Keywords: fasteners, Yijin hardware engineer explained in detail the selection principle of stainless steel wire for fasteners and several methods of identification
Most stainless steel materials can be made into steel wire or bar for fastener production, there are Austenitic stainless steel, ferrite stainless steel, martensitic stainless steel, precipitation hardened stainless steel. About the selection of materials for stainless steel wire fasteners, screw master is like this: fasteners are mechanical basic parts, great demand, usually bolts, screws, rivets, and other bolts in order to ensure safety or generally do not consider the influence of temperature in a bad environment or other dangerous conditions. Common materials are carbon steel, low alloy steel, and non-ferrous metals. But in certain situations, fastener materials need to meet severe corrosion or high strength conditions, there is much stainless steel and ultra high strength stainless steel emerge.
Through the comprehensive and comprehensive consideration of these five aspects, the final determination of the selection of fastener grades, varieties, specifications, material standards.
Stainless steel material selection mainly from the following five aspects to consider.
- The fastener material in the mechanical properties, especially the strength of the requirements.
- Requirements for corrosion resistance of materials under working conditions.
- The working temperature of the material heat resistance (high-temperature strength, oxidation resistance) requirements.
- Requirements for processing properties of materials in terms of production technology.
- Other factors, such as weight, price, and purchase, should be taken into account.
Austenitic stainless steel
Austenitic stainless steel refers to stainless steel with an austenitic structure at room temperature.
Austenitic cr-Ni stainless steels include the famous 18Cr-8Ni steel and the high Cr-Ni series steels developed by adding Elements such as Mo, Cu, Si, Nb, and Ti on the basis of Cr and Ni content.
The most commonly used austenitic stainless steel is fe-cr-ni alloy (that is, the United States AISI300 series); Fe-cr-ni-mn (American AISI200); Special austenitic stainless steel, and other three.
Commonly used grades are 302, 303, 304, 305 four grades, that is, the so-called “18-8” type austenitic stainless steel these four grades. Both corrosion resistance and mechanical properties are similar. The starting point of selection is the production process method of fasteners, and the method depends on the size and shape of fasteners but also depends on the number of production.
Model 302 is used for machined screws and self-tapping bolts. In order to improve the machinability, a small amount of sulfur is added to model 303 stainless steel for the use of bar processing nuts.
Type 304 is suitable for processing fasteners with hot upsetting processes, such as longer size bolts, large diameter bolts, which may be beyond the scope of the cold upsetting process.
Model 305 is suitable for cold heading processing of fasteners such as cold forming nuts, hex bolts.
Type 309 and 310, which have higher Cr and Ni content than type 18-8 stainless steel, are suitable for fasteners that work at high temperatures.
Type 316 and 317, which contain the alloying element Mo, thus their high-temperature strength and corrosion resistance are higher than type 18-8 stainless steel.
Type 321 and type 347, type 321 contains relatively stable alloy element Ti, type 347 contains Nb, which improves the resistance to intergranular corrosion. Suitable for fasteners that are not annealed after welding or in service at 420 ~ 1013℃.
Ferritic stainless steel
Ferrite stainless steels (400 series) contain 15%~30% chromium and have a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb, and other elements. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, and so on. It is mainly used for manufacturing parts that are resistant to atmospheric, water vapor, water, and oxidizing acid corrosion. Generally can be divided into ordinary ferrite stainless steel and high purity ferrite stainless steel two categories.
1. The ordinary ferrite
This type of steel includes low, medium, and high chromium content. Low chrome ferrite stainless steel, containing chromium about 11% ~ 14%, such as 00Cr12, 0Cr13Al China. AISI400, 405, 406MF from the United States. This kind of steel has good toughness, plasticity, cold deformation, and weldability. Because the steel contains a certain amount of chromium, aluminum, it has good oxidation resistance and stainless property. The 405 can be used as a petroleum refining tower, tank lining, steam turbine blade, high-temperature sulfur corrosion resistant device, etc. Used for home and office supplies, etc. 409 used for automobile exhaust muffler system and cold and warm water pipes. Chromite ferrite stainless steel, the chromium content of 14% ~ 19%, such as China’s 1Cr17, 1Cr17Mo. American AISI429, 430, 433, 434, 435, 436, 439. This kind of steel has good rust resistance and corrosion resistance. Its work hardening coefficient is small (N ≈2), has a good deep drawing performance, but poor ductility. Used for building decoration, car decoration, kitchen equipment, gas burner, nitric acid industrial plant components, etc. 434 for cars, exterior decoration of buildings. 439 used for gas water heater, coal, gas pipeline hose, etc. High chromium ferrite stainless steel containing 19% ~ 30% chromium, such as China’s Cr18Si2, Cr25, the United States of America’s AISI442, 443, 446. This kind of steel has good oxidation resistance. 442 in the atmosphere intermittent use, the upper temperature is 1035℃, continuous use of the highest temperature is 980℃. 446 has better antioxidant properties.
Type 430 ordinary chromium steel, has better corrosion resistance and heat resistance than type 410, magnetic, but it can not heat treatment to strengthen, suitable for corrosion resistance and heat resistance slightly higher, the strength of general stainless steel fasteners.
2, High purity ferrite
This kind of steel contains very low carbon, nitrogen; High chromium, molybdenum, titanium, niobium, and other elements. Such as China’s 00Cr17Mo, 00Cr18Mo2, 00Cr26Mol, 00Cr30Mo2 foreign 18-2, Cr26Mol, 25Cr-5Ni-4Mo-Nb, Monit, AL29-4, AL29-4-2. This kind of steel has good mechanical properties (especially toughness), weldability, resistance to intergranular corrosion, pitting corrosion, crevice corrosion, excellent stress corrosion cracking resistance. For example, 18-2 has good corrosion resistance in nitric acid, acetic acid, and NaOH, and its pitting resistance in 3%NaCl and FeCl3 is equal to or greater than 18-8 austenitic steel, and its SCC resistance is far greater than 18-8 steel. 26CrMo steel is resistant to corrosion in many media, especially organic acids, oxidizing acids, and strong alkalis. It has good pitting resistance in a strong chloride medium. Stress corrosion cracking does not occur in chloride, hydrogen sulfide, excessive sulfuric acid, and strong alkali. 30CR-2Mo has a higher resistance to pitting and crevice corrosion while maintaining stress corrosion resistance. A small amount of nickel is added to steels to improve their properties in reducing acids.
Martensitic stainless steel
The mechanical properties of stainless steel can be adjusted by heat treatment. Colloquially, it is a kind of hardening stainless steel. Typical brand is Cr13 type, such as 2Cr13,3Cr13,4Cr13, etc. It has high hardness after quenching, different tempering temperature has different strength and toughness combinations, mainly used for steam turbine blades, tableware, surgical instruments. According to the difference in chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium-nickel steel. According to the different structure and strengthening mechanisms, can also be divided into martensite stainless steel, martensite, and half austenite (or half martensite) precipitation hardening stainless steel and martensite aging stainless steel, etc.
Standard martensitic stainless steel is: 403, 410, 414, 416, 416(Se), 420, 430, 431, 440A, 440B and 440C, magnetic; The corrosion resistance of these steels comes from “chromium”, which ranges from 11.5 to 18%. The higher the chromium content, the higher the carbon content, to ensure martensite formation during heat treatment. The three types 440 stainless sheets of steel mentioned above are rarely considered for applications requiring welding, and the type 440 filler metal is not readily available.
Models 410 and 416 can be heat treated to strengthen, hardness 35 ~ 45HRC, good machinability, for general purpose heat and corrosion-resistant fasteners. Type 416 has a slightly higher sulfur content and is free-cutting stainless steel. Model 420, ≧ 0.15% sulfur content, improved mechanical properties, heat treatment can be strengthened, the maximum hardness value of 53 ~ 58HRC, used for fasteners requiring higher strength.
Standard martensite steel improved with adding elements such as nickel, molybdenum, vanadium, etc, are mainly used in the standard steel limited allowed working temperature to rise to higher than 1100 k, when adding these elements, carbon content increases, with the increase of carbon content, the welding material hardening problems to avoid cracking in the heat-affected zone become more serious.
Precipitation hardened stainless steel
Precipitation Hardening stainless steel refers to the addition of different types and amounts of strengthening elements on the basis of the chemical composition of stainless steel, and the precipitation hardening process precipitates different types and amounts of carbides, nitride, carbonitride, and intermetallic compounds. A class of high-strength stainless steel, PH steel, increases the strength of steel and maintains sufficient toughness. Precipitation hardened stainless steel can be divided into martensite type, semi-austenite type, and austenite type according to the metallographic structure of its matrix.
Precipitation hardened stainless steel has high strength, high toughness, high corrosion resistance, high oxidation resistance, excellent formability, weldability, and other comprehensive properties.
17-4pH, PH15-7Mo, they can obtain higher strength than the usual type 18-8 stainless steel and are therefore used for high strength, corrosion-resistant stainless steel fasteners.
A-286, A non-standard stainless steel, has higher corrosion resistance than the commonly used 18-8 stainless steel, as well as good mechanical properties at increased temperature. Used as high strength, heat and corrosion-resistant fasteners can be used to 650 ~ 700℃.