Heat Treatment Technology of Automobile Fasteners

With the development of the automotive industry, the requirements for automotive fasteners are getting higher and higher. China’s automotive high-strength fasteners are the main varieties of bolts, there are 8.8, 9.8, 10.9 and 12.9 grade 4 strength levels, these strength levels of fasteners have to be heat-treated to improve the overall mechanical properties of the product to achieve the specified tensile strength and flexural strength ratio, heat treatment process technology is the key to the production of automotive fasteners.

1. Factors affecting the heat treatment of automotive fasteners

Automotive fastener heat treatment is generally known as tempering (quenched and then tempered by high temperature) treatment, tempering of raw materials, furnace temperature, furnace atmosphere, quenching medium, heat treatment quality inspection have strict requirements, only to meet the basic requirements in order to reduce the deviations caused by the material, annealing without protective atmosphere caused by decarburization, cold heading process produces cracks, quenching medium caused by quenching cracking and distortion.

1.1 Heat treatment equipment

The late 1980s, China’s automotive fasteners, most of the high-strength bolts heat treatment using salt bath furnace quenching, salt bath furnace liner is limited, each time only 30-50kg due to the poor working environment of salt bath furnace heat treatment, quenching and tempering process is close, easy to cause mixed phenomenon, customers in the use of brittle breakage or simply can not be used. Because the heat treatment equipment is too backward, some automotive fastener enterprises can only support the production of 8.8 grade and the following strength grade bolts. Into the 1990s, due to the rapid development of the automotive industry, automotive high-strength fasteners in urgent need of localization. Heat treatment equipment has been developed from salt bath furnace, drop-in roller resistance furnace, exothermic shock bottom furnace to heat absorption or N2CH3OH cracking atmosphere of the cast chain furnace. Due to the substantial increase in the life of the mesh belt, the continuous mesh belt furnace with radiation tube heating has the largest amount, at the same time, the mesh belt furnace has the advantages of fabric in the furnace, uniform heating, stable quality of parts, good working environment, can reduce parts bumping, less gas consumption, low energy consumption, etc.
According to the requirements of the process, some companies in the continuous mesh belt furnace production line before cleaning equipment to increase the phosphorus function to remove the phosphate layer on the surface of the fasteners before heating; for large-size high-strength bolts, in order to prevent the quenching bump and quenching distortion, the choice of Unicase series box-shaped multi-use furnace processing; to reduce costs, some companies continuous mesh belt furnace heating zone using gas heating, insulation zone using In order to reduce costs, some companies use gas heating in the heating zone and electric heating in the holding zone, giving full play to the advantages of gas heating and electric heating, and choosing the type of carrier gas and carburizing agent for the furnace according to local conditions to achieve the purpose of energy saving and emission reduction.
The heat treatment process of continuous mesh belt furnace production line: feeding a cleaning – heating – quenching – cleaning – tempering – coloring – off line, can realize carbon potential control, automatic operation. In the carburizing process, oxygen probe zoning carbon potential control technology can be used to further optimize the quality of the shallow carburizing layer, so that the quality of the product has achieved a qualitative leap.
The problem of heat treatment equipment is the lack of quenching medium cooling performance measurement, carbon potential control is not stable, the furnace temperature calibration cycle is too long.

1.2 Raw material

Automotive high-strength fasteners with cold heading steel wire varieties of carbon steel and alloy steel. In addition to high-strength large hexagonal head bolts, according to the requirements of GB/T1231, the material used in the specifications below M30 is specified, other bolts either national or international standards do not specify the material to be used, only guiding the need to add alloying elements.
In fact, the selection of raw materials is a very important link. For example, a bolt factory in the United States PACCar project to produce American standard 1〃 – 14 grade 8 (equivalent to the national standard 10.9 grade) bolts, using 40Cr steel as raw materials, the product is used for heavy truck brakes, the requirements of the connection is very high. The finished product failed the load retention test in the first inspection and the wedge load test in the second inspection. After analyzing the material performance, it was difficult to meet the requirements of each index. After changing to 42CrMoA steel production, the process control was strengthened and qualified products were produced.

2. Heat treatment process control

2.1 Heat treatment process development principles

Automotive high-strength fasteners to obtain good mechanical properties of tempered soxhlet, tempered taustenite organization, the premise is to ensure that the quenching of the heart to get martensite organization, which is closely related to the hardenability of steel. Whether it is carbon steel or alloy steel, in the case of fully hardened, fasteners by high temperature tempering to get the same hardness of the finished product, their mechanical properties such as strength, plasticity and toughness are similar. In other words, 45 steel and 40Cr steel are completely hardened and tempered to the same hardness, their strength, plasticity and toughness are roughly the same, but if not completely hardened, even if the hardness after tempering and tempering after hardening the same, the yield strength, elongation after break, section shrinkage and impact strength are lower, the degree of reduction with the reduction of the degree of hardening and increase.
The same material products, when the cross-section size is different, hardenability is also different, although the same tempering process, its mechanical properties are not the same. The larger the cross-section, the worse the hardenability and the worse the mechanical properties. At the same time, the same material raw materials, its chemical composition is allowed to fluctuate within a certain range, especially carbon steel will be different because the steel mills in smelting technology, standards and different, using the same process to produce the finished product is bound to have performance differences.
Quenching heating temperature is mainly based on the chemical composition of the steel, combined with specific processes to determine. Steel chemical composition is the main factor in determining the quenching temperature, according to the different quenching medium, the quenching heating temperature used is also different. Sub-eutectic steel quenching temperature selection in the above; carbon steel in Ac3 above 30-50 ℃; alloy steel in the benefit of Ac3 above 50-80 ℃. For the heart of the hardened fasteners, due to the combined effect of tissue stress and thermal stress, the maximum tensile stress generated will be near the surface of the part, may cause quenching crack, the quenching risk size and the quenching medium used, for bolts, water quenching diameter of 8-10mm, oil quenching diameter of 20-39mm. in the dangerous size of the bolt quenching, must take appropriate measures to prevent quenching crack.
Quenching heating temperature, heating time must be appropriate to ensure that the quenching volume fraction of at least 90% of the organization for martensite, to have preheating measures, can take the following 100 ℃ temperature preheating, so that the quenching organization is uniform, martensite transformation is sufficient, the heart less free ferrite or granular carbide.
In order to control the fastener strength and guarantee the load in the qualified range, tempering should pay attention to the difference of raw material material, furnace type, upsetting process (cold heading and warm forging), thread length (full tooth and half tooth), medium (water quenching and oil quenching). Hardness and strength requirements can generally be adjusted by tempering temperature in the high temperature range. On the basis of increasing the lower hardness limit, a higher value should be selected for the tempering temperature so as to completely eliminate stress. Some alloy materials contain more elements sensitive to type 2 temper brittleness, and appropriate cooling methods must be adopted after tempering to avoid staying in the high temperature region for a longer time and producing type 2 temper brittleness.

2.2 Adding hydrogen repellent treatment

The energy-saving and lightweight development of the car, the design of the engine and power system has put forward higher requirements, it is not only the fasteners used bolts, nuts itself weight reduction, 8. 8 and above fasteners high reinforcement also plays a very important role, but with the increase in strength, fracture caused by hydrogen embrittlement becomes a very prominent problem, in the heat treatment process to carry out hydrogen repellent treatment.
Research shows that the actual use of fasteners in the natural environment of hydrogen embrittlement fracture is mainly quenched and tempered martensitic steel, occurring in high strength materials with yield strength > 620 MPa and hardness HRC value > 31. The higher the tensile strength, the more sensitive it is to hydrogen embrittlement and the easier it is for the material to absorb hydrogen and the less likely it is to repel it.
It is generally believed that hydrogen embrittlement fracture will occur only when the strength reaches 1050MPa or more. GB/T52671 stipulates that when the HV value of heart or surface hardness >320, hydrogen embrittlement should be tested by test and hydrogen repellent treatment, that is to say, the plated bolts with performance grade 10.9 and above should adopt low hydrogen process and carry out hydrogen repellent. From foreign information, the strength threshold for hydrogen embrittlement fracture of automotive fasteners may drop to 1000MPa
The influence of metallographic organization on the hydrogen sensitivity of carbon steel and alloy steel is obvious. Tempered martensite, upper bainite (coarse), lower bainite (fine), sothite, pearlite, austenite sensitivity to hydrogen is reduced in order. Tempered martensite is the most sensitive to hydrogen embrittlement, so the generation of tempered martensite tissue can be reduced by adjusting the heat treatment process during heat treatment. For example, the use of isothermal quenching will make the occurrence of hydrogen embrittlement fracture threshold increased by about 100MPa, this is because isothermal quenching generated by the lower bainite organization on the sensitivity of hydrogen embrittlement than tempered martensite low.
Automotive fastener tempering heat treatment is carried out at high temperatures, in order to reduce the oxidation of the material surface during the heat treatment process, a protective atmosphere is often added to the heating furnace. If the protective atmosphere contains hydride, there is a risk of hydrogen absorption during the heat treatment process, increasing the risk of hydrogen embrittlement of the fastener.
In addition, the heat treatment process is closely related to the effect of hydrogen repulsion after plating, and the residual stresses formed by quenching have a significant effect on hydrogen embrittlement. The more fully the residual (tensile) stress is eliminated, the better the effect of hydrogen repulsion after plating, and the higher the threshold of hydrogen embrittlement fracture, which is 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.
To prevent hydrogen embrittlement, 1000-1300 MPa high strength fasteners must be hydrogen driven after plating. In hydrogen repelling, the fasteners are heated to a certain temperature after plating and kept for a period of time, so that the hydrogen in the material will gather to form hydrogen molecules and escape. There are 2 main reasons for incomplete hydrogen repelling, one is that the hydrogen is not repelled in time, and the other is that the hydrogen repelling time is too short.

2.3 Strengthen the determination of fastener heart tissue

In the process of heat treatment of automotive fasteners, due to the quenching heating temperature, the hardenability of the material, quenching cooling rate and other factors will produce ferrite tissue in the center of the fastener. Excessive ferrite tissue will reduce the hardness and strength of the bolt, affecting the use of fasteners. 8.8 grade high-strength bolts are generally manufactured using SWRCH35K, ML35, 35 steel, bolts after tempering, the microstructure of the heart has more or less some ferrite. In actual production, for bolts with a large amount of ferrite in the heart, when the hardness of the heart is lower than the product requirements, it is often used to increase the tensile strength to control the product quality. When the bolts are not quenched enough, some companies ensure the tensile strength by reducing the tempering temperature, which leaves a hidden danger to the quality of the bolts.
In order to control the quality of heat treatment of automotive bolts, it is necessary to determine the ferrite content of the heart of the bolt. Automotive fasteners such as 8.8 grade bolts, respectively, check the hardness of the head or tail after quenching and tempering, the heart of the bolt ferrite has not been required, resulting in the bolt being stretched or pulled off during assembly. An engine tensioning wheel fastening bolt, material ML35 strength grade 8.8 in the assembly is often stretched, the failure of the bolt analysis found that the bolt heart contains a large amount of ferrite. For such bolts with high assembly torque requirements, in the heat treatment process, not only to control the hardness of quenching and tempering, while the core ferrite control within 3 levels, to eliminate the assembly was stretched or pulled off.
The guaranteed load test method is to put the bolt on a universal testing machine to do a tensile test, apply the guaranteed load specified in the standard and continue for 15s to measure the absolute elongation caused by applying the guaranteed load to the bolt. In the U.S. SAEJ429, it is clearly stipulated that grade 8 (equivalent to the national standard 109) or more, high-strength bolts used in the automotive industry must be tested for guaranteed load, and the absolute elongation is controlled to within 12.7μm. As the absolute elongation is very fine, the surface of the fine burr, sharp particles will have an impact on the measured value, in addition to the installation of fixtures will also bring a certain measurement deviation.SAE J429 clearly stipulates that the first error exceeds the standard, but also the second test. If the 2nd result fails, it can be judged that the bolt does not meet the requirements; if the 2nd result passes, it can be judged that the bolt meets the standard requirements.
The determination of the heart tissue of tempered fasteners can refer to GB/T13320-2007 box steel die forging metallographic organization rating chart and assessment method. The test results show that for SWRCH35K steel manufacturing 8 condensed bolts, the volume fraction of ferrite tissue in the heart should be within 12%, the minimum hardness HRC value of 27, for SCM435 steel manufacturing 10.9 grade bolts, the volume fraction of ferrite tissue in the heart should be controlled within 5%, the minimum hardness HRC value of 34.5 medium carbon alloy steel is mainly used to manufacture 9.8 10.9, 12.9 high-strength bolts, for bolts after quenching the determination of the heart ferrite: 9.8, 10.9 engine bracket fastening bolts without fatigue life requirements, the heart ferrite 1-3 qualified; 9.8, 10.9 engine rod bolts with friction factor requirements, the heart ferrite 1-2 qualified, for 12.9 bolts, the heart ferrite level 1 qualified. For 12.9 grade bolts, the core ferrite grade 1 is qualified.

3. Heat treatment process technology concerns

The rapid development of automotive fasteners has brought challenges and opportunities to the production enterprises. Compared with the world’s advanced level, the overall level of heat treatment process technology of fasteners in China is still lagging behind, the gap is increasing, especially in environmental protection, energy consumption, quality and many other aspects of the situation is serious, therefore, it is necessary to improve the level of automotive fastener technology, the author believes that more attention should be paid in the following areas.

3.1 The use of annealing-free cold heading steel and non-quenched steel

The use of annealing-free cold heading steel and non-quenched steel manufacturing automotive fasteners is a new research topic in recent years.
In order to ensure the cold forming performance of automotive fasteners, cold heading steel needs to be pre-annealed by spheroidization. Non-annealed cold heading steel can be directly used for cold heading forming by adjusting the chemical composition of the steel and controlling the rolling and cooling during the rolling process to partially spheroidize the pearlite and reduce the deformation resistance during cold forming. Such as the production of horse steel eWRCH35K steel due to the exemption of spheroid annealing, both significant energy-saving effect, and shorten the production cycle, is an energy-saving new materials.
Fasteners with non-tempered steel is mainly cold hardening non-tempered steel, steel with trace amounts of strong carbon (nitride) forming elements (V, Nb, Ti), in the drawing and cold heading process, its toughness can reach or close to 8.8, 9.8 level. GB/T309822-2009 “fasteners mechanical properties of fine grain non-tempered steel bolts, screws and studs” has been released, to non-tempered steel fasteners The promotion and use of non-tempered steel fasteners provides strong technical support. Baosteel FM08MnS, iFM08MnSV and MFT08 steel produced by Masteel, eliminating the need for spheroid annealing before cold-drawing and quenching and tempering after forming, reducing the tendency of decarburization of the threaded part and improving the yield, with obvious economic benefits.

3.2 Improvement of heat treatment process technical documents

Heat treatment process is a special process that cannot guarantee the product quality by subsequent inspection and test, therefore, there must be a reasonable system for its control to realize. First, the factors that may cause process failure should be screened, and the formulation of heat treatment process technology should be done well; second, a perfect heat treatment process management system should be established, and the process documents should clearly mark out the operation methods, process parameters and inspection methods of the heat treatment process, while special emphasis should be placed on the furnace loading method and furnace loading volume to address the feasibility of the operation and the correctness of the method, the raw material material material and the process condition of the fasteners to be treated The pre-heating process condition of the fasteners is inspected to ensure the rationality of the process parameters and the effectiveness of the treatment; furthermore, statistical techniques are used to analyze the heat treatment process, calculate and measure the heat treatment parameters, find the law of the influence of the heat treatment parameters on product quality according to the changes in the process quality, and then use the heat treatment expertise to continuously improve the heat treatment technology to ensure that the product quality is once delivered for inspection Maximize the passing rate.

3.3 Research and development of new heat treatment equipment

Develop new heat treatment equipment, further improve furnace temperature and carbon potential control technology and improve furnace gas uniformity, reduce quenching distortion and quenching workpiece bump, improve the cooling effect and cooling uniformity of the quenching cooling tank, pay attention to the supporting before and after cleaning, oil and water separation, waste heat utilization, energy saving and emission reduction technology, without affecting product quality, reduce heat treatment production costs.

4. Conclusion

Automotive fastener production enterprises should focus on the update of heat treatment equipment, the selection of suitable raw materials, and constantly improve the heat treatment process system in production, strengthen the hydrogen drive treatment and heart tissue detection, as soon as possible to reduce the gap between automotive high-strength fasteners and world-class products, and improve the grade of China’s automotive fasteners.