For reliable longevity and to bear extreme weight, structural steel must be of the right composition. Iron and carbon are two of the most vital components used by steel mills when making structural steel. The carbon lends strength to the iron ore, which is the source for the iron in steel and is quite soft on its own. To achieve load-bearing capacity, structural steel must have a higher carbon content by weight, and manufacturers can increase the amount of carbon according to the level of strength and ductility its application requires. Most construction purposes only have the need for low-carbon, or mild, structural steel, which contains between 0.04 and 0.30% carbon by weight. Medium- and high-carbon structural steel requires from 0.31 to 1.50% carbon by weight, making this steel suitable for mechanical engineering applications.
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Structural steel can also contain levels of manganese, phosphorus, sulfur, and silicone, among other materials. While manufacturers can add additional metals such as chromium, titanium, and molybdenum to their steel compositions to achieve greater strength, this is typically best for non-structural steel as it can result in a brittle end-product.
Whatever the composition, manufacturers must test their structural steel for acceptable yield and tensile strengths. Part of what makes structural steel strong is its ability to yield under weight pressure without permanently changing shape. The point at which structural steel does irrevocably change shape is called its yield strength. Additional weight pressure will eventually bring the steel to its tensile strength limit, the point at which the steel actually breaks. Yield and tensile strength are measured in pounds per square inch (psi) and kilopounds per square inch (ksi).
For evaluating impact or energy absorption within structural steel, the Charpy impact test has standardized the process. Operators utilize a weighty hammer pendulum and a structural steel material sample to calculate how much energy that particular steel can absorb when the pendulum strikes it before the material reaches yield and tensile strength limits. The Charpy test can also incorporate temperature testing to mimic environmental temperature fluctuations.
Steel is one of the most versatile and useful materials on the planet. Steel mainly consists of iron (Fe) and carbon (C), but modern steel is more complex than that. Steel’s characteristics and strength are affected by the concentration of carbon and iron or the inclusion of other elements, which allows steel to be used in an infinite number of scenarios.
Most people believe that steel is just a set combination of iron and carbon, but there are actually over 3,500 different grades of steel! You can determine the grade of steel by analyzing the quantity of carbon in it, the other alloying elements it includes, and how it’s processed.
In this article, we’ll discuss the four different types of steel, how they’re classified, the various steel grades and the methods of heat treatment used to improve steel’s mechanical properties.
Steel is graded and classified into four groups:
What are these many forms of steel made of, and what purpose do they serve? Let’s find out!
Aside from carbon and iron, carbon steels contain only trace amounts of other components. Carbon steels are the most common of the four steel grades, accounting for 90% of total steel production! Carbon steel is classified into three subgroups based on the amount of carbon in the metal:
Find out more about carbon steels in our mild steel vs. carbon steel guide!
Companies frequently produce these steels in large quantities since they are relatively inexpensive and robust enough to be used in large-scale construction.
Alloy steels are made by combining steel with additional alloying elements such as nickel, copper, chromium and/or aluminum. Combining these elements improves the strength, ductility, corrosion resistance and machinability of the steel. These types of steel are available in various steel grades to meet different performance requirements, ranging from high-strength structural uses to applications requiring enhanced wear resistance.
Stainless steel grades are alloyed with 10–20% chromium along with nickel, silicon, manganese, and carbon. Because of their increased capacity to survive adverse weather, these steels have phenomenally high corrosion resistance and are safe to use in outdoor construction. Stainless steel grades are also commonly used in electrical devices.
For example, 304 stainless steel is widely sought after for its ability to withstand the environment while keeping electrical materials safe.
While different stainless steel grades, including 304 stainless steel, have a place in buildings, stainless steel is more often sought after for its sanitary properties. These steels are widely found in medical devices, pipes, pressure vessels, cutting instruments and food processing machinery.
Tool steels, as the name suggests, excel in cutting and drilling equipment. The presence of tungsten, molybdenum, cobalt and vanadium helps improve heat resistance and general durability. These types of steel are specifically engineered to withstand high temperatures and repeated stress, making them ideal for industrial applications. And because they hold their shape even under heavy use, they are the preferred material for most hand tools. Learn how tool steel is made here!
Beyond the four groups, steel can also be classified based on different variables, including:
Steel grading systems allow us to categorize steel varieties based on their use case. For example, the rate at which steel is cooled by steel manufacturers might affect its molecular strength. The length of time they can maintain steel at critical temperatures throughout the cooling process is also essential. Therefore, two sheets of steel with the same alloy content can have various grades depending on the heat-treatment technique.
The ASTM Grading System assigns each metal a letter prefix based on its general category (“A” for iron-based alloys and steel materials) and a sequentially allocated number corresponding to that metal’s unique qualities.
In contrast, the SAE Grading System employs a four-digit number. The first two figures represent the steel type and alloying element concentration, while the latter two digits indicate the metal’s carbon concentration.
Steel grading standards are frequently used by scientists, engineers, architects, automotive engineers, and government bodies to ensure the quality and consistency of materials. These standards provide consistent terminology for communicating the properties of steel in great detail and direct product manufacturers toward suitable processing and application techniques.
Steel grading systems consider chemical composition, treatment, and mechanical qualities to help fabricators choose the best product for their application. These systems categorize different types of steel into distinct steel grades, ensuring that the right material is used for specific functions. Aside from the actual percentage of carbon and other alloys in the material, the microstructure has a considerable impact on steel’s mechanical properties. For instance, mild steel grades are often selected for their balance of strength and ductility, while other grades might prioritize hardness or corrosion resistance.
It is critical to understand the meaning of microstructure and how steel microstructure can be modified through hot and cold forming as well as post-manufacturing. These methods can be used to create goods with unique mechanical qualities. Manipulation of chemical composition and microstructure, on the other hand, will result in a trade-off between distinct qualities.
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The microstructure of a substance is the way molecules are bonded together with forces acting between them. Heating and cooling operations shift the microstructure from one form to another, affecting the material’s characteristics.
The microstructure cannot be seen with the human eye but can be studied under a microscope. Steel can have a variety of microstructures, including:
The molecular structure of pure iron at normal temperature is referred to as ferrite. This microstructure will also be found in steel with very low carbon content. The ferrite’s distinguishing feature is a body-centred cubic (BCC) crystal structure. The molecules in BCC are more loosely packed than in other microstructures that contain more molecules per cube.
At room temperature, however, the quantity of carbon that may be supplied without affecting the ferrite microstructure is limited to 0.006%.
Austenite is a microstructure generated when iron-based alloys are heated over degrees Fahrenheit but below degrees Fahrenheit (982 degrees Celsius). If the correct alloy, such as nickel, is present in the steel, the material will retain its microstructure even after cooling.
Austenite is distinguished by its face-centred cubic (FCC) crystal structure. The molecules in austenite are more densely packed than those in ferrite. Austenite, a common stainless steel microstructure, can contain up to 2% carbon.
When carbon steel is heated to austenite temperatures and cooled without any alloy to maintain the austenite shape, the microstructure reverts to ferrite.
However, if the carbon level exceeds 0.006%, the excess carbon atoms bond with iron to create iron carbide (Fe3C), also known as cementite. Cementite does not form on its own since a portion of the material is ferrite.
Pearlite is a laminated material composed of alternating layers of ferrite and cementite. It happens when steel is progressively cooled, generating a eutectic combination. A eutectic mixture occurs when two molten materials crystallize at the same time. Under these conditions, ferrite and cementite form concurrently, resulting in alternating layers within the microstructure.
Martensite has a tetragonal crystalline structure that is body-centred. This microcrystalline form is achieved by rapidly cooling steel, which traps carbon atoms inside the iron lattice. The end product is a needle-like iron and carbon structure. Steel with a martensite microcrystalline structure is typically a low-carbon steel alloy with about 12% chromium content.
To prevent corrosion, molten steel must be shaped into its final shape and then finished. Steel is often cast in machine-ready shapes such as blooms, billets and slabs. Rolling is then used to shape the casts. Depending on the material and intended application, rolling can be done hot, warm or cold.
Compression deformation is done during rolling by using two work rolls. The rolls rotate quickly, pulling and squeezing the steel between them.
Cold forming is the process of rolling steel at temperatures lower than its recrystallization temperature. The pressure applied by the rolls on the steel generates dislocations in the material’s microstructure, resulting in grains in the substance.
As the number of dislocations increases, the steel gets harder and more difficult to deform. Cold rolling also creates brittleness in the steel, which can be remedied with heat treatment.
After rolling is complete, the steel pieces are finished using secondary processing techniques to increase corrosion resistance and improve their mechanical properties, such as:
Spheroidization happens when carbon steel is heated to °F (699°C) for 30 hours. The pearlite microstructure’s cementite layers are changed into spheroid shapes, resulting in the softest and most ductile form of steel. This process is commonly used to enhance the workability of steel material, especially in applications that require extensive forming or machining. It is particularly beneficial for mild steel grades, as it further improves their ductility and malleability.
Carbon steel is annealed by heating slightly beyond the upper critical temperature for an hour, then at a rate of around 36°F (2°C) per hour. This procedure yields a coarse pearlitic structure that is flexible and free of internal tensions.
Process annealing relieves stress in cold-worked, low-carbon steel (> 0.3% C). For one hour, the steel is heated to –°F (552–700°C). Before cooling, dislocations in the microstructure are corrected by reconstructing the crystal. Annealing is often applied to types of steel, like mild steel grades, to enhance their machinability and relieve stress from previous manufacturing processes. The resulting steel material becomes easier to shape and form.
High-carbon steel is heated above its upper critical temperature first. The temperature is then maintained, reduced to the lower critical temperature and maintained once again. After that, it is gradually cooled to room temperature. This procedure guarantees that the material has reached a uniform temperature and microstructure before proceeding to the next cooling stage. This process is typically associated with high-carbon steel.
For one hour, carbon steel is heated to the normalizing temperature. The steel has now entered the austenite phase. The steel is then cooled by air. Normalization results in a fine pearlitic microstructure with high strength and hardness. By normalizing, the steel material achieves improved uniformity in strength and hardness, making it suitable for applications that require consistent performance under stress.
In this process, medium or carbon steel is heated to the normalizing temperature, then quenched (rapid cooling in water, brine, or oil) to the upper critical temperature. The quenching process results in a martensitic structure, which is highly hard but fragile. This method is widely applied to specific types of steel where maximum hardness is required, such as in cutting tools or wear-resistant components. While quenching significantly increases the hardness of the steel material, it often requires additional tempering to reduce brittleness and enhance durability.
This is the most popular heat treatment since the outcome is predictable. Tempering is used across various types of steel to balance hardness and toughness, making the steel material less brittle after quenching. Quenched steel is reheated and chilled to temperatures below the lower critical point. Temperatures vary depending on the desired effect, with 298–401°F (148–205°C) being the most typical.
There is no universal “best” grade of steel, as the optimal steel grade for an application depends on many factors, such as the intended usage, mechanical and physical requirements, and financial limits.
Steel grades that are regularly used and deemed the top series from each type include:
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