All About Alloy Steel: Definition, Types, and Uses
Learn more about alloy steel and how it's important in manufacturing.
Alloy steel is a type of steel with alloying elements other than carbon added to improve its properties. Some of the common additions to alloy steel include: chromium, cobalt, columbium, molybdenum, manganese, nickel, titanium, tungsten, silicon, and vanadium. Alloy steels are known for their enhanced properties compared to plain-carbon steel, such as: corrosion resistance, hardness, strength, wear resistance, and toughness. Alloy steels are used to make fabrication tooling and end products across just about all industries. The exact grade and composition of the alloy steel can be tailored to the requirements of the application.
This article will discuss the various classes of alloy steels, including their compositions, their properties, and their applications.
What is Alloy Steel?
Alloy steel is an iron-based material, which, in addition to carbon, contains one or more intentionally added elements. The alloying elements are added to the steel to improve one or more of its physical and/or mechanical properties, such as: hardness, strength, toughness, high-temperature performance, corrosion resistance, and wear resistance. These elements typically comprise 1-50 wt% of the metal’s composition. There are many ways to group alloy steels. They can be grouped by their major alloying elements (e.g., stainless steels contain considerable amounts of chromium), or by the percentage of all alloying elements that the steel contains (e.g., high-alloys steel typically contains more than 8 % alloying elements, while low-alloy steel has less than 8 %).
What is the Composition of Alloy Steel?
Depending on the desired properties of the material, alloy steel can contain a wide variety and variable amounts of alloying elements. Each of these elements is added to enhance some properties of the steel, such as hardness or corrosion resistance. Typical alloying elements include: boron, chromium, molybdenum, manganese, nickel, silicon, tungsten, and vanadium. Other less common elements that may be added are: aluminum, cobalt, copper, lead, tin, titanium, and zirconium.
How Much Carbon is in Alloy Steel?
The carbon content of alloy steel will depend on the type of alloy steel used. Most steels have a carbon content of below 0.35 wt% carbon. Low-carbon steel designed for welding applications, for example, has a carbon content of below 0.25 wt%, and often, the carbon content is below 0.15 wt%. However, tool steels are a type of alloy steel with high carbon content, usually between 0.7 and 1.5.
How is Alloy Steel Made?
Alloy steel is made by melting the base alloys in an electric furnace at over 1600 °C for 8-12 hours. It is then annealed at over 500 °C to alter the chemical and physical properties and remove impurities. The mill scale (produced through annealing) is removed from the surface using hydrofluoric acid. Annealing and descaling are repeated until the steel is melted. Melted steel is cast for rolling and shaping into the final form, depending on the required dimensions.
In general, steel is created using one of two processes: an electric arc furnace (EAF) or a blast furnace. The blast furnace is the initial process of turning iron oxides into steel. Pig iron is produced in the blast furnace using coke, iron ore, and limestone. The EAF differs from the blast furnace in that it creates molten steel by melting scrap steel, direct reduction iron, and/or pig iron using an electrical current.
Where is Alloy Steel Used?
The application of alloy steel is very wide and depends on the type of alloy steel. Some alloy steels are used to manufacture pipes, particularly those for energy-related uses. While others are utilized in the production of corrosion-resistant containers, silverware, pots, pans, and heating components for toasters and other kitchen equipment. Alloy steels can be divided into two primary categories: low-alloy steels and high-alloy steels. The application of alloy steels is mainly determined by the category that they fall in.
Low-alloy steels are employed in a variety of industrial sectors due to their strength, machinability, and affordability. They can be found in ships, pipelines, pressure vessels, oil drilling platforms, military vehicles, and construction equipment.
High-alloy steels, on the other hand, can be expensive to produce and challenging to work with. However, they are perfect for automotive applications, chemical processing, and power generation equipment due to their high strength, toughness, and corrosion resistance.
What Are the Properties of Alloy Steel?
Alloy steel can have a wide range of properties, depending on the specific alloying elements and the amounts of them added to the steel. Some of the key properties associated with some alloy steels are: high performance, durability, high strength, good performance under harsh conditions, and corrosion resistance.
What Temperature is Required to Harden Alloy Steel?
Not all alloy steels are heat treatable. Examples of alloy steels that cannot be heat treated include ferritic and austenitic stainless steels. For steel to be hardenable enough, carbon is needed to harden it. Steels like martensitic steels, for example, can be hardened to their relatively high carbon content. For alloy steels that have enough carbon content to be hardened, the temperature required to harden the alloy steel is usually between 760-1300 °C (depending on the carbon content). Just like with other types of steel, hardening of alloy steel involves controlled heating at critical temperatures followed by a controlled cooling step.
What Are the Main Types of Alloy Steel?
Alloy steel can be divided into seven main categories, as shown in Table 1 below:
| Steel type | Classification |
|---|---|
Steel type Low-alloy steel | Classification Less than 8% alloying elements |
Steel type High-strength low alloy (HSLA) steel | Classification Low alloy steel |
Steel type High-alloy steel | Classification More than 8% alloying elements |
Steel type Stainless steel | Classification High alloy steel |
Steel type Advanced high-strength steel (AHSS) | Classification Low alloy and low carbon content |
Steel type Maraging steel | Classification Low carbon content |
Steel type Tool steel | Classification High carbon content |
1. Low-Alloy Steel
Low-alloy steels are those with alloying elements making up less than 8% of the composition of the metal. These alloying elements are added to improve the mechanical properties of the steel. For example: molybdenum improves the strength; nickel increases the metal’s toughness, chromium adds high-temperature strength, corrosion resistance, and hardness.
Low-alloy steel is widely used in the manufacturing and construction industry. Common uses for this steel include: military vehicles, construction equipment, ships, pipelines, pressure vessels, structural steel, and oil drilling platforms.
2. High-Strength Low Alloy (HSLA) Steel
High-strength low alloy (HSLA) steel, or microalloyed steel, offers both high strength and good atmospheric corrosion resistance. There are six main categories of HSLA steel: weathering steel, acicular ferrite steels, pearlite-reduced steels, dual-phase steels, control-rolled steels, and microalloyed ferrite-pearlite steels. Typically, copper, chromium, phosphorus, and silicon are used to boost corrosion resistance, while vanadium, niobium, titanium, and copper are used to increase strength. The great strength of HSLA steels can make them difficult to form. In certain situations, zirconium or calcium are added to make them easier to form.
HSLA is widely used in the automotive industry. Hot-rolled HSLA steel can be used for suspension systems, chassis, wheels, and seat mechanisms. Whereas cold-rolled HSLA steels can be used for reinforcements and seat brackets.
3. High-Alloy Steel
High-alloy steel is distinguished by its high alloy content of more than 8% of the total composition of the steel. It can be expensive to manufacture high-alloy steel and it can be challenging to work with. However, these grades are perfect for automotive applications, structural components, chemical processing, and power generation equipment due to their hardness, corrosion resistance, and toughness.
4. Stainless Steel
Stainless steel is one of the most well-known alloy steel and the most corrosion-resistant. It typically has some combination of nickel, chromium, and molybdenum as major alloying elements, which make up around 11-30 % of the steel’s composition. Three types of stainless steel exist: austenitic, ferritic, and martensitic.
Austenitic steels are typically used to contain corrosive liquids and machinery for the mining, chemical, architectural, or pharmaceutical industries. High quantities of nickel (up to 35%), molybdenum, chromium (16-26%), and niobium are found in austenitic steels, with up to 0.15% carbon. Austenitic steels often have the best corrosion resistance of all stainless steels. These steels also have high formability and strength and are usually desired for their properties at extreme temperatures.
Ferritic steel, used in industrial machinery and cars, is a grade of stainless steel with less than 0.10% carbon and more than 12% carbon. This grade of steel was developed to resist corrosion and oxidation, more specifically stress cracking corrosion. These steels are essentially incapable of being hardened through heat treatment, and they can only be slightly hardened by cold rolling.
Martensitic steels, mostly used for cutlery, have a typical chromium content of 11.6 to 18% with 1.2 % carbon and nickel sometimes added. As a group, the highest chromium content of martensitic steels is lower than the highest chromium content for ferritic and austenitic steels. Martensitic steels are recognized for their exceptional hardenability with mild corrosion resistance. This makes them ideal for cutlery, wrenches, surgical instruments, and turbines.
5. Microalloyed Steel
High-strength low alloy (HSLA) steels are often referred to as microalloyed steels.
6. Advanced High-Strength Steel (AHSS)
Advanced high-strength steel (AHSS) is primarily used in the automotive industry. This metal alloy is a key player in reducing the total weight of vehicles. It has unique properties, such as: high strength and optimized formability– making it ideal for automotive applications.
7. Maraging Steel
Maraging steel is a special kind of steel alloy with low carbon content. This ultra-high-strength steel has superior toughness and good ductility compared to most steel. In contrast to other steel alloys, maraging steel is hardened by the precipitation of intermetallic compounds, not by the presence of carbon. Maraging steel combines high strength and hardness with relatively high ductility thanks to the lack of carbon and the use of intermetallic precipitation. The principal precipitate types are Ni3Mo, Ni3Ti, Ni3Al, and Fe2Mo, which also occur in high-volume fractions. Maraging steels are mainly used in the aerospace sector as well as the manufacture of tools and weaponry.
8. Tool Steel
Tools steel is a term used to describe a range of carbon and alloy steels that are well-suited for the production of tools. These steels are distinguished by their hardness, resistance to wear, toughness, and resistance to softening at high temperatures. Tool steel’s ideal application is for tool manufacturing, including (but not limited to) machine dies and hand tools.
Is Alloy Steel Suitable for Injection Molding?
Yes, alloy steel is suitable for injection molding. Low-alloy steels are one of the materials used in the injection molding process. Alloy steel grade P20, for example, is used for molds that are used for common plastics as they do not cause corrosion or much wear.
Summary
This article presented alloy steel, explained what it is, and discussed how it plays a crucial role in manufacturing. To learn more about alloy steel, contact a Xometry representative.
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