Automotive high-strength fasteners have a series of technical characteristics: high-precision grade; harsh service conditions, it will withstand the impact of severe cold and extreme temperature difference with the host, and withstand high temperature and low temperature erosion; static load, dynamic load, overload, Heavy-duty and environmental media corrosion, in addition to the axial preloading tensile load, will also be subjected to additional tensile alternating loads, transverse shear alternating loads or composite bending loads during operation. The effect is sometimes subjected to impact loads; additional lateral alternating loads can cause loose bolts, axial alternating loads can cause fatigue fracture of bolts, axial tensile loads can cause delayed fracture of bolts, and high temperature conditions Creep of bolts, etc.
A large number of failed bolts indicate that the break in the transition between the head of the bolt and the stem during service is broken; the joint between the thread of the bolt and the rod is broken; and there is a slide along the threaded portion. Metallographic analysis: There are more undissolved ferrite on the surface and core of the bolt. The austenitization is insufficient in quenching, and the strength of the matrix is ​​insufficient and stress concentration is one of the important reasons for failure. To this end, it is very important to ensure the bolt cross-section and uniformity of the structure.
The function of the quenching oil is to quickly remove the heat of the red hot metal bolt and reduce it to the martensitic transformation temperature to obtain the high hardness of the martensite structure and the depth of the hardened layer. At the same time, it is necessary to reduce the deformation and prevent the bolt. Cracking. Therefore, the basic characteristics of quenching oil is "cooling characteristics", which is characterized by faster cooling rate in the high temperature stage and slower cooling in the low temperature stage. This characteristic is very suitable for the quenching requirements of high strength bolts of alloy structural steel ≥10.9 or higher.
The rapid quenching oil is thermally decomposed, oxidized and polymerized during use, resulting in a change in cooling characteristics. The trace moisture contained in the oil will seriously affect the cooling performance of the oil, resulting in a decrease in the brightness of the fastener after quenching and uneven hardness. Produces soft spots and even a tendency to crack. Studies have shown that the deformation problems caused by oil quenching are partly caused by water in the oil. In addition, the water in the oil accelerates the emulsification of the oil and promotes the failure of additives in the oil. When the water content in the oil is ≥0.1%, when the oil is heated, the water accumulated at the bottom of the oil tank may reach a boiling point, and the volume suddenly expands, which may cause the oil to overflow the quenching oil tank and cause a fire.
For the quenching oil used in the continuous mesh belt furnace, according to the quenching characteristic data accumulated in the 3-month interval test, it is possible to establish the oil stability and quenching characteristic map, determine the suitable service life of the quenching oil, and predict the performance of the quenching oil. Changes related to the problem, thereby reducing the rework or waste loss caused by changes in quenching oil performance, making it a routine control method for production. The depth of hardening directly affects the quality of the bolt after heat treatment. When the hardenability of the material is poor, the cooling rate of the cooling medium is slow and the bolt size is large, it is impossible to completely quench the martensite at the core of the bolt during quenching. Tissue reduces the level of strength in the heart area, especially the yield strength. This is clearly very disadvantageous for bolts that are subjected to a uniform distribution of tensile stress along the entire section. The lack of hardenability reduces the strength. The metallographic examination found that the pro-eutectoid ferrite and the reticular ferrite structure existed in the core, indicating that the bolt hardenability needs to be strengthened. It is well known that increasing the hardenability of the two ways, increasing the quenching temperature; increasing the quenching ability of the quenching medium can effectively increase the quenching depth of the bolt.
Houghton-QuenchK2000 has specially developed rapid quenching oil based on the original medium-speed quenching oil and Houghto-QuenchG. Houghto-QuenchK2000 further improves its hardenability, especially suitable for quenching and cooling of fasteners. Harden depth.
The quenching oil has a short steam film stage, that is, the oil is cooled at a high temperature stage. This feature is beneficial for 10B33, 45 steel ≤ M20 bolts, M42 nuts to obtain a deep hardened layer, and for SWRCH35K, 10B28 steel Thin ≤ M12 bolts, M30 nuts can ensure a small difference between the hardness of the core and the surface hardness. From the analysis of the cooling rate distribution, in addition to the fast cooling required in the middle and high temperature stages, the low temperature cooling speed of the oil has a greater effect on obtaining the depth of the hardened layer. The higher the low temperature cooling rate, the deeper the hardened layer tends to be. This is uniform for the entire section of the high-strength fastener, and it is required to obtain about 90% of the martensite structure before tempering in the hardened state. The indicators for evaluation include flash point, viscosity, acid value, oxidation resistance, residual carbon, ash, sludge, quenching cooling rate, quenching brightness and other nearly 20 indicators.
For larger size bolts or PAG quenching agent, which meets the quenching requirements of most products, PAG quenching agent is in the boiling stage in the martensite transformation zone. The cooling rate is large and there is a greater risk. At a cold speed of about 300 °C, the lower the cooling rate at this temperature point, the stronger the ability to prevent quenching, and the more steels are suitable. The convective cooling rate stability during use is the most important factor to ensure the quenching quality.
On the specimen of the early failure bolt, it can also be seen that there are crack defects on the thread near the fracture of the fracture bolt, mainly due to the improper rolling process of the bolt, and the metal plastic rheology merges during the cold forming process of the twisted thread. It is caused by folding; there are also micro-cracks with different depths in the bottom of the thread, and the stress-concentrated area is formed by processing the built-up edge. GB/T5770.3-2000 "Special requirements for bolts, screws and studs for fastener surface defects" stipulates that for a bolt with a stress above the diameter of the thread, the depth is not more than a quarter of the height of the thread profile is allowed. Folding and built-up edge of the thread bottom is an impermissible defect, and folding is one of the main causes of bolt breakage. The use of Fulton's extreme pressure lubricant for bolt threading can effectively prevent built-up edge and reduce stress concentration, which helps to improve the fatigue life of the bolt. 
Editor in charge: Hu Na
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