Vacuum heat treatment of automotive leaf springs

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Automobile leaf spring is a kind of elastic element, its function is to bear the action of the carriage and the load (static load), it can transmit the vertical load, ease and restrain the impact caused by the uneven road, and limit the vibration of the body and the wheels. As an elastic element, it not only has the functions of buffering, damping, and energy storage, but also bears the role of transmitting force and guiding. During the working process, the leaf spring bears the high impact load caused by the uneven road, and thus or unidirectionally circulates. Bending stress and vibration are also affected by the erosion of mud water and sediment. Therefore, the utility model has the advantages of simple structure, reliable use and convenient maintenance, so it is widely used in general trucks. The automobile leaf spring is made of alloy steel with a hardness of 380~460HBW. The leaf spring reaches the maximum strength characteristics, that is, a high elastic limit. After shot blasting, it is in a state of surface compressive stress, and then enters the preliminary machining stage.

The “probability-stress curve” of a heavy-duty vehicle shows that the stress of the leaf spring is 882-980Pa. The main failure forms of automobile leaf springs are corrosion fatigue fracture, stress corrosion fracture, brittle fracture, wear and stress relaxation, and permanent plastic deformation, etc. The hazards include parking for repair, large loss of leaf springs, and reduced driving comfort, etc. Therefore, It should be taken seriously to reduce the probability of failure, and ensure product quality is qualified in all links such as spring manufacturing and vacuum heat treatment. Therefore, the leaf spring should have high elastic limit, good hardenability, low decarburization sensitivity, high hardness and strength, certain impact toughness, good fatigue strength and maximum specific elastic work. Ear and middle fixation. When the leaf spring is deformed under the load, there is relative sliding between the plates to generate friction and friction at the supports at both ends, which can promote the attenuation of the vibration of the frame. However, the dry friction between the plates will transmit the impact of the wheel to the frame to a large extent, reducing the ability of the suspension to mitigate the impact, and causing the plates of the leaf spring to wear faster. Therefore, thicker graphite grease is applied between the sheets, and some plastic gaskets are sandwiched between the leaf spring sheets.

Material selection

In the working state, the automobile leaf spring absorbs a large amount of elastic work, and requires no permanent deformation, so it has a bending strength ratio and the largest possible elastic specific work. In order to obtain a leaf spring with high strength and a large fatigue limit, the leaf spring The spring steel should be made of alloy steel with a carbon content of %~%, so 60Si2Mn steel can be selected as the automobile leaf spring.

Vacuum heat treatment process of 60Si2Mn automotive leaf spring: cutting and blanking → machining → straightening → rolling ears → complete annealing → quenching → medium temperature tempering → quality inspection

Pre-heat treated – fully annealed

1.Purpose of complete annealing: to refine the grains, reduce the hardness of the steel, improve the cutting performance of the steel, and eliminate the internal stress.

2.Heating temperature: 750℃

3.Cooling method: furnace cooling

4.Annealing equipment: medium temperature pit type resistance furnace, the following table gives the parameters of the medium temperature pit type resistance furnace

The surface hardness (HBS) of the steel measured after annealing does not exceed 222.

Final vacuum heat treatment – quenching

1.Vacuum quenching purpose: to improve the strength and hardness of the leaf spring and obtain good mechanical properties. After vacuum quenching, the matrix structure is martensite, and the matrix hardness is greater than 63HRC.

2. Vacuum quenching temperature: The heating temperature is very important in the vacuum quenching process. If the temperature is too high, the austenite grains will be coarse, and the oxidation and decarburization will be serious, which will reduce its strength and fatigue limit; while the temperature is too low, it will cause microstructure. Inhomogeneous transformation, the appearance of undissolved ferrite, also reduces its strength and service life. The best vacuum quenching temperature is 865~875℃, and the obtained tensile strength, impact toughness and fracture toughness are the best. .

3.Heating method: The method of heating to the temperature means that when the furnace temperature is heated to the specified temperature, the workpiece is loaded into the vacuum heat treatment furnace for heating. The reason is that the heating speed is fast, saving time and facilitating mass production.

4.Vacuum quenching medium: 60Si2Mn alloy spring steel has a low critical cooling rate, so vacuum oil quenching can be used, the oil is 5# mechanical oil, and the oil temperature is between 20~80℃.

5.Holding time: The heating holding time of 60Si2Mn automobile leaf spring is related to the thickness of the spring, as shown in the following table.

6.Microstructure after quenching: The metallographic structure after quenching is grade 5 martensite

 

Equipment selection: The RVGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment 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 annealing furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.