Metallurgy

Metallurgy

Metallurgy is a domain of materials science and materials engineering that studies the physical and chemical behavior of metallic elements and their alloys. Metallurgy concerns the chemical, physical, and atomic properties and structures of metals and the principles whereby metals are combined to form alloys. Metallurgy is used to separate metals from their ore.

Metallurgy is subdivided into ferrous metallurgy (the metallurgy of iron and its alloys also known as black metallurgy) and non-ferrous metallurgy (the metallurgy of aluminium, copper etc.). Ferrous metallurgy involves processes and alloys based on iron while non-ferrous metallurgy involves processes and alloys based on other metals.

Alloys

Fe-Fe3C Phase Diagram
In the figure, there is the iron–iron carbide (Fe–Fe3C) phase diagram. The percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties. The percentage of carbon determines the type of the ferrous alloy: iron, steel or cast iron. Source: wikipedia.org Läpple, Volker – Wärmebehandlung des Stahls Grundlagen. License: CC BY-SA 4.0

Alloying is a common practice because metallic bonds allow joining of different types of metals. For example, austenitic stainless steels, including Type 304 stainless steel (containing 18%-20% chromium and 8%-10.5% nickel), have a face-centered cubic structure of iron atoms with the carbon in interstitial solid solution.

Ferrous alloys, those in which iron is the principal constituent, include steel and pig iron (with a carbon content of a few percent) and alloys of iron with other metals (such as stainless steel). Ferrous alloys are known for their strength. Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. The simplest ferrous alloys are known as steels and they consist of iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type). Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. Due to its very-high strength, but still substantial toughness, and its ability to be greatly altered by heat treatment, steel is one of the most useful and common ferrous alloy in modern use.

Processing of Metals

Historically, processing of metals posses one of key domains in materials science. Materials science is one of the oldest forms of engineering and applied science and the material of choice of a given era is often a defining point (e.g. Stone Age, Bronze Age, Iron Age). Processing of metals involves the production of alloys, the shaping, the heat treatment and the surface treatment of the product. Determining the hardness of the metal using the Rockwell, Vickers, and Brinell hardness scales is a commonly used practice that helps better understand the metal’s elasticity and plasticity for different applications and production processes. The task of material engineers is to achieve balance between material properties such as cost, weight, strength, toughness, hardness, corrosion, fatigue resistance, and performance in temperature extremes. To achieve this goal, the operating environment must be carefully considered. In a saltwater environment, ferrous metals and some aluminium alloys corrode quickly. Metals exposed to cold or cryogenic conditions may endure a ductile to brittle transition and lose their toughness, becoming more brittle and prone to cracking. Metals under continual cyclic loading can suffer from metal fatigue. Metals under constant stress at elevated temperatures can creep.

Metalworking Processes

Processing of metals in the solid state can be divided into two major stages:

  • Hot workingHot working. First, the raw material in the form of large ingots or billets is hot-worked, usually by rolling, forging, or extrusion, into smaller shapes and sizes. These processes occur at a temperature above that at which recrystallization occurs. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high. For hot-working operations, large deformations are possible, which may be successively repeated because the metal remains soft and ductile.In general, metals are shaped by processes such as:
    • Forging
    • Rolling
    • Extrusion
    • Drawing
    • Casting
  • Cold working. Cold working is a metalworking process, which occurs below the recrystallization temperature. Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is plastically deformed, dislocations move and additional dislocations are generated. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. The dislocation density in a metal increases with deformation or cold work because of dislocation multiplication or the formation of new dislocations. The more dislocations within a material, the more they will interact and become pinned or tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the material. This process is known as cold working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. It is a process of making a metal harder and stronger through plastic deformation. Cold forming techniques are usually classified into four major groups:
    • Squeezing
    • Bending
    • Drawing
    • Shearing

Thermal Processing of Metals

Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. There is a number of phenomena that occur in metals and alloys at elevated temperatures. For example, recrystallization and the decomposition of austenite. These are effective in altering the mechanical characteristics when appropriate heat  treatments or thermal processes are used. In fact, the use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.

  • Annealing. The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. In this process, metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking; it is also easier to sand, grind, or cut annealed metal.
  • Quenching. The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.
  • Tempering. The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
  • Aging. Age hardening, also called precipitation hardening or particle hardening, is a heat treatment technique based on the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix to enhance The strength and hardness of some metal alloys.  Precipitation hardening is used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.
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See above:
Metals