Predicting, Preventing Core Gas Defects in Steel Castings
Modeling and analyzing core gas evolution and metal solidification behavior can aid in the prediction and prevention of porosity caused by core gas.
L. Xue, M. Carter, A. Catalina, Z. Lin, C. Qiu and C. Li
(Click here to see the story as it appears in the September issue of Modern Casting.)
Porosity is a common but serious casting defect. One type of porosity is a result of core gas that has evolved and become trapped in the casting during solidification. To reduce or eliminate core gas-related defects, detailed information is needed regarding the core gas generation, flow and venting in the core, and the metal flow and solidification behavior in the mold. In a recent study, numerical simulations were conducted based on a prototype design for a steel casting for Caterpillar. Core gas and porosity defects calculated in the simulations were analyzed and compared with the real casting results.
The gases dissolved during solidification can be caused by hydrogen or nitrogen in the initial liquid or core gas decomposed from the sand core and vented to the liquid, and they play a major role in porosity formation in castings. For all the analytical models developed to predict porosity defects in castings, most are based on tracking the evolution of dissolved gases in the initial liquid. Due to the complicated physics involved, modeling the core gas evolution in castings is difficult. However, without the consideration of core gas, predictions of porosity defects are insufficient.
For example, in a previous study, porosity defects in a steel casting were predicted. One of the three main regions that showed porosity in the actual casting was missed in the simulation. The missed porosity region might have resulted from core gases. As the quality requirements of parts become more stringent, the ability to precisely predict defects, including core gas-related defects, becomes more important.
Simulating the casting process involves a wide variety of models, such as fluid dynamics, heat transfer and solidification. A general purpose computational fluid dynamics package might study gas generation and flow and venting in the core but typically will not track the core gas evolution in the liquid metal. By analyzing the core gas venting, metal flow and solidification behavior, possible core gas defects in the castings still can be extrapolated.
Two typical models can be employed to predict macroporosity formation in metals due to shrinkage. A hydrodynamic model can predict the evolution of velocity and pressure in the solidifying metal. Despite being an accurate tool to study the porosity formation phenomena, this model may be computationally costly because at each time step, the numerical algorithm involves the complete solution of momentum and energy equations. The time step, controlled by various stability criteria associated with fluid flow, also may be short compared to the total solidification time of the casting. The latter may be as long as hours for large sand castings.
Another shrinkage model based on only the solution of the metal and mold energy equations (not fluid flow equations) can predict porosity by evaluating the volume of the solidification shrinkage in each isolated liquid region at each time step. This volume then is subtracted from the top of the liquid region in accordance with the amount of liquid metal available in the cells from which the fluid is removed. The top of a liquid region is defined by the direction of gravity. The relevance of this approach is supported by the fact, that in many situations, fluid flow in the solidifying metal can be ignored. Porosity formation in that case is primarily governed by metal cooling and gravity. Feeding due to gravity often occurs on a time scale much shorter than the total solidification time.
When the solid fraction reaches a sufficiently high value for a dendritic structure to exist throughout the bulk of the metal, further liquid flow is impossible without extremely high pressure gradients. The zero flow point is called the solid fraction for rigidity, or the critical solid fraction. For modelling microporosity, it is assumed this last stage of solidification accounts for microporosity.
Microporosity can exist only in a computational element containing a solid fraction exceeding the solid fraction for rigidity. The volume shrinkage in an element is computed from the change in density using a conservation of mass relation. The mixture density is assumed to be a linear function of solid fraction.
According to the model, the maximum shrinkage porosity (volume fraction) possible is equal to the density at the solidus temperature minus the density at the liquidus temperature, divided by the density at the solidus temperature. However, the maximum microporosity is much less, because it is associated only with solidification occurring above the critical solid fraction.
In computational fluid dynamics software, gas is considered ideal and has a fixed composition with a specific gas constant. The specific gas constant can be deduced from experiments in which the gas is collected in a fixed volume apparatus and the gas pressure is measured. The specific gas constant can be computed from the total collected standard volume and the initial mass of the binder.
The density of the core gas simultaneously satisfies the mass transport equation and follows the ideal gas assumption. Because the core gas is compressible, thermal expansion can occur, increasing the flow of the initial gas in the core as the temperature increases, even in the absence of gas sources. The temperature of the gas generated is assumed to be equal to the local core temperature. This is a good approximation because the heat capacity of the solid core material is very large compared to that of the gas.
The exchange of gas at boundaries of the core material is treated as boundary conditions for the core gas model. For instance, if the core surface is exposed to air, then gas may flow across the boundary in either direction depending on the pressure difference. If there is liquid metal at the core surface, gas is allowed to pass out of the core when its pressure is greater than the pressure of the metal at that location, but no metal is allowed to enter the core. If the metal has already solidified at the surface of the core, no gas is allowed to flow across the boundary at that location. At core print surfaces, where a core surface is in contact with another solid part of the mold, gas does not normally flow unless channels have been cut into the mold to allow for venting. For this scenario, the core gas model has an option for allowing venting at the print surfaces.
The geometry of the casting/riser assembly used in the simulation is shown in Fig. 1. The steel casting was poured at a metal temperature of 1,853 K. The metal was prefilled with a uniform pouring temperature to simplify the simulation. The casting weighed 301.6 lbs. (136.8 kg) and measured 28.1 x 8.7 x 9.3 in. (0.715 x 0.22 x 0.235 m).
The core was a polyurethane cold box silica sand core. To simulate an extreme case, no venting was allowed at the print surfaces. The grain size of the core was 180 μm, and the binder weight fraction was 1%. The core and the mold had the same thermal conductivity and density-specific heat, which were temperature dependent and shown in Figs. 2 and 3.
The meshed domain was 43.3 x 25.5 x 23.6 in. (1.1 x 0.65 x 0.6 m). The simulations were set to finish after all metal was solidified. To test the correctness of the parameter settings, the simulations were first run with a rapid solidification shrinkage model.
The porosity defects in the three main porosity regions on the middle-plane longitudinal cross section of the casting are shown in Fig. 4. The calculated result of the rapid solidification shrinkage model is shown in Fig. 4b. As can be seen from the figure, the model provides qualitatively satisfactory results for regions A and B.
The same parameters were used to run full simulations with the first principles shrinkage model and core gas model. For the rapid solidification shrinkage model, total solidification time was 6,040 seconds, whereas the first principles model took 2,500 seconds. The first principles model took less time because the liquid metal convection transfers heat more efficiently. For castings with large cross sections, convective heat transfer has to be considered. If the cross sections are relatively small, rapid solidification shrinkage modeling can produce good results.
As can be seen in Fig. 4c, the first principles model correctly predicted the microporosity in regions A and B, with a smaller porosity area than predicted by the rapid solidification shrinkage model. This is understandable considering the fluid flow in the first principles model. However, like the rapid solidification shrinkage model, the first principles model failed in predicting the porosity in region C. The porosity in region C might have resulted from gases evolved from the core. To verify this hypothesis, the core gas surface mass flux to the metal and the metal solid fraction contour at two different times during the solidification of the first principles model were plotted (Figs. 5 and 6).
Due to the high pressure of core gas, it vents into the metal. However, as the metal cools down, part of the core surface is sealed by the solidified metal. As the solidification front closes in, the core gases that vented to the metal at the final stages will be trapped in the metal, forming microporosity. As demonstrated in Fig. 5, core gases vented to the two liquid pockets. Since the two liquid pockets were isolated by the high solid fraction region, core gases that vented to the lower liquid pocket could not have escaped and were trapped at the solidification front, forming microporosity. This location coincided with the trapped location in region C. Core gases vented to the upper liquid pocket either escaped through the free surface or were trapped inside the metal, forming severe microporosity in the last solidified region, which is region A. Even though the core gas was not tracked explicitly in the metal, the possible locations of the porosity can still be extrapolated based on the core gas venting and metal solidification behavior.
This article is based on the paper “Numerical Simulation of Core Gas Defects in Steel Castings” (14-018)presented at the 2014 AFS Metalcasting Congress.