Steel alloys are a commonly used term in the world of construction and machinery. To understand the term steel alloy or alloy steel, you must first know that steel is itself an alloy of iron. Iron mixed with small amounts carbon forms steel. This steel when mixed with other metals gives you steel alloy, one of the important raw materials used in various industries. It is used for construction of ships and pipelines, automotive applications, chemical processing, etc.
What is an alloy?
Alloy is a substance made by combining two or more metals together. For example, sterling silver is an alloy of silver and copper.
There are different kinds of steel used for various applications. The four main categories of steel include – carbon steel, tool steel, stainless steel and alloy steel.
- Carbon steel – Made of iron and carbon, it is the most produced steel in the world. Carbon steel is classified into three subcategories: low carbon steel (0.03-0.15% carbon), medium carbon steel (0.25%-0.50% carbon), and high carbon steel (0.55%-1.10% carbon). As the amount of carbon increases, the steel becomes harder and difficult to bend and weld.
- Tool steel – This steel contains a mix of carbon, tungsten, chromium, vanadium, and molybdenum. They are known for their hardness and resistance to wear and tear making the tool steel perfectly suited for use in machining and tool making.
- Stainless steel – Stainless steel is an alloy of iron with chromium, nickel, and molybdenum. These elements make up 11-30% of stainless-steel alloy composition. This is the most widely known steel alloy that is commonly used in food handling, food processing, medical instruments, hardware, and appliances as it has high corrosion resistance.
What is alloy steel?
Alloy steel is made by adding metal and non-metal elements to carbon steel. These alloys showcase different environmental, chemical and physical properties in conjunction with the elements mixed to make the alloy. Depending on the composition of elements in the alloy, it can have different mechanical properties.
Common elements in alloy steel
There are more than 20 elements that can be mixed with carbon steel to produce steel alloys. Given below are some of the common elements used in the production of steel alloys:
- Aluminium – makes steel lighter and is used to make intricate parts
- Lead – improves steel’s machining characteristics
- Chromium – makes steel resistant to scratching, staining, rusting, and denting
- Manganese – increases high-temperature strength, wear resistance and ductility and is used to cast ore crushers or railways crossings
- Copper – creates precipitation hardening properties
- Nickel – increases corrosion resistance and strength
- Silicon – increases magnetism and strength
- Vanadium – increases corrosion, shock resistance and toughness
- Tungsten – increases strength and hardness
Some other elements used to make alloy steel include nitrogen, boron, sulfur, cobalt, molybdenum, titanium, bismuth, selenium, tellurium, zirconium and niobium.
To make alloy steel, elements are melted at over 1600 °C for 8-12 hours in an electric furnace. It is then annealed at over 500 °C to alter the chemical and physical properties and remove impurities. Annealing and descaling are repeated until the alloy steel is melted. Molten steel is then send for rolling and shaping into its final form.
Types of alloy steel
There are different types of steel alloys that feature different physical and mechanical properties. Given below are the two main types of alloy steel and their properties:
- High alloy steel:
High alloy steel is characterised by a high percentage of alloying elements. It contains elements (not including carbon or iron) that make up more than 8% of its composition. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass which gives stainless steel increased corrosion resistance.
- Low alloy steel
This steel is only modified slightly with other elements to provide slight advantages in hardenability, strength, and free machining. The carbon content of low alloy steel is lowered to around 0.2% to help it retain its strength and improve formability. It is commonly used for making military vehicles, construction equipment, ships, pipelines, pressure vessels, structural steel, and oil drilling platforms.
Applications of alloy steel
Here are a few uses of alloy steel in various industries:
- Automobile parts: Alloy steels are used to make complex-shaped automotive steel parts that have requirements of deep drawing suitability and strength. It is also applied to some automobile stamping parts with more complicated shapes.
- Building bridges: Bridges are an epitome of strength and aesthetics. Alloy steel helps bridges conform to these two factors. The addition of metal elements increase strength and corrosion resistance and ease of construction.
- Ships, pipelines, pressure vessels and oil drilling platforms: Alloy steels are hard and show increased corrosion resistance and therefore is an excellent material for building structures exposed to water.
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