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Vacuum Hardening of thin-walled small-sized parts
Advantages of Vacuum Hardening in Double Chamber Vacuum Oil Quenching Furnace
With the innovation of vacuum heat treatment equipment and the progress of vacuum heat treatment process, quenching with double-chamber vacuum oil quenching furnace is the main direction of heat treatment development. The double-chamber vacuum oil quenching furnace has high thermal efficiency, which can realize rapid and automatic temperature rise, heat preservation, and vacuum hardening to ensure that the surface of the parts is free from oxidation, decarburization, and carburization during the vacuum hardening process. At the same time, the parts can be degreased, degassed, and decomposed. Oxide, and the deformation after vacuum hardening is small, green and environmentally friendly, and pollution-free, which is the development direction of future quenching.
Disadvantages of Vacuum Hardening in Double Chamber Vacuum Oil Quenching Furnace
However, the quenching transfer time of the double-chamber vacuum oil quenching furnace is long. Generally, the transfer time of parts from the heating chamber to the quenching oil tank is about 20-25s. The effect is poor, and it is difficult to guarantee the hardness requirement.
Vacuum hardening requirements for thin-walled pinion parts
The carbon content of T9A steel strip is high, and the cooling rate is high during vacuum hardening. If the cooling rate is slow, cementite will be precipitated from austenite, and a large amount of austenite will also transform into pearlite. The content of martensite after vacuum hardening is low, and the hardness cannot meet the requirements.
Vacuum heat treatment is fully automatic control. The transfer time of the parts from the heating chamber to the quenching process is determined by the properties of the vacuum furnace. The transfer time is relatively long (20-25s) when quenched in a vacuum furnace, so the steel strip after quenching cannot be guaranteed. Hardness requirements. The thickness of the steel strip is only 0.6mm, continuous heat dissipation during quenching transfer, part of the cementite dissolved in the austenite will be precipitated, and then part of the austenite will be transformed into pearlite, and the cooling speed of the steel strip will be accelerated when immersed in oil , the remaining austenite transforms into martensite. The temperature during vacuum hardening can no longer meet the quenching requirements, so the quenching effect cannot be achieved.
In order to solve the problem of high heat loss during the quenching transfer process, a set of quenching tooling is specially designed in this paper. The picture below is the quenching tooling diagram of T9A steel strip. The tooling is assembled from an upper cover of a protective box and a lower cover of a protective box. Holes are evenly punched on the large plane of the upper and lower covers of the protective box, and a $5.5 hole is opened every 5mm along the length and width directions. The upper and lower covers of the protective box are made of No. 10 steel plate with a thickness of 10 mm.
Before vacuum hardening, spread the T9A steel strip parts evenly on the lower cover of the protective box, and fasten the upper cover of the protective box to put the parts into the protective box, so as to reduce the heat loss during the batch quenching transfer of parts. The vacuum heat treatment process is fully automated, and the steel strip is put into a protective box for vacuum hardening. The hardness is all higher than the required hardness after quenching, and the hardness after tempering also meets the requirements.
The steel belt is spread in the protection box. After the heat preservation is over, the protection box is transferred from the heating chamber to the cooling chamber and enters the quenching oil tank during the transfer time. Since there is no air in the vacuum furnace for conduction heat dissipation and only radiation heat dissipation, the thickness of the steel plate can be guaranteed. The outer wall radiates heat in the cooling chamber, and the steel belt and the inner wall of the protective box radiate to each other with almost no heat loss. The number of holes and the size of the holes on the upper and lower covers of the protection box make the heat dissipation of the steel strip negligible, thus ensuring that the temperature of the steel strip meets the quenching requirements during vacuum hardening.
During the process of immersing in oil, the protective box loaded with steel strip enters the quenching oil tank from top to bottom, and the tumbling quenching oil moves from bottom to top, and enters the box from the lower hole of the protective box to quickly wash the steel strip, and the quenching oil quickly washes the protective box After filling, it is quickly extruded from the upper hole to take away the heat, and the quenching oil conducts strong convection heat dissipation on the steel strip, so that the steel strip can achieve the purpose of rapid cooling during the quenching process.
(1) Put the T9A thin steel strip into the quenching tool for vacuum hardening. The average hardness after quenching can meet the requirements, and there is no increase or decarburization.
(2) The quenching temperature of T9A steel is low (790 ℃), close to the eutectoid transformation point temperature of 727 ℃, and does not contain alloying elements (Cr, Mo, W, V, etc.) that form carbides, so that the supercooled Austrian The tenite is extremely unstable, and the C curve shifts to the left, which shortens the start time and end time of pearlite transformation during cooling, and a higher cooling rate is beneficial to increase the hardness.
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