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Basic knowledge of fastener products: types, specifications, materials, and processing techniques

May 8, 2024

Basic knowledge of fastener products: types, specifications, materials, and processing techniques
Fasteners are a type of mechanical parts used for fastening connections and are widely used. It is used in a wide range of industries and can be seen in various machinery, equipment, vehicles, ships, railways, bridges, buildings, structures, tools, instruments, chemicals, instruments and supplies. It is the most widely used mechanical basic part.

It is characterized by a wide variety of specifications, different performances and uses, and a high degree of standardization, serialization and generalization. Therefore, some people call a type of fasteners with national standards standard fasteners, or simply standard parts.

Basic knowledge of fastener products (specifications)

The parameters that need to be indicated when describing fasteners are: product name (standard), specifications, material, strength level, and surface treatment. Such as: DIN912, M4-0.7x8, SCM435, grade 12.9, black.

1. Product name (standard)

Note: For screws that do not have standards and are non-standard parts, drawings need to be provided. For example, DIN912, the Chinese name is: hexagon socket cylindrical head screw, this is the product name. However, the most accurate way is to call it a standard, because GB70 also has such a product name; but the two standards are not consistent in size in many places. The most influential standards in the world are: German standards (DIN), international standards (ISO), Chinese national standards (GB), American standards (ANSI), and Japanese standards (JIS).

2. Specifications

Generally, the name of the screw is the diameter of the thread * the length of the screw. Tooth pattern diameter, commonly used metric systems include: M2, M3, M4, M5, M6, M8, M10, M12, etc.; commonly used American systems include: 4#-40, 6#-32, 8#-32, 10#- 24, 1/4-20, 5/16-18, 3/8-16, 1/2-13, etc. The screw length refers to the effective length of the embedded object. For example: the total length of countersunk head screws is loaded, the length of half the head is added to semi-countersunk head screws, and the length of cylindrical head screws does not include the head size. For example: for specifications, it is best to add the tooth pitch in the full name. For example, M4-0.7x8, the outer diameter of the 4-finger tooth is 4mm, 0.7 means that the distance between the two tooth peaks is 0.7mm, and the effective length of the 8-finger tooth embedded in the object is 8mm. For the sake of simplicity, we do not write the tooth distance. We default the coarse teeth to the standard teeth because they are the most common; in this way, there is no need to mark them. This is only available in the metric system, and American products still have to mark the tooth pitch.

Here we focus on the specifications of American screws, such as 6#-32*3/8. 6# is the outer diameter of the thread, which is close to 3.5mm; 32 is the 32 threads per inch of thread length (equivalent to the threads of metric screws). distance); 3/8 is the length of the screw (specifically the same as the metric screw). There are two formulas to remember here: tooth outer diameter A#=(Ax0.013+0.06)x25.4(mm), 1 inch=25.4mm. Among them, 2#=2.2mm, 4#=2.9mm, 6#=3.5mm, 8#4.2mm and 10#=4.8mm are the data that must be memorized. The number of teeth corresponding to the screws of each specification must also be memorized: 2#-56, 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18 , 3/8-16, 1/2-13 (American standard teeth). Note: US-made UNC teeth are standard teeth, and UNF are fine teeth. We default coarse teeth to standard teeth.

3. Material

The materials in order of the most common are: carbon steel, stainless steel, stainless iron, copper, aluminum, etc.

Carbon steel is further divided into low carbon steel (such as C1008/C1010/C1015/C1018/C1022), medium carbon steel (such as C1035), high carbon steel (C1045/C1050), and alloy steel (SCM435/10B21/40Cr). Generally, C1008 material is used to make ordinary grade products, such as 4.8 grade screws and ordinary grade nuts; C1015 is generally used to make eye screws; C1018 is generally used to make machine screws, and of course it is also used to make self-tapping screws; C1022 is generally used to make self-tapping screws. ; C1035 uses grade 8.8 screws; C1045/10B21/40Cr uses grade 10.9 screws; 40Cr/SCM435 uses grade 12.9 screws. For stainless steel, SS302/SS304/SS316 are the most common. Of course, a large number of SS201 products are also popular now, and even products with lower nickel content are called non-authentic stainless steel products. They look similar to stainless steel in appearance, but their anti-corrosion properties are very different.

4. Strength level

Strength grade mainly refers to carbon steel fasteners. Common strength grades of carbon steel screws are: 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. Nuts are correspondingly: level 4, level 6, level 8, level 10, and level 12. Generally, screws below grade 8.8 are called ordinary screws, while screws above grade 8.8 (including grade 8.8) are high-strength screws. The difference is that high-strength screws require quenching and tempering heat treatment.

5. Surface treatment

Surface treatment is mainly to increase the anti-corrosion performance, and some also take into account the color, so it is mainly for carbon steel products, which generally require surface treatment. Common surface treatments include: blackening, galvanizing, copper plating, nickel plating, chromium plating, silver plating, gold plating, Dacromet, hot dip galvanizing, etc.; there are many types of galvanizing, including blue and white zinc, blue zinc, white Zinc, yellow zinc, black zinc, green zinc, etc. are also divided into environmentally friendly and non-environmentally friendly. Each plating type has a variety of coating thicknesses to meet different salt spray test effects.

Basic knowledge of fastener products (function)

1. Functional aspects:

1. Screw torque requirements: External hexagonal screws can withstand a relatively larger torque, internal hexagonal screws can withstand a smaller torque, and cross-recessed screws can withstand an even smaller torque (so such screws are generally ordinary grade screws). 2. When assembling external hexagonal bolts, adjustable wrenches, torx wrenches and open-end wrenches are generally used. The assembly efficiency of the adjustable wrench is low, but it is very versatile and is suitable for external hexagonal screws with various head specifications; the torx wrench is the most efficient, but it is not suitable for some occasions. A torx wrench only has 2 heads, so it can only be used for There are two types of external hexagonal screws on the head; the open-end wrench is similar to the torx wrench, but it can be used with an extended sleeve. The smaller the size of the external hexagonal screw, the higher the requirements for the edges and corners of the external hexagonal bolt, otherwise the head will easily slip when subjected to (wrench) force. In order to save materials, Wenzhou people invented the external hexagonal cavity. The hexagonal screws outside the pocket are light in weight and have a thin head. They are prone to slipping under force, and the head may be screwed off.

3. Assemble the hexagonal screws with an hexagonal wrench. This has high requirements for the inner hexagonal hole. If the hole is larger, the wrench will slip, and if the hole is smaller, the wrench cannot be inserted. The smaller the inner hexagonal size, the higher the requirements for the hole. For some large-sized hexagon socket screws, as long as one of the opposite sides of the hexagon is qualified, it can be assembled normally with a wrench; for some small-sized hexagon socket screws, such as M2 hexagon socket tightening, insert the wrench and use a little force. The hexagon will slip, so if one of the opposite sides is slightly larger, it will not pass. Therefore, the wrench of M2, M2.5, and M3 hexagon socket screws (especially tightening products) is easy to slip during the assembly process. 4. Cross-recessed screws are assembled with a screwdriver and do not require too much force, so the strength only needs to be 4.8. Occasionally, some screws that require high strength only need carburizing and heat treatment. 5. When using the product, we generally recommend that the screw grade be one grade higher than the nut grade, which will provide the most economic benefits. For example, grade 8.8 screws are matched with grade 4 nuts; in this way, you only need to change the nuts next time.

2. Heat treatment:

Heat treatment is mainly for carbon steel screws, mainly quenching and tempering heat treatment and carburizing heat treatment to meet the requirements for screw strength in different environments. 1. Quenching and tempering heat treatment: Products with strength levels of 8.8 and above are all products of quenching and tempering heat treatment. The characteristic of this heat treatment is that the hardness is relatively uniform inside and outside. When the same material is heat treated, the higher the hardness, the worse the toughness. Therefore, there is a need for safe matching. While the hardness is met, the toughness is also guaranteed. 2. Carburizing heat treatment: This treatment is basically required for self-tapping screws, which are characterized by a very hard surface and a relatively soft core because they need to be drilled into a hard iron plate. Self-tapping nails have a higher risk. For example, self-tapping nails often encounter broken ends. The possible reasons are: ① Hydrogen embrittlement; ② The hardness is too high or too low, causing twisting and breaking; ③ The cross groove is too deep; ④ The head is too thin ; ⑤ There is no R angle at the head and neck joint, resulting in stress concentration; ⑥ Irregular operation, etc.

3. Risks of hydrogen embrittlement:

1. Generally, for products with hardness >32HRC, there is a risk of hydrogen embrittlement in electroplating. Therefore, the risk of hydrogen embrittlement will occur when electroplating all products above grade 10.9 (including grade 10.9) and products that have undergone carburizing heat treatment (self-tapping nails). 2. Hydrogen embrittlement means that when the product is electroplated, H+ enters the interior of the metal to form bubbles, causing the screw to continue to break on the spot during use, but delayed fracture occurs within 24 hours. 3. Products with hydrogen embrittlement risk need to be sent to a dehydrogenation furnace within 4 hours after electroplating and stored at 200 degrees Celsius for about 8 hours; this is called dehydrogenation treatment. 4. Hydrogen embrittlement treatment methods can greatly reduce the risk of hydrogen embrittlement, but it cannot be completely avoided. Therefore, when it is necessary to ensure 100% no risk of hydrogen embrittlement, it is strictly prohibited to use electroplating products; instead, use surface treatment processes such as Dacromet and sandblasting. .

4. Fastener performance and processing technology development direction:

1. It has high strength while ensuring toughness; 2. It can reduce the weight while maintaining the same size; 3. It can make the volume smaller while maintaining its mechanical strength; 4. It can enhance its toughness while maintaining its strength; 5. When the appearance is satisfactory, the anti-corrosion ability can be enhanced; 6. The accuracy limit is improved; any of the above existing high processing cost processes, new processes that can meet the requirements at relatively low cost will open up the future of our fastener industry. new ambience.