Understanding the Types of Steel, Steel Grades, and Uses
As the backbone of many industries, steel is remarkably versatile, strong, and durable. Used to manufacture heavy equipment, kitchen utensils, electronics, appliances, and more, contractors benefit from steel every day.
Steel is an alloy primarily composed of iron and carbon, but its properties vary widely depending on its composition and treatment. This results in four main types of steel, each with its own purpose.
In this blog, we'll explore the various types of steel, their grades, and the crucial role they play in different applications.
The Four Major Types of Steel
The different steel classifications are based on how they're composed. Iron can be combined with carbon and a wide variety of other elements, creating steels that include carbon steel, stainless steel, alloy steel, and tool steel. Below, we will break down each one and its uses.
Carbon Steels
Carbon steels are the simplest form of steel, primarily composed of iron and carbon. This form of steel is known for its exceptional strength and affordability. These steels are divided into three subtypes:
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Low Carbon Steels: Low carbon steels, often referred to as mild steels, contain a minimal amount of carbon (typically less than 0.3%). This low carbon content makes them soft and malleable. They are suitable for applications like sheet metal, wire, and automotive panels due to their ease of forming and welding.
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Medium Carbon Steels: Medium-carbon steels contain a moderate amount of carbon (between 0.3% and 0.6%). They strike a balance between strength and ductility. Medium-carbon steels are often used in applications like shafts, gears, and railway tracks, where a combination of strength and toughness is required.
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High Carbon Steels: High-carbon steels contain a higher carbon content (above 0.6%) and are known for their hardness and wear resistance. These steels are used in cutting tools, springs, and components that require excellent abrasion resistance. They will not contain more than 2.0% of carbon.
Carbon steel is the primary form of steel used for everyday purposes and accounts for 90% of all steel that is manufactured.
Alloy Steels
Alloy steels contain additional alloying elements besides iron and carbon, which impart specific properties to the steel. Possible elements include manganese, silicon, nickel, titanium, aluminum, chromium, and copper. Below is how the alloying steels and elements work together to produce a better product:
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Chromium: Adds toughness to the steel. Makes it incredibly resistant to wear and tear.
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Silicon: Used to enhance magnetic properties in steel. This makes them great for generators, motors, and transformers. It can also be used as a deoxidizer and reduce the risk of defects.
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Nickel: Increases the strength and toughness of the steel. Often used to reduce corrosion.
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Aluminum: Very lightweight and able to resist high temperatures, aluminum-influenced alloy steels are commonly used in hot exhaust systems and generators.
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Cobalt: Added to increase heat resistance. Great for high-speed power tools. Also can enhance cutting capabilities.
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Vanadium: Similar to cobalt, vanadium increases strength, toughness, and wear resistance. Great for high-power tools.
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Manganese: Creates impact-resistant steel with more strength and hardenability. Used in structural applications as well as safes, bulletproofing, and more security applications.
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Molybdenum: Increases strength at higher temperatures and increases resistance to corrosion. Common applications include high-pressure industries like underwater, oil, and gas.
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Copper: Because of the electrical conduction properties that copper provides, it is common in electrical applications.
Stainless Steels
As mentioned above, chromium is one of the alloys that can be added to steel to give it new properties. The main properties of stainless steel to care about are its resistance to corrosion and staining.
To make stainless steel, chromium is needed at a rate of around 10.5% to 30%. The higher the chromium content, the more shiny the finished product will have, as well as being more resistant to corrosion.
Stainless steels come in various subtypes, such as:
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Austenitic Stainless Steels: Austenitic stainless steels are non-magnetic and highly formable due to their face-centered cubic (FCC) crystal structure. They contain high levels of nickel and chromium, making them corrosion-resistant. Austenitic stainless steel is used in kitchenware, pipes, and chemical tanks. This type of stainless steel is easier to clean and durable.
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Ferritic Stainless Steels: Ferritic stainless steels are magnetic and are characterized by their body-centered cubic (BCC) crystal structure. They contain chromium, which provides corrosion resistance. These steels are commonly used in automotive exhaust systems and appliances.
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Martensitic Stainless Steels: Martensitic stainless steels are known for their hardness and strength. They have a BCC crystal structure and are often used in applications where high strength and corrosion resistance are required, such as knives, surgical instruments, and turbine blades.
Tool Steels
Tool steels are, as their name implies, designed for manufacturing high-speed tools that create a lot of heat. This is because tool steels can withstand high temperatures and resist wear. Common elements used in creating tool steels include tungsten, chromium, and vanadium.
There are many types of tool steels, each engineering and designed for particular tools in specific settings. The elements that are added change depending on the end use. Some of the different end uses include:
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High-Speed Tool Steels: High-speed tool steels are used for cutting tools like drills and saw blades. They can maintain their hardness and sharpness at high operating speeds, making them essential in machining and cutting applications.
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Hot-Work Tool Steels: Hot-work tool steels are ideal for forging and casting. They maintain their strength and hardness even at elevated temperatures, making them suitable for shaping materials at high heat.
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Cold-Work Tool Steels: Cold-work tool steels are used in cold-forming applications like stamping and bending. They retain their hardness and wear resistance even at lower temperatures, ensuring durability in manufacturing processes.
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Air-Hardening Steels: High in chromium, this metal can be exposed to extremely high heat without distorting.
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Oil-Hardening Steels: Made to be slip-resistant, oil-quenched steel is used to produce knives and shears.
Tool steels are harder than other types, making them more durable.
Understanding the Different Types of Steel Grades
Steel is classified beyond the four major types using various grading systems. Two global steel grading systems are commonly used:
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AISI/SAE Steel Grades: The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) assign a four-digit code to classify steels based on their chemical composition and application.
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ASTM Steel Grades: The American Society for Testing and Materials (ASTM) system categorizes steel based on its mechanical properties, such as tensile strength and yield strength.
These grading systems provide valuable information for engineers, manufacturers, and designers when selecting the right steel for a particular application.
Understanding Steel's Microstructure
The microstructure of steel also plays a crucial role in determining its properties. Let's delve into some key aspects:
Ferrite and Austenite
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Ferrite is a phase with a body-centered cubic (BCC) crystal structure. It is magnetic and is often found in low-carbon steels. Ferrite is ductile (able to retain its toughness in different shapes) and offers good formability, making it suitable for applications where ease of shaping is important.
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Austenite is non-magnetic and has a face-centered cubic (FCC) crystal structure. It is more formable than ferrite and is often found in stainless steel. Austenitic stainless steels, in particular, are highly formable and corrosion-resistant, making them ideal for a wide range of applications.
Cementite, Pearlite, and Martensite
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Cementite is a hard, brittle phase that forms in high-carbon steels. It consists of iron and carbon and is extremely wear-resistant. While cementite provides hardness, it can also make the steel brittle, so it's typically found in small amounts in most steel alloys.
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Pearlite is a microstructure that results from the combination of ferrite and cementite. It appears as alternating layers or lamellae and provides a balance of strength and ductility. Pearlitic steels are often used in structural applications.
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Martensite is very hard and brittle and forms when steel is rapidly cooled, such as during quenching. It has a body-centered tetragonal (BCT) crystal structure and is characterized by its high hardness. Martensitic steels are used in applications where hardness and wear resistance are essential, such as in knife blades and industrial equipment.
Understanding these microstructures is essential for engineers and metallurgists to tailor steel properties to specific applications through heat treatment and alloy composition adjustments.
The Dynamics of Steel: Hot and Cold Forming
Understanding how steel behaves under different temperatures during forming is crucial in various industries. The choice between hot forming and cold forming depends on the specific requirements of the application:
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Hot forming involves shaping steel at high temperatures, typically above the recrystallization temperature. This process makes the steel more malleable and easier to deform. Hot forming is often used in processes like forging and rolling, where large deformations are required.
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Cold forming is done at lower temperatures, typically room temperature or slightly above. Cold-formed steel retains its strength and hardness, making it suitable for applications where dimensional accuracy and tight tolerances are essential. Processes like cold rolling and stamping are common examples of cold forming.
The choice between hot and cold forming depends on factors such as the material being used, the desired properties of the final product, and the manufacturing process's requirements.
Steel's Transformations: Heat Treatment Methods
Steel heat treatment is a critical process that alters steel's microstructure and properties. Here are some essential methods:
Spheroidizing to Full Annealing
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Spheroidizing: Spheroidizing is a heat treatment process that aims to soften steel, making it more machinable. This is particularly important for materials that will undergo extensive machining or forming processes. Spheroidizing involves heating the steel to a temperature just below its critical point and holding it there for an extended period. This process causes cementite particles to aggregate into rounded shapes, making the steel softer and more ductile.
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Full Annealing: Full annealing is a heat treatment process that involves heating the steel to its critical temperature and then slowly cooling it in a controlled manner. This process relieves internal stresses, refines the microstructure, and makes the steel softer and more ductile. Full annealing is often used to improve the formability and machinability of steel.
From Isothermal Annealing to Tempering Quenched Steel
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Isothermal Annealing: Isothermal annealing is a heat treatment process in which the steel is heated to a specific temperature and then held at that temperature for a fixed duration. This process is used to achieve specific microstructural changes, such as the transformation of austenite to pearlite or bainite. Isothermal annealing can be used to enhance the toughness and ductility of steel
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Tempering: Tempering is a heat treatment process that is typically applied to quenched steel. Quenching involves rapidly cooling the steel to achieve a hardened microstructure, often martensite. However, martensite can be excessively hard and brittle. Tempering is performed by reheating the quenched steel to a lower temperature and holding it there for a specific time. This process reduces hardness and brittleness while improving toughness. The specific temperature and time of tempering can be adjusted to achieve the desired balance between hardness and toughness, making tempered steel suitable for a wide range of applications.
Conclusion: The Versatility and Significance of Steel
Steel is an integral part of our lives, from the buildings we live and work in, to the tools we use daily. Understanding the different types, grades, microstructures, and heat treatment methods allows us to harness the full potential of this remarkable material across various industries.
Whether it's in construction, manufacturing, or even the kitchen, steel continues to shape the modern world. Its adaptability and versatility make it a cornerstone of technological progress and industrial innovation, ensuring that it remains an essential material in the years to come.
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