Metallurgical Model Research Articles

Critical deposition height for sustainable restoration via laser additive manufacturing

Laser material deposition based restoration of high-value components can be a revolutionary technology in remanufacturing. The deposition process induces residual stresses due to thermomechanical behavior and metallurgical transformations. The presence of tensile residual stresses in the deposited layer will compromise the fatigue life of the restored component. We have developed a novel fully coupled metallurgical, thermal and mechanical (metallo-thermomechanical) model to predict residual stresses and identified a critical deposition height, which ensures compressive residual stresses in the deposited layer. Any lower deposition height will result in tensile residual stresses and higher deposition height will result in excessive dilution (substrate melting). We have validated the model using neutron and micro-focus X-ray diffraction measurements. This study highlights that the critical deposition height corresponds to the minimum cooling rate during solidification. It addresses one of the major outstanding problems of additive manufacturing and paves a way for “science-enabled-technology” solutions for sustainable restoration/remanufacturing.

Minimizing the Cycle Time of a Roller Hearth Furnace for Hot-Forming Die-Quenching

In most hot forming die quenching (HFDQ) processes, roller hearth furnace parameters, including roller speed, blank layout, and zone temperatures, are adjusted by trial-and-error. The resulting solution is always suboptimal and often leads to incomplete austenitization. This paper presents an optimization procedure for an industrial roller hearth furnace for minimizing cycle time, structured around a thermo-metallurgical model used to determine the total heat load, dropout temperature, and austenite phase fraction. Imposed constraints include minimum fraction austenite and burner capacity, and the optimization problem is solved using nonlinear programing. The outcome highlights the potential of improving process efficiency through design optimization, and the importance of high-fidelity metallurgical modelling.

Design of the austempering heat treatment of a ductile iron differential case aided by computer simulation

Abstract Austempered ductile iron is frequently employed in the fabrication of automobile parts due to its good mechanical properties, and its simplicity and low cost of the manufacturing process. The heat treatment design is an important stage, in which parameters such as temperatures and durations are chosen according to the required microstructure, part dimension, and initial characteristics of ductile iron. A coupled thermo-mechanical-metallurgical model is employed in this work to facilitate the heat treatment design of a ductile iron differential case. The thermal-mechanical model is solved by using a general-purpose software, and the metallurgical model is implemented into it by means of user-defined subroutines. To perform the austempering heat treatment simulation of the part, experimental results were required to complete some basic parameters of the model. In order to select appropriated values of thermal cycle parameters, the numerical model was employed to analyze the influence of austenitizing temperature, and austempering time and temperature in the microstructure, dimensional change of the part, cooling rate, and minimum required time of the heat treatment to obtain a full ausferritic matrix. A good performance of the computational tool was found by comparison of numerical and experimental results.

Dependency of phase transformation on the prior austenite grain size and its influence on welding residual stress of S700 steel

The austenite grain size (AGS) before decomposition is a crucial factor for the development of microstructure. However, this dependency is seldom discussed due to the difficulty of observing the grain growth of austenite during welding. In the current work, a grain growth algorithm is combined in a thermodynamics-based metallurgical model for the first time to analyse the influence of prior austenite grain size (pAGS). The phase volume fractions predicted at different cooling rates and pAGSs are compared with the experimental results of the continuous cooling transformation (CCT) diagram. To further investigate the influences of pAGS and microstructure on residual stress, experiments of bead-on-plate welding are conducted at three heat inputs, in which plates of S700 steel are operated by the arc welding process. The geometries after welding, chemical composition in the fusion zone (FZ) and the parameters of the double ellipsoidal heat source are calibrated using the software SimWeld. These geometries are imported to ABAQUS to create a Finite Element (FE) model. The validated metallurgical model together with the grain growth algorithm is implemented in the subroutine ABAMAIN to provide a thorough prediction of microstructure. With the knowledge of temperature and phase distributions, a coupled thermo-metallo-mechanical FE model is established to predict the residual stress distributions. The material properties are assigned by interpolating the individual phase property with its volume fraction. By comparing the results predicted by the model assuming constant pAGS, the influence of the pAGS on the residual stress is manifested. Moreover, simulations using overall material properties are also conducted. The stress distributions in the middle of plate surface are plotted along with the volume fractions of product phases to analyse the sensitivity of the residual stress to microstructure.

Effects of Process Related Variations on Fillablity Simulation of Thin-Walled IN718 Structures

Simulation tools have improved significantly and are now capable of accurately predicting mould filling behavior. The quality of prediction is highly dependent on material properties and set-up of boundary conditions for the simulation. In this work material properties were measured and casting conditions were analyzed to accurately replicate the casting process in simulation. The sensitivity of the predictions to minor process variations commonly found in foundries was evaluated by comparing simulation and cast samples. The observed discrepancies between simulation and cast samples were evaluated and discussed in terms of their dependency on process variations. It was concluded that the simulation set-up was capable of reasonable predictions and could replicate the asymmetry of the filling however did not accurately predict the absolute value of the unfilled area. It was discovered that asymmetric flow due to variations in the orientation of the casting mould during filling could have greater influence on the predictions than the actual variation in fill time. The quality of simulation is dependent on equipment and techniques used in the foundry as well as the metallurgical model to simulate the process.

EFFECT OF MICRO-ALLOYING ON QUENCHING BEHAVIOUR OF STEELS FOR BACK-UP ROLLS

The use of micro-alloyed steels for back-up rolls manufacturing gives the possibility to obtain advantages associated with the benefit of the application of micro-alloying elements and thermo-mechanical treatments. In this paper the effect of alloying elements has been evaluated aimed to improve steel hardenability and at the same time to reduce the fabrication cost. 3% Cr and 5% Cr steels are considered with a reduced Mo content. Analysis of alloying on hardenability is performed by means of metallurgical models and on laboratory scale. Results show a higher hardenability in the case of 5% Cr steels. Moreover, such family of steels also show a dependence on prior austenitic grain size. In both the steel families no warnings are detected in terms of residual austenite presence after quenching.

Austempering heat treatment of ductile iron: Computational simulation and experimental validation

The three-step austempering heat treatment of ductile iron parts in this paper is simulated by employing a coupled thermo-mechanical-metallurgical model. The thermo-mechanical formulation is solved in the macroscale using the finite element method to predict the evolution of temperatures and deformations in the whole part. The metallurgical model deals with phase transformations that occur during the heat treatment, which are analyzed in the microscale. For each phase transformation the microstructure features such as type, distribution, and shape and size of phases are considered employing representative volume elements. The performance of the proposed model is tested by simulating experimental heat treatments reported in the literature. By means of comparison with experimental results, the model proved to be able to predict correctly the evolutions of temperature and dimensional changes, the minimum austenitization and austempering times, and the final phase fractions. It is concluded that this model could be a useful computational tool in the design of austempered ductile iron parts as well as the austempering heat treatment process.

A numerical kinematic model of welding process for low carbon steels

A numerical metallurgical integrated model, based on Bhadeshia microstructure model, is developed to predict microstructure development of welding low carbon steels. The new model integrates the thermodynamic kinematics equations and provides the start and finish temperatures during continuous cooling, the transformation kinetics, as well as, the resultant volume fractions of each micro-constituents. Further, it is integrated into finite element (FE) commercial package ABAQUS and the laser welding process of DP600 blanks is numerically simulated. The temperature-dependent thermal properties are adopted to calculate the temperature field and history, which are used as input to describe the kinematics of phase transformation. Knowing the chemical composition in each node, the process of austenization and austenite-to-allotriomorphic ferrite/Widmannstätten ferrite/pealite/bainite/martensite are modelled, respectively. The results obtained using our proposed model are compared with those obtained using Kirkaldy model for S355 steel. Furthermore, the results predicted by both models are compared with experimental data. In addition, the predicted volume fractions are validated using experimental data at selected locations. The proposed thermo-metallurgical model serves as a useful tool to forecast the transformation products during and after welding.

Metallurgical design of micro-alloyed high strength steels for forgings

To fulfill the industrial demand of forged steels with high tensile properties and microstructural requirements coupled with reduced cost, the possibility to increase the properties of C-Mn steels by means of precipitation strengthening as achieved by micro-alloying (and without the addition of expensive elements such as Mo and Cr) has been evaluated. In order to do that, the effect of V addition has been exploited by means of metallurgical modelling followed by a laboratory ingot manufacturing. Heat treatment that has been designed aimed to achieve the target tensile properties. Results show that ASTM A694 F70 grade requirements can be fulfilled by 0.15% V addition and a proper heat treatment in a ferrite-pearlite microstructure, representative of a forged component.

Investigation into the evolution of identification of metallurgical process mathematical models when creating real automatic control systems

Investigation into the evolution of identification of metallurgical process mathematical models when creating real automatic control systems

Temperature Field Evolution during Flash Butt Welding of Railway Rails

Flash butt welding (FBW) of railway rails was investigated in this work. For this purpose samples of R260 rail steel and 60E1 profile were instrumented and subsequently welded on a Schlatter GAA 100 welding machine under industrial conditions. The intention is to gain in depth process knowledge by more accurately depicting thermal cycles for an entire welding sequence in the immediate proximity of the weld as well as in the heat affected zone (HAZ). A detailed characterization of the single stages of the heat up phase of the process is important. Additionally, the secondary welding voltage was measured simultaneously during the experiments to characterize the transient heat input. Moreover, these data were used in the analysis of the temperature signals to better cope with electrical interferences. Additionally, a finite element (FE) model of this FBW process was developed in the present work. Its implementation and solution is realized with the help of ESI’s FE-software SYSWELD. A strong coupled thermo-electrokinetical and metallurgical calculation routine was used. The model comprises the transition resistance at the welding surfaces as the main heat source to the process. Temperature dependent material properties and a corresponding metallurgical model based on an experimental CCT diagram of the rail steel R350HT are implemented in the simulation as well.

A Microscale Model for Ausferritic Transformation of Austempered Ductile Irons

This paper presents a new metallurgical model for the ausferritic transformation of ductile cast iron. The model allows predicting the evolution of phases in terms of the chemical composition, austenitization and austempering temperatures, graphite nodule count, and distribution of graphite nodule size. The ferrite evolution is predicted according to the displacive growth mechanism. A representative volume element is employed at the microscale to consider the phase distributions, the inhomogeneous austenite carbon content, and the nucleation of ferrite subunits at the graphite nodule surface and at the tips of existing ferrite subunits. The performance of the model is evaluated by comparison with experimental results. The results indicate that the increment of the ausferritic transformation rate, which is caused by increments of austempering temperature and graphite nodule count, is adequately represented by this model.

Residual stresses induced by electron beam welding in a 6061 aluminium alloy

Electron beam welding fusion line was performed on 6061-T6 aluminium plates. The thermal histories encountered in the heat affected zone were measured to calibrate a thermal finite elements model. This model has been used as entry parameter of a metallurgical model that predicts the precipitation dissolution induced softening, as well as residual elastic strains. Local deformations measured with a neutron diffraction experiment show a good agreement with the coupled modelling approach for all strain components and the “M” shaped residual strain curves, characteristic of age hardening alloys weld joints, is well reproduced.

FE Simulation Models for Hot Stamping an Automobile Component with Tailor-Welded High-Strength Steels

Ultra-high-strength in sheet metal parts can be achieved with hot stamping process. To improve the crash performance and save vehicle weight, it is necessary to produce components with tailored properties. The use of tailor-welded high-strength steel is a relatively new hot stamping process for saving weight and obtaining desired local stiffness and crash performance. The simulation of hot stamping boron steel, especially tailor-welded blanks (TWBs) stamping, is more complex and challenging. Information about thermal/mechanical properties of tools and sheet materials, heat transfer, and friction between the deforming material and the tools is required in detail. In this study, the boron-manganese steel B1500HS and high-strength low-alloy steel B340LA are tailor welded and hot stamped. In order to precisely simulate the hot stamping process, modeling and simulation of hot stamping tailor-welded high-strength steels, including phase transformation modeling, thermal modeling, and thermal-mechanical modeling, is investigated. Meanwhile, the welding zone of tailor-welded blanks should be sufficiently accurate to describe thermal, mechanical, and metallurgical parameters. FE simulation model using TWBs with the thickness combination of 1.6 mm boron steel and 1.2 mm low-alloy steel is established. In order to evaluate the mechanical properties of the hot stamped automotive component (mini b-pillar), hardness and microstructure at each region are investigated. The comparisons between simulated results and experimental observations show the reliability of thermo-mechanical and metallurgical modeling strategies of TWBs hot stamping process.

Micro-alloyed steels for forgings: effect of nano-precipitation on mechanical properties

The aim of the present paper deals with the production of heavy steel forgings of micro-alloyed steels. Improvements of mechanical properties of forgings up to level by sheets, strips and tubes have been achieved by mean of application of micro-alloying elements and thermo-mechanical treatments. The effect of vanadium addition has been assessed by means of metallurgical modelling step followed by a laboratory demonstration phase by means of ingot manufacturing. Heat treatment has been designed aimed to achieve the desired target tensile properties. Results show that ASTM A694 F70 grade requirements can be fulfilled by 0.15% V addition and a proper heat treatment in a ferrite-pearlite microstructure, representative of a forged component.

Micro-alloyed steels for forgings: effect of nano-precipitation on mechanical properties

The aim of the present paper deals with the production of heavy steel forgings of micro-alloyed steels. Improvements of mechanical properties of forgings up to level by sheets, strips and tubes have been achieved by mean of application of micro-alloying elements and thermo-mechanical treatments. The effect of vanadium addition has been assessed by means of metallurgical modelling step followed by a laboratory demonstration phase by means of ingot manufacturing. Heat treatment has been designed aimed to achieve the desired target tensile properties. Results show that ASTM A694 F70 grade requirements can be fulfilled by 0.15% V addition and a proper heat treatment in a ferrite-pearlite microstructure, representative of a forged component.

Numerical Simulation of Austempering Heat Treatment of a Ductile Cast Iron

This paper presents a coupled thermo-mechanical-metallurgical formulation to predict the dimensional changes and microstructure of a ductile cast iron part as a consequence of an austempering heat process. To take into account the different complex phenomena which are present in the process, the stress-strain law and plastic evolution equations are defined within the context of the associate rate-independent thermo-plasticity theory. The metallurgical model considers the reverse eutectoid, ausferritic, and martensitic transformations using macro- and micro-models. The resulting model is solved using the finite element method. The performance of this model is evaluated by comparison with experimental results of a dilatometric test. The results indicate that both the experimental evolution of deformation and temperature are well represented by the numerical model.

Implementation of a damage evolution law for dual-phase steels in Gurson-type models

This paper is a contribution to the phenomenological modeling of damage evolution in DP steels in the framework of Gurson's approach. It is based on recent results of X-ray tomography in-situ tensile tests and subsequent one-dimensional metallurgical void nucleation models proposed in [C. Landron et al., Scripta Materialia 63 (2010) 973–976]. A macroscopic void nucleation law for DP steels is proposed, covering a wide range of stress triaxialities. The respective effects of nucleation, growth and coalescence are clearly separated. Validations with respect to experimental porosity measurements were performed for several monotonic loading cases and for two loading sequences involving large strains and strain-path changes.

A thermal–metallurgical model of laser beam welding simulation for carbon steels

A coupled thermal–metallurgical model is developed to predict the temperature fields and spatial distribution of volume fraction of phases during laser beam welding of 1020, 1045, and 1060 steels. The classical transient heat conduction model is used to calculate the temperature fields during laser beam welding. For phase transformation, the austenization, the austenite-to-pearlite/ferrite transformation, the austenite-to-bainite transformation, and the austenite-to-martensite transformation are modeled respectively. All of these transformation models are solved by the finite element method (FEM) based on the simulated temperature fields. The thermal properties of the three steels are determined by the linear interpolation base of the phase fractions, and thermal properties for each pure phase. The temperature fields and spatial distribution of phases are predicted by 3D finite element method (FEM) code which is developed by the authors to solve the thermal–metallurgical models. In addition, comparison between the coupled model and the pure conduction model without considering phase transformations is carried out to study the influence of phase transformation on temperature fields during welding. According to the comparison, the temperature of the coupled model is higher than the pure conduction model in the temperature region above 1000 °C, but the temperature profiles are very similar at the temperature region under 1000 °C. The predicted volume fractions of 1020 and 1060 steels are close to experimental results. However, there is an obvious difference between predicted and experimental results of the phase fraction of 1045 steels.

Hot Rolled Coil Property Heterogeneities due to Coil Cooling: Impact and Prediction

The importance of coil cooling conditions on mechanical properties uniformity of HSLA and AHSS steel grades is discussed. It is namely shown that hot rolled coil under conventional industrial production can be cooled non-uniformly. That is why to predict correctly the final steel microstructure and mechanical properties of hot-rolled products an accurate description of not only run-out table condition but also of coil cooling should be done. Two solutions to provide accurate description of coil cooling were tested. First one is to use 2D finite element (FE) thermal model. When coupled with the ArcelorMittal metallurgical model to predict hot-rolled microstructure and properties (TACSI) it matches well the industrial data within +/-15-20MPa both for the tensile and yield strength. However, this approach is recognized to be heavy and time consuming. A second solution, a new 2D coil cooling simplified model incorporated in TACSI model, proved to be quite efficient, as it leads to performances similar to the more detailed first solution. Moreover, it is able to compute the coupling between the thermal evolution of the hot band and the kinetics of phase transformation during coil coiling and cooling, and will enable a better evaluation of the final mechanical properties especially for the grades for which the phase transformation is not completed before hot band coiling.