Introduction
Alloy structural steels are referred to as medium-carbon steels that are alloyed with elements such as manganese, silicon and molybdenum to strengthen them. Their use for buildings, railway bridges and shipbuilding is an example. Alloy structural steels are generally divided into three major categories, namely low-alloy steels, medium-alloy steels and high-alloy steels.
Chemical Composition
The main elements of alloy structural steel are iron, carbon and silicon, accounting for about 99% of the total mass, and other elements are titanium, manganese, sulfur, phosphorus, chromium, boron, nickel, molybdenum, aluminum, copper and vanadium. The most commonly used alloy structural steel contains the following elements: Carbon (C): 0.2-2.0%; Silica (Si): 0.17-0.37%; Manganese (Mn): 0.6-1.2%; Sulfur (S): ≤0.04%; Phosphorus (P): ≤0.035%; Chromium (Cr): 0.3-1.0%; Boron (B): ≤0.0025%; Nickel (Ni): ≤0.2%; Molybdenum (Mo): 0.15-0.2%; Aluminum (Fe) 0.15-0.3%; Copper (Cu): 0.1-0.35%; Vanadium (V): 0.05-0.1%; In addition, manganese is a very important chemical element of alloy structural steel. It improves the strength of the steel and increases its ability to be heat treated.
Heat Treatment
The alloy structural steel needs to be heat treated before and after processing. Usually, the quenching and tempering treatment is adopted, which needs to be heated to 850-900°C and then quenched with water or oil to obtain a certain mechanical properties. The quenching process is used to reduce the hardness and brittleness of the steel, while the tempering process is used to improve the toughness and plasticity so that the steel has a better mechanical properties. After quenching and tempering, the strength, hardness and impact toughness of the steel can be greatly improved.
Mechanical Properties
The mechanical properties of alloy structural steel are related to the chemical composition of the steel. The mechanical properties of the steel increases with the increase of the carbon content, but the strength and hardness increases faster than the toughness when the carbon content increases. The strength and hardness of the steel will drop after tempering, and the toughness and plasticity will increase significantly. The mechanical properties of the alloy structural steel includes: yield strength (the greatest amount of stress which the material can withstand before begin to plastic deformation), tensile strength (the greatest stress when the steel sample is subjected to tension), elongation (the degree of elongation of the steel under tension), reduction of area (the percentage reduction of the area of the steel sample under tensile strength), impact toughness (the ability of steel to resist impact force), and hardness (the ability of steel to resist permanent deformation).
Grade and Applications
The grade of the alloy structural steel can be divided into low-alloy structural steel, medium-alloy structural steel and high-alloy structural steel.
The low-alloy structural steel is mainly used for ships, vehicles, bridges, construction and other structures. Some of these steels are AISI 1020, AISI 1030 and AISI 1045.
The medium-alloy structural steel is mainly used for pressure vessel, boiler and special steel. Some of these steels are AISI 4140, AISI 4320, AISI 4340, AISI 4130, AISI 4140, AISI 4145 and AISI 4145.
The high-alloy structural steel is mainly used for special machinery die sets and other industrial applications, such as AISI 8620, AISI 8630, AISI 9310, AISI 9320 and AISI 9315.
Conclusion
Alloy structural steel is an important structural material in various civil engineering, shipbuilding and other industrial applications. It has good strength, toughness and plasticity and has higher requirements for its chemical composition and heat treatment process. In order to obtain the best performances of the steel, the alloy structural steel must be strictly in accordance with the grade requirements and heat treatment process requirements.
