Structural Steel
What is structural steel?
Structural steel is a grade of steel designed primarily for load‑bearing and load‑receiving structures. It is an iron-based alloy whose key properties are sufficient yield strength, toughness, and good weldability. Structural steels are widely used in building frames, bridges, mechanical engineering steel structures, as well as in the frames of machines and equipment. In machine shops, structural steel is a common material for various welded assemblies, plate structures, and machine parts where strength and cost‑effectiveness are required.
Structural steel differs from, for example, tool steel or stainless steel in that its properties are optimized specifically for load‑carrying capacity and structural safety, rather than for wear resistance or corrosion resistance.
Designations and standards for structural steel
The designations of structural steels are based on the European EN standard. Typical grades include, for example, S235 and S355. The letter S refers to the word structural, i.e. structural steel, and the number gives the minimum yield strength of the material in megapascals (MPa).
For example, the yield strength of S235 steel is at least 235 MPa and for S355 steel at least 355 MPa. Additional markings, such as J2, indicate impact toughness at a given temperature. In a machine shop, this information is crucial especially when designing welded structures or determining whether a material is suitable for dynamically loaded applications.
When dimensioning structural steels, concepts such as yield strength, tensile strength and fatigue strength are often used to define the safety margins of the structure.
Chemical composition and carbon content of structural steel
Structural steels are usually low‑alloy steels with a relatively low carbon content. Typically, the carbon content remains below about 0.25 percent, which improves weldability and reduces the risk of cracking.
As the carbon content increases, the hardness and strength of the steel rise, but at the same time toughness and weldability deteriorate. For this reason, higher‑carbon steels such as spring steel or quenched and tempered steel are not considered traditional structural steels, even though they are used in machine shops for mechanically loaded parts such as shafts and gears.
Alloying elements such as manganese and silicon can be used to influence strength and impact resistance. Structural steels can be heat treated if necessary, but most often they are used in the as‑rolled delivery condition.
Structural steel in machine shops and machining
In mechanical engineering, structural steel is one of the most common machinable materials. It is machined, for example, by milling, turning and drilling when manufacturing mounting plates, frame structures, supports, and basic machine components.
The machinability of structural steel is generally good, but it depends on the steel grade and strength class. Because of its higher strength, S355 is slightly more demanding to machine than S235, which can affect the choice of cutting speed, feed, and tool material. With proper cutting parameters and suitable carbide tools, good surface finish and dimensional accuracy can nevertheless be achieved.
For welded structures, the properties of the heat‑affected zone and any required stress‑relief annealing must also be taken into account if the structure is heavily loaded.
Applications of structural steel
Structural steel is widely used in both the construction industry and mechanical engineering. Typical applications include steel frames, beams, plate structures, machine frames, parts of lifting equipment, and various load‑bearing frame structures.
S235 is well suited for lighter structures and general‑purpose use, whereas S355 is chosen for applications requiring higher load‑carrying capacity without significantly increasing the cross‑sectional area. The correct material choice directly affects the safety, weight, and cost of the structure.
Summary
Structural steel is a key material for load‑bearing structures and steel structures in mechanical engineering. Its most important properties are defined yield strength, good weldability, and sufficient toughness. The most common grades, such as S235 and S355, are specified based on their minimum yield strength. Proper selection and appropriate machining of structural steel ensure the safety, durability, and cost‑effectiveness of structures in the machine shop.
