From the tallest skyscrapers to your kitchen cutlery, steel is essential for many projects. Understanding the differences between steel types helps you choose the right metal for your next project.
The type of iron and its alloying elements determine a steel grade. Two comprehensive grading systems have been developed by standards organizations to help engineers and fabricators accurately categorize steels.
1. Carbon Steel
Steel is one of the most widely used metals on the planet. It’s a key component of everything from building materials to home appliances and is the foundation for transportation networks and telecommunication infrastructure. While many think of steel as a pre-set combination of iron and carbon, the fact is that it can be manipulated in any number of ways to create a wide variety of different steels with specific properties and uses.
Depending on the percentages of these elements and the addition of other alloying materials, carbon steel can be created to be malleable and ductile or hard and durable. This type of steel is typically used in construction materials and automotive components, but it’s also the backbone for things like cutting tools that retain their sharp edge and masonry nails that can be driven into concrete blocks or bricks without breaking (although if they are mistreated, these types of carbon steel can become very brittle).
When you see a grade designation on a piece of steel, this will indicate what specific properties are required for the application. Two of the most comprehensive grading systems include the alphanumeric classifications of ASTM and the four-digit numerical classification of the Society of Automotive Engineers (SAE). The first digit indicates the general categorization of the steel and the last digit indicates the specific properties (such as yield strength or tensile strength) that the steel is required to have for the given application.
2. Stainless Steel
As its name implies, stainless steel is a type of steel that resists corrosion. It does this by creating a thin layer of chromium on its surface that creates a barrier that prevents iron from interacting with oxygen at the steel/air interface. This makes it highly resistant to rust and other oxidants, which is why many metals manufacturers use it in manufacturing equipment, building components, and even consumer products like sinks, refrigerators, and kitchen utensils.
Despite its name, not all stainless steel is the same. In fact, there are more than 100 different metallurgical grades of stainless steel. These get broken down into distinct metallurgical “families” based on their alloy content and physical properties, such as toughness, weldability, and corrosion resistance.
The most common type of stainless steel is 304, which contains 18% chromium and 8% nickel. Also known as 18/8 steel, this grade is often used in food preparation and cutting equipment and can be found in automotive and aerospace structural parts.
Another popular option is 430 stainless steel, which is low on nickel and uses molybdenum, niobium, aluminum, and titanium as alloying elements to enhance toughness. These grades offer good corrosion resistance, weldability, and ductility, making them suitable for a wide range of industrial applications. Then there’s precipitation hardening stainless steel, which has a high chromium and nickel content along with additions of aluminum, copper, and niobium to allow it to be hardened through solution and aging treatment processes.
3. Alloy Steel
Steel is one of the most popular and versatile industrial materials in existence. It is used to create many of the items we use on a daily basis, from automobiles to construction elements, and it comes in numerous variants for different uses. But despite how ubiquitous it is, many people are still unaware of the intricacies and differences between various types of steel.
To address this issue, a number of organizations have worked to classify and categorize the different types of steels for the reference of fabricators and consumers alike. Most notable among them are the American Society for Testing and Materials (ASTM) grading system, which assigns each metal a letter prefix based on its general category and then a sequentially assigned number that indicates the variations in its physical properties, and AISI/SAE’s unified steel grade classification system.
Alloy steels are created when carbon steel is mixed with one or more alloying elements, such as manganese, silicon, nickel, chromium, titanium, vanadium and aluminum. These additions create a wide range of attributes that can greatly improve the material’s strength, formability or corrosion resistance.
For example, adding aluminum can make an alloy of steel more ductile and weldable, while increasing its hardness and tensile strength. The type of heat treatment used in the cooling process also plays a role in the final quality of an alloy of steel. For example, an alloy of steel with the code 1214 can be made more machinable by including the additional symbol ‘L’ in the classification code, which denotes that the metal has been hardened with lead.
4. Tool Steel
Unlike other grades, tool steels feature very precise chemical compositions and manufacturing processes that make them ideal for the job they need to do, like metal forming. They also have very high strength and resistance to wear. They may contain chromium, cobalt, molybdenum, nickel and vanadium in varying amounts to provide specific properties such as hardness, toughness and corrosion resistance.
The two main numbering systems for steels are the American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE). Both have several different grades that can be used in various applications depending on the requirements. The AISI and SAE use four-digit numeric codes to classify steels.
For example, type O grade alloys are general purpose tool steels that can hold an edge after sharpening and machining. These types also tend to be oil quenched, which allows for a slower cooling process that reduces the likelihood of distortion during the heat treatment production phase.
Other categories of tool steel include cold work, high speed and impact-resisting. Alloys such as tungsten, molybdenum and chromium are added to these types to promote hardening during the heat treatment process. These alloys can also improve the hardness of the final product while increasing its durability. The impact-resisting category of tool steel offers optimum resistance to chipping and breakage, making it popular for punches and trim dies.