Von Richards

Three-Dimensional Nonlinear Finite Element Analysis of Hot Radial Forging Process for Large Diameter Tubes

A nonlinear coupled finite element model is developed to predict the behavior of large diameter tubes subjected to mechanical and thermal loadings during hot radial forging process. The model is formulated in a three-dimensional (3D) framework to account for both axial and circumferential effects. This model considers both material and geometric nonlinearities. A rate-dependent material model is used to describe the viscoplastic behavior of the workpiece subjected to high temperature and large strain. A tubular workpiece with the mandrel inside and four forging dies outside is modeled in commercial finite element code. A subroutine is developed and implemented to simplify the modeling process for radial forging simulation. Numerical results presented include residual stress, plastic strain, and temperature distribution along the axial and hoop directions in the deformed workpiece. Results are also presented for contact force to evaluate the performance of the die in the forging process. Finite element model predictions are compared with experimental and two-dimensional (2D) axisymmetric simulation results available in literature. A variety of case studies are conducted for hot radial forging process using the developed 3D model.

First principle study of cobalt impurity in bcc Fe with Cu precipitates

The addition of cobalt was experimentally observed to increase the strength and impact toughness of Cu precipitation hardened steel. In order to understand the mechanism of this strengthening, we studied the effect of cobalt in the bulks and surfaces of bcc Fe and bcc Cu, as well as at the Fe/Cu interface by ab initio density-functional approach. We investigated the cobalt distribution between the Fe matrix and Cu precipitate, and found that cobalt is rejected from the core of the Cu particle. The calculated elastic constants and stacking fault energies show that cobalt does not produce any solid solution softening or hardening in bcc Fe. However, cobalt segregated in the interfacial region increases the cleavage fracture energies and cleavage stress of the Fe/Co/Cu interface. The compressive stress, which arises near the interface due to strong Fe–Co bonds, may serve as a barrier for dislocation motion through the interface resulting in additional hardening.

Corner Strength of Investment Casting Shells

During the investment casting process, the shell is subjected to high internal pressure and thermal stress, particularly during pattern removal and when pouring steel into the free standing ceramic shell. Most testing methods investigate the properties of the ceramic shell in flat regions while cracks typically form in the sharp corners and edge regions. The corners and edge regions have different structure and thickness when compared to flat regions and experience large mechanical stress during processing. In this study, experimental methods were combined with finite element modeling to predict failure stress in the internal corner regions of the shell. The model takes into consideration the mechanical properties of the ceramic shell to determine the stress developed during loading. The effect of shell porosity on stress concentration in sharp corners was evaluated. A general equation was developed to predict the force necessary for crack formation in the shell based on various geometric variables. The results from the model were experimentally verified and the failure stress in flat and corner regions of the shell were compared in order to develop an improved equation.

Thermo-Chemistry of Non-Metallic Inclusions in Ductile Iron

Non-metallic inclusions formed during multi-stage ductile iron melt treatment play an important role in solidification, structure, and casting quality. Professor Carl Loper made fundamental studies on the role of inclusions in heterogeneous nucleation of graphite. In this article, a comparison of inclusions in steel and cast iron was done using thermodynamic calculations (FACTSAGE software) and automated SEM/EDS inclusion analysis (ASPEX system). Statistics of non-metallic inclusions (size, shape, composition) were studied in the iron and steel samples collected by quenching after different stages of melt treatment as well as from castings. The suggested methodology of inclusion analysis has the potential to be applied together with adaptive thermal analysis (ATAS) for solving practical problems such as decreasing shrinkage defects in ductile iron.

Optimization of Melt Treatment for Austenitic Steel Grain Refinement

Refinement of the as-cast grain structure of austenitic steels requires the presence of active solid nuclei during solidification. These nuclei can be formed in situ in the liquid alloy by promoting reactions between transition metals (Ti, Zr, Nb, and Hf) and metalloid elements (C, S, O, and N) dissolved in the melt. Using thermodynamic simulations, experiments were designed to evaluate the effectiveness of a predicted sequence of reactions targeted to form precipitates that could act as active nuclei for grain refinement in austenitic steel castings. Melt additions performed to promote the sequential precipitation of titanium nitride (TiN) onto previously formed spinel (Al2MgO4) inclusions in the melt resulted in a significant refinement of the as-cast grain structure in heavy section Cr-Ni-Mo stainless steel castings. A refined as-cast structure consisting of an inner fine-equiaxed grain structure and outer columnar dendrite zone structure of limited length was achieved in experimental castings. The sequential of precipitation of TiN onto Al2MgO4 was confirmed using automated SEM/EDX and TEM analyses.

Examination of Spheroidal Graphite Growth and Austenite Solidification in Ductile Iron

Microstructures of a ductile iron alloy at different solidification stages were captured in quenching experiments. Etched microstructures showed that spheroidal graphite particles and austenite dendrites nucleated independently to a significant extent. Growth of the austenite dendrite engulfed the spheroidal graphite particles after first contacting the nodule and then by forming an austenite shell around the spheroidal graphite particle. Statistical analysis of the graphite size distribution was used to determine the nodule diameter when the austenite shell was completed. In addition, multiple graphite nucleation events were discerned from the graphite particle distributions. Majority of graphite growth occurred when the graphite was in contact with the austenite. Circumferential growth of curved graphene layers appeared as faceted growth fronts sweeping around the entire surface of a spheroidal graphite particle which was at the early growth stage. Mismatches between competing graphene growth fronts created gaps, which divided the spheroidal graphite particle into radially oriented conical substructures. Graphene layers continued growing in each conical substructure to further extend the size of the spheroidal graphite particle.

Age Strengthening of Gray Iron-Kinetics Study

Kinetics of gray cast iron strengthening at room and elevated temperature (100°C, 182°C and 285°C) were studied using 100 specimens cast from one industrial heat of gray iron. Tensile strength and Brinell hardness was measured during age strengthening. Peak aging was observed at shorter times for higher temperatures and over-aging was observed at 182°C and 285°C. Statistically significant data was used for evaluating aging kinetics and the determination of activation energies for precipitation.Tensile strength-temperature-time curves were described using Arrhenius and Avrami-Johnson-Mehl kinetics and an attempt was made to create a predictive model for age strengthening in gray iron. Earlier literature and a current study with another foundry indicate improvement of machinability with aging. The proposed model will help in optimizing the process for maximum tool life.

Age Strengthening of Cast Irons: Review of Research and Literature

Since 1997, AFS has supported a number of studies to improve the understanding of the age strengthening in gray cast iron. This has resulted in a series of publications and presentations that address aging behavior in a piecemeal fashion. The purpose of this paper is to knit together those pieces in the context of the literature on age strengthening in ferrous alloys. The conclusion reached from these studies is that the age strengthening is a nitride precipitation process described by Avrami-Johnson-Mehl kinetics.

Geological Sequestration of CO2 by Hydrous Carbonate Formation with Reclaimed Slag

The concept of this project is to develop a process that improves the kinetics of the hydrous carbonate formation reaction enabling steelmakers to directly remove CO2 from their furnace exhaust gas. It is proposed to bring the furnace exhaust stream containing CO2 in contact with reclaimed steelmaking slag in a reactor that has an environment near the unit activity of water resulting in the production of carbonates. The CO2 emissions from the plant would be reduced by the amount sequestered in the formation of carbonates. The main raw materials for the process are furnace exhaust gases and specially prepared slag.

Curing kinetics of ceramic slurries used in investment casting with ice patterns

Abstract Ice patterns can be used to make ceramic investment moulds for metal castings. Using ice means that the ceramic slurries must be poured around the pattern and cured at sub freezing temperatures. Success of this process depends significantly on the curing kinetics of the slurries. This paper describes the experimental results of an investigation into the curing kinetics of slurries with differing compositions. The variables considered include volume percent solids, temperature and the volume of the catalyst. Spindle speed was also included as an experimental variable to see if the viscosity exhibited shear rate dependency. The Taguchi method of experimental design was used to reduce the number of trial runs. The curing kinetics of the slurries was studied by measuring the viscosity variation of the slurries with time. The data suggests that there is an initiation and propagation stage and a reactant depletion stage during the slurry gelation process. Three models are evaluated for fitting the time dependence of the viscosity data: a simple exponential in time, a Dougerty–Krieger exponential where there is a defined end point time, and a two stage model developed by superposing a simple exponential for the second stage with an Avrami kinetics model for the first stage. The results indicate that the two stage model can best describe the curing. Significance and effects of the process variables are discussed. For stage one, the characteristic time is strongly dependent upon the amount of catalyst, the spindle speed and the solids loading. For stage two, the characteristic time is strongly dependent upon the amount of catalyst, as well as temperature. A cylindrical ice part is used as an example to investigate the dimensional accuracy. The measured outer diameters of the castings are very close to the nominal outer diameter.