High Strength Low-Alloy Steels (HSLA) – All You need to know
High strength low-alloy steel (HSLA) refers to alloy steel that have high strength-to-weight ratio. Compared to carbon steel grades, HSLA steels have better corrosion resistance as well as greater mechanical properties. HSLA steel is not defined by its alloying elements or composition. Rather, the improvement of mechanical properties by an alteration of the microstructure determines whether a metal can be defined as HSLA.
What are HSLA steel grades?
High strength low-alloy steel provides better mechanical properties compared to carbon steel. Generally speaking, grain size is reduced to reduce pearlite structure, increasing the material’s yield strength. Typical elements that are added to achieve this are titanium, copper, niobium and vanadium. The carbon content of HSLA steel can be anywhere between 0.05 and 0.25% (in mass content) in order to retain formability and weldability.
Various alloying elements can be added for different effects other than just strengthening. For example, nitrogen can be added to improve wear protection and resistance to localized corrosion. Other elements include, but are not limited to, nickel, chromium, molybdenum and calcium. However, as low-alloy steels, the combined amount of alloying elements (excluding carbon) does not exceed the limit of 2% in mass content.
Due to its altered microstructure, HSLA material does not rust as fast as carbon steel because of its ferrite structure. That does not mean that the metal can not rust at all. Rust prevention, or the spreading thereof, can be influenced by using certain alloying elements, such as chromium, which forms a protective layer of chromium oxides instead of iron oxide.
Micro-alloyed steels contain very small amounts of alloying elements (0.05 – 0.15%), meaning they are very low-alloyed. Without any heat treatment, its yield strength is only 500 to 750 MPa, which is close to the yield strength of carbon steel (415 MPa). Weldability can be improved by reducing the carbon content to 0.05%. Micro-alloyed grades can be cold-or hot-worked to achieve greater ductility and mechanical strength. On the plus side, the material is not prone to crack due to quenching, nor do it have to be straightened.
Weathering steels are high strength low alloy steels that are known for their high resistance to corrosion and abrasion compared to other grades. The term ‘weathering’ is derived from the word ‘weather’ because this type forms a layer on its surface for protection against weather influences. This protective layer develops due to different alloying elements: carbon, silicon, manganese, phosphorus, sulfur, chromium, copper, vanadium, and nickel.
The reason why such grades have a high corrosion resistance is not because it does not rust. Quite on the contrary, the metal has to rust to form its protective layer. Carbon steels form iron oxides on their surface which spreads not only on the surface, but can also cause cavitation, meaning that oxidation can penetrate deeper into the material. The alloying elements of weathering grades, however, form strong oxides (rust) on the surface, preventing any deeper corrosion. This is also why such grades can be recognized by their rusty color.
Pearlite refers to the atomic structure of materials. It is made up of alternating strips of ferrite (body-centered cubic) and cementite (orthorhombic structure). Perlitic steels are known for their high hardness and high yield strength. To form a fully pearlitic structure, there has to be at least 0.8% carbon content. Due to the high content of carbon, pearlite grades are more susceptible to abrasive wear and cutting force. Pearlite-reduced grades aim to improve these mechanical properties by producing fine grain ferrites. Therefore, there’s little to no pearlite in the microstructure.
Dual-phased steels have a ferritic-martensitic structure. Therefore, they have a high ultimate tensile strength and low initial yielding stress. Further, they are more malleable than microalloyed steels and show great fatigue resistance. Furthermore, they are deoxidized, meaning all the oxygen is removed from the material during the steelmaking process, reducing gas porosity greatly. Dual-phase steels are often used for automotive parts such as wheels.
Enhancing carbon steel to properties of HSLA
HSLA grades show greater mechanical properties compared to simple carbon steel. However, their properties can be enhanced to the same levels as HSLA grades or even exceed those by means of Borocoat® treatment. Our boronizing technique improves surface hardness even on unalloyed steels and can provide greater corrosion and outstanding wear resistance.