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Bolt and Fastener vacuum Heat Treatments process

Fasteners are known as the “rice of industry” and they are universal basic components. Bolts are the most commonly used and widely used fasteners. The materials used to make fasteners mainly include steel, stainless steel, titanium alloys and non-ferrous metals. At present, except for a small number of large-sized products that are manufactured by warm heading (warm extrusion) and cutting, most of them are manufactured by cold heading (cold extrusion) molding process. The mechanical properties of bolts largely depend on the quality of the material from which the fastener is made.

In fastener manufacturing, the correct selection of fastener materials is an important step, because the performance of fasteners is closely related to their materials. Improper or incorrect material selection may result in unsatisfactory performance, shortened service life, or even accidents or processing difficulties, high manufacturing costs, etc. Therefore, the selection of fastener materials is a very important link.

The greater the degree of hardenability of the steel, the greater the yield-strength ratio after quenching and tempering. The increase in the yield-strength ratio means that the potential of the steel can be better utilized, which is important for meeting the tensile strength and yield ratio specified by the product. High-strength bolts have more practical significance.

Compared with carbon steel with the same carbon content, alloy steel used for hardening and tempering of high-strength bolts often has higher plasticity and toughness at the same strength; or has higher strength at the same plasticity; or strength, plasticity, They have high toughness and are heat-resistant and can withstand service in high-temperature environments below 500°C. Because in the hardened and tempered state, in addition to solid solution strengthening ferrite, the alloying elements more importantly improve the tempering stability of the steel. After tempering at the same temperature, the carbides of the alloy steel are relatively dispersed, thereby increasing the strength of the steel. higher. If tempered to the same hardness and strength as carbon steel, the tempering temperature of alloy steel should be higher, so that the stress can be eliminated more completely.

High-strength fasteners must be tempered according to technical requirements. Heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the tensile strength value and yield ratio specified by the product.

The vacuum heat treatment process has a crucial impact on high-strength fasteners, especially their intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced vacuum heat treatment technology and equipment are necessary.

Due to the large production volume and low price of high-strength bolts, and the threaded part has a relatively fine and precise structure, vacuum heat treatment equipment is required to have large production capacity, high degree of automation, and good vacuum heat treatment quality. High-strength fasteners are all automatically controlled from loading – cleaning – heating – vacuum hardening – cleaning – vacuum tempering – coloring to off-line, effectively ensuring the quality of vacuum heat treatment.

ML25B is a low carbon alloy cold heading steel. The reason why ML25B is selected for grade 8.8 high-strength bolts is due to considerations of bolt hardenability and service conditions. For ML35 and SWRCH35K cold heading steels with a carbon content between 0.33% and 0.38%, the heat treatment of M16 and above bolts often suffers from quenching impenetrability and an increase in undissolved ferrite in the core, causing early failure during service.

After quenching low-carbon alloy steel with a carbon content of 0.25, almost all low-carbon martensite can be obtained, also known as dislocation martensite.

Bolt vacuum heat treatment process parameters

①The suitable vacuum quenching temperature for ML25B steel high-strength bolts is 870~885℃, and the hardness after vacuum quenching is greater than 44HRC.

② After quenching and tempering, ML25B steel is vacuum tempered at 410~430℃, and its mechanical properties can meet the technical requirements of grade 8.8 bolts. Considering the comprehensive mechanical properties of high-strength bolts, the tempering temperature is 430℃; but for grade 9.8 bolts , cannot meet the minimum tempering temperature requirement of 425°C in the standard.

③In order to avoid quenching cracking of ML25B steel, high-strength bolts with specifications below M10 can be vacuum oil quenched. The hardness after vacuum oil quenching can also reach 44~45HRC. The tempering stability of ML25B steel is poor, so the tempering temperature can be appropriately lowered. For example, during quenching and tempering, the quenching temperature should be increased by about 5°C and the tempering temperature should be lowered by about 5°C. If the mechanical properties do not meet the standard, it is mainly caused by the lax control of the vacuum heat treatment process, especially the insufficient holding time. As long as the heating austenitization is ensured uniformly, the high-strength bolts can be guaranteed to have good comprehensive mechanical properties. Cooling medium temperature and tempering process are secondary.