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Vacuum heat treatment process of ball screw

The lead screw is a slender part composed of teeth with different tooth shapes spiraling around the slender cylindrical axis. It transmits equal parts precision and force distance through the helix formed by the threaded teeth, thereby realizing rotary motion and linear motion. mutual conversion. According to the sliding characteristics of the screw during operation, it can be divided into two categories: sliding screw and ball screw. In recent years, with the development of machinery manufacturing industry, ball screws have gradually replaced sliding screws due to their small frictional resistance and extremely high transmission efficiency, and are widely used in various lathes, milling machines, boring machines and other industrial equipment and Manufacture of precision instruments.
Bearing life is the most important performance index of screw material. The fatigue life of the screw bearing is a statistical concept, that is, it is expressed by the relationship between the failure probability and the number of cycles under a certain load condition. In addition to fatigue life, the lead screw must also meet the process requirements of high speed, heavy load, precision and longevity. Therefore, the lead screw is required to have technical indicators such as high strength and toughness, high surface hardness and corrosion resistance, good hardenability, high dimensional accuracy, and good dimensional stability.
Hardening and Vacuum Tempering of Ball Screws
1 Hardening method of ball screw
For lead screw materials, the higher the hardness, the greater the wear resistance and the longer the service life. As a vacuum heat treatment process that greatly improves the strength and hardness of steel, hardening is the most important link in the vacuum heat treatment process of screw materials. With the progress of vacuum heat treatment technology in recent years, more and more hardening technology has been applied to the production of bearing steel, and achieved certain results.
(1) Carburizing and quenching Carburizing of low carbon alloy steel includes solid, liquid, gas carburizing, carbonitriding, ion carburizing, pulse vacuum carburizing, etc. The purpose is to obtain a high carbon layer on the surface of the steel. , to achieve the effect of wear-resistant surface, strong and tough core, and not easy to produce brittle fracture. After carburizing, it is best to cool it a little, and pre-cool it to 820~840 ℃ and then oil quenching. Due to the fine-grained structure on the surface after carburizing, the alloy carbides have not yet been precipitated from the austenite, so air cooling from 930 °C to 830 °C can reduce the network precipitation of alloy cementite; after 830 °C oil quenching, the carbon content is higher The surface layer can be retained in martensite and retained austenite, without network alloy carbides.

(2) Intermediate frequency surface hardening After pre-normalizing and annealing, the lead screw can be heated in an intermediate frequency induction coil, and then tempered at low temperature after surface hardening. This process can generally increase the hardness of the workpiece to above HRC50, and the surface is hardened, the core has good toughness and strength, and the quenching deformation is not large. It is suitable for the manufacture of various types of lead screws, but the process is cumbersome.
Generally, medium carbon steel and medium carbon alloy steel (such as 35, 45 steel) are quenched after medium frequency induction heating, and the hardness can reach about 48~56HRC; and for GCr15 steel, after this process, the surface structure is martensite + residual Austenite + carbide, the hardened layer can reach about 20mm, and the hardness can reach 750HV, which is very suitable for the manufacture of large-sized lead screws.
(3) Heating and quenching along the raceway is not suitable for large ball screws with deep raceways (8~12mm) and large pitch, and the method of heating and quenching along the outer circle is no longer applicable, so high-frequency induction heating along the raceway can be used. , the method of immersion hardening. The process principle is shown in the figure, the induction coil is bent into two semi-circular arcs, the top is upturned, and the two tails are inserted into the secondary poles of the high-frequency or intermediate-frequency transformer, which are cooled by water. When the lead screw with raceway is loaded into the immersion tank, the lead screw rotates once, and the thread advances one tooth along the clip. In this way, the small diameter clearance along the raceway arc also heat-hardens a raceway. The induction coil is two semi-circular arcs, the former is used for preheating, and the latter is reheated. When the heated raceway rotates and advances, after the raceway heated to the austenite state leaves the induction heating coil, the heating stops, and the aqueous solution in the groove immediately enters the heated raceway to form hardening. This cycle is progressive, so that all the screw raceways are hardened.

2 Low temperature vacuum tempering of ball screws
The ball screw generally adopts the low temperature vacuum tempering process. The common tempering specification is 200±10℃ for 2~4h. After the low temperature tempering, the hardness is required to be between HRC58~62; -60 ℃, after maintaining the cold treatment for more than 2 hours, it is only raised to room temperature and tempered. For the carburized and hardened lead screw, after vacuum tempering at 200~220℃, the retained austenite and martensite are transformed, and the alloy cementite is precipitated in granular form, which has good wear resistance and toughness.
Summary
(1) Due to its long life and high efficiency, ball screws have gradually replaced sliding screws and have been used in the manufacture of various machine tools and mechanical equipment. In order to improve the fatigue life of the lead screw material, inclusions and defects in the steel should be strictly controlled and eliminated, and the material purity and organizational uniformity of the steel should be improved. Because of its excellent comprehensive properties, GCr15 steel has been widely used in the manufacture of ball screws; some alloy tool steels, such as 9Mn2V, etc., have gradually become increasingly popular due to their good hardenability and hardenability and high tempering stability. Used in the manufacture of ball screws.
(2) Pre-vacuum heat treatment of the screw material before hardening is a necessary process. For low-carbon carburized and hardened screws, high-temperature normalizing can completely transform pearlite and ferrite into austenite at high temperature. And in the process of air cooling, flaky pearlite + ferrite (P+F) is formed to eliminate band segregation; for medium carbon alloy steel and high carbon chromium bearing steel, spheroidizing annealing is required to improve the structure and eliminate the network Fe3CⅡ , and obtain 2~3 spherical pearlite. For some steels with high tempering stability (such as 9Mn2V, etc.), the quenching and tempering spheroidization can be directly carried out without normalizing, thereby reducing the processing time and improving the work efficiency. At the same time, after the workpiece is normalized and spheroidized and the subsequent machining process, stress relief annealing treatment is required to eliminate residual stress and reduce quenching deformation.
(3) Carburizing and hardening of low carbon steel can obtain a high carbon layer on the surface of the steel, achieving the effect of wear-resistant surface, strong and tough core, and not easy to produce brittle fracture; the medium frequency surface hardening process can increase the hardness of the workpiece to HRC50 or above, and the surface is hardened, the core of the screw has good toughness and strength, and the quenching deformation is not large; for large ball screws with deep raceways and large pitch, it is suitable to use high-frequency induction heating along the raceway, liquid immersion quenching Hard method; the laser raceway hardening process can rapidly heat the surface of the workpiece and realize self-excited quenching, which greatly improves the surface hardness of the workpiece and reduces the working time. After hardening, the lead screw material is generally subjected to a low temperature tempering process to improve the toughness and wear resistance of the material.
Selection of vacuum tempering equipment: The RVT series vacuum tempering furnace produced by SIMUWU is a high-quality product for the vacuum tempering process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum tempering process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.