What Is HSLA Steel?

HSLA stands for high-strength low-alloy steel. It is a type of carbon steel that has small amounts of alloying elements added to its chemical composition. The alloying elements are used primarily to increase the strength of the steel. In addition to being able to provide increased strength over carbon steel, HSLA steel can also be made to have higher toughness and be more responsive to heat treatment. The alloying elements can also be used to increase the corrosion resistance of the steel.

How Is HSLA Steel Made?

HSLA steel is made in a fashion similar to other types of steel. Iron ore and coal are combined in a furnace which melts materials burns away some of the impurities. Varying amounts and types of alloying elements are then added to the molten mixture, depending on the grade of HSLA steel. Once the proper chemical composition has been achieved, several other steps are performed to ensure there is a minimal level of contaminants in the HSLA steel. The steel is then allowed to solidify into the form of a large rectangular ingot. The HSLA steel ingot is then worked down to the final dimensions.

How Does HSLA Steel Work?

HSLA steel can have many different advantages over standard carbon steel. The addition of alloying elements increases the strength and hardness because the atoms of the elements help to block dislocation movement in the microstructure of a carbon steel. Alloying elements such as tungsten, vanadium, silicon, nickel, molybdenum, and manganese are known to increase the strength and hardness of carbon steel. Nickel is especially useful for increasing the toughness.

Corrosion resistance can also be increased in HSLA steels. Alloying elements such as copper, nickel, and chromium are able to augment the corrosion resistance of steel. This is accomplished because the copper, nickel, and chromium in HSLA steel are more apt to oxidize than the iron. This protects the iron from forming iron oxide, or rust.

Common Grades of HSLA Steel

There are many different grades of HSLA steel available because of the variety alloying element combinations that can be used. The chemical composition and grade should be dependent on the intended application of the HSLA steel.

Weathering steel is a type of HSLA Steel. It is commonly used in structural applications where a coating or layers of paint are not applied, such as bridge construction.

Another popular type of HSLA steel is the North American grade ASTM A36. ASTM A36 is a general purpose HSLA steel commonly used when erecting steel structures. It is affordable, weldable, and machinable. This versatility, combined with its excellent mechanical properties, is what makes it such a prevalent choice for structural applications.

HSLA steel is not solely used for structural applications. It frequently is used in oil and gas transmission pipelines as well. API 5L Grade X70 is one of the most common materials used for modern pipelines. The “70” in the name refers to the 70,000 psi minimum yield strength that American Petroleum Institute requires it to have.

Of course, there are numerous other grades of HSLA steel available. Some grades are more readily weldable than machinable, while others are made for abrasion resistance, or to be precipitation hardened. Best engineering judgement should be used when selecting an appropriate HSLA steel for a job.