Carbon steel screws - Over 90% of the screws are made of carbon steel because they have good processing properties and are easy to obtain and inexpensive. Carbon steel screws have more than 100 strength grades, and are mostly used for special purposes. There are not many grades that are generally applied in engineering. The strength grade of carbon steel screws is divided into three categories: low carbon steel (carbon content <0.3%), medium carbon steel (carbon content 0.3~0.6%) and alloy steel. Alloy steel is divided into low alloy steel (content of alloy elements <8%) and high-alloy steels (content of alloy elements>8%), high-carbon steels (carbon content>0.6%) are not suitable for manufacturing screws due to their high strength and difficulty in processing. At present, the most cited by the industry is the SAE J429 screw grading system. There are 10 grades from low carbon steel grade 1 to alloy steel grade 8, among which the more important grades are also cited in ASTM specifications, such as A307, A449, A325, and A354. And A490 and so on. The grading system for metric carbon steel screws described in ISO 898/I is very similar to SAE J429. ASTM F568 is a replica of ISO 898/I and describes the classification of screws commonly used in North America.
Low-carbon steel screws commonly used material chemical composition is AISI 1006, 1008, 1016, 1018, 1021 and 1022, such screws equivalent to SAE Class 1, ASTM A307 Class A, ASTM F568 Class 4.6, with good processability, can be Cold working improves strength and can also be surface hardened and welded. Grade A307 Grade B is used for fittings and flanges. Except for increasing the upper limit of tensile strength, other properties are the same as Grade A307 A. The purpose of setting the upper limit of tensile strength is to damage the cast iron flange before it breaks when the screw is over-locked, thus protecting the more expensive lines, valves, etc.
Medium carbon steel screws can significantly increase their tensile strength through heat treatment. The commonly used materials are AISI 1030, 1035, 1038 and 1541. These materials have good processability, but when the carbon content increases, the processing difficulty becomes higher. Because the tools and molds used for machining are easy to wear, the service life is reduced. Therefore, normal processing or spheroidizing treatment is usually performed before processing to reduce strength and facilitate turning. For example, if the carbon content is less than 0.5%, annealing and normalization can make the distribution of martensite more uniform and improve the turning performance. If the carbon content is more than 0.5%, it can be spheroidized to improve the turning performance.
The strength of the heat-treated screw is directly related to the size of the screw. When the chemical composition of the screw is the same and the heat treatment method is the same, the larger the size, the lower the strength. For example, the strength of the SAE grade 5 and the ASTM A449's imperial screw is large. The size is lower than the size. However, ISO 8.8 and 9.8 metric screws are not exactly the same. 9.8 screws with a diameter of 16 mm or less have higher strength, but 8.8-grade screws have higher strength. The use of medium carbon steel can produce 8.8 grade 24 mm screw strength, if the production of 24 mm or more, you need to use alloy steel, such as grade 10.9, and the heat treatment after the strength will be more ideal.
The heat-treated medium carbon steel screws have a higher tensile strength per unit cost than other metals, while the yield strength calculated per unit of tensile strength is the lowest, indicating excellent ductility and the best balance between materials. This is why SAE Class 5, ASTM A449, ASTM A325, F568 8.8, and 9.8 are the most commonly used screw strength progressions because of their cost, manufacturing convenience, and mechanical properties.
When the content of manganese in carbon steel is greater than 1.65%, silicon content is greater than 0.6%, copper content is greater than 0.6%, or chromium content is less than 4% (if greater than 4%, it is close to stainless steel), or contains a trace amount of aluminum, copper, boron, cobalt , molybdenum, nickel, titanium, vanadium, zirconium, or other added elements to produce a certain degree of influence, this time is called alloy steel. The commonly used alloy steel compositions are AISI 1335 (manganese steel), 4037 (molybdenum steel), 4140 (chrome molybdenum steel), 4340 (nickel-chromium-molybdenum steel), 8637 (nickel-chromium-molybdenum steel), and 8740 (nickel-chromium-molybdenum steel), As long as you understand its mechanical properties, you know why it is so widely used.