Ferrous Metal Casting
Metal characteristics and composition for casting design
A list of ferrous metals contains iron and its alloys, including all steels. Any solid metal that can be melted can be cast. Foundries are the factories that do this casting work, developing expertise with a handful of metals and methods, and designing standard products to maximize value and efficiency in production.
Metals and casting methods influence each other: the best casting choice for a product is influenced by how its metal will behave in molten, cooling, and solid states. For these dependencies, a foundry’s specialties are part of the determination of what sort of products they make. The foundry doing diecast children’s toys is not generally the same foundry that produces high quality engine parts.
Ferrous metals are defined as those metals that contain iron. Non-ferrous metals do not.
One of the major distinctions in specialization is whether foundries work with ferrous metals, non-ferrous metals, or both. The definition of a ferrous metal is any metal that contains iron; non-ferrous metals do not. Ferrous metallurgy represents roughly 90% of worldwide production of metal. Gray iron is the most common metal cast in foundries. Outside of the foundry, steel is the ferrous alloy most used in industry, construction, and transportation.
Foundries that specialize in common casting methods like sand casting usually work in metals chosen for specific qualities such as ease of melting and pouring, detail capture within the mold, predictable behavior while cooling, and readiness for machining or finishing.
Ferrous metals and their properties
Iron’s signature attributes are that it’s dense, strong when mixed with carbon, plentiful and easy to refine, highly prone to corrosion, and magnetic. Alloying iron with other elements in different ratios can mitigate or eliminate one or more of these factors.
Hundreds of ferrous alloys are well-known. They are specified by the proportions of each element in their makeup, as well as directions on their melting and finishing. Ferrous alloys with carbon are usually named iron or steel, and can contain any number of other elements, from aluminum to vanadium, based on their specification. These metals are usually chosen for their mechanical properties. Engineers and designers might be interested in their yield strength, toughness, ductility, weldability, elasticity, shear, and thermal expansion, all of which describe how a material will behave under specific stressors.
These distinctives aspects of iron can be changed in alloys, which mix iron with other elements. Stainless steel is a good example, with some alloys of stainless being both non-magnetic and non-corrosive. A common way of telling if a metal is steel is to put a magnet against it, since the iron in the alloy will cause the magnet to stick; however, people who have tried to stick magnets to their stainless steel fridge know this is not a foolproof test. Though the iron is still present in this ferrous alloy, a high percentage of nickel changes the microstructure of the steel enough to prevent a magnetic reaction. Stainless steel can rust, though it is much more corrosion resistant than other types of iron alloys. This is due to the addition of chromium. Chromium protects against rust through a process called passivation, in which the top molecules of the metal oxidize but stay strongly bonded to the metal below, forming an impenetrable shell.
In ferrous metals, iron and steel are the most common casting materials.
Iron
Cast irons are a category of iron alloys with carbon content greater than 2%. They are relatively inexpensive, dense irons. When they are heated and cast, they have a much higher flowability at lower temperatures than steel, meaning that they can flow into and fill parts of a complex mold with greater efficiency. Cast irons also shrink at half the rate of steel while cooling.
Basic cast irons have good compression properties, but they are brittle: they will fracture before they bend or distort. This vulnerability can mean that brittle grades of cast iron are not used for designs with extruding or elaborate details, or with very sharp edges, as these features may chip.
The mechanical properties of ferrous metals like steel make for strong load-bearing wheels. Gray Iron is the most common type of cast iron currently produced, featured in everything from manhole covers to disc brakes on cars. It gets its name from the color it takes upon fracture, which is gray due to the presence of graphite as the carbon additive. Gray iron is 2.5–4% carbon by weight, and additionally contains 1–3% silicon, which stabilizes the graphite. It has many of the attributes of basic cast iron, in that it is inexpensive and has a high flowability compared to steel when molten, but the graphite’s presence allows the iron to be somewhat less brittle, allowing easier machining. Gray iron is still inflexible: it bends very little before breaking.
Ductile Iron is a form of cast iron in which the carbon added is a spherical (nodular) graphite. Ductile iron is usually 3.2–3.6% carbon by weight, and has silicon and other elements. Higher ferrite levels mean it builds up on cutting tools during machining, so it is often used in primarily cast manufacture, where very high flowability makes it a great choice for finely detailed work. The spheroid shape of the graphite that gives ductile iron higher impact resistance and tensile strength than either cast or gray iron, making detailed or edged designs feasible. Ductile iron is a relative newcomer in the specification of irons, as it was first discovered in 1943.
Steel
Steels of all sorts are also sometimes cast. In general, steel has a carbon content of less than 2.14% by weight, and is often alloyed with other elements. Steel has stronger mechanical properties than cast irons, but what is gained in toughness is lost in flowability. Molten steel needs to be much hotter than molten iron to flow into detailed molds, and the high temperatures required to work with steel are challenging to manage and can handicap the design and finish of the object that comes out of the mold. As with all castings, different parts of a piece can cool at different rates, and this differential causes stress within the product: because steel shrinks more extensively and rapidly than cast irons, these stresses need greater management in cast steel.
These challenges mean that steel can be much more labor-intensive to cast well. It requires expert attention during all stages of its production. Still, the high mechanical strength of the final product can make a steel alloy the clear choice for some applications, with machining providing end stage finishes.
Other iron alloys
Other alloys of iron exist outside these common types, and are used in specific applications where their mechanical behaviors are useful. For example, elinvar is a nickel-iron alloy that does not expand and contract in heat, and is used in very small parts in clocks and other precision devices.
The definition of non-ferrous metals are those that don’t contain iron—like these pure precious metals.