Grading systems standardize how the properties of steel are measured and described. They are essential for global trade and use of a versatile material.
Steel is an alloy of iron, carbon, and various other elements. Steels are the most widely used family of metals because they can be manufactured inexpensively, in large quantities, and to very precise specifications.
A steel classification system is necessary due to widespread use in engineering. The broad category of steel is broken down by relative carbon content, with mild steel at the low end, and carbon steel at the high end. These groups are further broken down into specified grades. The most important criteria for grading steels are:
- Chemical Composition
- Finishing Method
- Heat Treatment
- Mechanical Properties
1. Chemical Composition
Chemical composition is often used as the primary basis for classifying steels. The first – and roughest – distinction is between mild steel and carbon steel.
Mild steels have extremely low carbon content, ranging from near-zero up to 0.25%. That carbon content is the most important factor governing mild steel’s mechanical properties – low carbon content results in greater ductility, while higher carbon content produces a stronger steel.
Mild steels also contain 0.4 - 0.7% manganese for sulfur stabilization, 0.1 - 0.5% silicon for deoxidation, and various other elements.
The most common category of mild steels are the low carbon (less than 0.08% carbon, with no more than 0.4% manganese) variations used for forming and packaging.
Carbon steels have a higher carbon content than mild steel, to the extent that they rely on carbon content for their structure. The American Iron and Steel Institute (AISI) defines carbon steel as “steel that has properties made up mostly of the element carbon and which relies on the carbon content for structure.” They are much stronger, but less ductile, than mild steel.
Carbon steels are by far the most frequently used – more than 85% of the steel produced in the United States is carbon steel.
Most commercially available carbon steels are classified into one of three groups representing their carbon content: plain carbon, low-alloy, and high-alloy.
Plain Carbon Steels
Because they rely on carbon for their grain structure and, by extension, physical properties, plain carbon steels are defined almost entirely by their carbon content. They are broken down into the sub-categories of low, medium, high, and ultra-high carbon.
Low carbon steel typically contains 0.04 - 0.30% carbon. It is the most common grade among all carbon steels. This category machines easily, welds nicely, and is much more ductile than higher-carbon steel. Typical uses include automobile body panels, plate, wire products, forgings, seamless tubes, and boiler plate.
Medium carbon steel has a carbon range of 0.31 - 0.60% and a manganese range of 0.60 - 1.65%. It can be used in the quenched and tempered condition, with accompanying increases in hardness and tensile strength. The uses of medium carbon steels include shafts, axles, gears, crankshafts, couplings, forgings, rails, railway wheels, and rail axles.
High carbon steel contains 0.60 - 1.0% carbon and 0.30 - 0.90% manganese. It is difficult to weld; pre-heating, post-heating, re-heat treating, and sometimes even heating during welding is often necessary to produce acceptable welds. High-carbon steel is used for spring materials and high-strength wires.
Ultra-high carbon steel is an experimental alloy possessing carbon levels between 1.25 and 2.00%. It can be tempered to a great hardness level. Just like high carbon steel, welding can be tough, and controlled heat treatments are necessary to prevent cracking and to maintain physical properties. Ultra-high carbon steel is used for special purposes like knives, axles or punches.
Low and High Alloy Steels
Low Alloy Steels add alloying elements such as nickel (Ni), chromium (Cr), and molybdenum (Mo) to attain specific properties. Depending on the alloying elements used, there can be great variation between different low alloy steels.
The total alloy content can range from 2.07% to approximately 10.5%. For many low-alloy steels, the primary function of the alloying elements is to increase hardenability (the degree to which the steel will harden during heat treatment). Alloy additions can also be used to reduce environmental degradations under certain specified service conditions.
High Alloy Steels are usually a type of stainless steel, the most widely consumed commercial high alloy steel. Stainless steels contain at least 10.5% chromium, along with other alloys such as nickel to alter other properties (such as strength or weldability) in addition to corrosion resistance.
The microstructure of steel can be altered through controlled heating and cooling.
Microstructure is the small-scale structure of materials, and is only observable under a microscope. Steel can adopt several distinct microstructures: ferrite, pearlite, martensite, cementite, and austenite.
Ferrite is the structure of pure iron and steels with very low carbon content. It has a body-centred cubic (BCC) crystal structure. It is soft and ductile.
This is the structure of iron at high temperatures (over 1670°F). It has a face-centred cubic (FCC) crystal structure. It is usually not present at room temperatures, but is the structure from which other structures are formed when the material cools from high temperatures.
Cementite is a compound of iron and carbon, iron carbide (Fe3C). It is hard and brittle – its presence in steels causes an increase in hardness and a reduction in ductility and toughness.
Pearlite is a laminated structure formed of alternate layers of ferrite and cementite. It combines the hardness and strength of cementite with the ductility of ferrite. It is the key to the wide range of the properties of steels. The laminar structure also acts as a barrier to crack movement, increasing toughness.
Martensite is a very hard needle-like structure of iron and carbon. It is only formed by very rapid cooling from the austenitic structure. It needs to be modified by tempering before acceptable properties reached.
Microstructure is a useful tool for defining steel grades because it has a dramatic effect on the physical properties of metal, including strength and ductility (see section 5). Moreover, it is possible for two samples with the same alloy content to have different microstructures depending on the finishing method and heat treatments used (see sections 3 and 4).
3. Finishing Method
Rolling has the double purpose of creating a more useful shape and strengthening the steel.
After the molten metal is cast, it must be formed into its final shape and finished to prevent corrosion. Steel is usually cast into machine-ready forms: blooms, billets, and slabs. The cast shapes are then formed by rolling. Rolling can be carried out hot, warm, or cold depending on the material and target application.
Hot forming occurs over the recrystallization temperature of the metal, while cold forming occurs below it. During rolling, compression deformation is accomplished by using two work rolls. The rolls rotate rapidly to simultaneously pull and squeeze the steel between them.
Because it occurs above the all-important recrystallization temperature, hot forming breaks up the grain structure, which then re-forms in a more uniform and even distribution. Cold forming produces an improved surface finish and boosts the steel’s strength by strain hardening.
After rolling is completed, the steel pieces are finished using secondary processing techniques to prevent corrosion and improve mechanical properties.
Hot forming breaks up the grain structure, which can then re-form in a more uniform distribution with lower internal stress.
- surface treatment
- heat treatment
4. Heat Treatment
The microstructure of steel can be altered through controlled heating and cooling, so heat treatment is used in steel production to achieve a more desirable microstructure.
Steels change phase at specific temperatures. Heat treatment is based on understanding and manipulating these transformation points.
- Normalizing Temperature
Austenite is the phase from which other structures are formed, so most heat treatments begin by heating the steel to a uniform austenitic phase (1500 – 1800°F).
- Upper Critical Temperature
The temperature below which cementite or ferrite begin to form. Depending on carbon content, that point rests between 1333 and 1670°F.
- Lower Critical Temperature
The point of austenite-to-pearlite transformation. Austenite cannot exist below the lower critical temperature (1333°F).
The rate of cooling from the normalizing, upper and/or lower critical temperatures determine the resulting steel microstructure at room temperature.
Sheet steel is stored and shipped in large rolls.
Heat Treatment includes a range of processes, including annealing, quenching and tempering. Ductility and strength of steel have an inverse relationship, so these treatments can either increase ductility at the expense of strength, or vice versa.
Spheroidite forms when carbon steel is heated to approximately 1290°F for 30 hours. It produces the softest and most ductile form of steel.
- Full Annealing
Carbon steel is annealed by first heating slightly above the upper critical temperature, maintaining that temperature for an hour, then cooling at a rate of approximately 36°F per hour. This process produces a coarse pearlitic structure; it is ductile, with no internal stresses.
- Process annealing
Proccess annealing releives stress in cold worked, low carbon steel (> 0.3% C). The steel is heated to 1025 – 1292 °F for one hour.
- Isothermal annealing
High carbon steel is heated above the upper critical temperature, maintained, cooled to the lower critical temperature, again maintained, then gradually cooled to room temperature. Isothermal annealing eliminates temperature gradient.
Carbon steel is heated to the normalizing temperature for one hour; The steel completely enters the austenite phase. The steel is then air cooled. Normalizing creates a fine pearlitic microstructure with high strength and hardness.
Medium or high carbon steel is heated to the normalizing temperature, then quenched (rapid cooling by submersion in water, brine, or oil) to the upper critical temperature. The quenching process produces a martensitic structure; it is extremely hard, but brittle.
- Tempering quenched steel
This is the most common heat treatment becase its outcome can be calculated with precision. Quenched steel is reheated to a temperature below the lower critical point, then cooled. Temperatures vary according to intended outcome, but the 298 to 401 °F range is most common. This process restores some toughness to the brittle quenched steel by allowing some spheroidite to form.
5. Mechanical Properties
While steels are primarily categorized by their carbon and alloy contents, heat treatment, and microstructure, mechanical properties are the measurement with the most practical significance. In other words, the other criteria are considered important because of their impact on mechanical properties.
Mechanical properties are measured in accordance with international standards like the ASTM (American Society for Testing and Materials) or SAE (Society of Automotive Engineers). The most important mechanical properties for steel are:
The ability of a material to withstand abrasion; carbon content determines the maximum hardness obtainable in steel, or hardenability.
The amount of force necessary to deform a material. Higher carbon content and hardness result in steel with higher strength.
The ability of a metal to deform under tensile stress. Lower carbon content and less hardness result in steel with higher ductility.
The ability to withstand stress. High ductility is associated with better toughness.
- Corrosion resistance
The resistance to oxidization (rusting).
The ease with which the steel can be shaped by cutting, grinding, or drilling. Machinability is influenced by hardness, strength, thermal conductivity, and thermal expansion.
The ability of steel to be welded without defects. Weldability is primarily dependent on chemical composition and heat treatment.
For more information on the mechanical properties of steel and the various methods for testing those properties, see Cast Steel.
6. Quality Descriptors
Steel is used in many structural and otherwise sensitive applications. Without a standardized language for communicating quality, it would be extremely difficult, bordering on impossible, for consumers to navigate the diverse steels on the market. This need led to the development of fundamental quality descriptors.
The quality descriptors are applied to steel products in broad categories such as “merchant quality”, “industrial quality” or “structural quality”. These labels mark certain steels as suitable for specific applications and fabrication processes, allowing for faster market navigation and decision making. Steels are placed in specific categories based on several different factors.
- Internal soundness
- Chemical composition and uniformity thereof
- Degree of surface imperfections
- Extent of testing during manufacturing
- The number, size, and distribution of inclusions
Steel Grading Systems
Specifications, such as those issued by ASTM, AISI, and SAE (among many), provide a standard language for engineers, specifiers, fabricators and consumers to efficiently communicate the properties of a given steel. Grading is often very specific, including everything from chemical composition, physical properties, heat treatment, fabrication process, and form.
The AISI/SAE numbering system uses a 4-digit number for classification. The first two numbers indicate the steel type and alloying element concentration, and the last two numbers indicate carbon concentration.
For example, SAE 5130 describes a steel containing 1 % Chromium and 0.30% Carbon.
Letter prefixes are used as quality descriptors (such as merchant quality).
The ASTM system uses a descriptive letter followed by a sequential number. For example, A indicates a ferrous metal, and 53 is the number assigned to galvanized carbon steel. ASTM A53 would have the following properties:
- Chemical Composition, Max %
- Carbon: 0.25 (Grade A), 0.30 (Grade B)
- Manganese: 0.95 (Grade A), 1.20 (Grade B)
- Phosphorous: 0.05
- Sulfur: 0.045
- Mechanical Properties
- Tensile Strength, UTS: 330 MPa or 48,000 psi (Grade A), 414 MPa or 60,000 psi (Grade B)
- Tensile Strength, Yield: 207 MPa or 30,000 psi (Grade A), 241 MPa or 25,000 psi (Grade B)
- Form & Treatment
- Pipe NPS 1/8 – NPS 26
- Galvanized steel
- Black and hot-dipped
- Zinc coated
- Welded and seamless