Introduction
All metals can be classified into the two broad groups of ferrous and nonferrous metals, ferrous metals being ones that contain iron. Iron is the basis of such classification because of it proliferance; it is a very cheap metal that is easy to refine yet is tremendously versatile due to the many different phases and properties it can have by itself or when combined with carbon or other alloying metals. This document discusses primarily the engineering aspect of metallurgy.
Ferrous Alloys
Metals containing iron as a primary constituent can be subdivided into two general groups including steels and cast irons. Of course, these subdivisions can be further split into many classifications still. Of all steels, the primary classifications are:
- low carbon steels, which contain less than 0.25wt% carbon (hypoeutectoid)
- medium carbon steels which contain between 0.25% and 0.6% carbon (again, hypoeutectoid)
- high carbon steels which contain between 0.6% and 1.4% carbon (hypoeutectoid, eutectoid, or hypereutectoid)
The Society of Automotive Engineers has a classification splits hairs with these classifications, specifying a numbering system for various steels of the form XXYY, where XX represents the alloying metals in the steel and YY represents hundreths of a weight percent carbon in the alloy. For example, a 1025 steel would contain 0.25% carbon and no alloying agents (identified by the 10). Common metals included in steel include:
| SAE Grade | Contents |
|---|---|
| 10YY | plain steel with 0.YY wt% carbon |
| 11YY | resulfurized steel (for machineability) |
| 15YY | Manganese 10-20% |
| 40YY | Molybdenum 0.2-0.3% |
| 43YY | Nickel (1.65-2%) + Chromium (0.4-0.9%) + Molybdenum (0.2-0.3%) |
| 44YY | Molybdenum 0.5% |
Going back to the classifications of low, medium, and high-carbon steels, here are some examples of applications and compositions/alloys of these steels:
| Plain Steel | High Strength, Low Alloy (HSLA) | Plain Steel | Heat Treatable | Plain Steel | Tool Steel | Stainless Steel |
|---|---|---|---|---|---|---|
| eg, 1010 | eg, 4310 | eg, 1040 | eg, 4340 | eg, 1095 | eg, 4190 | eg, 304 |
| No other alloying metals | Alloyed with chromium, vanadium, molybdenum, and/or nickel | No other alloying metals | Alloyed with chromium, nickel, and/or molybdenum | No other alloying metals | Alloyed with chromium, vanadium, molybdenum, and/or tungsten | Alloyed with chromium, nickel, and/or molybdenum |
| Used in auto frames | Used in structural members and I-beams | Used in crank shafts, bolts, hammers, and blades | Used in pistons, gears, and high-wear applications | Used in high-wear applications | Used in drills, saws, and dies | Used in high-temperature applications such as in gas turbine engines and furances |
As more carbon is added to steels, the alloy enters the cast iron regime, where there is between 3% and 4.5% carbon by weight. These cast irons are often combined with elemental silicon in order to decompose the cementite (iron carbide, Fe3C) phase and therefore the pearlite phase into iron and graphite. The advantage of these cast irons are that they are cheap, hard, brittle, and as its name implies, easy to pour. They find their uses in applications such as cast-iron engine blocks and such, and can be broken down into several categories:
- Gray cast iron is formed upon slow cooling of a silicon-rich cast iron and is characterized by the occurrence of graphite flakes in the structure which add as stress concentrators. These produce favorable vibrational dampening and wear resistance at the expensive of strength and ductility; while the tensile stress is weak, grey cast irons still possess fair compressive strength.
- Ductile cast iron is made when magnesium or cerium is added to spheroidite, a product of annealing pearlitic iron. Ductile iron, as its name implies, is soft and ductile because of these additions.
- White cast iron contains less than 1% silicon which results in more residual cementite (Fe3C), increasing hardness at the expense of ductility.
- Malleable cast iron is the product of annealing white cast iron at between 800°C and 900°C and is characterized by graphite rosettes within the metal. Malleable cast iron is more ductile than white cast iron.
Nonferrous Alloys
Nonferrous alloys vary more widely in their primary metallic constituent and therefore have a wide range of properties.
Copper Alloys
Metals based around copper carry significant engineering interest. Copper takes a face-centered cubic structure in a pure state and is therefore very ductile. Copper is an excellent thermal and electrical conductor, and it possesses moderate resistance to corrosion.
The addition of zinc produces brass which is even more resistant to corrosion and may take either the α phase (which is face-centered cubic and ductile) or β′ phase (body-centered cubic and harder). The addition of tin to copper produces bronze which often also contains aluminum, nickel, zinc, and/or silicon, which is a stronger than pure copper.
Aluminum Alloys
Aluminum is also the base for many alloys and takes an FCC structure while unalloyed. Aluminum alloys are remarkably lightweight due to aluminum's density of 2.7 g/cc (compared to the 7.9 g/cc density of steel or 4.0 g/cc of alumina) and has very good resistance to corrosion and oxidation due to its self-passivation. Aluminum is often strengthened with the addition of alloying metals.
Other Structural Alloys
Some less-common alloys include those based from magnesium, which has become popular as a material for bicycle frames due to its extremely low density (1.7 g/cc) and rigidity due to its hexagonal close-packed structure. Titanium alloys are more dense than these lightweight alloys (titanium's density is 4.5g/cc), but are very strong and have remarkable strength to weight ratios.
Refractory Alloys
Many metals possess strong metallic bonding and strong elastic moduli which, along with their low thermal expansion, make exceptional in high-temperature applications. Common refractory metals include
- Tungsten (W), which melts at around 3400°C
- Tantalum (Ta), which melts at around 3000°C
- Molybdenum (Mo), which melts at around 2600°C
- Niobium (Nb), which melts at around 2400°C
To put things into perspective, pure iron melts at around 1500°C, and the surface of the sun is around 6000°C. Thus, these refractory metals have melting points that exceed those of steels by twofold or more.
Superalloys
Superalloys are nickel, cobalt, or iron-based alloys that are used in environments that demand extremely low creep and high resistance to oxidation at temperatures exceeding 1000°C. For example, almost all high-end gas turbine blades (i.e., the insides of jet engines) are single crystal nickel-based superalloy blades coated with a zirconia thermal (plasma) spray. They operate at extremely high temperatures and are subject to extremely high stresses, so exceptional creep resistance is required to prevent the turbine blades from deforming during operation.
Noble Metals
Noble metals are also of interest due to their rarity in nature (which makes them appealing to jewelers) and inertness in many chemically reactive environments (which makes them appealing to material scientists). These metals include gold, silver, platinum, iridium, osmium, rhenium, and palladium, and they all resist oxidation.
Metal Processing
Transforming metal stock into a useful shape is generally done by either forming, casting, or joining, and postforming processes such as hardening may be required to fully tailor a metal's properties to a specific applications. Rather than discuss each method of shaping metal, below is a brief explanation of the various terms that are common in metalworking.
- Metal forming
- Hot forging involves stamping a piece of hot metal into a die to shape it. Cold forging is similar, sans the hot part.
- Drawing is used to make tubes and wires and involves pulling hot metal through a die or orifice.
- Extrusion is similar to drawing, except pressure is used to force the metal through a die or orifice.
- Rolling is when metal (such as in the form of an ingot) is squeezed through two rollers. Like forging, this can be hot or cold.
- Metal casting
- Sand casting involves pouring liquid metal into a sand mold and then breaking away the sand after cooling; this is used in the molding of engine blocks and large parts due to its relatively low cost.
- Investment casting is the casting of liquid metal into refractory plaster molds which are formed around wax models.
- Die casting uses a two-piece refractory die to mass produce high volume pieces.
- Continuous casting is used for simple objects such as ingots and, as the name implies, is a continuous process.
- Metal Joining
- Powder metallurgy is much like ceramic powder processing where a compact of metal powder is sintered into a dense body.
- Powder forging is much like hot forging, except instead a collection of powder is stamped under enough stress to cause fusion of the particulates.
- Welding is used to join large pieces of metal and often causes localized microstructural changes due to the intense heat conducted to the regions around the weld
Another important concept to recognize are the hardening phenomena that occur in alloys which is at the core of why alloying is such a useful process. Because metals yield by the movement of dislocations through their lattice, inhibiting dislocation movement hardens metals and makes them less ductile. Solid solution hardening occurs because alloyed atoms in the primary metal's crystalline structure induce stresses in the lattice; these localized tension/compression zones set up regions where large and small atoms prefer to move, respectively. Thus, as a dislocation migrates through a such region, the dislocation has to either overcome this natural inclination for atoms to stay in these preferred location or drag these stressed locations along with it.
A related phenomenon called precipitation hardening occurs when an alloy is cooled below its limit of solid solution stability and a second phase begins to precipitate out. By controlling how long the metal is held in this two-phase region, the size of these second phase nucleii can be controlled, thereby allowing for relatively precise control over the hardening of the alloy.