When the chromium content of the iron alloy is more than 12%, it is called stainless steel. Since chromium is a non-corrosive element, the stainless steel has good corrosion resistance, and the higher the chromium content, the better the corrosion resistance. All stainless steels contain carbon in addition to iron and chromium. Carbon can increase hardness, but it has an adverse effect on corrosion resistance. Because chromium and carbon form carbides, chromium in the middle of carbide has no antioxidant properties. When the carbon content is increased, the chromium content also needs to be increased, otherwise the corrosion resistance will be deteriorated, so most of the stainless steel has a low carbon content, and the carbon content must be strictly controlled. In addition, all stainless steels contain other alloying elements, each with its own characteristics. For example, nickel is the most important element, which can significantly improve corrosion resistance, low temperature brittleness and high temperature strength. In addition, molybdenum, copper, silicon, aluminum, selenium, sulfur, antimony, cobalt, titanium, etc. are all important alloying elements, and their components can be controlled and formulated to obtain the desired mechanical properties.
Stainless steel is not easy to rust because the metal surface naturally forms an invisible oxide film that prevents oxidation. When the screw processing process is such as forging and turning, the surface may be contaminated by tiny metal particles generated by the processed die, and there may be contamination if there is subsequent heat treatment. If the screw is not cleaned and delivered for use after it is finished, the appearance looks like a rusty appearance, not the product is rusted, but it is actually caused by impurities or impurities that are buried in the surface. Therefore, the stainless steel screws must be pickled before delivery, and the surface will quickly form an oxide film and remove the surface contaminants.Stainless steel is divided into four categories: austenite, ferrite, martensite and precipitation hardened steel. Each type has its own advantages and disadvantages. Austenitic stainless steel is the most widely used. About 80% of stainless steel screws are made of it. Its microstructure is mainly austenite, with chromium and nickel as the main alloying elements. As long as it is cold-rolled, its mechanical properties can be improved. . Commonly used content is 18% chromium, nickel 8% is called 18-8 or 300 series, corrosion resistance is better than ferrite and martensitic stainless steel, and it is not magnetic, and has high strength at very low temperature or elevated temperature. And good toughness. Austenitic stainless steels include 301, 302, 303, 303 Se, 304, 305, 384, XM 7, 316, 321 and 347 and the like.
303 and 303 Se (17/19% chrome, 8/10% nickel) are easy to turn and are not suitable for cold heading. 304 (18/20% chrome, 8/10.5% nickel, 0.08% or less carbon) is suitable for cold, hot enthalpy, and good corrosion resistance. It is commonly used to manufacture hot-machined screws with complex shapes and large dimensions. 305 (17/19% chromium, 10.5/13% nickel) reduces work hardening speed and is easy to form by cold heading. 384 (15/17% chromium, 17/19% nickel, 0.08% carbon) is specially used for cold forging nut and Phillips groove screw processing. Due to the high nickel content, the work hardening speed can be reduced. After 384 cold heading, it is still non-magnetic, but other austenitic stainless steels will be somewhat magnetic after cold heading and must be annealed to restore non-magnetic properties. XM7 (17/19% chromium, 8/10% nickel, 3/4% copper) is improved from 302, has better cold heading and is less expensive than 305 and 384. 316 (16/18% chromium, 10/14% nickel, 2/3% molybdenum, 0.08% or less carbon), because it contains molybdenum, it is excellent in halogen corrosion resistance, and it is still higher than other austenitic stainless steels at elevated temperatures. High tensile strength and creep strength. 321 (17/19% chromium, 9/12% nickel) and 347 (17/19% copper, 9/13% nickel) are stable stainless steels, which still have good corrosion resistance at temperatures up to 820 °C. Manufacture of aerospace industry or screws used in environments where high temperatures or chemicals contaminate the environment.
The microstructure of ferritic stainless steel is mainly ferrite, accounting for 5% of stainless steel screws. Only chromium is the main alloying element, which is magnetic and has better corrosion resistance than martensite. The content of other elements is very small, and this stainless steel is characterized by rust and corrosion resistance. All use 430 (14/18% chromium, 0.12% carbon) to make screws, mostly used for cold heading and hot enthalpy. Adding sulfur to 430F can improve turning performance. If economics, material costs, and corrosion resistance are considered, it is more appropriate to use ferritic stainless steel to make screws. Both ferrite and austenitic stainless steel cannot be quenched and hardened. Only the drawing and cold rolling can be used to improve the strength and hardness, but the ductility is deteriorated, so annealing is usually applied to remove the residual stress and restore the ductility.
The microstructure of martensitic stainless steel is martensite, which accounts for about 10% of stainless steel screws. It is made of chromium as the main alloying element. It has magnetic properties and can be quenched and hardened to obtain the highest mechanical properties. It is graded with SAE 5, 8 and ASTM A449. , A354 BD level approximation. However, the corrosion resistance is worse than that of austenitic and ferritic stainless steels. Screws are commonly used in 400 series materials, such as 410, 416, 416 Se and 431.
The strength of 410 (12.5/13.5% chromium, 0.15% or less carbon) is similar to that of SAE grade 5 or A449. After heat treatment, it can increase the strength, and it is easy to be cold and hot. Because of its low chromium content, it is the cheapest among all stainless steels. If SAE 5-grade galvanized or cadmium-plated screws have insufficient corrosion resistance, they can be replaced with 410.
416 and 416 Se (12/14% chromium, 0.15% carbon, sulfur or arsenic), turning performance may be the best of all stainless steels, mechanical properties are the same as 410. 431 (15/17% chromium, 1.25/2.5% nickel, 0.2% carbon) is commonly used in the manufacture of aerospace screws. It has high strength, good corrosion resistance, easy cold heading and enthalpy, and its mechanical properties are no less than SAE Class 8 and ASTM A354 BD level.
Precipitated hardened steel screws account for 5% of stainless steel screws, and their use has gained more and more attention. It has corrosion resistance comparable to austenite and high strength comparable to martensite. For example, 630 (15.5/17.5% chromium, 35% nickel, 0.07% carbon, 0.15/0.45% é’¶ é’½) is also known as 17-4PH is the most commonly used precipitation hardened steel for the manufacture of screws, in addition to high strength, It is also malleable and can withstand high and low temperatures.
in conclusion
This paper mainly introduces the composition and mechanical properties of carbon steel screw materials commonly used in industry. Others such as aluminum alloy, copper alloy, nickel alloy, titanium alloy or super alloy also have different characteristics and uses. (Finish)related suggestion:
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