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. LIPING XUE AND MELISSA CARTER, FLOW SCIENCE INC., SANTA FE, NEW MEXICO; ADRIAN CATALINA, ZHIPING LIN AND CAIAN QIU, CATERPILLAR INC., PEORIA, ILLINOIS; AND CHUNSHENG LI, CATERPILLAR INC., CHAMPAIGN, ILLINOIS
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 Cater- pillar. Core gas and porosity defects calculated in the simulations were analyzed and compared with the real casting results. Te gases dissolved during solidi-
P
fication 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.
orosity is a common but serious casting defect. One type of porosity is a result
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. Te 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 solidifica- tion behavior, possible core gas defects in the castings still can be extrapolated.
Solidification Macroshrinkage 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 algo- rithm involves the complete solution of momentum and energy equations. Te time step, controlled by various stability criteria associated with fluid flow, also may be short compared to the total solidification time of the casting. Te latter may be as long as hours for large sand castings. Another shrinkage model based
Fig. 1. The geometry of the casting and riser assembly used in the simulation is illustrated.
on only the solution of the metal and mold energy equations (not fluid flow equations) can predict porosity by evaluating the volume of the solidifica- tion shrinkage in each isolated liquid region at each time step. Tis volume then is subtracted from the top of the liquid region in accordance with the
September 2014 MODERN CASTING | 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70