What is Alloy Steel – Definition

What is Alloy Steel – Definition

What is Alloy Steel – Definition

In general, alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties.

Steel is an alloy of iron and carbon, but the term alloy steel usually only refers to steels that contain other elements— like vanadium, molybdenum, or cobalt—in amounts sufficient to alter the properties of the base steel. In general, alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Stainless steels are specific group of high-alloy steels, that contain a minimum of 11% chromium content by mass and a maximum of 1.2% carbon by mass. Alloy steels are broken down into two groups:

  • Low-alloy Steels. Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels resulting from additions of such alloying elements as nickel, chromium, and molybdenum, manganese, and silicon. The role of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.
  • High-alloy Steels. Steels with alloying greater than 5 wt% are typically classified as high-alloy steel. Stainless steels are the major types of high-alloy steels, but two other types are ultrahigh-strength nickel-cobalt steels and maraging steels. Stainless steels are defined as low-carbon high-alloy steels with at least 10.5% chromium with or without other alloying elements.

Alloy Steel Advantages

Whether your project requires advanced corrosion resistance, machinability, strength, or another quality, there is an alloy steel that provides the features you need. With added heat treatment alloy steels can provide a wide range of beneficial qualities including:


  • Enhanced corrosion resistance
  • Increased hardenability
  • Superior strength and hardness

High & Low Alloy Steel Differentiating Qualities

A high alloy steel has alloying elements (not including carbon or iron) that make up more than 8% of its composition. These alloys are less common, because most steel only dedicates a few percent to the additional elements. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass. This ratio gives stainless steel more corrosion resistance, with a coating of chromium oxide to slow down rusting. Meanwhile, low alloy steel is only modified slightly with other elements, which provide subtle advantages in hardenability, strength, and free-machining. By lowering the carbon content to around 0.2%, the low alloy steel will retain its strength and boast improved formability.

Common Steel Alloying Elements

When it comes to steel, there are many different elements that can be added to the base material, allowing the purchaser to tweak variances until the right alloy is found. Common alloying elements include the following:

  • Manganese: Used in tandem with small amounts of sulfur and phosphorus, the steel alloy becomes less brittle and easier to hammer.
  • Chromium: A small percentage (0.5% - 2%) can help to harden the alloy; larger percentages (4% - 18%) have the added effect of preventing corrosion.
  • Vanadium: With only .15%, this element can boost strength, heat resistance, and overall grain structure. Mixed together with chromium, the steel alloy becomes much harder, but still retains its formability.
  • Nickel: Up to 5%, this alloying element will improve the steel’s strength. In excess of 12%, it provides impressive corrosion resistance.
  • Tungsten: Boosts heat resistance, so the melting point is higher. Also improves the structural makeup of the steel.

Alloying Agents in Alloy Steels

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or silicon for instance) greatly increases its mechanical strength.

Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties.

  • Carbon. Carbon is a non-metallic element, which is an important alloying element in all ferrous metal based materials. Carbon is always present in metallic alloys, i.e. in all grades of stainless steel and heat resistant alloys. Carbon is a very strong austenitizer and increases the strength of steel. In fact, it is the principal hardening element and is essential to the formation of cementite, Fe3C, pearlite, spheroidite, and iron-carbon martensite. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. If it is combined with chromium as a separate constituent (chromium carbide), it may have a detrimental effect on corrosion resistance by removing some of the chromium from solid solution in the alloy and, as a consequence, reducing the amount of chromium available to ensure corrosion resistance.
  • Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. The high-speed tool steels contain between 3 and 5% chromium. It is ordinarily used for applications of this nature in conjunction with molybdenum.
  • Nickel. Nickel is one of most common alloying elements. About 65% of nickel production is used in stainless steels. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate. Nickel based alloys (e.g. Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base elements for superalloys, which are are a group of nickel, iron–nickel and cobalt alloys used in jet engines. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.
  • Molybdenum. Found in small quantities in stainless steels, molybdenum increases hardenability and strength, particularly at high temperatures. The high melting point of molybdenum makes it important for giving strength to steel and other metallic alloys at high temperatures. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel. It retards the transformation of austenite to pearlite far more than it does the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels.
  • Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels.
  • Tungsten. Tungsten produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels resulting from additions of such alloying elements as nickel, chromium, and molybdenum, manganese, and silicon. The role of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions. Low-alloy steels may be classified into four major groups:

  • low-carbon quenched and tempered (QT) steels
  • medium-carbon ultrahigh-strength steels
  • bearing steels
  • heat-resistant chromium-molybdenum steels

Types of alloy steel

There are multiple subcategories of alloy steel. These include:

  • Low-alloy steel
  • High-strength low alloy (HSLA) steel
  • High-alloy steel
  • Stainless steel
  • Microalloyed steel
  • Advanced high-strength steel (AHSS)
  • Maraging steel
  • Tool steel

Low alloy steels generally contain less than 8 wt.% non-iron elements, whereas high-alloy steels contain more than 8 wt.% non-iron elements [2]. Both typically have superior mechanical properties in comparison to carbon steels

Properties of alloy steel

Alloy steels can contain a wide variety of elements, each of which can enhance various properties of the material, such as mechanical thermal and corrosion resistance. Elements added in low quantities of less than around 5 wt.% tend to improve mechanical properties, for example increasing hardenability and strength, whereas larger additions of up to 20 wt.% increase corrosion resistance and stability at high or low temperatures.

The effects of adding various elements to steel, along with the typical amounts in weight fraction, is summarised in the table below.

Element Symbol wt. % Function
Aluminium Al 0.95–1.30 Alloying element in nitriding steels
Bismuth Bi Improves machinability
Boron B 0.001–0.003 Improves hardenability
Chromium Cr 0.5–2.0 Improves hardenability
4–18 Corrosion resistance
Copper Cu 0.1–0.4 Corrosion resistance
Lead Pb Improves machinability
Manganese Mn 0.25–0.40 Prevents brittleness in combination with sulfur
>1 Increases hardenability
Molybdenum Mo 0.2–0.5 Inhibits grain growth
Nickel Ni 2–5 12–20 Increases toughness Improves corrosion resistance
Silicon Si 0.2–0.7 Increases strength and hardenability
2 Increases yield strength (spring steel)
Higher % Increases magnetic properties
Sulfur S 0.08–0.15 Improves machinability (free-machining steel properties)
Titanium Ti Reduces martensitic hardness in Cr steels
Tungsten W Increases hardness at high temperatures
Vanadium V 0.15 Increases strength while maintaining ductility, promotes fine grain structure