Heat treatment technology and development of automotive fasteners

With the development of automobile industry, the requirements of automobile fasteners are also higher and higher. The main types of high strength fasteners in China are bolts, which are 8.8, 9.8, 10.9 and 12.9. All these fasteners of strength level need heat treatment to improve the comprehensive mechanical properties of the products, and reach the specified tensile strength and yield strength ratio. Heat treatment technology is the key to produce automobile fasteners.

Factors affecting heat treatment of automobile fasteners

The heat treatment of automobile fasteners is generally called quenching (after quenching and then tempering at high temperature). The tempering has strict requirements on raw materials, furnace temperature, furnace atmosphere, quenching medium and heat treatment quality inspection. Only if the basic requirements are met, the segregation caused by materials, decarbonization caused by annealing unprotected atmosphere, cracks generated in cold heading process and quenching medium can be reduced Quenching cracking and distortion.

Heat treatment equipment

At the end of 1980s, most of the high-strength bolt heat treatment of automobile fasteners in China was quenched by salt bath furnace, and the inner tank of salt bath furnace was limited. Only 30-50kg can be installed at a time. Because of the poor working environment of salt bath furnace heat treatment and close quenching and tempering process, it is easy to cause mixed loading. Customers may break or cannot use it at all. Due to the backward heat treatment equipment, some automobile fastener enterprises can only produce bolts with strength grade 8.8 and below. In the 1990s, due to the rapid development of automobile industry, the high-strength fasteners of automobile need to be localized. The heat treatment equipment has been developed from salt bath furnace, drop injection type roller resistance furnace and exothermic shock bottom furnace to a chain casting furnace with heat absorption or n2ch3oh cracking atmosphere. Because of the great improvement of the life of the belt, the continuous mesh belt furnace used for radiation tube heating is the most. At the same time, the net belt furnace has the advantages of uniform distribution and heating in the furnace, stable quality of parts, good working environment, less gas consumption and low energy consumption.
According to the requirements of the process, some enterprises add dephosphorization function to the front cleaning equipment of continuous mesh belt furnace production line, remove the phosphating layer on the surface of fasteners before heating; for large-scale and high-strength bolts, in order to prevent collision and quenching distortion during quenching, unicase series box type multi-purpose furnace is used for treatment; in order to reduce cost, some enterprises use gas plus gas to reduce the cost Electric heating is adopted in heat and heat preservation area, which gives full play to the advantages of gas heating and electric heating, and selects the type of carrier gas and carburizing agent for furnace according to local conditions to achieve the purpose of energy saving and emission reduction.
Heat treatment technology of continuous mesh belt furnace production line: charging, cleaning, heating quenching cleaning tempering coloring down line, can realize carbon potential control and automatic operation. In carburizing process, the technology of carbon potential control by oxygen probe can be used to further optimize the quality of shallow layer, which makes the quality of products leap forward.
The problems of heat treatment equipment are the lack of cooling performance measurement, unstable carbon potential control and long calibration period of furnace temperature.

Raw materials

The cold heading steel wire rod used for high strength fasteners of automobile has carbon steel and alloy steel. Except for high strength and large hexagon head bolts, according to the requirements of gb/t1231, the materials used for M30 specifications are specified, and no other bolts are specified in national or international standards, but only the alloy elements to be added are indicated in a guiding way.
In fact, raw material selection is an important link. For example, when a bolt factory undertakes the American paccar project to produce American Standard 1 ″ – 14 grade 8 (equivalent to national standard 10.9) bolts, 40Cr steel is used as raw material, which is used for truck brake, and the requirements for connecting parts are very high. The finished product load keeping test is unqualified during the first inspection and the wedge load test is unqualified during the second inspection. After analyzing the properties of materials, it is difficult to meet the requirements of various indexes. After 42CrMoA steel is used, the process control is strengthened to produce qualified products.

Control of heat treatment process

Principles for heat treatment process formulation

The high strength fasteners of automobile should obtain the tempered soxhlet and torchyman with good mechanical properties, if the martensite structure is obtained at the center of the car during quenching, which is closely related to the hardenability of steel. When the fasteners are tempered at high temperature to obtain the same hardness, the mechanical properties of the fasteners, such as strength, plasticity and toughness, are similar when the carbon steel or alloy steel is fully quenched. That is, when 45# steel and 40Cr steel are completely quenched and tempered to the same hardness, their strength, plasticity and toughness are roughly the same. However, if not fully quenched, even if the hardness after tempering is the same as that after quenching, the yield strength, elongation after fracture, section shrinkage rate and impact strength are lower, and the reduction degree increases with the decrease of hardenability.
When the cross-section size of the same material is different, the hardenability is different. Although the same tempering process is adopted, the mechanical properties of the products are different. The larger the section, the worse the hardenability and the worse the mechanical properties. At the same time, the chemical composition of raw materials of the same material can fluctuate in a certain range, especially carbon steel will be different due to the differences in smelting technology and standards in each steel plant. The finished products produced by the same process must have different performance.
The quenching temperature is mainly determined according to the chemical composition of steel and the specific process. The chemical composition of steel is the main factor to determine the quenching temperature. According to the different quenching medium, the quenching heating temperature is different. The quenching temperature of hypoeutectoid steel is above, carbon steel is 30-50 ℃ above AC3, alloy steel is 50-80 ℃ above eca3. For the fastener hardened in the center, due to the comprehensive effect of the structural stress and thermal stress, the maximum tensile stress will be near the surface of the part, which may cause the quenching. The dangerous dimension of quenching is related to the quenching medium used. For the bolt, the diameter of water quenching is 8-10 mm, and the diameter of oil quenching is 20-39mm. When the bolts of dangerous size are quenched, proper measures must be taken to prevent cracking.
The heating temperature and heating time must be appropriate during quenching. To ensure that the microstructure with a volume fraction of at least 90% after quenching is martensite, preheating measures shall be taken. Preheating at the temperature of 100 ℃ below can be taken to make the structure uniform after quenching, the martensite transformation is sufficient, and the free ferrite or granular carbide is less in the center.
In order to control the strength and load of fasteners within the qualified range, the differences of raw material, furnace type, upsetting process (cold heading and warm forging), thread length (full tooth and half tooth), medium (water quenching and oil quenching) should be paid attention to during tempering. Hardness and strength requirements can be adjusted by tempering temperature in high temperature range. On the basis of improving the lower hardness value, the higher tempering temperature should be selected to eliminate the stress completely. Some alloy materials contain more sensitive elements of temper brittleness in the second class. After tempering, proper cooling method must be adopted to avoid staying in high temperature area for a long time, and producing the second type tempering brittleness.

Increase hydrogen removal treatment

The development of energy saving and lightweight of automobile has put forward higher requirements for the design of engine and power system. It is not only the weight reduction of bolts and nuts used, but also plays an important role in the high reinforcement of fasteners of grade 8.8 and above. However, with the increase of strength, the fracture caused by hydrogen embrittlement becomes a very prominent problem. In the process of heat treatment Hydrogen removal treatment is required.
The results show that the hydrogen embrittlement fracture of the fastener in the natural environment is mainly martensitic steel which is quenched and tempered, which occurs in the high strength materials with yield strength >620mpa and hardness HRC value >31. The higher the tensile strength, the more sensitive to hydrogen embrittlement, the easier the material is to absorb hydrogen, and the less easy to drive hydrogen.
It is generally believed that hydrogen embrittlement fracture will occur when the strength reaches 1050mpa or above. GB/T 52671 stipulates that when HV value of the surface hardness of the careful part or surface is more than 320, hydrogen embrittlement shall be tested and hydrogen driven treatment shall be carried out through the test. That is, electroplating bolts with performance grade 10.9 and above shall be hydrogen driven by low hydrogen process. From the foreign data, the strength threshold of hydrogen embrittlement fracture of automobile fastener may decrease to 1000MPa.
The influence of metallographic structure on hydrogen sensitivity of carbon steel and alloy steel is obvious. The sensitivity of tempered martensite, upper bainite (coarse), lower bainite (fine), Soxhlet, pearlite and austenite to hydrogen is decreased in turn. The tempering martensite has the strongest sensitivity to hydrogen embrittlement, so the formation of tempered martensite structure can be reduced by adjusting the heat treatment process. For example, the threshold of hydrogen embrittlement fracture will be increased by 100MPa by isothermal quenching, because the sensitivity of the lower bainite structure to hydrogen embrittlement is lower than that of tempered martensite.
The heat treatment of automobile fastener is carried out at high temperature. In order to reduce the oxidation of material surface in the heat treatment process, the protective atmosphere is often added to the heating furnace. If the hydrogen in the protective atmosphere contains hydride, it is possible to absorb hydrogen during heat treatment and increase the risk of hydrogen embrittlement of fasteners.
In addition, the heat treatment process is closely related to the hydrogen displacement effect after electroplating, and the residual stress formed by quenching has obvious influence on hydrogen embrittlement. The more sufficient residual stress is removed, the better the hydrogen displacement effect is after plating, and the higher the threshold of hydrogen embrittlement fracture is, which is the most obvious in the production of high strength bolts. Is09581 takes the elimination of residual stress as one of the important measures to reduce the risk of hydrogen embrittlement.
In order to prevent hydrogen embrittlement, the high strength fasteners of 1000-1300mpa must be hydrogen driven after plating. During hydrogen drive, the plated fasteners are heated to a certain temperature and kept for a while, so that the hydrogen in the material can gather to form hydrogen molecules and escape. There are two main reasons for incomplete hydrogen flooding: one is not timely hydrogen flooding, the other is that the time of hydrogen drive is too short.

Strengthen the determination of the core tissue of fasteners

In the process of heat treatment, the ferrite structure will be produced in the center of the fastener due to the influence of quenching temperature, hardenability of materials and quenching cooling speed. Too much ferrite structure will reduce the hardness and strength of bolts, and affect the use of fasteners. Grade 8.8 high strength bolts are generally made of swrch35k, ML35 and 35 steel. After the bolts are tempered, there are more or less ferrite in the microstructure of the center. In practice, for the bolts with a large number of ferrite in the center, when the hardness of the parts is lower than the product requirements, the quality of the products is often controlled by increasing the tensile strength. When the bolt is not quenched enough, some enterprises can guarantee the tensile strength by reducing tempering temperature, which leaves hidden danger to the quality of bolts.
In order to control the heat treatment quality of automobile bolts, it is necessary to measure the ferrite content in the center of the bolts. For automobile fasteners such as grade 8.8 bolts, the hardness of head or tail after quenching and tempering shall be checked respectively. The ferrite at the center of bolt has not been required, which leads to the bolt being lengthened or pulled off during assembly. The bolt of an engine tightening wheel is usually elongated when assembling, and the material is ML35 strength grade 8.8. The analysis of the failure bolt shows that there are a lot of ferrite in the bolt core. In the process of heat treatment, the hardness of quenching and tempering is not only controlled, but also the ferrite in the center is controlled within 3 levels, which prevents the bolts from being elongated or pulled during assembly.
The method of guaranteed load test is to test the bolt on the universal testing machine, apply the guaranteed load specified in the standard and measure the absolute elongation caused by the bolt applying the guaranteed load for 15s. In saej429, it is clearly stipulated that the high-strength bolts used in the automobile industry must be tested for load keeping, and the absolute elongation shall be controlled within 12.7 μ M. Because the absolute elongation is very small, the surface fine burr and sharp particles will have an impact on the measurement value, and the installation fixture will also bring some measurement deviation. Saej429 clearly stipulates that the first error is beyond the standard, and the second test can be carried out. If the second result is unqualified, it can be judged that the bolt does not meet the requirements; if the second result is qualified, it can be judged that the bolt meets the standard requirements.
For the determination of the core structure of the tempered fasteners, refer to GB / T 13320-2007 metallographic structure rating chart and evaluation method for box steel die forgings. The results show that for 8-point bolts made of swrch35k steel, the volume fraction of ferrite structure in the core should be within 12%, the hardness HRC value is 27, and for the 10.9 grade bolts made by SCM435 steel, the volume fraction of ferrite structure in the core should be controlled within 5%, and the lowest hardness HRC value is 34.5 medium carbon alloy steel is mainly used to manufacture 9.8, 10.9, 12.9 high strength bolts. For bolts Determination of ferrite in the core after quenching: for the engine support fastening bolts of grade 9.8 and 10.9 without fatigue life requirements, the core ferrite Grade 1-3 is qualified; for the 9.8, 10.9 engine connecting rod bolts with friction factor requirements, the core ferrite grade 1-2 is qualified, and for 12.9 grade bolts, the core ferrite grade 1 is qualified.

The focus of heat treatment technology

The rapid development of automobile fasteners brings challenges and opportunities to the production enterprises. Compared with the advanced level in the world, the overall level of heat treatment technology of fasteners in China is still backward, and the gap is increasing, especially in environmental protection, energy consumption, quality and other aspects. Therefore, it is necessary to improve the technical level of automobile fastener technology. The author thinks that more attention should be paid to in the following aspects.

Use annealed cold heading and non quenched steel

It is a new research topic to manufacture automobile fasteners by using annealed cold heading steel and non quenched steel.
In order to ensure the cold forming performance of automobile fasteners, the cold heading steel needs to be spheroidized annealing in advance. The non annealed cold heading steel can be used directly in cold heading forming by adjusting the chemical composition of the steel, controlling rolling and cooling during the rolling process, making pearlite partially spheroidized and reducing the deformation resistance in the cold forming process. For example, the ewrch35k steel produced by Ma steel has remarkable energy saving effect and shortened production cycle because of the no spheroidizing annealing, and it is a new energy-saving material.
The non quenched and tempered steel is mainly cold hardening type non quenched steel. The steel is added with the micro strong carbon (nitrogen) forming element (V, Nb, Ti). The toughness of the steel can reach or close to the level of 8.8 and 9.8 in the process of drawing and cold heading. Gb/t3098 22-2009 “mechanical properties of fasteners” bolts, screws and studs of fine grain non tempered steel have been issued, which provides strong technical support for the promotion and use of non quenched and tempered steel fasteners. The mft08 steel produced by fm08mns, ifm08mnsv and Ma steel produced by Baosteel has eliminated spheroidizing annealing before cold drawing and quenching and tempering treatment after forming, reducing decarburization tendency of thread part, improving the yield of finished products, and obvious economic benefits.

Improve the technical documents of heat treatment process

Heat treatment process is a special process which can not guarantee the quality of products through subsequent inspection and test. Therefore, it must be controlled by a reasonable system. First, the factors that may cause process failure shall be identified and the work of heat treatment process technology shall be made; secondly, a perfect heat treatment process management system shall be established. The process documents shall clearly mark the operation method, process parameters and inspection methods of the heat treatment process, and at the same time, the furnace charging method and charging quantity shall be emphasized to solve the feasibility and method of operation The accuracy, the raw material and the pre heating process condition of the fasteners to be treated are inspected to ensure the rationality of the process parameters and the effectiveness of the treatment; moreover, the statistical technology is used to analyze the heat treatment process, calculate and determine the heat treatment parameters, and find out the influence rules of the heat treatment parameters on the product quality according to the change of the process quality, and then use the heat treatment specialty Knowledge of heat treatment technology is constantly improved to ensure the maximum quality of products.

Develop new heat treatment equipment

Develop new heat treatment equipment, further improve furnace temperature carbon potential control technology and improve furnace gas uniformity, reduce quenching distortion and collision of workpiece during quenching, improve cooling effect and cooling uniformity of quenching cooling tank, pay attention to matching technologies such as front and rear cleaning, oil-water separation, waste heat utilization, energy saving and emission reduction, and reduce heat treatment production without affecting product quality Production cost.

Conclusion

The automobile fastener manufacturer should pay attention to the renewal of heat treatment equipment, select the appropriate raw materials, improve the heat treatment process system, strengthen hydrogen drive treatment and center organization and inspection in production, reduce the gap between high-strength fasteners and world-class products as soon as possible, and improve the grade of automobile fasteners in China.

Source: China Fasteners Supplier: www.epowermetals.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.)

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