SPECIAL ALLOYS: WHAT ARE THEY?
Many materials are used in industry, but one of the most common is metal, as it combines very important qualities such as the ability to conduct electricity, malleability, and ductility. These latter qualities mean it can be reshaped at will under certain specific conditions without breaking or losing its firmness. These chemical elements come from nature: they are found in the earth’s crust and are easy to obtain, which is why they have been used since we first learned of their existence. Different types of steel are used in industry depending on the chemical alloying elements that are added to obtain the right qualities for the application in question.
What is an alloy?
Alloys are not pure metals but the combination of two or more elements, one of which must be a metal. The result of this mixture is a material whose properties are significantly different from those of the constituent elements. For this reason, there is a wide range of characteristics that are achievable depending on the specific compound, and an equally wide range of applications. Alloys have the following characteristics:
- They have a metallic luster
- Conductors of electricity and heat.
- They have mechanical properties such as hardness, ductility and
- They do not have a unique melting point.
Ferrous and steel alloys (i.e. those with an iron base or a steel base) are the most commonly used because they offer a wide range of properties and are easy to produce. However, their density is relatively high, as electrical conductors they are weak and can corrode in common media, which has led to the search for other types of alloys that are more resistant.
Types of alloys
As mentioned above, there are different types of alloys. The requirement they must meet is that the base element must be a metal.
Iron alloys are mainly used in the construction industry. Depending on their composition, a distinction can be made between:
- Carbon steels: These contain small amounts of carbon, manganese and silicon, plus sulfur and phosphorus in even smaller quantities.
- Alloy steels: As the name suggests, these are steels containing alloying elements. The purpose of adding alloying elements is twofold: to make them harder and to improve their corrosion resistance.
Copper alloys are also widely used because they are heavier than iron alloys. Unlike the above, their creep temperature is high, but the strength to weight ratio is lower than aluminum and magnesium alloys.
Light alloys are those whose hardness is similar to steel alloys, but whose weight and density are lower. They include the following:
- Aluminum alloys. Aluminum is a metal that stands out for its durability. It is used in the aeronautical and automotive industries when weight is very important.
- Magnesium alloys. These are extraordinarily light and their density is even lower than that of aluminum.
- Titanium alloys. Titanium is a difficult material to extract and process, but its properties are extraordinary. Its major limitation is its chemical reactivity at high temperatures when in contact with other materials. However, it is resistant to corrosion at room temperature and its properties are not altered in the atmosphere or in marine or industrial environments. This explains its use in aircraft and special vehicles and in the petrochemical industry.
Low-melting-point alloys are those that melt below 150ºC. They are also called fusible alloys and are often used as coolants, since they remain stable under heating and have a higher thermal conductivity than other coolants. They are used in steam boilers, machining parts and fire sprinklers, for example. Of this type of alloys, those of zinc stand out, since it is a material used to coat and protect against corrosion.
What are “special alloys”?
Special alloys are those that are made from materials such as copper or brass, but have small amounts of other elements added.
Low-alloy coppers are divided into:
- Non-precipitation-hardened alloys: These contain small amounts of silver, iron, tin or zinc, which increase the strength and softening temperature, making it more durable.
- Precipitation-hardened alloys: Precipitation hardening releases the constituents in a controlled manner, so that precipitate clusters are formed to improve the strength of the component.
Iron increases mechanical strength and resistance to erosion corrosion. On the other hand, electrical and thermal conductivity decrease. When cold-worked and subjected to heat treatment, the mechanical strength improves without affecting its conductivity. These types of alloys are used in electronic and electrical engineering devices.
“Special brasses” have a zinc or copper base. If certain percentages of manganese, tin, aluminum, silicon, cobalt or nickel are added to these alloys, the properties change completely.
Nickel is a metal widely used in steels due to its resistance to corrosion in basic, and in many acidic media. This explains its common use in chemical engineering plants and also in the food industry.
There are many alloys in which nickel is a principal element, but just a few stand out for their use in industry:
- Monel: nickel 68%, copper 30% and iron 2%. This is a corrosion-resistant material used in gas turbines.
- Inconel: 80% nickel, 14% chromium and 6% iron. The result of this alloy is a corrosion-resistant material capable of maintaining its properties at elevated temperatures. It is used as a sheath for electrical heating elements in industrial plants.
Metals and alloys have very interesting characteristics that make them suitable materials for various engineering applications. They can be used as structural materials, as they are resistant to rust and temperature changes. Advances and different combinations have made it possible to further develop and optimize the properties of alloys, and new ones are just waiting for the ideal applications where their unique properties can be put to use.