Effect of alloying elements on mechanical properties
Adding manganese to carbon steel increases strength and hardening energy, but when the manganese content is too high, ductility and weldability deteriorate. When the copper content is more than 0.2%, the corrosion resistance can be increased, and the nickel can increase the strength and toughness at a low temperature, that is, increase the impact strength at a low temperature, and also increase the corrosion resistance. Chromium can increase hardenability, wear resistance and corrosion resistance, and can also increase the strength at high temperatures. Molybdenum can increase high temperature tensile strength and creep strength, and can also reduce temper brittleness. Boron can increase the hardenability, and the addition of boron during the heat treatment of low carbon steel is more obvious. Two or more alloying elements are added to the carbon steel, and not only the properties of each element are combined, but the main performance is to increase the hardenability, that is, to increase the depth of the hardened layer after quenching.
Boron steel is a low-carbon martensite. Due to its low carbon content and good formability, it does not need to be annealed before the screw head, so it can prolong the life of the die. Commonly used are 10B18, 10B21 and 10B22, with high hardening energy enough to meet SAE level 8 requirements, and those with slightly higher carbon content such as 10B30 for higher strength. Boron steel can be said to be quite practical. It can replace medium carbon heat-treated steel, but it is not completely flawless. Because the tempering temperature is low, the stress relaxation is poor when the temperature rises, and the screw manufacturer must strictly control the heat treatment. It is larger than other heat-treated steels with more alloying elements.
Weathering steels contain elements such as copper, manganese, nickel, and chromium. These steel screws are used in structural engineering, such as bridges, buildings, and towers. They have good corrosion resistance and do not require surface treatment.
SAE Class 8, ASTM A354 BD, A490 and F568 10.9 are high strength alloy steel screws with similar mechanical properties. SAE Class 5.2, Class 8.2, ASTM A325 2 and A490 2 are boron steels, which are considered as substitutes for grades 8.8, 9.8 and 10.9. A325 3 and A490 3 are weather resistant steel screws.
Related information: Carbon steel screws - fastener materials selection (3)
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