Foundry castings of cast iron workpieces are produced in molds made out of sand bonded with resin and shaped by wrapping the desired pattern with the resulting mix, either manually or using automated or semi-automated machinery. The cast iron process makes use of sand cores to produce intricate part shapes or hollow parts. Iron castings are used across a broad range of industries such as the automotive industry where cast iron is used for the engine block, in agriculture where much of the heavy machinery is produced in cast iron and in the production of rolling mills. Following is a summary of the cast iron categories and methods of production and a brief explaination on how simulation based engineering is revolutionizing this timeless industry.
Some of the more notable characteristics of cast iron are its lower melting point than steel which makes it cheaper to produce, its good wear resistance, its excellent machinability (with the exception of white cast iron); its tensile strength (which does vary for the different types of cast iron), and the ease with which intricate designs can be produced.
Whereas the first iron castings date back several centuries BC, the modern element composition of cast iron is a much more recent invention. Today, a number of different types of cast iron or iron alloys are produced; each one of them is classified by its graphite morphology which is a direct result of the melt inoculation methods used and the resulting solidification rates. Among the different categories of cast iron we can distinguish:
- Grey cast iron also known as lamellar graphite cast iron which is produced from pig iron (scrap iron or steel is often used too) and carbon and is economical to produce because of its low melting point. The principle elements of grey iron are carbon, which gives it its lamellar graphitic microstructure, and silicon. The combined effect of the two elements on the microstructure is known as the carbon equivalent (CE). Two other essential elements are manganese and sulfur which have their own optimum ratios established for high quality grey iron. The flaky nature of the graphite gives grey cast iron a lower ultimate tensile strength (UTS) than other cast iron types but gives it a higher thermal conductivity. Grey cast iron has virtually no elongation. Although it is possible to heat treat grey castings, they are mostly used in their as-cast condition
- Ductile cast iron which uses a carbon equivalent ratio that produces spheroidal graphite, which causes less surface tension than lamellar graphite, and has a dampening effect on tensions. Ductile iron has superior material characteristics to grey iron with the added benefit of its ductility. Its production requires the addition of some rare earth minerals. As the graphite precipitates during the solidification process ductile iron expands slightly
- Compact graphite iron (CGI) also known as vermicular graphite which is characterized by the shape of the graphite which falls somewhere in between the flakes of the graphite in grey iron and the spherical shape of that in ductile iron. Its compressed graphite shape gives it a greater tensile strength than grey iron. The process of making CGI is very similar to that of ductile iron; its nodularity depends on its Mg content, inoculation and the conditions under which it is cooled
- White or cementite cast iron which differs from the other cast iron types in that it uses carbon in the form of carbide and not graphite. It is produced with an accelerated cooling process with the use of metal inserts. The resulting cast iron is not easily machineable, is mainly used to manufacture small parts and does not have a broad industrial use
There are two types of furnaces that can be used to produce cast iron:
- an induction furnace: this type of furnace relies on the effect of the eddy current to raise the metal temperature until it liquefies
- a cupola furnace: this is a cylindrical furnace which melts the metal by igniting layers of coke and then allowing the entry of pressurized air. When the furnace is heated to a high enough temperature the solid metal liquefies
Iron casting can be prepared in several different types of production plants such as:
- plants where sand molds are produced manually using patterns. Normally handmade mold casts are used for the production of very particular and large workpieces
- semiautomatic manufacturing plants where medium sized parts are produced in small production runs that are cast with a ladle
- automated production plant where long runs of small to medium cast iron parts are produced
- disamatic plants that use disamatic molding machines to produce flaskless molds for vertical casting
Simulation Based Engineering for Iron Casting
The quality of a cast part depends largely on the composition of the iron and the filling and solidification processes of the molten iron alloy, both of which are influenced by the product process design. When a workpiece is cast for the first time it is hard to obtain an excellent quality part in the initial castings. This is why carrying out a virtual simulation of the workpiece casting at the design stage will decrease the number of prototypes needed, save time and allow the end product to come to market faster.
A fluid-dynamics simulation of the casting process can identify:
- the effects of turbulence that can be the cause of the formation of oxides and inclusions
- the excessively high velocity speeds at the start of the casting that can cause the formation of foam as well as erode the sand cast itself
- vortices that are the cause of air entrapment
Using software modules specifically programmed to analyze the microstructure of iron cast it is further possible to asses:
- the transformation of the graphite based on the solidification process used and the type of iron
- the yielding effect on the molds
- the effect of humidity in the molds
- the sequence of phase changes until the workpiece reaches the temperature of the final phase
- the micro-structural characteristics and mechanical properties of the workpiece after the final casting phase
What are the Advantages of Virtual Simulation in the Design Phase of an Iron Casting?
The main purpose of virtual simulation is to improve workpiece quality while reducing manufacturing costs and time-to-market of the finished product. The way to achieve this is to produce a complete virtual engineering study from the design phase through its entire production process. This is made possible by passing the parameters output from the manufacturing process phase (tensile strength, microstructure and mechanical characteristics of the workpiece) to the FEM code for a structural analysis.
Thanks to these analyses several improvements can be made to the process design, such as designing a reduced size sprue and reduce the number of feeders by changing their positions. Other improvements could include an optimized drag and cope design as it relates to the filling, solidification and cooling of the molten alloy.
Some of the advantages of using virtual simulation in the design phase of an iron casting are:
- reduced scrap iron
- fewer prototypes
- fewer design changes following the initial prototype
- increased manufacturing efficiency
- faster time-to-market for the end product
- reduced costs resulting from a reduction in raw materials, labor and energy
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