Centerline Porosity Research Articles

Prediction of Periodic Centerline Porosity and Pulse Marks by CFD and Experimentation for Continuously Cast Copper

For 8 mm OFCu rod, the formation of pulse mark defects on the exterior of the rod and periodic macroporosity (> 1 mm diameter pores) occurring internally along the centerline of the castings have been observed and noted to limit the maximum attainable withdrawal rates (~ 4 m/s). Conversely, the same casting defects have been witnessed for slower rates; therefore, investigations were performed to investigate how the casting motions (withdrawal, dwell, and pushback) formed these defects and could be prevented. Characterization of the physical properties of the cast rods by tensile testing and analysis by optical and computed tomography (CT) imaging revealed correlations to the outer pitch length marks on the rods and confirmed relationships to casting motions and pitch lengths evaluated from metallurgical equations. Computational fluid dynamic modeling using Ansys Fluent v.R1 was applied to quantify the localized formation and dissipation of periodic hotspots internally within the die for the different motions. A mechanism leading to periodic porosity was identified, which was irrespective of the average casting withdrawal rates and enabled prediction of the location, frequency, and magnitude of the macroporosity defect.

Counter-Gravity Sand Casting of Steel with Pressurization During Solidification

Counter-gravity filling and pressurization during solidification have been separately shown to improve casting quality in sand cast steel. Studies and industry practice show that counter-gravity filling increases the yield and reduces reoxidation inclusions. Other studies show pressurization during solidification has the potential to increase the feeding effectivity of risers, which leads to a reduction in centerline shrinkage. In the present study, a method for sand casting steel is developed that combines both practices. Casting simulations are used to determine key events during the process. Counter-gravity filling of a sand mold with liquid steel is performed by linearly decreasing pressure to a value of 0.053 MPa and holding this pressure for 40 s. After the vacuum pressure is released, the entire system is pressurized to a maximum pressure of 0.35 MPa, 220 s after the start of the filling process. The casting method is applied to cylindrical bar castings. Radiographic and dye penetrant inspection of the bars shows no detectable centerline porosity forms when the casting system is pressurized, unlike the gravity-poured control castings.

Casting of aluminum alloy clad strip by an unequal diameter twin-roll caster equipped with a scraper

A single-roll caster equipped with two scrapers and an unequal-diameter twin-roll caster equipped with two scrapers are proposed for casting clad strip without centerline porosity or segregation. Although usually found in roll cast 5182 strip, no centerline porosity or segregation occurred in the 5182 strip, which was the overlay clad strip cast by both types of roll casters. The interface between the 5182 strips and the 3003 base strip was clear. The two strips consisting of layers of the clad strip were bonded strongly at the interface. Both roll casters could cast the clad strip at 30 m/min. This casting speed is much higher than that for the conventional twin-roll caster for aluminum alloys.

Porosity Defect Remodeling and Tensile Analysis of Cast Steel.

Tensile properties on ASTM A216 WCB cast steel with centerline porosity defect were studied with radiographic mapping and finite element remodeling technique. Non-linear elastic and plastic behaviors dependent on porosity were mathematically described by relevant equation sets. According to the ASTM E8 tensile test standard, matrix and defect specimens were machined into two categories by two types of height. After applying radiographic inspection, defect morphologies were mapped to the mid-sections of the finite element models and the porosity fraction fields had been generated with interpolation method. ABAQUS input parameters were confirmed by trial simulations to the matrix specimen and comparison with experimental outcomes. Fine agreements of the result curves between simulations and experiments could be observed, and predicted positions of the tensile fracture were found to be in accordance with the tests. Chord modulus was used to obtain the equivalent elastic stiffness because of the non-linear features. The results showed that elongation was the most influenced term to the defect cast steel, compared with elastic stiffness and yield stress. Additional visual explanations on the tensile fracture caused by void propagation were also given by the result contours at different mechanical stages, including distributions of Mises stress and plastic strain.

Physical modeling and numerical simulation of V-die forging ingot with central void

Numerical simulation and physical modeling performed on small-scale ingots made from pure lead, having a hole drilled through their centerline to mimic porosity, are utilized to characterize the deformation mechanics of a single open die forging compression stage and to identify the influence of the lower V-die angle on porosity closure and forging load requirements of large cast ingots. Results show that a lower V-die angle of 120° provides the best closure of centerline porosity without demanding the highest forging loads or developing unreasonably asymmetric shapes that may create difficulties in multi-stage open die forging procedures.

Influence of Mould and Insulation Design on Soundness of Tool Steel Ingot by Numerical Simulation

The solidification of 4. 4 t cold work tool steel ingot type X210Cr12 was simulated by Magma software. By the reduction of ingot height, solidification modeling and pouring of a new 3 t ingot were performed and decreasing the porosity formation potential in 3 t ingot in comparison with 4. 4 t ingot on the basis of Niyama criterion was observed which was in good accordance with experimental data. In order to produce sound ingot, a new 2. 8 t ingot mould was designed which includes some parameteric changes in mould such as mould slope, slenderness ratio, mould concavity radius, fillet radius of mould internal corners and feeding diameter to ingot upper diameter ratio. Furthermore, the effects of insulating between kokil and feeding ring and also insulating the outer surface of feeding ring as well as insulating the outer surface of one third of kokil upper part on centerline porosity formation were investigated in both 2. 8 and 4. 4 t ingots. The results show that the ingot which was produced in new designed 2. 8 t mould has a better Niyama pattern and the centerline porosities were eliminated.

Open Die Forging of Large Shafts with Defects – Physical and Numerical Modelling

The aim and scope of this paper is centered to analyze the influence of the geometry of V-shaped dies on the closure of internal centerline porosity defects in ingots during multistep open-die forging. The investigation is performed with small scale physical models made from lead using V-shaped dies with 90º and 120º and a reference pair of flat parallel platens. Holes drilled through the center of these preforms are produced to mimic centerline porosity in full scale cast ingots and intermediate rotation of the preforms replicate a multi-stage forging sequence under laboratory testing conditions. The presentation is supported by finite element modelling using an in-house developed computer program and the overall investigation shows that better results in closure of centerline defects are obtained with a V-shaped die with 120º die angle.

Elimination of Hot Tears in Steel Castings by Means of Solidification Pattern Optimization

A methodology of how to exploit the Niyama criterion for the elimination of various defects such as centerline porosity, macrosegregation, and hot tearing in steel castings is presented. The tendency of forming centerline porosity is governed by the temperature distribution close to the end of the solidification interval, specifically by thermal gradients and cooling rates. The physics behind macrosegregation and hot tears indicate that these two defects also are dependent heavily on thermal gradients and pressure drop in the mushy zone. The objective of this work is to show that by optimizing the solidification pattern, i.e., establishing directional and progressive solidification with the help of the Niyama criterion, macrosegregation and hot tearing issues can be both minimized or eliminated entirely. An original casting layout was simulated using a transient three-dimensional (3-D) thermal fluid model incorporated in a commercial simulation software package to determine potential flaws and inadequacies. Based on the initial casting process assessment, multiobjective optimization of the solidification pattern of the considered steel part followed. That is, the multiobjective optimization problem of choosing the proper riser and chill designs has been investigated using genetic algorithms while simultaneously considering their impact on centerline porosity, the macrosegregation pattern, and primarily on hot tear formation.

A Casting Yield Optimization Case Study: Forging Ram

This work summarizes the findings of multi-objective optimization of a gravity sand-cast steel part for which an increase of casting yield via riser optimization was considered. This was accomplished by coupling a casting simulation software package with an optimization module. The benefits of this approach, recently adopted in the foundry industry worldwide and based on fully automated computer optimization, were demonstrated. First, analyses of filling and solidification of the original casting design were conducted in the standard simulation environment to determine potential flaws and inadequacies. Based on the initial assessment, the gating system was redesigned and the chills rearranged to improve the solidification pattern. After these two cases were evaluated, the adequate optimization targets and constraints were defined. One multi-objective optimization case with conflicting objectives was considered in which minimization of the riser volume together with minimization of shrinkage porosity and limitation of centerline porosity were performed.

Morphology and Segregation in Continuously Cast High Carbon Steel Billets

Morphology and centerline macrosegregation in continuously cast high carbon steel billet samples were investigated in order to establish the casting behavior of a six strand curved mold billet caster without electro-magnetic stirring. Several billet samples were collected from the continuous casting shop of Tata Steel, India and experimental observations were correlated with the operating parameters of the caster. Macrostructural examination revealed predominantly columnar structure associated with high degree of segregation and porosities in all billets cast above 21 °C of tundish superheat. Centerline porosity was practically absent in the billet cast below 21°C. These billets showed prominent V-segregation and less prominent centerline segregation. Transition from U-segregation to V-segregation was observed around 21°C superheat. An attempt has been made to study the effectiveness of secondary cooling by measurements of the secondary dendrite arm spacing at various locations in billet samples. Finally, degree of segregation of constituent solute elements were correlated among themselves and applicability of one of the simple segregation models to the centerline macrosegregation has been tested.

CO2 shielding gas effects in laser welding mild steel

The most widely used shielding gases for laser welding of steels are helium and argon. Helium produces significantly more penetration than argon in penetration-mode laser beam welding. Another gas that has been proposed as an alternative to these inert gases is carbon dioxide. The benefits of using carbon dioxide as a shielding gas for laser welding are that it costs less than helium and argon and that it provides nearly the same penetration as helium. The major drawback to its use as a shielding gas for mild steel is that it can cause porosity and other weld discontinuities. In this investigation helium and carbon dioxide were investigated as shielding gases in the laser welding of a fully killed 1018 steel, a semikilled 1018 steel, and an unkilled 1018 steel. Helium was generally found to produce discontinuity-free welds, although small quantities of very fine centerline porosity were observed in the fully killed and semikilled steels. Carbon dioxide was found to provide an average of 70% of the penetration obtained with helium. The surface appearance and formation of porosity in welds made using carbon dioxide shielding was found to be dependent on the amount of deoxidizing element in the weld pool. While porosity-free welds were formed in the semikilled steel (0.22% Si), low silicon or other deoxidizer content resulted in poor surface appearance and entrapped porosity in both the fully killed and unkilled steels in this investigation, presumably due to the formation of carbon monoxide in the weld pool. The addition of ferrosilicon powder to the weld pool effectively controlled the formation of carbon monoxide in these welds and resulted in porosity-free welds with carbon dioxide shielding. The results of this investigation indicate that carbon dioxide can be used as a cost-effective shielding gas for penetration-mode welding of carbon steels. It was found that sufficient levels of deoxidizing elements must, however, be available in the molten pool to control porosity.

Centerline porosity in plate castings

Centerline porosity in horizontally cast steel plates has been examined analytically. The presence and extent of porosity has been calculated on the basis of heat flow in the system and interdendritic fluid flow. The calculated distribution of porosity was found to agree closely with measured values reported in the literature. Several solidification parameters have been proposed to indicate the onset of porosity. The present investigation shows that the parameter G/R1/2 is the best of the parameters suggested, where G andR are the centerline temperature gradient and growth velocity, respectively.

Improved Melting and Casting Procedures for a Cobalt-Base Magnetic Alloy

One component of a very intricate central station telephone switching system contains a hard magnetic alloy of the composition cobalt-12 wt% iron-4 wt% gold. Problems were encountered during the final stages of the fabrication of the alloy to final form (0.00050 in.×0.0054 in. tape) and during subsequent assembly of this product. Extensive failure analyses were performed to identify the nature of the problems, and for the most part, the causative factors were traced to the induction melting and casting practices employed for initial synthesis of the alloy. The major problems addressed during the melting and casting research were inclusions and centerline porosity in the ingots prepared for subsequent fabrication. Since the most likely source of inclusions can be related to melting crucibles, sprues, runners, gates, etc., initial attention was directed toward these areas. Sprues, runners, and gates were eliminated, and high-density, stable crucible was employed. Massive copper molds were used and ceramic hot tops were eliminated. Although several molding variables were investigated, centerline porosity (secondary piping) was eliminated only by very controlled pouring rates to effect directional solidification. In conclusion some final yield data that relate to the improved vacuum melting and casting techniques developed are presented.