Semi-solid Steel Research Articles

Effect of compression temperature on deformation of CaO–CaS–Al2O3–MgO inclusions in pipeline steel

Plain strain deformation trials were carried out on samples of pipeline steels at 1473 K, 1673 K and 1723 K, respectively. The deformation of inclusions in the solid steel at different temperatures and in the semi-solid steel was studied. The composition of inclusions changed from 60.62%Al2O3–16.10%CaO–10.27%MgO–13.01%CaS before deformation to 60.59%Al2O3–13.03%CaO–12.74%MgO–13.63%CaS, 59.69%Al2O3–7.04%CaO–11.51%MgO–21.76%CaS, and 68.26%Al2O3–22.56%CaO–6.68%MgO–2.5%CaS with corresponding deforming temperatures of 1473 K, 1673 K and 1723 K. While the average aspect ratio of inclusions increased from 1.28 to 2.23, 1.32, and 1.35, respectively. Thermodynamic calculations performed by FactSage 7.0 verified the composition transformation from CaO to CaS during the solidification and cooling process of the steel. A kinetic model was used to calculate the dynamic transformation of the inclusion composition at compression temperatures. The inclusion transformation ratio from CaO to CaS increased from 38.28% at 1473 K to 50.50% at 1673 K. For the deformation in the solid steel at the temperature below 1673 K, the thickness of the hard phase CaS increased with the soaking temperature, while the hardness of the steel matrix decreased. The larger hardness difference between inclusions and the steel matrix led to a higher aspect ratio of inclusions after deformation. For the deformation in the semi-solid steel, the small difference of hardness between the soft inclusion phase and the soft steel matrix resulted in a low aspect ratio of inclusions in the semi-solid steel after deformation.

Near Solidus Forming (NSF): Semi-Solid Steel Forming at High Solid Content to Obtain As-Forged Properties

Near solidus forming (NSF) of steels is a novel process under the umbrella of semi-solid forming technologies midway between classical hot forging and semi-solid technologies. This article presents the work done at Mondragon Unibertsitatea to develop this technology and demonstrates the great potential of the NSF process. The study proves the capability of the process to reduce raw material consumption by 20%, reduce forming loads from 2100 t to 300 t, and reduce forming steps from three to one, to obtain as-forged mechanical properties, as well as the excellent repeatability of the process. The work demonstrates that manufacturing commercial steel components in a single step using several off-the-shelf alloys is possible thanks to the flowing pattern of the material, which enables near-net shaping. In the first part of the article, a general overview of the semi-automated near solidus forming cell, together with a description of the NSF manufacturing trials, is provided, followed by the presentation and discussion of the results for the selected steel alloys.

Wear and Corrosion Behavior of Functionally Graded Semi-Solid 9Cr18Mo Steel

A kind of 9Cr18Mo multidiameter shaft part was fabricated through a designed thixoforging set-up. As compared with conventional heat treatment specimen, wear and corrosion behavior of 9Cr18Mo thixoforging specimen were studied. The results showed the surface of 9Cr18Mo thixoforging specimen demonstrated fine wear and corrosion resistance. Thixoforging specimen exhibited functionally graded property with inner austenite particles and fine dendrite eutectic skin layer. At the skin layer, the former solidified liquid led to the concentration of alloying elements such as Cr and Mo. Besides, eutectic structure of secondary austenite and M7C3carbide helped to improve the hardness. The high hardness of M7C3carbide and high concentration of Cr, Mo elements at the edge skin layer contributed to reducing the delamination wear, abrasive wear and the appearance of oxidation. Even at higher wear load, relative sliding occurred at the wear surface, which avoided crack formation and chunk spalling. As compared with conventional heat treatment specimen, corrosion current density of thixoforging surface reduced, while corrosion potential increased. Thixoforging surface also exhibited passivation behavior and the corrosion resistance increased slightly. High concentration of Cr and Mo at the skin layer avoided the formation of chromium depleted zone. In view that wear and corrosion failure tend to occur at the surface of specimen, the thixoforging specimen with a wear and corrosion skin layer might provide a possibility for expanding the application fields of 9Cr18Mo steel.

A new route for semi-solid steel forging

Forging in semi-solid state significantly extends the possibilities of classical hot forging. In order to fully exploit its potential, the process requires a specific and demanding environment, penalizing its industrial deployment. In this context, an alternative route is proposed. In the proposed process, semi-solid zones at the heart of the material coexist with surrounding solid zones within the part. The outcome is an optimized process where the benefits of thixoforging are reached at a significant extent within the classical process framework of hot forging. The paper investigates this proposal up to a full-scale proof-of-concept in an industrial setting.

Dilatancy in semi-solid steels at high solid fraction

The study of mushy-zone deformation in steels is important for limiting defect formation in continuous casting. Here, we use in situ synchrotron radiography to quantify the shear deformation mechanisms of an equiaxed carbon steel at a solid fraction >0.9 and to understand how these mechanisms lead to casting defects. We show that the grain assembly undergoes shear-induced dilation (Reynolds' dilatancy) which opens liquid-filled fissures and cracks at high solid fraction. We further show a complex interaction between grain rearrangement, grain deformation and local coarsening, where rearrangement reduces the grain-grain contact area and coordination number which alters the stress network and also drives coarsening as grains move apart.

Crystallization and Ripening Phenomena in Semi-Solid Steels

Rheological properties, crystallization and ripening phenomena have been investigated for three different steel grades in the semi-solid state. For each steel grade detailed investigation has been performed via differential scanning calorimetry to quantify the solid fraction as a function of temperature. The experimental procedure for all samples has been as follows: the completely liquid samples were cooled with a constant rate under constant shearing to temperatures corresponding to solid fractions of 30 and 40%. Tracking the viscosity during this procedure allowed analyzing the crystallization kinetics and final ripening process, which differ for the different steel grades. Evaluating the viscosity data on the base of a combination of convection dominated ripening model and a modified Krieger-Dougherty equation gives valuable information of the microstructural evolution. This is validated by additionally quenching samples.

Simulation the flow of semi-solid steel alloy using an enhanced model

Simulating steel semi-solid metal forming close to a real industrial forming is necessary to reduce potential run-time errors and costs. Simulating requires modeling of semi-solid behaviour. In this work a model based on micromechanical analysis with consideration the viscoplastic response along with an enhanced Herschel-Bulkley model has been proposed to predict the flow behaviour of semi-solid M2 high speed steel. This model is introduced to overcome the limitations of the previously used models. The extracted model parameters from the steady state flow stress and step-change of shear rate experiments were then calculated and fitted to the experimental rheology results of continuous cooling. The model was then implemented linking into the computational fluid dynamics software (FLOW 3D Ver.9.3). A T-Shape die used to investigate the flow front of semisolid alloy. Also the extracted parameters at low shear rate in the step change experiments used for simulation of the compression process of M2 tool steel at high solid fraction by the proposed model and in the ABAQUS 6.9 software. The experimental results of compression test shows a well agreement with simulations results indicative of the performance of the proposed model to predict rheological properties and flow behaviour of semi-solid states especially for the tool steel systems in the wide range of liquid fraction.

Web System Dedicated to Parallel Computation for Modeling of Mushy Steel Deformation

Abstract The paper presents web base system for an application of parallel object-oriented programming technique in modelling of rolling process of steel plates with semi-solid zone. It also throws light on the problem of semi-solid steels yield stress relationship, one of the main input data of the simulation, and on application of inverse solution, the only possible method of development of the stress-strain curves at extremely high temperatures. Due to limitations of available computer resources a very accurate computation can sometimes be impossible or the time performance can be a barrier for practical application of complex sequential models. Taking advantage of parallel computing the authors have developed an algorithm allowing for fast computation using multiple processors, which is the main subject of the presented paper.

Thermomechanical Modelling and Simulation of C38 Thixoextrusion Steel

The present paper focuses the modelling and the simulation of a direct thixoextrusion test achieved on C38 semi-solid steel. Many parameters related to thermal, mechanical, material features are involved but are currently unknown. Consequently to validate the modelling and the simulation, it is important to get various experimental informations during the test and to correlate them with simulated results. In a previous paper (Becker et al, 2008), the force-displacement curve, the temperature within the die, the macro and micro structure obtained for different process parameters during thixoextrusion of C38 were investigated. In this work, those results are correlated to those obtained by simulations of the processing. The simulations were performed using the commercial software Forge®. The thermal modelling is based on the heat equation and the thermal boundary conditions involving the heat losses, the thermal conduction within the semi-solid slug and the die and the plastic dissipation as heat source. The latent heat associated to the liquid-solid phase transformation is not considered here. The constitutive equation of the material is given by a multi-scale modelling based on micromechanics and homogenization techniques, labelled as micro-macro modelling (Favier et al, 2009). Friction is modelled using the usual modified Tresca equation. The parameters of the model are determined (i) using literature results and (ii) to match various experimental measurements obtained during the test and described in Becker et al (2008) such as the die temperature during the test and the load-displacement curve. Comparisons between experimental and simulated reveal the presence of complex temperature field and the presence of zones having very low viscosities. These zones contribute actively to the semi-solid material flow.

Multiscale Modelling of Mechanical Properties of Steel Deformed in Semi-Solid State – Experimental Background

The paper reports the results of experimental and theoretical work leading to the construction of a multiscale mathematical model describing the phenomena accompanying the steel deformation in semi-solid state as well as at extra-high temperatures. Conducted experiments and simulations confirms the need to seek new methods to obtain precise characteristics in the context of detailed computer simulations. The investigations presented in the current paper has shown, that temperature distribution inside the controlled semi-solid volume is strongly heterogeneous and non-uniform. Axial-symmetrical model (core of the old methodology) does not take into account all the physical phenomena accompanying the deformation. Finally, the error of the predicted strain-stress curves can still be improved. The proposed solution of the presented problem is application of both fully three-dimensional solution and more adequate solidification model taking into consideration evolution of forming steel microstructure. Contrary to the current model the new approach should allow to better capture the physical principles of semi-solid steel deformation in micro-scale. Additionally, the new complex methodology should allow to transfer the characteristics of the material behaviour between the micro-and macro-scale. As a consequence the final results should be more precise and accurate.

Flow behaviour and rheological properties of M2 high-speed steel in semi-solid state

In this research, the rheological properties and flow behaviour of M2 high-speed steel have been investigated in low and high solid fractions by compression and rheometry tests. First series of experiments were started by partial remelting of rolled-annealed M2 billet in an argon-controlled atmosphere and holding for an appropriate time to obtain a globular microstructure and then conducting the rapid compression test on the prepared sample. Load versus deformation data were recorded using a high-frequency data acquisition system during the tests. Deformation mechanism of the semi-solid steel alloy and its correlation to rheological properties were then investigated. In the second series of experiments, the material was sheared continuously while cooling down from liquidus state to a semi-solid temperature. The rheology tests were performed using a self-developed concentric cylindrical viscometer under different cooling rates (25 °C/min–35 °C/min) and shear rates. The results revealed that when solid fraction exceeds about 40%, the viscosity increases suddenly and reaches a viscosity of about 20 Pa s. This solid fraction is a critical point when designing the process for industrial applications such as thixocasting.

An Enhanced Herschel–Bulkley Model for Thixotropic Flow Behavior of Semisolid Steel Alloys

An enhanced model based on Herschel–Bulkley model has been proposed. Modifications are based on the attempts to overcome the limitations of the previously used models. The proposed model predicts thixotropic time-dependent flow behavior at medium shear rates, while reaching toward Newtonian viscosity at higher shear rates, which is a typical behavior of semisolid metal slurries. Rheology tests on M2 high-speed steel were then conducted using a self-developed high-temperature Searle-type rheometer for three different solid fractions. All of the tests were performed in an electric furnace with an argon-controlled atmosphere. Two series of experiments were performed to evaluate the rheological behavior of the material: steady-state flow stress experiments to determine the equilibrium flow curves, and step-change of shear-rate experiments to determine the time-dependent characteristics of the material. The model parameters were then derived using the experimental results and calibrated by curve fitting of experimental data. The model was then linked to a commercial CFD software, and simulation of the process was conducted to evaluate the model. The results show that the model fits well with the experimental data and is capable of simulating a wide range of shear rates compared with typical Herschel–Bulkley model.

In situ study of granular micromechanics in semi-solid carbon steels

The granular micromechanics of semi-solid steel at ∼80% solid are studied by synchrotron radiography. A particulate soil mechanics approach to image analysis shows that deformation occurs by the translation and rotation of quasi-rigid grains under the action of contact forces, and that the changes in directional fabric and grain–grain contacts occur by mechanisms similar to those of highly compacted soils including “locked sands”. Grain-scale phenomena are then linked to the macroscopic displacement and strain fields and it is shown that shear-induced dilation is a fundamental response at both the grain and macro scales. Based on this, recommendations are made on future rheology experiments.

The Influence of Reheating Profile on the Final Microstructure of the Semi-Solid S600 and K100 Tool Steels

Semi-solid processing of steel alloys has attracted the attention of researchers recently. Many attempts have been carried out for the industrialization of the process. High temperature nature of the steel semi-solid processing compared to other semi-solid alloys has been a problem in studying different process parameters like reheating parameters and microstructures continuously and simultaneously. In this research in-process and final microstructures of semi-solid steel alloys M2 and K100 under different reheating cycles and different parameters have been studied. All experiments are carried out in a resistance furnace and in an argon controlled atmosphere to avoid any oxidation and decarburization. During the reheating cycle, surface and core temperature of the samples have been recorded and compared with the oven temperature. The result of such a comparison can be used in other experiments where there is no access to sample for its temperature control or it is very difficult to measure the sample temperature like in rheology tests. Partial reheating and quenching has been used to determine the in-process microstructure of the alloy. For each set of experiments particle shape factor has been derived and its relation to other parameters has been studied. Finally appropriate reheating cycle and parameters for each alloy has been determined.

The Rheological Behavior of HS6-5-2 Tool Steel for Non-Isothermal Processing

The rheological behavior of semi-solid tool steel is investigated using a conventional rheometer under cooling rate and shear rate. The processing of steels in semi-solid state is still at development stage due to the high processing temperature involved. The candidate grades of steel for thixoforming are high speed, high carbon, stainless steel and ductile iron. TheHS6-5-2 tool steel has a wide semi-solid range of over 200 C and the required spheroidal microstructure achieved by partial re-melting from as supplied state without any mechanical agitation, indicated the potential of this grade to be investigated in semi-solid processing. For non-isothermal processing an increase of solid fraction related to solidification due to thermal exchanges is observed. In the first series of experiment after melting the feed stock in an alumina cup, the alloy is sheared continuously by a rotating rod with a constant shear rate under three different cooling rates to the 55% corresponding liquid fraction. In the second series of experiment three different shear rates were applied under a constant cooling rate. The apparent viscosity of the slurry as a function of cooling rate and solid fraction is calculated from the applied torque to the outer cylinder. The results show that the slurry viscosity decreases with increasing shear rate and increases with increasing cooling rate and solid fraction. Additionally for a given solid fraction, shear thinning behavior is observed.

High Temperature Flow Behaviour of Semi-Solid Steel Alloys

Semi-solid processing is an attractive and an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. Its key is the spheroidal solid particles suspended in a liquid matrix which in turn eventuate in a complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during semi- solid forming processes is still neither completely characterized and nor fully understood, especially for high melting point alloys like steels. Many attempts have been made for understanding the time- dependent flow behaviour of semisolid materials which is essential for the simulation of semi-solid processes. During processing, the structure of a semi-solid material changes with the processing history due to agglomeration and dissagglomeration of particles or particle–particle interactions. This kind of processing condition can be experimented using rotary rheometers. In the case of steel alloys, conventional rheometers cannot be directly used because of their high temperature processing conditions. For this purpose, a self-developed rotary rheometer has been employed to study the rheology of steels up to 1500 oC. Steel samples are melted in an Alumina cup and sheared with an Alumina rod in a resistance furnace under isothermal condition and also under argon controlled atmosphere. The torque and rotary speed signals are used to calculate the viscosity and shear rate data. For rheology investigations, shear rate jump experiments and rest experiments have been carried out using the rheometer for different steel alloys. The flow of SSM is modelled using the Herschel–Bulkley fluid model. The viscosity and the power-law index are assumed to be functions of the solid volume fraction and a structural parameter that changes with the processing history. The evolution of the structural parameter is described by a first-order kinetic differential equation. The model is in a good agreement with the experimental results which can then be used for simulation purposes.

Crystalline <I>γ</I>-Al<SUB>2</SUB>O<SUB>3</SUB> Physical Vapour Deposition-Coating for Steel Thixoforging Tools

The process of thixoforming, which has been part of many researches during the last decades, combines the advantages of forging and casting for the shaping of metallic components. But due to the high temperatures of semi-solid steel alloys high demands on the tools are requested. To resists the thermal and mechanical loads (wear, friction, thermal and thermomechanical fatigue) protecting thin films are necessary. In this regard crystalline gamma-Al2O3 deposited via Physical Vapour Deposition (PVD) is a promising candidate: It exhibits high thermal stability, high oxidation resistance and high hot hardness. In the present work the application of a (Ti, Al)N/gamma-Al2O3 coating deposited by means of Magnetron Sputter Ion Plating in an industrial coating unit is presented. The coating was analysed by means of Rockwell test, nanoindentation, and Scanning Electron Microscopy (SEM). The coated tool was tested in thixoforging experiments with steel grade X210CrW12 (AlSI D6). The surface of the coated dies was examined with Scanning Electron Microscope (SEM) after 22, 42, 90 and 170 forging cycles.

Thixotropy in Semisolid Steel Slurries under Rapid Compression

Thixotropy in Semisolid Steel Slurries under Rapid Compression

Direct observation of deformation in semi-solid carbon steel

The dynamics of globule motion are studied in situ during the deformation of globular semi-solid steel (Fe–2C–0.9Mn–0.5Si in mass%). Samples with globular morphology and 55–65% effective solid fraction were deformed in direct shear at a shear rate of ∼10 −3 s −1 during time-resolved synchrotron X-ray imaging. Globule rearrangement involving dilation was the main deformation mechanism. Agglomeration is shown to significantly influence rearrangement by increasing the effective size of structural units.

Near‐net shaping of geometrically complex diving knife by semisolid steel casting using ceramic mould system

A steel thixocasting experiment was carried out using self‐heating alumina moulds operating at ∼1000°C. An industrially manufactured diving knife was selected as workpiece geometry to examine the ability of near‐net shaping complex steel structures by semisolid processing. Results show that a significant reduction in machining effort can be achieved, and engineered surfaces of a characteristic size of at least less than 30 μm can be generated in situ without additional surface treatment. It is concluded that the intended decrease in forming forces and the increase in shape complexity in comparison to forging can be accomplished using highly preheated ceramic moulds. Considerable construction work on tool implementation has to be carried out to improve performance of the alumina moulds with respect to the applied process forces.