2,114 research outputs found

    Influence of pre-existing martensite on the wear resistance of metastable austenitic stainless steels

    Get PDF
    The effect of pre-existing martensite on the sliding wear behavior of a commercial metastable austenitic stainless steel was investigated. Two different steel conditions were considered: annealed (with a fully austenitic microstructure) and cold rolled, consisting of mixtures of austenite and martensite. Wear tests were carried out using ball on disc technique at constant velocity and different sliding distances. Correlation between microstructure and wear mechanisms was performed by X-ray diffraction, electron back-scattered diffraction and focus ion beam. Results show that wear resistance decreases at increasing the amount of pre-existing martensite. In this sense, more strain-induced martensite developed for cold rolled samples, hardening the surface and consequently reducing wedge formation, which induced material removal from the surface. The detailed analysis of the wear track demonstrated the formation of an ultrafine-grain layer just below the surface, not only for annealed but also for cold rolled steel.Peer ReviewedPreprin

    Stability of martensite with pulsed electric current in dual-phase steels

    Get PDF
    Softening frequently occurs in dual-phase steels under isothermal tempering of martensite. Recently, non-isothermal tempering is implemented to decrease the softening process in dual-phase steels. Here, we have discovered using high power electropulsing treatment can significantly enhance the strengthening effects via the formation of ultrafine-grained ferrite with nano-cementite particles in tempered martensitic-ferritic steels. To the best our knowledge, electropulsing treatment is a proper candidate to retard even to recovery the softening problems in the tempering of martensite in comparison with other isothermal and non-isothermal tempering methods

    Fundamentals of materials modelling for hot stamping of UHSS panels with graded properties

    Get PDF
    The aim of this study is to develop the fundamentals of materials modelling to enable effective process control of hot stamping for forming UHSS panels with graded properties for optimised functional performance. A selective heating and press hardening strategy is adopted to grade the microstructural distribution of a press hardened component through differential heat treatment of the blank prior to forming. Comprehensive material models, to enable prediction of austenite formation and deformation behaviours of boron steel under hot forming conditions, as well as the dynamic response of a press hardened part with tailored properties in collision situations, have been developed based on experimental investigations and mechanism studies. The research work is concerned with four aspects: feasibility of the selective heating and press hardening strategy, austenite formation in boron steel during selective heating, thermo-mechanical properties of boron steel under hot stamping, and mechanical properties of boron steel with various microstructures at room temperature. Feasibility studies for the selective heating and press hardening strategy were carried out through a designed experimental programme. A lab-scale demonstrator part was designed and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Test pieces were formed under various heating conditions to obtain demonstrator parts having variously graded microstructures. Microstructural distributions in the as-formed parts were determined through hardness testing and microstructural observation. Ultimately, the structural performance of the parts was evaluated through bending tests. Heat treatment tests were performed to study the formation of austenite in boron steel during selective heating. Characterisation of the effects of heating rate and temperature on transformation behavior was conducted based on the test results. A unified austenite formation model, capable of predicting full or partial austenite formation under both isothermal and non-isothermal conditions, was developed, and determined from the heat treatment test results. Hot tensile tests were performed to study the thermo-mechanical properties of the austenite and initial phase (ferrite and pearlite) of boron steel. The viscoplastic deformation behaviours of the both phase states were analysed in terms of strain rate and temperature dependence based on the test results. A viscoplastic-damage constitutive model, capable of describing the thermo-mechanical response of boron steel in a state corresponding to hot stamping after selective heating, was proposed. Values of constants in the model for both the austenite and initial phase were calibrated from the hot tensile test results. Dynamic and quasi-static tensile testes combined with hardness testing and microstructural observation were carried out to study the mechanical properties of press hardened boron steel with various microstructures at room temperature. Based on the results, the strain rate sensitivity of the martensite and initial phase of boron steel was characterised; the relationships between mechanical properties (true ultimate tensile strength, 0.2% proof stress, elongation, and hardness) and phase composition (volume fraction of martensite), for boron steel with various microstructures, were rationalised. Finally, a viscoplastic-damage constitutive model, capable of predicting the mechanical response of a press hardened boron steel part with graded properties being subjected to crash situations in automobiles, were developed, and determined from the test results.Open Acces

    Modelling of phase transformation in hot stamping of boron steel

    No full text
    Knowledge of phase transformations in a hot stamping and cold die quenching process (HSCDQ) is critical for determining physical and mechanical properties of formed parts. Currently, no modelling technique is available to describe the entire process. The research work described in this thesis deals with the modelling of phase transformation in HSCDQ of boron steel, providing a scientific understanding of the process. Material models in a form of unified constitutive equations are presented. Heat treatment tests were performed to study the austenitization of boron steel. Strain-temperature curves, measured using a dilatometer, were used to analyse the evolution of austenite. It was found that the evolution of austenite is controlled by: diffusion coefficient, temperature, heating rate and current volume proportion of austenite. An austenitization model is proposed to describe the relationship between time, temperature, heating rate and austenitization, in continuous heating processes. It can predict the start and completion temperatures, evolution of strain and the amount of austenite during austenitization. Bainite transformation with strain effect was studied by introducing pre-deformation in the austenite state. The start and finish temperatures of bainite transformation at different cooling rates were measured from strain-temperature curves, obtained using a dilatometer. It was found that pre-deformation promotes bainite transformation. A bainite transformation model is proposed to describe the effects of strain and strain rate, of pre-deformation, on the evolution of bainite transformation. An energy factor, as a function of normalised dislocation density, is introduced into the model to rationalise the strain effect. Viscoplastic behaviour of boron steel was studied by analyzing stress-strain curves obtained from uni-axial tensile tests. A viscoplastic-damage model has been developed to describe the evolution of plastic strain, isotropic hardening, normalised dislocation density and damage factor of the steel, when forming in a temperature range of 600°C to 800°C. Formability tests were conducted and the results were used to validate the viscoplastic-damage model and bainite transformation model. Finite element analysis was carried out to simulate the formability tests using the commercial software, ABAQUS. The material models were integrated with ABAQUS using VUMAT. A good agreement was obtained between the experimental and FE results for: deformation degree, thickness distribution, and microstructural evolution

    Cracking and Earing Phenomenon in Deep-Drawn Stainless Steel Alloys: Role of Transformation Kinetics, Microstructure, and Texture

    Get PDF
    The enhancement of formability of advanced high-strength TRIP-assisted steel alloys is a challenging assignment for industrial application due to the cracking phenomenon. The critical factor governing the cracking behavior is residual-stress concentration resulting from the inhomogeneous plastic deformation and microstructural evolution during the forming processes. Martensitic phase transformation kinetics, constituent phases, and crystallographic texture in TRIP-assisted steel alloys are correlated to the microstructure evolution, resulting in phase-specific stress concentration. In the current study, we are aiming at understanding the fundamental mechanisms responsible for the cracking phenomenon and thus improving the formability of TRIP-assisted steel alloys. Four stainless steel (SS) alloys were used in the current study to provide the variables in stability in austenite phase and constituent phases. There are two main objectives: (1) the constitutive behavior of the SS alloy plates during tensile loading, and to provide a basic understanding of the SS alloy behavior, and (2) the correlation of microstructure and its evolution to the cracking behavior in deep-drawn SS alloy cups and formability of the SS alloys during deep-drawing process. Firstly, the effect of phase transformation kinetics, constituent phases, and crystallographic texture on the phase-specific stress partitioning and plastic anisotropy was investigated in SS alloy plates subjected to uniaxial tension using synchrotron x-ray diffraction (S-XRD) and in-situ neutron diffraction. Secondly, the correlation of microstructure and its evolution to the cracking phenomenon and formability of TRIP-assisted steel alloy during deep-drawing process was studied using S-XRD. The results show that the phase-specific stress partitioning behavior is significantly affected by the martensitic phase transformation and constituent phases, resulting in the residual stress concentrating in α’-martensite responsible for the cracking phenomenon in TRIP steel. However, the residual stresses in α’-martensite could be reduced in the duplex TRIP steel due to the local stress partitioning between ferrite and α’-martensite, leading to a better formability in duplex TRIP steel. The textures are correlated to the transformation kinetics, thus influencing the martensite phase fraction and cracking behavior in the TRIP-assisted steel alloys. This study provides the basic idea to improve the formability of high-strength TRIP-assisted steel alloys by manipulating the microstructure to tailor the stress partitioning behavior and plastic anisotropy

    Meso-scale modelling of deformation, damage and failure in dual phase steels

    Get PDF
    Advanced high strength steels (AHSS), such as dual phase (DP) and transformation induced plasticity (TRIP) steels, o er high ductility, formability, and strength, as well as high strength-to-weight ratio and improved crash resistance. Dual phase steels belong to a family of high strength grades which consist of martensite, responsible for strengthening, distributed in a ductile ferrite matrix which accommodates the deformation throughout the forming process. It has been shown that the predominant damage mechanism and failure in DP steels depends on the ferrite and martensite grain sizes and their morphology, and can range from a mixture of brittle and ductile rupture to completely ductile rupture in a quasi-static uniaxial tension test. In this study, a hybrid nite element cellular automata model, initially proposed by Anton Shterenlikht (2003), was developed to evaluate the forming behaviour and predict the onset of instability and damage evolution in a dual phase steel. In this model, the nite element constitutive model is used to represent macro-level strain gradients and a damage variable, and two di erent cell arrays are designed to represent the ductile and brittle fracture modes in meso-scale. In the FE part of the model, a modi ed Rousselier ductile damage model is developed to account for nucleation, growth and coalescence of voids. Also, several rate-dependent hardening models were developed and evaluated to describe the work hardening ow curve of DP600. Based on statistical analysis and simulation results, a modi ed Johnson-Cook (JC) model and a multiplicative combination of the Voce-modi ed JC functions were found to be the most accurate hardening models. The developed models were then implemented in a user-de ned material subroutine (VUMAT) for ABAQUS/Explicit nite element simulation software to simulate uniaxial tension tests at strain rates ranging from 0.001s-1to 1000s-1, Marciniak tests, and electrohydraulic free-forming (EHFF). The modi ed Rousselier model could successfully predict the dynamic behaviour, the onset of instability and damage progress in DP600 tensile test specimens. Also, the forming limit curve (FLC) as well as the nal damage geometry in DP600 Marciniak specimens was successfully predicted and compared with experiments. A hybrid FE+CA model was utilized to predict the major fracture mode of DP600 and DP780 sheet specimens under di erent deformation conditions. This hybrid model is able to predict quasi-cleavage fracture in ultra- ne and coarse-grained DP600 and DP780 at low and high strain rates. The numerical results showed the capabilities of the proposed model to predict that higher martensite volume fraction, greater ferrite grain sizes and higher strain rates promote the brittle fracture mechanism whereas ner grain sizes and higher temperature alter the dominant fracture mechanism to ductile mode

    Advances in Low-carbon and Stainless Steels

    Get PDF
    This Special Issue of Metals was dedicated to recent advances in low-carbon and stainless steels. Although these types of steels are not new, they are still receiving considerable attention from both research and industry sectors due to their wide range of applications and their complex microstructure and behavior under different conditions. The microstructure of low-carbon and stainless steels resulting from solidification, phase transformation, and hot working is complex, which, in turn, affect their performance under different working conditions. A detailed understanding of the microstructure, properties, and performance for these steels has been the aim of steel scientists for a long time. This Issue received quality papers on different aspects of these steels including their solidification, thermomechanical processing, phase transformation, texture, etc., and their mechanical and corrosion behaviors

    Dynamic impact response and corrosion behavior of coarse- and ultrafine-grained AISI 321 austenitic stainless steel

    Get PDF
    The excellent corrosion-resistance of metastable AISI 321 austenitic stainless steel makes it a choice material in the fabrication of nuclear and chemical plants, pressure vessels, automobile and aircraft components, etc. However, AISI 321 is characterized by low-yield strength and poor tribological properties that hinder its widespread application. Therefore, it is important to improve its yield-strength to expand its structural applications without compromising its excellent corrosion resistance. In this study, the effect of grain refinement via cryo-rolling followed by annealing on the strength and corrosion resistance of AISI 321 austenitic stainless steel is investigated. The mechanical behavior of the as-received coarse-grain and refined alloy (fine-grain and ultrafine-grain) were investigated at high (dynamic impact) and low (quasi-static compression) strain rates using the split Hopkinson pressure bar and Instron R5500 mechanical testing machine, respectively. The corrosion resistance of coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) specimens were also investigated using electrochemical methods. Scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), and electron-backscattered diffraction (EBSD) were used for the microstructural and textural characterization of various specimens of the alloy before and after plastic deformation. The optimum thermomechanical process conditions for developing UFG structure in the AISI 321 steel is cryo-rolling to 50 % reduction of plate thickness followed by process annealing at 1023 K (750 ℃) for 600 s (10 minutes). The hardness of the UFG steel specimens is determined to be ~195 % higher than that of the as-received (CG) AISI 321 steel. The developed UFG specimens have strong intensity of ζ-fibre ({110}) texture, which is attributed to pseudo-texture memory effect in AISI 321 steel. The mechanism for pseudo-texture memory in AISI 321 steel is proposed. The yield strength of the UFG AISI 321 steel is ~400 and ~200% higher than those of the CG specimens under both quasi-static and dynamic deformation conditions, respectively. Slip and twinning are the active deformation mechanisms in CG specimens. Both are highly suppressed in the UFG specimens due to spatial restriction effect. During plastic deformation, γ-FCC to martensite (αʹ-BCC) phase transformation occurred, which is more favored in the UFG specimens and at low strain rates. The co-existence of martensitic phase transformation paths with and without an intermediate phase (HCP ɛ-martensite) is confirmed in AISI 321 steel during plastic deformation under both quasi-static and dynamic loading conditions. Irrespective of grain size, Shoji-Nishiyama, Kurdjumov-Sachs and Burgers orientation relationships exist between the γ and ɛ, γ and αʹ, and ɛ and αʹ phases, respectively. Thus, the phase transformation sequence follows both FCC γ → BCC αʹ and FCC γ → HCP ɛ → BCC αʹ path. The stable end-orientation of the austenite phase in compression is [110]||CD texture while that of the martensitic phase is [100]||CD with spread towards [111]||CD texture. Under dynamic impact load, UFG specimens exhibit lower critical strain and strain rate at which shear strain localization (adiabatic shear bands) occurs. EBSD analysis revealed the development of equiaxed ultrafine-grained structure (average grain sizes of ~0.17 μm in CG and ~0.14 μm in UFG specimens) inside transformed shear bands by rotational dynamic recrystallization mechanism. The five strengthening sources that contribute to strain hardening in AISI 321 steel are determined to be: (a) grain boundary strengthening, (b) deformation-induced martensite transformation, (c) deformation twinning acting as a barrier to dislocation motion (d) dislocation-dislocation interactions, and (e) dislocation interaction with titanium carbides. On the stability of the austenite phase in AISI 321 steel, EBSD analyses confirmed the evolution of both thermally- and deformation-induced martensite that is grain size and orientation-dependent. The results of corrosion studies show that the excellent corrosion resistance of AISI 321 steel is not compromised by strength enhancement through grain refinement. Although the presence of TiC particles in AISI 321 is not detrimental to its corrosion resistance, that of TiN particles is

    Influence of a Pronounced Pre-Deformation on the Attachment of Melt Droplets and the Fatigue Behavior of Laser-Cut AISI 304

    Get PDF
    Laser cutting is a suitable manufacturing method for generating complex geometries for sheet metal components. However, their cyclic load capacity is reduced compared to, for example, milled components. This is due to the influence of the laser-cut edge, whose characteristic features act as crack initiation sites, especially resolidified material in the form of burr and melt droplets. Since sheet metal components are often formed into their final geometry after cutting, another important factor influencing fatigue behavior is the effect of the forming process on the laser-cut edge. In particular, the effect of high degrees of deformation has not yet been researched in detail. Accordingly, sheets of AISI 304 were processed by laser cutting and pre-deformed. In the process, α’-martensite content was set to be comparable despite different degrees of deformation. It was found that deformation to high elongations caused a large part of the melt adhesions to fall off, but those still attaching were partially detached and thus formed an initial notch for crack initiation. This significantly lowered the fatigue strength
    • …
    corecore