What is alloy steel? What are the classifications of alloy steel? Find the answer in this article.
Alloy steel is steel that contains in addition to Fe and C also other alloying elements. The alloying elements added to steel include Ni, Cr, Mo, Ti, Mn, and others.
The purpose of adding alloying elements is to get the properties we want according to the application, including:
- Increases toughness and tensile strength without losing its ductility.
- Retains its properties at high temperatures.
- Increases corrosion and fatigue resistance.
- Increase hardness.
- Increase machine capability.
- Increase elasticity.
Effect of Elements on Steel
Carbon (C)
Adds: strength and hardness (maximum at C = 0.9%), tough ability.
Reducing: melting point, expansion, weldability, and forging.
Phosphorus (P)
Adds: runny, brittle, cold, heat resistant.
Reducing: elongation, impact resistance.
Sulfur (S)
Adds: viscosity, cutability, hot brittle.
Reducing: withstands blows.
Nitrogen (N)
Adds: strength, and fragility.
Reducing: durable (age).
Hydrogen (H)
Adds: fragility.
Reducing: notch resistant.
Silisium (Si)
Adds: elasticity, strength, corrosion resistance, and graphite separation in cast iron.
Reducing: weldability.
Manganese (Mn)
Adds: strength, total hardenability, impact resistance, and wear resistance.
Reducing: capable of machining, and separation of graphite in cast iron.
Nickel (Ni)
Adds: strength, ductility, corrosion resistance, total hardenability, and electrical resistance.
Reducing: expansion.
Chrome (Cr)
Adds: hardness, heat resistance, strength, wear, and rust resistance.
Reducing: stretching.
Vanadium (V)
Adding: durable, hardness, ductility, heat resistance.
Reducing: sensitivity to high temperatures.
Molybdenum (Mo)
Adds: hardness, durability, and heat resistance.
Reducing: expansion and malleability.
Cobalt (Co)
Adds: hardness, heat resistance, cutting strength.
Reducing: ductile.
Wolfram (W)
Adds: hardness, strength, corrosion resistance, heat resistance.
Reducing: expansion.
Aluminum (Al)
Adds: nitriding (the ability of elements to be penetrated by nitrogen), hardness.
Reducing: the weight of the object.
Classifications of Alloy Steel
Low Alloy Steel
Low alloy steel is an alloy steel with a 10% alloying element content that has the same carbon content as carbon steel but less alloying elements. Alloying elements can increase strength without reducing ductility, fatigue strength, and resistance to corrosion, wear, and heat. Ship materials, bridges, railroad wheels, steam boilers, and gas tanks are all made from low alloy steel.
Low alloy steel is classified into three types based on its composition: strong steel, low-temperature resistant steel, and heat-resistant steel. Strong steel has a tensile strength of 379-620 MPa, good weldability (C less than 0.2%), tough and excellent mechanical properties, manganese (1%), copper (0.5%), and Si can form solid solution on ferrite, Nb, Ti, V, N (0.2%) can inhibit grain growth during hot rolling and form precipitates, and Al and Si for deoxidation. This steel can be used to make spring steel.
High Alloy Steel
Is an alloy steel with an alloying element content greater than 5%. Stainless steel, manganese steel, and tool steel are all examples of high-alloy steel. High-alloy steels are used for specific applications that necessitate material properties not found in low-alloy steels.
1. Stainless steel
Has a higher chrome content than 11%. There are four types of stainless steel: ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, and duplex stainless steel.
Ferritic stainless steel contains the main alloying elements Fe and Cr, has a ferrite microstructure, is non-heat treatable, can be hardened and strengthened by cold working, and is magnetic. This steel is used in a variety of applications, including glass molds, high-temperature valves, and combustion chambers. This category includes types such as AISI 409 and AISI 446.
The main alloying elements in austenitic stainless steel are Fe, Cr, Ni (Cr>16%, Ni>3.5% with Mn), and the microstructure is composed of the austenite phase. This steel is non-heat treatable, can be hardened and strengthened through cold working, is not magnetic, has the best corrosion resistance, and is the most commonly produced. This steel is then used to make cryogenic steel, as well as food and chemical industry process equipment. AISI 304 and AISI 316L are two examples of austenitic steel types.
Martensitic stainless steel has the main alloying elements Fe and Cr, a martensitic phase in the microstructure that can be hardened and strengthened by heat treatment and is magnetic. This material is used in a variety of applications, including bearings and surgical tools. This category includes types such as AISI 410 and AISI 440A.
Precipitation hardenable stainless steel is another name for duplex stainless steel. The main alloying elements in this steel are Fe, Cr, Ni, Al, and Mn. The microstructure is made up of a mixed phase (ferrite and martensite or ferrite and austenite). The phase transformation from austenite to the second phase causes this steel to harden. This steel is used in the manufacture of spring steel and pressure vessels. Materials of this type include, for example, AISI 17-7PH.
2. Tool Steel
Used for a variety of purposes during the machining process. This steel is used following heat treatment. These steels are classified into several groups based on the type of heat treatment used. Tool steels of the W type are hardened by immersion in water. To describe tool steel that has been hardened by immersion in oil. Type A tool steel that has been hardened in free-air cooling. Tool steel is used for a variety of purposes, including cutting tools and dies. High-speed steel is an example of this type of steel.
3. Manganese Steel / Hadfield Steel
It has a 13% higher manganese content and a 1% higher carbon content. At room temperature, this steel has an austenitic microstructure. It has a high hardness mechanical property, and when deformed, it becomes harder because the austenite structure becomes martensite or harder. This steel is used for a variety of purposes, including dredging bowls on heavy equipment, prison trellises, and railroad frogs.