An alloy steel is a steel that is alloyed with various elements to improve its mechanical properties. Typically, these elements are added at a small percentage of usually no more than 5%. Technically, every steel is an alloy, but not all steels are labeled ‘alloy steel’. Steels with this label are alloyed with other alloying elements in addition to carbon. Common alloying elements include manganese, chromium, nickel, silicon, boron, and molybdenum.

Low alloy steels are the most common, with only slight chemical modification to subtly improve the properties of the steel. A high alloy steel is often defined as having alloying elements that make up more than 8% of the materials composition. Typically, low alloy steels are used to increase strength and hardenability and high alloy steels are used for special properties such as temperature stability.

Mechanical Property Improvement

As we’ve already mentioned, alloy steels have improved mechanical properties based upon the alloying elements used.

One of the most popular mechanical properties that are altered in alloy steels is the strength of the material. Manganese, silicon, copper, and nickel are common alloying elements used to increase the strength of an alloy steel, as they form solid solutions in ferrite. Tungsten, molybdenum, chromium, and vanadium can also increase strength in alloy steels by forming second-phased carbides.

Corrosion resistance is another mechanical property that is often increased by the use of alloying elements. Chromium, copper, and nickel are often used to do this, which allow these alloys to be used in more extreme environments. Alloy steels can also have properties that help ease the fabrication process, for example, bismuth and lead help to improve machinability.

Common Alloy Steels

While there are a lot of different alloying elements that can be used to improve the mechanical properties of steel, certain combinations are used often and there are different types of alloy steel that are more popular than others.

High strength low alloy (HSLA) steel is an alloy that provides greater atmospheric corrosion resistance and high strength. There are six different classifications of HSLA steel, with varying alloying elements used. Typically, vanadium, niobium, titanium, and copper are used to provide the increased strength, and copper, chromium, phosphorus, and silicon are used to increase corrosion resistance.

What is a High Strength Low Alloy Steel (HSLA Steel)?

High strength low alloy steel (HSLA steel) is an alloy that provides improved mechanical properties and greater atmospheric corrosion resistance than traditional carbon steel. These types of steels differ from ‘normal’ alloy steels as they are not designed to meet a specific chemical composition but to meet specific mechanical properties.

The Chemical Composition of HSLA Steel

The chemical composition of high strength low alloy steels consists of a low carbon content of between 0.05% – 0.25% for sufficient formability and weldability, and a manganese content of up to 2%.

The remaining chemical constituents can vary depending on the product thickness and mechanical property requirements, and small quantities of chromium, molybdenum, nickel, copper, vanadium, niobium, nitrogen, zirconium, and titration can be used in different combinations.

Strength is added to the HSLA steel with the addition of vanadium, niobium, copper, and titanium. HSLA steels can reach yield strengths of greater than 275 MPa or 40 ksi, but because of this high strength, high strength low alloy steels usually require 25% – 30% more power to form than traditional carbon steels.

Increased corrosion resistance is given to high strength low alloy steel by the addition of silicon, copper, chromium, and phosphorus. Formability is improved with the inclusion of zirconium, calcium, and other rare earth elements as they provide sulfide-inclusion shape control.

Classifications of HSLA Steel

There are six different classifications of high strength low alloy steels, which are all designed to provide specific combinations of mechanical properties to suit the application requirements.

Inclusion-Shape-Controlled Steel

This type of HSLA steel includes the addition of calcium, zirconium, titanium, or other rare earth elements, which change the shape of the sulfide inclusions from elongated stringers to small, dispersed, globules that are almost globules. This change provides this type of steel with improved through-thickness toughness and ductility.

Microalloyed Ferrite-Pearlite Steel

Microalloyed ferrite-pearlite steel contains strong carbide or carbonitride-forming elements, in very small quantities – often less than 0.10%. These elements could be titanium, niobium, and/or vanadium, and they provide the HSLA steel with precipitation strengthening, grain refinement, and the possibility of transformation temperature control.

Dual-Phase Steel

These steels have a ferrite microstructure that contains uniformly distributed, small sections of martensite. This microstructure provides these high strength low alloy steels with ductility, high tensile strength, low yield strength, high rate of work hardening, and good formability.

Acicular Ferrite Steel

These HSLA steels are characterized by a very fine high strength acicular ferrite structure. They are low carbon steels, with high yield strengths, excellent weldability and formability, and good toughness.

As-Rolled Pearlitic Steel

As-rolled pearlitic steels often include carbon-manganese steels, but they may also have small additions of other alloying elements to provide enhanced strength, formability, weldability, and toughness.

Weathering Steel

Weathering steel, also known by the trade name Corten, is a type of HSLA steel that exhibit a high level of atmospheric corrosion resistance and tensile strength. This type of high strength low alloy steel is commonly used in outdoor structures such as bridges due to these properties.

Steel Alloy Advantages

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

Enhanced corrosion resistance
Increased hardenability
Superior strength and hardness
Unique alloyed features

Common Alloying Elements Used by Steel Suppliers

Steel is commonly alloyed by suppliers in order to provide consumers with the most effective metal for their application. This blog post will outline some of the most common alloying elements used by steel suppliers.

Nickel (Ni) Nickel is used by steel suppliers to promote an austenitic microstructure. Austenitic stainless steel is created with nickel compositions of more than 8% in addition to over 18% chromium composition. Austenitic stainless steel benefits from greatly improved corrosion resistance, as the corrosion state is reduced, making these steels useful in acidic environments. Nickel can also be used for martensitic grades, which improves weldability when combined with a reduced carbon content. The use of nickel also improves the mechanical properties of steel, increasing the toughness and ductility.
Chromium (Cr) Chromium is the most common alloying element for steel, and it is used in percentages of over 11% for stainless steel. This element provides stainless steel with its high level of corrosion resistance and prevents oxidation in many conditions.
Molybdenum (Mo) Like chromium, molybdenum greatly improves the resistance of the steel to corrosion. Molybdenum also promotes a ferritic microstructure, but it increases the risk of the formation of secondary phases in ferritic, duplex, and austenitic steels. This element can also improve the tensile strength, hardenability, and toughness of steel.The hardenability is improved as molybdenum lowers the quench rate required during the heat treatment process to make hard steel. The element also reduces the risk of steel pitting, as the resistance to chloride-induced corrosion is improved.
Vanadium (V) Carbide and nitrides are formed at lower temperatures when vanadium is added. These carbides are used to keep the grain size of the steel small, as they block the formation of the grains. The finer grain structure helps to increase the ductility of the steel.Vanadium increases the hardness and strength of martensitic steels due to the effect it has on the type of carbide present.
Manganese (Mn) Manganese is often used to improve the hot ductility of steels. It is employed to help with the heat treatment process of steels to improve hardness and strength. A quench usually has to be performed rapidly to obtain the desired mechanical properties. This fast quench can cause the process to become unstable.

Typical Steel Alloy Applications

Steel alloys can be forged into various shapes, including pipes, tubes, plates, sheets, coils, bars, rods, wires, forged fittings, buttweld fittings, flanges, fasteners, and more. Steel alloy industrial applications include the following: