There are many types of steel available, and each has its own purpose. The way they are classified depends on how much carbon is mixed with iron, as well as their other alloys and treatment.
The two most comprehensive grading systems used in the industry are the ASTM and AISI/SAE numbering system. These use four-digit numeric classifications that highlight a steel’s base carbon or alloy content.
1. Carbon Steel
Steel grades are generally categorized into four groups – Carbon, Alloy, Stainless, and Tool. Carbon steels contain only trace amounts of elements besides iron and carbon. They are the most common type of steel, making up 90% of the world’s steel production. Carbon steels are further broken down into three subcategories – Low Carbon Steels/Mild Steels, Medium Carbon Steels, and High Carbon Steels.
Almost all carbon steels are designed for welded applications, and many are also used in machine tools because they have good weldability. They are mainly comprised of iron with low amounts of carbon, but do contain small amounts of manganese, phosphorus, sulfur, and silicon. These alloys are added to improve tensile strength and weldability, but may influence crack susceptibility in high-temperature conditions and require special treatment when welding.
When it comes to the grading of these metals, standards organizations like AISI and SAE have developed systems that use four-digit numbers to categorize carbon and alloy steels. The first two digits denote the steel type and the last two digits indicate the concentration of the main alloying element (usually aluminum or nickel).
There are also additional symbols that can be part of these classification codes, but they are far too numerous to go into here. These designations indicate any special requirements or conditions that are dictated by the standard to which the steel belongs.
2. Stainless Steel
In the modern industrial world, stainless steel is used widely due to its wide array of beneficial properties. It is extremely hygienic, easy to clean and durable, making it an essential component in many different industries. The material is also highly recyclable and versatile, with an extensive range of applications in manufacturing and construction.
Like carbon steel, stainless steel is categorized by an alloy grade system to differentiate the various variants of the metal. The system is generally classified by both a letter and numeric code, with the letters indicating general alloy categories and the numbers describing variations in physical properties. The ASTM and SAE grades are two of the most common alloy grading systems.
The grading system is further subdivided into categories and families that are based on their crystalline structure, microstructure and hardening mechanism. Within each group, there are several variants, each with a unique chemical composition and appearance. The most popular grades of stainless steel are 304 and 316.
Each steel grade has a variety of characteristics that make it useful for different purposes. For example, stainless steel that contains nickel adds to the corrosion resistance of the material in acidic environments while molybdenum increases strength. When identifying the appropriate steel for your project, it is important to understand these distinctions to ensure that you are choosing the best possible alloy.
3. Alloy Steel
Steel is a versatile metal that can take on many different properties thanks to the elements it’s combined with. Different alloys, as well as the way it is heated and cooled, can change its characteristics, making it ideal for a wide range of applications.
In terms of its use, steel is a popular material for construction projects. It is made into flat products like plates and strips, which can then be used to build up structures by welding them together to create walls and roofs. It’s also found in long products such as girders, structural sections, bars, rails, rods and wires.
Alloy steels contain a greater percentage of alloying elements than carbon steels. This means they have a higher level of strength, but it can be difficult to work with due to its increased malleability. Nonetheless, the versatility of alloy steels makes them ideal for large diameter flange manufacturing.
The rate at which manufacturers cool their alloy steels can play a significant role in its final characteristics, as it impacts how strong it is on a molecular level. This is why they are classified into four categories by a numbering system developed by American Iron & Steel Institute (AISI) and Society of Automotive Engineers (SAE). The naming convention for these grades involves a letter designating the specific application followed by a numerical value, which indicates the mechanical property dictated by the standard.
4. Tool Steel
In the steel world, tool steel refers to a variety of different metals that are used to make molds and cutting tools. These steels tend to have high hardness that allows them to hold their shape and resist deformation and cracking. They also have great toughness that allows them to withstand impact and wear. These steels can be categorized into unalloyed and low alloyed and then further subdivided by quenching methods into oil, air and water hardening steels.
The main alloying elements for these types of steel include tungsten, chromium, vanadium and molybdenum. These alloys increase the hardenability of the steel so it can withstand greater stresses during heat treating without risk of distortion or cracking. These steels also have a good surface hardness that is important for cutting and grinding applications.
Depending on the application and the steel fabricator, there are other alloys that can be added to enhance its properties as well. For example, if the metal is used in plastic injection molding, or press hardening applications, then additional additives may be added to improve thermal conductivity to shorten cycle times and boost productivity. These additional alloys can include chromium, molybdenum, copper or tungsten. It is important for fabricators to know which alloys are used and the specific grades so they can ensure that they get the right type of steel for their particular applications.