolling, wire line, reinforcing wire in bays, V-microalloyed C-Mn-Si-steel, microstructure.

Characterizing Localized Microstructural Deformation of Multiphase Steel by Crystal Plasticity Simulation with Multi-Constitutive Law

Characterizing Localized Microstructural Deformation of Multiphase Steel by Crystal Plasticity Simulation with Multi-Constitutive Law

Hierarchically Patterned Multiphase Steels Created by Localised Laser Treatments

Hierarchically Patterned Multiphase Steels Created by Localised Laser Treatments