Automobile fastener variety optimization and material and heat treatment

Automobile fastener variety optimization and material and heat treatment

According to statistics, there are currently about 240 commonly used standards for automotive standard parts, including tube sealing components, pipe line connecting fixtures, washers, screws, nuts and bolts, etc., of which 115 standards and metal fastening Related, accounting for about 48%. With the continuous development of the automotive industry, the automakers have increased the cost of production and management to curb the increasing number of metal fasteners (hereinafter referred to as automotive fasteners), from structural elements, materials, heat treatment and surface treatment. In terms of the benchmarking of fasteners, this paper discusses this.

Auto Fastener Variety Optimization

Materials and Heat Treatment

Nut production process

How are our common steel nuts produced

How do you distinguish between stainless steel screws, galvanized and nickel-plated screws

Auto Fastener Variety Optimization
Bolt

For the hex head bolt + spring washer assembly, the anti-loose effect is better when the bolt pre-tightening force is lower, but the spring washer is not used for important parts, so the main use precision for the bolt with spring washer is ±40. The pneumatic wrench of about % is tightened, the assembly torque and the axial force difference are large, the spring washer is often in the flattened state, and even the risk of ringing is increased. The actual anti-loose performance of the bolt+spring washer assembly is uncontrollable and cannot meet the product design requirements. . For hex flange bolts, there are the following advantages.

1. Under the same thread specifications, the hexagonal flange bolt support area is larger than the hexagon bolt, which can better disperse the support surface pressure and avoid deformation by the joint;
2. Under the condition of the same friction coefficient, the anti-loose effect of the flange face bolt is obviously better than that of the hexagon head bolt;

In order to avoid the friction between the end of the sleeve and the connected part to damage the surface of the connected part, the flange face bolt is more economical than the hexagonal head bolt assembly with a flat washer.
Hex flange face bolts are preferred. Hexagon head bolts, hex head bolts + spring washers, hex head bolts + spring washers + flat washers, hex head bolts + flat washers are restricted.

Screws

The screw is screwed in the form of inner hex, cross recess, inner six-flower, round head, cylindrical head flange, flat round head, flat round head flange, pan head, pan head flange There are 8 kinds of commonly used countersunk heads, countersunk heads and semi-sunk heads.
As the requirements for assembly efficiency are also increasing, it is preferred to use hexagonal socket screws to optimize the standard structure and gradually limit the use of hexagon socket screws and Phillips screws.

Nut

Hexagon flange nuts are used in the same way as hexagonal flange bolts. If the structure allows, hex flange nuts are preferred. For parts with special anti-loose requirements, consider using an effective torque type lock nut, such as an all-metal lock nut and a non-metallic insert lock nut. Since the all-metal lock nut is locked by the deformation of the thread, it is not suitable for the parts that are frequently disassembled; the non-metallic insert lock nut has good reusability, but according to the standard requirements of the fastener, in addition to the engine, Its application temperature is ≤120 °C. It is important to note that the effective torque type lock nut needs to overcome the nut deformation or the additional torque caused by the non-metallic insert during installation. Therefore, it is necessary to confirm the torque, and it is possible to assemble the clamping force according to the torque value of the ordinary nut. Not enough, there is a risk of use.

Thread

Because the load capacity and the anti-loose ability of the fine thread are higher than that of the coarse thread, when selecting the larger size threaded fastener, the fine thread should be selected as much as possible, and the variety of the threaded fastener can also be reduced. It can be seen from Table 1 that there are only coarse teeth products below M12, fine teeth products above M12, and M12 and above in commercial vehicle thread fasteners coexist with thick and thin teeth, and there is room for optimization, see Table 1.
Table 1 Comparison of coarse and fine thread pitch (mm)

Coarse teeth	M5X0.8	M6X1	M8X1.25	M10X1.5	M12X1.75	M14X2	M16X2	M18X2.5	M20X2.5	M24X3
Fine teeth	/	/	/	/	M12X1.25	M14X1.5	M16X1.5	M18X1.5	M20X1.5	M24X1.5

Washer

In order to improve assembly efficiency and reduce the risk of missing and misplaced, in principle, gasket products are not allowed to exist separately. The use of various types of gaskets is recommended as follows.

1. Flat washer is mainly used to improve the contact state, increase the bearing area and maintain the friction coefficient of the supporting surface is stable; 
2. Spring washer uses the elastic to generate the axial pre-tightening force, which can alleviate the axial force attenuation, but it is difficult to effectively control due to the anti-loose performance. The bolt is easy to bear the eccentric load and has the risk of damage; the 3 toothed elastic washer has twisted teeth, the hardness is higher after heat treatment, the teeth are elastically deformed during assembly, and the support surface is partially embedded to form a loosening effect, and the toothed washer is connected. Sites should be used with caution.
3. Surface treatment: automotive fasteners include bolts, nuts, and washers. Most of them must be surface treated to protect them from corrosion, improve appearance, or achieve certain special functions, such as screw fasteners that control locking torque. For example, the use environment and corrosion resistance requirements of a domestic automobile fastener are shown in Table 2.

Table 2 The use environment and corrosion resistance requirements of automotive fasteners

Use environment	Use environment example	Plating type	Anti-corrosion requirements	Application example
Highly corrosive environment, directly affected by mud and sand	Chassis, engine bottom, skirt, etc.	Zinc-nickel alloy	720h no red rust	Frame and body connection bolts, steering knuckle and control arm, brake caliper and steering knuckle connection bolts. Engine fixing bolt, fuel tank connecting bolt
		Zinc aluminum coating	720h no red rust
Medium corrosive environment, external body area not directly affected by mud and sand	Engine compartment, door lock, gas spring bolt, wiper motor and continuous dry assembly mounting bolt	Galvanized coating	96h no white rust 240h no red rust	Connecting bolts at the door and the lock ring, suspension bolts for the hood, radiator, condenser and intercooler
		Zinc aluminum coating	480h no red rust
Low corrosion requirements environment, cockpit internal bolts	Seat mounting bolt	Electroplated zinc	72h no white rust 168h no red rust	Instrument, seat and guide rail, mounting strap, parcel attachment bolt
		Zinc aluminum coating	240h no red rust
Oil resistant	Engine, gearbox	Oxidation	8h no red rust	Engine, gearbox and oil contact bolts in confined spaces
		Anti-corrosion phosphating	6/48/72h no red rust
Protective decoration	Bolts for exterior and interior parts	Decorative chrome	No change in 48h CASS test	High-end car wheel nuts, etc.
Connecting with different metals, it is easy to produce contact corrosion environment	Contact corrosion conditions, such as connecting aluminum alloy bolts	Zinc-nickel alloy	720h no red rust	Steel fasteners for connecting aluminum alloy parts, etc.
		Zinc aluminum alloy	720h no red rust
There is also a high-humidity corrosive environment, connecting bolts at the spark plug (copper plating), assembly coating (painting anti-rust oil), etc.

Electroplating zinc

The best anti-corrosion performance is galvanized yellow passivation, followed by galvanized green passivation, galvanized black passivation and galvanized blue-white passivation corrosion resistance is generally 8μm, yellow passivated white rust time 72h, red rust Time 144h; black and white passivated white rust time 6h, red rust time 72h.
In practical applications, we need to pay attention to the following three aspects. With the gradual tightening of environmental protection, the use of trivalent chromium passivation, zinc-aluminum coating and other environmentally friendly methods for automotive fasteners is the future trend; cars with a maximum tensile strength greater than 1000 MPa (corresponding to a hardness value of 33.5 HRC, 332 HV) After the fastener is electroplated, it needs to be driven by hydrogen before passivation to reduce the risk of delayed fracture; when the chromate passivation film is exposed to the environment above 70 °C for a long time, the corrosion resistance will be destroyed, so the ambient temperature is high. The area, galvanized passivation is used with caution.

Zinc aluminum coating

The zinc-aluminum coating has no hydrogen embrittlement and meets environmental protection requirements. The neutral salt spray test can reach 720 hours. The color of the coating is black and gray. Adding a lubricant to the coating fluid can change the coefficient of friction. Bolts of grade 10.9 and above are preferred. In addition, the following aspects should also be considered when using. Zinc-aluminum coating and substrate adhesion strength is not as good as galvanizing, there is powder drop in use, so it can not be used inside the transmission parts, it is not recommended for bolts that need to be repeatedly disassembled. In addition, for large-size bolts and nuts, the barrel plating is prone to scratches and scratches, and the corrosion resistance is reduced. It should be considered when selecting; for fasteners with conductive performance, the external diameter of the external thread is less than M6 and the internal thread is less than M10. The fasteners are not suitable for zinc-aluminum coating to ensure the screw-in and normal assembly.

Zinc-nickel alloy

Compared with galvanizing, zinc-nickel alloy has greatly improved anti-corrosion ability. Under the condition of 8μm coating, after passivation and sealing treatment, it can reach 240h surface without white rust, 1000h without red rust; in addition, it also meets high temperature resistance. Requirements for use. Since zinc-nickel alloys still have a slight tendency to hydrogen embrittlement, for automotive fasteners with tensile strength greater than 1000 MPa, the quality risk is reduced, and necessary verification is required before use.

Copper plating

The melting point of copper is around 1083 °C. In the high temperature environment, in order to avoid the sintering of the screw, the surface treatment chooses copper plating, especially the automotive fasteners around the engine exhaust manifold.

Materials and Heat Treatment

Automotive high-strength bolts generally refer to grade 8.8 or higher. They not only require high tensile strength and yield ratio, but also have high low-temperature impact properties. One of the difficulties in manufacturing is high-strength bolt quenching and tempering treatment. In the material selected, SWRCH35K, 10B21, 10B33, 35CrMo, 42CrMo or 20MnTiB steel is used, as shown in Table 3. As we all know, the mechanical performance test results of high-strength fasteners are the key product quality indicators, and are also important indicators related to safety. The main problem of SWRCH35K and 10B21 steel is poor hardenability, effective control of the process and process of high-strength bolt quenching and tempering, which plays a vital role in mechanical properties.
In order to heat-treat the high-strength bolts of automobiles, steel is controlled from the following three aspects. 1 The carbon content is controlled at the upper and upper limits, which not only improves the toughness of the steel but also reduces the tendency to segregation. 2 alloy elements are controlled at the upper limit to increase the hardenability of the steel and improve the toughness. 3 Minimize the content of harmful residual elements P and S to ensure the purity of steel. Car fastener grades and materials, see Table 3.
Table 3 Automotive fastener grades and materials

Car fastener name	Specifications and diameter	Material grade	Use level
Bolts, screws, studs	≤M12	SWRCH38K, 10B28	Level 8.8
	≤M20	10B33, 40Cr	Level 8.8
	≤M24	SCM435, 35CrMo, 20MnTiB	8.8 to 10.9
	≤M36	ML40CrMo, 40CrNiMo, ML45CrMo	10.9
	≤M36	35CrMoV, 30Cr2Ni2Mo	12.9 level
Nut	≤M16	SWRCH35K, 45,	Level 8
	≤M36	10B33, 40Cr, 35CrMo	Level 8-10
	≤M36	42CrMo	10 to 12

The internal organization and performance of the bolt during the quenching and tempering process cannot be monitored in real time, which is one of the difficulties in quality control. Before loading the furnace, the bolt head mark should be carefully checked to ensure that the information of the treated bolt is accurate and not lost after heat treatment and can be recognized. The quenching heating process is strictly controlled, the carbon potential is accurate, and the time for each batch of bolt quenching is recorded. The surface hardness of the workpiece should be tested after quenching the medium. 10B21, 20MnTiB steel should be greater than 43HRC; SWRCH35K, 45, 10B33 steel should be greater than 48HRC . The microstructure after quenching is fine needle-shaped martensite, which is evaluated according to JB/T9211-2008 “Machine grade of medium carbon steel and medium carbon alloy structural steel”. The quenched martensite is grade 3 to 5, which meets the technical requirements. The surface and core quenching hardness uniformity is not more than 3HRC.

Nut production process

A nut is a part that is screwed together with a bolt or screw for fastening, and one component that must be used in manufacturing machinery. There are many kinds of nuts. We often have the national standard, German standard, British standard, American standard, and Japanese standard nut.
(1) There are many types of threaded teeth developed by various countries. The common types of threaded teeth are as follows:

• M – metric coarse teeth, fine teeth, very fine teeth
• UNC – Joint Coarse (Inch)
• UNF – Joint Fine Teeth (Inch)
• W——Wei’s teeth, coarse teeth, fine teeth (JIS)

In addition, there are 8 thread series (8-UN) and 12 thread series (12-UN) in the inch system.
(2). Thread accuracy level:

• The metric system (DIN, ISO, GB) is divided into 6H and 6G grades. Usually it is 6H. Metric Japanese Standard (JIS)
• It is divided into three levels: I, II, and III. Usually the situation is level II.
• Nut material

Nuts are divided into carbon steel, high strength, stainless steel, plastic steel and other types according to different materials. According to the different standard numbers of the country corresponding to the product attributes, it is divided into ordinary, non-standard, (old) national standard, new national standard, American system, English system and German standard. Different sizes, the threads are not divided into different specifications. Generally, the national standard and German standard are indicated by M (for example, M8 and M16), and the US and English systems use fractions or # to indicate specifications (such as 8#, 10#, 1/4, 3/8).

How are our common steel nuts produced?

The general nut manufacturing process has the following steps:
1. Steel stamping forming

2. tapping (machining thread)

3. Heat treatment (higher grade nuts require heat treatment, low grade nuts do not require heat treatment)

4. After the above production process, the nut has been basically formed.

5. After the above steps, the nut also needs the final step of the process – surface treatment (that is, the surface treatment method we want to process, such as blue and white zinc, color zinc, black, etc.)

High-strength bolt manufacturers judge and analyze the surface treatment method of the nut. The quality judgment of the surface treatment of the nut is mainly to check whether the corrosion resistance of the nut is effectively increased. If it increases, whether the increase is in line with the expected requirements. After the nut is processed through the surface, the appearance of the nut will be more beautiful, and the corrosion resistance of the nut will be improved, thereby increasing the usability and reliability of the nut. There are three ways to handle the nut:
Electroplating the nut. Everyone knows that the nut will rust under long-term use. Electroplating is to soak the part of the nut rusted in an aqueous solution. This aqueous solution is specific. This aqueous solution contains a metal compound. The surface of the nut can be treated by passing current. Nut plating generally includes galvanizing, copper, nickel, chromium, copper-nickel alloy, and the like.

1. The nut is heated, that is, hot dip galvanizing, we know that some of the nuts are made of carbon steel, the carbon steel nuts are immersed in the molten zinc liquid, and then heated, when heated The temperature must be higher. Only in this way is it beneficial to the surface treatment. Generally, the heating temperature is 510 degrees Celsius. When the nut is used for a long time, it will also produce iron-zinc alloy. After heating, the substance will change and change. It is a non-ferrous purified zinc, which completes the surface treatment of the nut, which increases the service life and corrosion resistance of the nut.
2. The third way is to use mechanical plating to carry out the surface treatment of the nut. Simply speaking, mechanical plating is accomplished by combining physical properties and chemical properties. It is only after the metallized powder-like de-stimulation nut has been rusted. The surface is then cold-treated and the coating is applied to the surface of the nut. This coating protects the nut and improves the age and corrosion resistance of the nut. This is called coating, which is mainly used for screws and nuts. 

How do you distinguish between stainless steel screws, galvanized and nickel-plated screws?

First by color:

• Stainless steel is the primary color of iron.
• Galvanized screws are white, colored, and black.
• The nickel plated screws are silver.

1. Stainless steel screws are made of stainless steel. The color of stainless steel is mainly iron and carbon alloy, so the color is close to the color of iron;

2. The surface of galvanized screws is galvanized with white zinc, colored zinc, black zinc, etc., so the color of galvanized screws is generally white, color, black;

3. the color of nickel-plated screws is related to nickel, generally showing a very bright silver.

Different methods of distinguishing stainless steel screws, galvanized and nickel-plated screws

1. Distinguish by magnet: stainless steel does not have magnetism under normal conditions. Stainless steel screws cannot be absorbed by magnets. Zinc-plated and nickel-plated screws can be adsorbed by magnets.
2. Distinguished by oxidant: metal nickel has a strong passivation ability, when the plating can quickly form a very thin passivation on the surface, so the surface of the nickel-plated screw has a passivation film, the galvanized surface is a layer Zinc, stainless steel has no surface and can be distinguished by strong oxidizing agents.
3. Distinguish by strong acid and alkali: stainless steel contains chromium and nickel, has strong anti-corrosion performance, is not corroded by strong acid and alkali, has a passivation film on the surface of nickel-plated screw, and is slowly eroded by strong acid and alkali, galvanized screw Being eroded the fastest.

The way to distinguish between stainless steel screws, galvanized and nickel-plated screws are to look at the color of the screws. The color of the stainless steel screws is close to the primary color of the iron. The galvanized screws are white, colored, black, and the nickel-plated screws are very bright silver: In addition to this, magnets, oxidizers and strong acids and alkalis can be used to distinguish between stainless steel screws, galvanized and nickel-plated screws.
In addition, hot-dip galvanized screws have the following advantages:

• 1. Low processing cost: the cost of hot dip galvanizing rust is lower than that of other paint coatings;
• 2. Durable: in the suburban environment, the standard hot-dip galvanized rust thickness can be maintained for more than 50 years without repair; in urban or offshore areas, the standard hot-dip galvanized rust layer can be maintained for 20 years without repairing; 
• 3. Good reliability: the galvanized layer and the steel are metallurgically combined and become a part of the steel surface, so the durability of the coating is relatively reliable;
• 4. The toughness of the coating: The galvanized layer forms a special metallurgical structure that can withstand mechanical damage during transportation and use;
• 5. Comprehensive protection: each part of the plated parts can be plated with zinc, even in the depressions, sharp corners and hidden places can be fully protected;
• 6. Save time and effort: the galvanizing process is faster than other coating construction methods and can avoid the time required to paint on the construction site after installation.

In addition, galvanized screws have a mechanical performance rating of up to 8.8, while stainless steel screws can only be up to -70, such as A2-70, A4-70, which is equivalent to 7 and lower than galvanized screws.
Therefore, the use of galvanized screws is wider than that of stainless steel screws, and the amount is also higher than that of stainless steel screws.
However, it should be noted that the galvanized screw is not a complete replacement for the stainless steel screw, and the corrosion resistance is longer than the stainless steel screw. Therefore, it is still necessary to select the appropriate material according to the occasion of use.
For the difference between stainless steel screws, galvanized and nickel-plated screws, you may encounter the following problems.
Question: I would like to ask the master of electroplating. Why do screws generally require nickel plating of 3-5 microns, while galvanizing requires a film thickness of 5-8 microns? Will it affect the pass regulation? What is the contrast of salt spray? Can you answer the details in detail?
Answer 1: The thickness of the coating is not specified, and it is determined according to customer requirements or salt spray requirements. The thicker coating will affect the pass-stop, the key is to control the thread tolerance. In terms of salt spray, zinc plating is more resistant to corrosion than nickel plating.

Answer 2: The thickness of the plating and the type of plating depend on the conditions and environment used by the customer. The criteria for determining the thickness and corrosion resistance are determined by the buyer and the seller.

Source: China Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(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]

Please notice that you might be interested in the other technical articles we’ve published:

• Fasteners exports ushered in transformation: high-end products cost hundreds of thousands of times
• What are flange bolts
• What are the differences between bolts, screws and studs
• What are fasteners
• What is a fastener

References:

• https://www.yaang.com