Understanding Steel Types and Grades

Steel is one of the most widely used metals globally. It Is an alloy with a basic composition that consists of iron and carbon (in small amounts). Different alloying elements can be added to the iron and carbon mixture to alter the properties of steel, such as its corrosion resistance, strength, etc. Steel comes in a wide variety of grades that can be classified based on its composition (e.g., carbon content, alloying elements, etc.). It can be used for anything from construction and structural elements like bridges, to home appliances like washing machines and refrigerators.

This article discusses in detail steel and its types and grades, the classification of those steels, and the different systems used to grade steels. 

What Are the Different Types of Steel?

Steel comes in different types, distinguished by their composition and properties. The different steel types are mentioned and discussed below:

1. Carbon Steel

Carbon steel is the most basic and widely used type of steel. It accounts for about 90% of all steel that is made. It is characterized by its composition of primarily iron and carbon. Carbon steel also has a maximum alloying element percentage of 2% with compositional limits of 4.65% manganese, 0.6% copper, 0.6% silicon, 0.05% sulfur, and 0.04% phosphorus with no other elements deliberately added in. It is divided into three subcategories based on carbon content: low, medium, and high. Low-carbon steel, also known as mild steel, contains up to 0.3% carbon, making it ductile and malleable for easy forming. The low carbon content also makes it easy to weld. Low-carbon steel is a good choice for construction materials and automotive body panels. Medium-carbon steel, with 0.3% to 0.6% carbon, balances ductility and strength, making it suitable for gears, rails, and structural components. High-carbon steel, with more than 0.6% carbon, is prized for its hardness and wear resistance and is typically used in cutting tools and springs. Typically, they do not contain more than 2.1% carbon. Ultra-high or very high carbon steel has a carbon content above 1.0% and below 2.1%.  

To learn more, see our full guide on the Definiton of Carbon Steel.

2. Alloy Steel

Alloy steel has other elements in its composition besides iron and carbon. These elements are added to enhance the steel’s mechanical properties to meet specific industrial needs. Elements such as manganese, silicon, nickel, and chromium augment the steel's strength, toughness, and corrosion resistance. The amount of alloying elements can vary from as little as 1% to as much as 50%. Alloy steel is divided into low-alloy, medium-alloy, and high-alloy steels, based on the percentage of alloying elements. 

Low alloy steels are characterized by their modest alloying element contents, having less than 1.8% nickel, less than 6% chromium, and less than 0.65% molybdenum. Their tensile strength varies significantly, ranging from 450–620 N/mm2 to 850–1000 N/mm2. Medium alloy steels have an alloying element percentage between 5–12%. This category includes nickel steels ideal for structural components like axles and shafts, nickel-molybdenum steels that can be case-hardened for use in parts such as cams and bearing races, and nickel-chrome-molybdenum steels, which are known for their high strength and excellent fatigue resistance. 

High alloy steels, on the other hand, contain over 12% alloying elements and offer specialized properties such as the corrosion resistance found in stainless steels with 13–18% chromium content, the wear resistance of austenitic steels with over 11% manganese, and the exceptional heat resistance and strength seen in certain high alloy steels, making them suitable for demanding applications across various industries. In general, alloy steel finds use in such sectors as the automotive, pipeline construction, and power generation industries.

To learn more, see our full guide on the Definiton of Alloy Steel.

3. Stainless Steel

Renowned for its corrosion resistance and aesthetic appeal, stainless steel incorporates a significant chromium content that forms a passive oxide layer, protecting the steel from staining and corrosion. Stainless steel is categorized into duplex, austenitic, ferritic, and martensitic groups, each with distinct properties. Austenitic stainless steels are non-magnetic, highly formable, and used extensively in kitchenware, chemical processing equipment, and medical devices. Ferritic stainless steels are magnetic and used in automotive and appliance applications, while martensitic steels offer hardness and strength for cutlery and tool manufacturing.

To learn more, see our full guide on Stainless Steel Metals.

4. Tool Steels

Tool steels are ideal for manufacturing various tools, such as drill bits, cutting tools, dies and molds, punches and chisels, knives, blades, etc. They are designed for durability and resistance to wear and deformation. These steels are alloyed with elements like tungsten, molybdenum, chromium, cobalt, and vanadium to withstand high temperatures and mechanical stress. Tool steels are classified into several types based on their hardenability, hot hardness, and shock resistance. Each type is tailored for specific tooling applications, such as: cutting, shaping, and molding materials. The different types of tool steels are: air-hardening, water-hardening, oil-hardening, high-speed, hot-working, and shock-resisting.

To learn more, see our full guide on the Definiton of Tool Steel.

5. Weathering Steel (COR-TEN® Steel)

Weathering steel, known commercially as COR-TEN® steel, is engineered to resist corrosion in outdoor applications, forming a protective rust-like appearance over time without corroding. Alloyed with small amounts of chromium, nickel, and copper, it offers increased resistance to atmospheric conditions. This steel is used in bridges, sculptures, and other outdoor installations.

What Are the Different Types of Steel Grades?

Steels are classified into different grades based on steel grading systems such as those maintained by ASTM and SAE. These systems are discussed in the upcoming sections. 

1. ASTM Grading System

The ASTM (American Society for Testing and Materials) grading system is an alloy naming system that classifies materials based on their properties and qualities, such as: their chemical composition, mechanical properties, and intended use. It covers a wide range of steel types, including: carbon steel, alloy steel, stainless steel, and tool steel, among others. This system uses a letter prefix and a sequentially assigned number to classify steels based on their material type and specific properties. The letter indicates the category of steel (e.g., "A" for iron and steel materials), while the numbers specify the exact material according to ASTM standards. The numbers are assigned arbitrarily only for identification and have no direct link to different properties that a material might have. 

2. SAE Grading System

The Society of Automotive Engineers (SAE) Grading System, often used in conjunction with the American Iron and Steel Institute (AISI) system, primarily focuses on classifying steels based on their chemical composition. The SAE system uses a four-digit number to designate the steel grade. The initial two digits specify the type of steel and its primary alloying components, and the final two digits denote the percentage of carbon present in the steel, expressed in hundredths of a percent. For example, in the SAE system, a steel grade designated as 1040 means it is carbon steel (the "10" series) with approximately 0.40% carbon content.

What Is Steel?

In its simplest form, steel can be defined as an alloy that is made primarily from iron and carbon. However, all steels (carbon steel, alloy steel, tool steel, stainless steel, and weathering steel)  can also have small amounts of other elements included in their composition. The added elements impart different properties and characteristics to the steel, resulting in the different commercial steel types and grades. Some of the most common elements that are added to steel to alter its properties include: manganese, chromium, and nickel. 

How Does Steel Differ From Other Metals?

The term "metals" can refer to any pure metal or metallic alloy. Steel is an alloy based on the element iron with a small amount of carbon and varying amounts of other possible elements. The unique feature of steel is the combination of iron with a small amount of carbon. Steel is a type of metal, but not all metals are steel. 

What Are the Properties of Steel?

Some of the properties and characteristics associated with steel are listed below:

1. Strength: Steel, although it varies from steel type to steel type, typically has high strength, making it an ideal material for buildings, bridges, and vehicles. Its strength can be adjusted through the addition of alloying elements and heat treatment processes.
2. Toughness: The toughness of some steels means they can absorb energy and withstand impacts without fracturing. However, it’s important to note that steel is not uniformly tough; some grades can be quite brittle. In Particular, ferritic steels have a ductile-brittle transition temperature. Below this critical temperature, ferritic steels shift from exhibiting ductile behavior, where they can deform significantly before fracture, to a brittle behavior, where they are more likely to crack or break under stress without significant deformation.
3. Hardness: This property indicates steel's resistance to deformation and wear. The hardness of steel can vary greatly depending on its alloy composition and heat treatment.
4. Fatigue Strength: Steel can withstand repeated cycles of loading and unloading without failing. This makes it ideal for components subjected to cyclic stresses, such as bridges and machine parts. If the cyclic stress is kept below a certain "fatigue limit," which is different for different steels, it can have an "infinite" fatigue life. 
5. Corrosion Resistance: While basic carbon steel is prone to rust when exposed to moisture and oxygen, alloying elements like chromium can be added to produce stainless steel, which exhibits enhanced corrosion resistance.
6. Heat Resistance: Certain types of steel, such as certain stainless steel grades (e.g. 309, 310, and 330) and chromium-molybdenum (Cr-Mo) steels, are designed to maintain their strength and structural integrity at high temperatures. These steels are suitable for use in boilers, engines, and turbines.
7. Weldability: The ability of steel to be welded is critical for manufacturing and construction. The weldability of steel depends on its chemical composition, with certain alloys being more conducive to welding than others.
8. Conductivity: While not as conductive as metals like copper, steel can conduct electricity and heat, albeit to a lesser extent. The conductivity of steel varies with its composition and structure.
9. Magnetic Properties: Ferritic steels are ferromagnetic, martensitic ones are less so, and austenitic steels are not or barely. The ferromagnetic properties of some steels are exploited in electric motors, transformers, and various electronic devices.
10. Machinability: Most steels are easily machinable, depending on the grade. 
11. Weldability: Most steel grades are easily weldable, although some might require specialized welding procedures.
12. Recyclability: Steel is recyclable. Due to its value, more than 60% of steel is recycled globally.

How Are Steel Grades Classified and Categorized?

Steel grades are classified and categorized based on their chemical composition and specific properties. The primary methods include:

1. Carbon Content: Steel is divided into categories based on its carbon content: low (up to 0.3%), medium (between 0.3 and 0.6%), high (0.6-1.0%), and very high (1.0-2.1%) carbon content).
2. Alloying Elements: Beyond carbon, the addition of elements like manganese, silicon, nickel, chromium, and molybdenum differentiates alloy steels from carbon steels and contributes to specific properties such as: strength, ductility, and corrosion resistance.
3. AISI/SAE Numbering System: This naming system uses a four-digit code to designate steels, in which the first two digits indicate the type of steel and its primary alloying components, and the last two digits represent the carbon content in hundredths of a percent.  
4. ASTM Specifications: These cover a wide range of steel types and forms, focusing on: test requirements, manufacturing processes, and application-specific properties.
5. Additional Classifications: Other ways to categorize steels include groupings like high-strength low-alloy (HSLA) steels, stainless steels, and tool steels, based on their unique characteristics and intended use.

How Is Steel Typically Manufactured?

Steel production occurs through one of two primary smelting processes: the blast furnace method or the electric arc furnace method.

In the blast furnace process, a mixture of iron ore and coke (coal that has been specially treated) is heated with air in a furnace. Lime is added to facilitate the reduction of iron ore to metallic iron, producing what is known as pig iron. This pig iron is then transferred to a basic oxygen furnace, where it is converted into molten steel by adding oxygen, which removes impurities.

Alternatively, the electric arc furnace method begins with the direct reduction of iron ore using natural gas, producing iron metal. This iron is then melted in an electric arc furnace, where large electrodes create high-temperature arcs to melt the metal. Alloying elements are added during this phase to achieve the desired steel composition.

Following either smelting process, the molten steel is cast into various shapes and sizes by a continuous casting process in a hot strip mill. The steel is then rolled to form different products, which can be either hot-rolled or cold-rolled depending on the required properties of the final product. Additional finishing processes, such as tempering or annealing, are applied to produce the specific grade of steel needed for various applications.

What Are Some Common Applications of Steel in Daily Life?

Common applications of steel in our daily lives include:

1. Construction: It is used in the framework of buildings, bridges, and tunnels. It is also found in roofing, beams, and reinforcements.
2. Transportation: Cars, trucks, trains, ships, and airplanes utilize steel in their frames, bodies, and engines.
3. Appliances and Tools: Refrigerators, washing machines, cutlery, and tools are often made of steel.
4. Medical Equipment: Surgical instruments and medical equipment, such as MRI machines make use of steels, especially stainless and tool steels.
5. Storage: It can be used for home storage solutions, such as steel cabinets and shelving units. 
6. Screws & Hinges: Many screws and hinges are made from steel. 

How Does Xometry Select Steel Grades for Manufacturing?

Xometry selects steel grades for manufacturing based on the specific requirements of the project, including: mechanical properties, application environment, manufacturability, and cost. Factors considered include:

1. Strength and Durability Needs: Depending on the structural demands of the part.
2. Corrosion Resistance: Necessary for parts exposed to harsh environments.
3. Machinability and Weldability: Based on the manufacturing processes to be used.
4. Client Specifications: Adhering to client-provided requirements or industry standards.
5. Cost-Effectiveness: Balancing material properties with budget constraints.

This approach ensures the selected steel grade optimally balances performance with cost and manufacturability for the intended application.

What Is the Difference Between Iron and Steel?

The primary difference between iron and steel is that iron is a chemical element, while steel is an alloy mainly composed of iron and carbon. Iron in its pure form is soft and prone to rusting, whereas steel, with a small amount of carbon added, is harder, stronger, and less likely to rust. The addition of carbon and other alloying elements to iron during the steel-making process significantly enhances its mechanical properties, making steel more durable and versatile for a wide range of applications.

Summary

This article presented steel types and grades, explained each of them, and discussed their various applications. To learn more about steel, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

Copyright and Trademark Notice

1. COR-TEN® is a trademark of the United States Steel Corporation (USS).

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