Producing steel of a quality that can meet all of industry’s needs requires classification to be considered broadly. The classification of steels can be outlined as follows:
Classification of Steels by Application
Among metals, steel has the widest range of applications. This aspect of steel has not only increased its production volume but also brought variety to that production. In a sense, it has made it possible to produce and classify steel according to its intended application. A typical example of classification by application is construction steel. Steel used in the reinforced-concrete sections of all kinds of structures is known as construction steel. Whatever field a steel is produced and used for, it takes that field’s name and is classified accordingly. For example: bolt-and-nut steel, spring steel, rail steel, and so on.
Classification of Steels by Chemical Composition
When classifying steels by chemical composition, the material’s carbon, phosphorus, sulfur, or silicon ratios are taken into account.
Classification by chemical composition is done under two headings.
- Plain carbon steels.
- Alloy steels.
Plain Carbon Steels
The cheapest and most effective alloying element in steel is carbon. Steel can contain up to 1.7% carbon. Every change in the carbon ratio also means a change in the steel’s properties.
Plain carbon steels are grouped into two categories, taking the eutectoid ratio into account.
Hypoeutectoid Steels
Mild steels. Carbon ratio: 0.1-0.2%
Low-carbon steels. Carbon ratio: 0.2-0.3%
Medium-carbon steels. Carbon ratio: 0.3-0.85%.
Hypereutectoid Steels
Steels with more than 0.85% carbon in their internal structure.
For more details, see: Plain Carbon (Non-Alloy) Steels and Their Classification
Alloy Steels
Steel is an alloy of iron and carbon. But this alone doesn’t make it an “alloy steel.” For a steel to be called alloyed, the nonmetals or metals present in its internal structure need to change the properties of the base alloy. If the metal or nonmetal added to the steel gives it a new property, that steel becomes an alloy steel.
The alloying elements added to steel impart their own properties to the steel. Steel is classified based on the number of new alloying elements.
- Simple (low) alloy steels. These are steels with one alloying element besides carbon in their internal structure. Examples of this type of steel include nickel steel, chromium steel, and vanadium steel.
- Double alloy steels. These are steels with two alloying elements besides carbon in their internal structure. Examples include chromium-nickel, chromium-molybdenum, and chromium-vanadium steels.
- Multi-alloy steels. Steels in this group have several alloying elements besides carbon in their internal structure. Examples include chromium-nickel-vanadium and chromium-tungsten-cobalt steels.
Effect of Carbon Content
A steel’s hardness and strength generally increase as its carbon content increases.
Alloying Elements
Alloying elements like chromium, nickel, and molybdenum give steel corrosion resistance and high-temperature strength.
Effect of Heat Treatment
The same steel can acquire very different mechanical properties depending on the heat treatment applied (annealing, quenching, etc.).
Classification of Steels by Quality
Since steel is a material, it’s inevitable that there will be quality differences among its types. As a result, we get a classification based on quality. Classifying steels by quality falls under three groups.
1. By formability: A steel’s quality affects forming operations like cutting, drilling, forging, and bending. Steels are classified based on these forming operations.
2. By structural properties: A steel’s structural properties reveal its resistance to corrosion, heat, and wear. Besides these, hardenability characteristics also depend on structural properties. For example: steels suitable for through-hardening, steels whose properties don’t change, and steels suitable for hardening.
3. By microscopic structure: When steels with different carbon ratios are examined under a microscope, their structural differences can be seen. Based on this, steels are grouped into three categories: ferritic, austenitic, and martensitic steels.
Classification of Steels by Hardening Medium
A steel’s suitability for hardening is linked to the carbon ratio in its internal structure. Steels with less than 0.20% carbon in their internal structure can’t be hardened. Steels outside this ratio can be hardened.
For steel to harden, it needs to go through heat treatment. In general terms, heat treatment means heating the steel and then cooling it in a controlled way. The cooling process is done very rapidly at first, then slowed down over time. The medium in which the cooling is done is called the hardening medium. Water, oil, and air are the most commonly used hardening media. Steels are classified according to the medium they’re hardened in — such as water-hardening steel, oil-hardening steel, and air-hardening steel.
Related Questions
Classification by application practically shows where the steel will be used (such as construction, springs, rails). Classification by chemical composition, on the other hand, provides the technical data needed for engineering calculations by specifying the steel’s carbon and other element ratios; these two different needs require two different classification systems.
Plain carbon steels don’t contain expensive alloying elements (like chromium or nickel) in their production — they’re based only on iron and carbon. This simple composition lowers production cost, making them more economical.
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