Metal Castings
How do we get molten steel into a form that we can use, and what is the best way of doing this in this module?
We will try to answer these questions by looking at casting.
In steelmaking, casting is required to solidify the molten steel ready for shaping or forming into a suitable profile.
Types of Casting
The three main ways of doing this are sand casting, continuous casting on Ingot casting.
Sand casting.
Sand casting is a process for making apart by pouring molten metal into a mold, having a cavity of the desired shape, and then allowing it to solidify
in the sand-casting sand mixed with water and suitable binders is the molding material. The molding material is packed around a pattern often made of wood. This pattern is slightly larger than the finished part due to the metal’s contraction on solidification and cooling.
When the pattern is removed from the molding material, a cavity having the same shape is the pattern remains.
The sand must be packed or round to the correct degree to avoid air or water vapor being trapped in the molten metal causing and soundness.
The cavities are usually created in two or more sections that are then fitted together to create a mold; the mold will have one or multiple holes in which to pour the molten metal.
There will be channels to direct the molten metal. There will also be a reservoir next to the cavity. When the molten metal in the cavity solidifies and consequently shrinks, any voids will be immediately filled with more molten metal from this reservoir.
The metal of the desired composition is heated well above its melting point to ensure good fluidity. If this is not done, the molten metal may solidify in the mold and consequently be unable to completely fill up.
Many casting grades of steel have higher silicon levels, as this helps to increase the fluidity of the molten metal.
After the metal has solidified, the finished casting could be shaken or knocked out of the mold, which can only be used once.
Advantages of Sand Casting
The advantages of sand castings include low cost, high flexibility.
Virtually any size casting can be produced, and it is a simple, expedient way to make a wide range of products.
Continuous Casting
Continuous casting, usually continuous casting, uses feedstock from the basic oxygen still process. A ladle of steel is teamed or pulled through a gas type of factory nozzle into a ton dish with special weirs and dams to aid in removing impurities.
This ton dish is a reservoir, and it feeds the steel through further gas-tight nozzles into several waters called copper molds.
The cooling is controlled on only the outer skin of the steel that is solidified. The molds are oscillated to ensure that the solidifying shell does not stick and break.
Flux is also added to aid lubrication. The steel shell grows as it is drawn down through a curved arrangement of support roles and water sprays.
It then emerges horizontally as a solid steel strand and is cut to length with automatic gas burners. Depending on the size, these solid strands are called billets, blooms, or slabs.
The billets, blooms, and slabs are typically processed through a rolling mill where the caste structure is refined. The initial set up of a continuous caster is expensive, but it is a very efficient process, has an excellent throughput, and has yield benefits over other casting types.
Ingot casting
Ingot casting is primarily used with the electric arc furnace. As with continuous casting, it is an intermediate process to solidify steel to enable further shaping in a forge rolling mill.
It is a process where molten steel is poured into a cast iron ingots mold, often cylindrical in shape. The finished ingots can weigh anything from a few kilos to hundreds of tons. The main ways of casting these ingots are either uphill teaming or vacuum casting.
Ingots that are uphill teemed use a ladle either directly or in a teeming car that is positioned over a feeder system to enable the ingots to be filled from the bottom of the mold. The label is positioned over a funnel called a trumpet.
It is opened, and the metal stream is established. The pouring rate is controlled throughout this teeming process. Powders with an extra thermic reaction are often added to the top of the ingot following teaming to generate heat to aid in feeding the ingots, minimizing voids.
An insulating material is also added to aid in this.
The mold is placed in a vacuum tank for ingots that are vacuum cast, and an intermediate ladle is positioned on top.
The ladle that is on the vacuum tank is filled with molten metal, and the tank’s pressure is pumped down to create a vacuum.
Pouring of the ingot to started and the intermediate ladle is fed with additional molten metal have required.
In the Ingots field, the pressure vented and the same exothermic and insulating powders are added to reduce voids in the ingot.
Although these powders air added some ferocity and vacancies would form within the material due to shrinkage and gas is liberated from the operation.
A large feeding pipe defect is cut from the ingots, but most ferocity in the ingots is acceptable since it will disappear through further processing in the rolling mills or forge.
In both continuous and ingot casting, the metal solidifies from the outside, progressing towards the center. Thus the ingot grain structure develops in columnar and points in the direction of solidification.
This grain structure development is typical of cast materials. Hot rolling or forging will break up the caste structure, and former smaller more uniforms wrought grain structure.
Ingot casting is better suited to one-offs and small batches. It is also able to produce large ingots easily and is the least complicated of the casting techniques.
Summary
In summary, sand casting advantages include low cost, high flexibility, virtually any size casting can be produced, and it is a simple, expedient way to make a wide range of products.
Ingot casting is used primarily with the electric arc furnace and is best suited for one-offs and small batches. It is also able to produce large ingots easily and is the least complicated of the casting techniques.
Continuous cast an ingot cast materials are further worked in a rolling mill or forge to refine the structure, whereas sand castings are not.