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Several common surface treatment methods of bolt and nut

According to the properties of nuts, there are national standard (GB), German standard (DIN), international standard (ISO), Japanese standard (JIS), American Standard (ASTM / ANSI) and other standards. Among them, national standard, German standard and Japanese standard are represented by M (for example, M8, M16), American system and English system are represented by score or × (for example, 8 × 10 × 10, 1 / 4, 3 / 8). Nuts which are screwed together with bolts or screws to play a tightening role. According to different materials, all production and manufacturing machinery must use a component, which is divided into carbon steel, stainless steel, non-ferrous metals (such as copper), etc.

These three materials are introduced in detail below: 

(I) carbon steel

We distinguish low carbon steel, medium carbon steel, high carbon steel and alloy steel by carbon content in carbon steel. 1. Low carbon steel

C% ≤ 0.25% is commonly referred to as A3 steel in China. Foreign countries are basically called 1008101510181022, etc. It is mainly used for grade 4.8 nuts, grade 4 nuts, small screws and other products without hardness requirements. (Note: 1022 material is mainly used for drilling tail nail. ) 2. Medium carbon steel C%: 0.25% – 0.45%.

At present, alloy steel is not used in the market. 4. Alloy steel: add alloy elements to ordinary carbon steel to increase some special properties of steel: such as 35, 40 chrome molybdenum, SCM435, 10b38. 

(II) stainless steel

Performance grade: 45, 50, 60, 70, 80 mainly consists of austenite (18% Cr, 8% Ni) with good heat resistance, corrosion resistance and weldability. A1, A2, A4 martensite and 13% Cr have poor corrosion resistance, high strength and good wear resistance. C1, C2, C4 ferritic stainless steel. 18% Cr has better upsetting property and better corrosion resistance than martensite. At present, the imported materials are mainly Japanese products. It is mainly divided into SUS302, SUS304 and SUS316 by level. In addition to stainless steel bolts, many of our commonly used carbon steel bolts require surface treatment, so what are the common treatment methods for bolts? The most common methods of bolt and nut surface treatment are as follows:

1. Zinc plating 

Electro galvanizing is the most commonly used surface treatment method for fasteners. It not only has low cost, but also has beautiful appearance. It can be electroplated into black and military green. However, one of the disadvantages of electro galvanizing is that its anti-corrosion performance is general, and its anti-corrosion performance is the lowest among the plating (coating) layers. Generally, the screws after electro galvanizing can pass the neutral salt spray test within 72 hours, and special sealing agent is also used, so that the salt spray test after electro galvanizing can reach more than 200 hours, but its price is expensive, 5-8 times that of general galvanizing.

20191209221722 42728 - Several common surface treatment methods of bolt and nut

The screw fastener is easy to produce hydrogen embrittlement during the surface treatment of galvanizing. Generally, we do not recommend the surface treatment of galvanizing for bolts above grade 10.9. Although the hydrogen can be removed by oven after galvanizing, the passivation film will be damaged when it is above 60 ℃, so the hydrogen removal must be carried out before passivation after galvanizing, but such operation will increase the processing cost and operability Poor performance, so generally if the customer does not require hydrogen removal, the manufacturer will not take the initiative to remove hydrogen, which also needs special attention. 

2. Surface phosphating 

Phosphating is cheaper than galvanizing, but its corrosion resistance is worse than galvanizing. After phosphating, screw fasteners should be coated with oil, because its corrosion resistance has a lot to do with the performance of the oil. After phosphating, apply general antirust oil, and the salt spray test can only take 10-20 hours. If some advanced antirust oil is applied, the screw fastener can take 72-96 hours, but the price is 2-3 times of phosphating oil. Most of the industrial screw fasteners are treated by phosphating + oiling, because its torque and pre tightening force have good consistent performance. In the assembly of parts and components, it can meet the expected fastening requirements, so it is widely used in industrial construction. Especially for the connection of some important parts: some high-strength bolts adopt phosphating, which can also avoid the problem of hydrogen embrittlement. Therefore, in the industrial field, the bolts above grade 10.9 are generally treated with phosphating surface. 

3. Blackening (oxidation) 

The surface of steel parts blackened, also known as bluing. Its principle is to oxidize the surface of iron and steel products rapidly, so as to form a dense oxide film protective layer. The commonly used methods of blackening treatment are the traditional basic heating blackening and normal temperature blackening. Room temperature blackening process is mainly completed by the following process steps:

  • 1) cleaning;
  • 2) degreasing: the workpiece must be completely immersed in the degreasing solution; the concentration pH of the degreasing solution is 12-14, the treatment time is 10-30min, and the degreasing powder is added when the concentration of the solution is lower than pH12;
  • 3) water washing;
  • 4) acid pickling: acid pickling solution concentration pH 2-4, treatment time 5-10min;
  • 5) water washing;
  • 6) blackening: the pH value of the pool solution was 2.5-3.5, and the treatment time was 10-12 min;
  • 7) water washing;
  • 8) drying;
  • 9) oil.
The advantages of blackening treatment are as follows:
  • 1. Blackening is safe without electricity, and 100% electricity is required for blackening with alkaline high temperature.
  • 2. Improve work efficiency: 1-2 minutes in total.
  • 3. The blackening cost is low, the equipment is simple and the operation is convenient; the blackening time is strictly controlled.
  • 4. Strong process adaptability: solved the problem that nodular cast iron cannot blacken. Blackening for wear and rust.

Blackening and oiling of screw fasteners are common coatings for industrial fasteners. Because it’s cheap, it’s cheaper than all the surface treatments. But the blackened surface treatment basically has no antirust ability, so it will rust soon if it is not oiled. Even when it is oiled, its salt spray test can only reach 3-5 hours. The consistency of torque and pre tightening force of screw fastener after blackening treatment is also poor. To improve the consistency of torque and pre tightening force, it can only be screwed in after applying grease on internal thread during assembly. 

4. Cadmium plating 

The corrosion resistance of cadmium plating for screw fasteners is very good, especially in marine atmospheric environment. However, the cost of waste liquid treatment in the process of cadmium plating is large and high, and the price is about 15 * 20 times that of galvanizing. Therefore, it is not used in general industries, but only used in some specific environments. 

5. Chromium plating 

The chromium coating of screw fastener is stable in the atmosphere, not easy to change color and lose luster, high hardness and good wear resistance. Chromium coating is generally used as decoration on fasteners, but it is rarely used in industries with high corrosion resistance. Because good chrome plated fasteners are as expensive as stainless steel, only when the strength of stainless steel is not enough, chrome plated fasteners can be used instead. In order to improve the corrosion resistance of chromium plating, copper and nickel should be plated before chromium plating. Chromium coating can withstand the high temperature of 1200 degrees Fahrenheit (650 ℃), but it also has the same problem of hydrogen embrittlement as galvanizing.

6. Silver and nickel plating on the surface 

The use of silver coating for screw fasteners can not only prevent corrosion, but also be used as solid lubricant for fasteners. Due to the cost, the nuts are usually silver plated, the bolts are not used, and sometimes the small bolts are also silver plated. Silver is tarnished in the air, but can work at 1600 degrees Fahrenheit. Therefore, people use its high temperature resistance and lubrication characteristics to work in high temperature fasteners to prevent oxidation of bolts and nuts. The nickel plating of fasteners is mainly used in places with good corrosion resistance and conductivity. Such as the outgoing terminal of vehicle battery, etc. 

7. Galvanizing (hot dip galvanizing and zinc infiltration) 

The surface treatment method of hot-dip galvanizing is: the zinc is heated to the liquid and the coating is thermally diffused. The thickness of the coating is 15 ~ 10 μ m, and it is not easy to control, but it has good corrosiveness, and is often used in engineering. In the process of hot-dip galvanizing, there are serious pollution, such as zinc waste and zinc vapor. Due to the thick galvanized coating of screw fasteners, it is often difficult to screw in the internal and external threads of fasteners. There are two ways to solve this problem. One is to tap the internal thread after plating, although the problem of screw connection is solved. But it also reduces the corrosion resistance. One is to make the thread larger than the standard thread by about 0.16-0.75mm (m5-m30) when tapping the nut, and then hot-dip galvanizing can solve the problem of screw in, but it affects the strength grade of the screw. Now, there is also a kind of anti loose screw – the use of the American “staples” internal thread can solve this problem. Because the internal thread and external thread have large tolerance when they are not tightened, they can be used to accommodate the thick coating, so they do not affect the screw in performance, while the anti-corrosion performance and strength remain the same and are not affected. Due to the temperature of hot-dip galvanizing process, it can not be used for fasteners above grade 10.9. Zincification treatment method: generally within the range of 10-190 μ m, the thickness of the zincification layer can be controlled according to the user’s requirements, with an error of ≤± 10%. For the workpiece with screw thread and matching requirements, interchangeability can be guaranteed, while the thickness of hot-dip galvanizing process is difficult to control, and interchangeability can not be guaranteed. In addition, large fitting clearance should be reserved for the workpiece to damage the binding strength of the fitting parts. The vacuum powder zincification overcomes the defects of hot-dip galvanizing, such as zinc tumor and burr, and makes the appearance of the zincification parts flat and smooth, even without any treatment after zincification, it is also very beautiful. The thickness of the zinc layer is uniform, and the thickness of the vacuum powder zinc layer is the same with that of other parts even in the thread, blind hole and corner. In the process of using, there will be no local premature corrosion, and it can prevent the tip discharge of high voltage transmission. This is a problem that can not be solved by the hot-dip galvanizing process, especially for the screw thread, blind hole and other parts. The zinc layer is a zinc iron alloy layer, which has a very compact structure and a firm bond with the steel matrix. The electrode potential is lower than that of iron and higher than that of zinc. The surface hardness is high and the wear resistance is good. Its hardness is more than one time higher than that of steel body and four times higher than that of hot-dip zinc layer. In the process of transportation, assembly and disassembly, the zinc layer is not easy to wear, scratch and fall off. The coating performance of zinc coating is good. The zinc layer can be directly painted, plastic coated or coated with polymer materials without any treatment. The bonding force of coating is 3-4 levels higher than that of hot-dip zinc, and the bonding force reaches the first level of national standard. The mechanical properties of the steel matrix material are not changed by the zinc infiltration process. The forming temperature of the zinc layer is lower than the transformation temperature of the steel, which does not damage the mechanical properties of the steel matrix, and there is no hydrogen embrittlement. It can deal with calcined parts, assemblies, special-shaped parts, high-strength steel parts, cast iron and steel castings (not broken), spring steel (not inelastic) and section steel (not deformed). 

8. Screw surface treatment Dacromet

20191209221251 60227 - Several common surface treatment methods of bolt and nut 

There is no hydrogen embrittlement in the surface treatment of Dacromet for screw fasteners, and the consistent performance of torque preload is very good. But its pollution is serious. In fact, it is most suitable for high strength fasteners with high anti-corrosion requirements without considering the problems of chromium and environmental protection.

Source: China Bolt and Nut Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

What are different types of fasteners?

There are three major steel fasteners used in industries: stainless steel, carbon steel, and alloy steel. The major grade used in stainless steel fasteners: 200 series, 300 series, and 400 series. Titanium, aluminum, and various alloys are also common materials of construction for metal fasteners.

How do you identify a fastener?

As a general rule, the length of fasteners is measured from the surface of the material, to the end of the fastener. For fasteners where the head usually sits above the surface such as hex bolts and pan head screws, the measurement is from directly under the head to the end of the fastener.

What do the numbers on metric bolts mean?

In addition, most metric bolts are identified by a number stamped on the bolt head, such as 4.6, 5.8 or 10.9. The number has nothing to do with the size, but does indicate the relative strength of the bolt. The higher the number, the stronger the bolt.

What is self tapping screw?

As the name implies, it simply means a screw that can be tapped by itself. The difference between self-tapping screw and general screw is that the general screw must have a machined screw hole to screw in. Self tapping screw is not necessary, while screwing in the tapping itself. In order to achieve their own tapping purposes, the hexagonal drill tail screw is generally pointed, in order to facilitate the squeezing into the object to be screwed.
In all cases, the larger the number, the larger the size. Consequently a #8 screw is larger than a #4 screw, just as a 3 inch bolt is larger than a 2 inch bolt. … Common threads per inch sizes are: 4, 4-1/2, 5, 5-1/2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 24, 28, 32, 36, 40, 48, 56, 72, 80.



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