Alloy steels have been the subject of tremendous interest due to their unique combination of strength and ductility, as well as their ability to be shaped and formed into a wide variety of components. Alloy steels are developed to fulfill specific requirements, such as increased strength, improved corrosion resistance, improved weldability, and improved machinability. Historically, alloy steels were developed by combining high-carbon steels with other alloying elements such as chromium, nickel, and molybdenum. Modern advances in metallurgy and manufacturing technologies have enabled the production of a multitude of modern alloy steels with a variety of microstructural characteristics that greatly increase their mechanical properties.
The melting point and crystallization properties of alloy steels differ from those of carbon steels. In general, alloy steels are seen to possess a higher melting point than the corresponding carbon steels, especially in the case of quenched and tempered alloys. This is because the alloying elements affect the lattice structure of the alloy, resulting in increased crystal lattice energy and, thus, a higher melting point. During solidification, alloy steels tend to form refined and fine-grained structures than carbon steels. This difference is attributed to the presence of alloying elements which inhibit crystal nucleation, resulting in smaller and more uniform grains. In addition to this, the presence of alloying elements can cause the alloy to form a variety of complex microstructures, such as bainite, martensite, and pearlite, which can significantly affect the mechanical properties of the steel.
In terms of strength, alloy steels are seen to possess a significant increase in strength as compared to carbon steel. The higher strength of the alloy is attributed to a combination of factors, such as increased solid solution hardening, increased cold-work hardening, and increased tempering due to the presence of alloying elements. The strength of the alloy can be affected by the choice of alloying elements and the type of microstructure present. For example, the presence of molybdenum in an alloy can increase its strength due to the formation of complex carbides and nitrides, while the presence of chromium can improve stress-reliability by increasing resistance to high-temperature oxidation.
The toughness of alloy steels is also seen to be higher than that of carbon steels. This is attributed primarily to the formation of a fine-grained microstructure and the presence of, for example, chromium, which is known to improve resistance to impact and fatigue loading. Additionally, alloy steels often possess a greater yield strength than a corresponding carbon steel, due to the formation of a fully ferritic matrix and precipitation or solution strengthening.
Due to their high strength, alloy steels are often used in the automotive, aerospace and defense industries where high strength and toughness is required. In these applications, alloy steels are often used in the form of quenched and tempered steels, wherein, the steel is subjected to a heat-treatment process that results in a combination of fine-grained microstructure of tempered bainite and martensite, along with complex precipitates at grain boundaries, which improves the strength, toughness and fatigue properties of the steel.
In conclusion, alloy steels offer a wide range of properties that are not found in carbon steels. They possess higher melting points, stronger mechanical properties, improved toughness, higher yield strength, and improved stress-reliability, which makes them the preferred choice of steels for a variety of structural applications.
