High speed steel vacuum heat treatment

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The vacuum heat treatment of high-speed steel has developed rapidly because of the vigorous promotion of green and low-carbon life. Many professional tool factories and heat treatment factories have successively established vacuum heat treatment workshops, and the quantity and variety of products subjected to vacuum heat treatment have increased significantly. All this is inseparable from the excellent characteristics of vacuum heat treatment.

Basic knowledge of vacuum

Putting the workpiece in a negative pressure space below normal atmospheric pressure for heating and heat preservation is called vacuum heating. This negative pressure space is called a vacuum. The degree of negative pressure is expressed by the degree of vacuum Tor (Torr) or Pa (Pa). According to the degree of vacuum, it is usually divided into 4 grades:

Low vacuum——105~102Pa

Medium vacuum——102~10-4Pa

High vacuum——10-4～10-5Pa

Ultra-high vacuum——less than 10-5Pa.

At present, the vacuum degree of vacuum heat treatment furnace is mostly 10³~10-4 Pa. When the workpiece is heated in the vacuum furnace, in addition to the residual air, there are many gas sources in the vacuum device, such as the gas released from the workpiece, the gas absorbed by the inner wall of the device, the gas released from the lining material, and the gas leaking into the device. Therefore, it must be exhausted with a vacuum pump to ensure the required vacuum degree. The higher the vacuum degree, the lower the gas pressure, the less the number of gas molecules in the furnace, and the less the impurity amount.

Characteristics of vacuum heating

When steel is heated in a vacuum, it will have characteristics that are not present when heated in other media, and obtain unique effects.

(1) Surface protection

When steel is heated in an oxidizing atmosphere containing oxygen, water vapor and carbon dioxide, oxidation and decarburization will occur, but when heated in a vacuum, the content of the oxidizing atmosphere is extremely low, that is, the partial pressure of oxygen is very low, which can make steel Avoid oxidation and decarburization.

(2) Surface purification

When the decomposition pressure of oxygen in the furnace is lower than the decomposition pressure of oxides, it can not only prevent oxidation, but also decompose and remove the existing oxides on the steel surface, so as to obtain a bright surface.

(3) Degreasing effect

The cutting coolant, lubricant, and anti-rust oil on the surface of the workpiece can be decomposed into hydrogen, carbon dioxide and water vapor when heated under vacuum, and discharged during the pumping process. This phenomenon is called degreasing. If the surface of steel parts is not seriously polluted in advance or the requirements are not too strict, it is not necessary to clean before vacuum heating.

However, in actual production, degreasing treatment is generally performed in advance to prevent or reduce the pollution of the vacuum system.

High Speed Steel Vacuum Heating Process

Warm up

During salt bath quenching, 1~2 temperature plateaus are generally used for preheating, namely 500~550℃; 820~880℃. However, during vacuum quenching, due to the slow heat exchange speed between the heating medium and the workpiece, it is necessary to add another high temperature average temperature platform. As shown in the figure:

Heating

Heating includes three processes: temperature rise, through-firing and heat preservation. The purpose is to obtain austenite with a certain degree of alloying while maintaining fine grains. In vacuum heat treatment, due to the slow heating speed, long soaking (holding) time, and the addition of a high temperature soaking platform, the heating temperature must be slightly lower than that of salt bath quenching, otherwise it is easy to produce grain growth and inhomogeneity. Depending on the shape and size of the workpiece, the requirements for use and the original structure of the material, the temperature of the salt bath is 10~30℃ lower than that of the salt bath.

Vacuum quenching process

(1) Vacuum oil quenching

In the early vacuum quenching of high-speed steel, there is a process of vacuum oil quenching, that is, closed oil-cooling quenching is realized in a double-chamber vacuum furnace. The oil tank is placed under the front chamber, and the oil temperature is controllable. The oil used for vacuum quenching is different from the general quenching oil, and its vapor pressure is required to be lower than 0.0133~0.00133Pa (10-4~10~5 Tom). This is to ensure that a large amount of evaporation does not occur at the commonly used vacuum level of 1.333Pa (10-2Torr), and to ensure that the front chamber still has sufficient cooling capacity when it is under negative pressure. The pressure of the oil surface has a great influence on the hardenability of the oil. When it is lower than 0.05 MPa, the hardenability of high-speed steel may be lost.

The disadvantage of vacuum oil quenching is that in addition to the loss of (gas quenching) due silver white and the need to remove oil stains, etc., it is mainly that the surface layer has trace carburization, the thickness is about tens of microns, and the carbon content near the surface can reach 2 %. Therefore, for high-speed steel tools, the process of vacuum oil quenching has been rarely used.

(2) Vacuum air quenching

Air jet quenching after vacuum heating can avoid the above-mentioned oil quenching surface carburization, oil pollution and other disadvantages caused by oil in and out, and obtain a silver-white surface and a surface layer without decarburization and carburization. The flow and pressure of the jet determines the cooling ability of a certain gas to the workpiece. When the injection device (fan power) is constant, the pressure is the decisive factor.

tempered

The tempering after vacuum quenching includes vacuum tempering or tempering in a pit furnace with a protective atmosphere, both of which are secondary hardening and tempering of high-speed steel. The mechanism is basically the same as that of the commonly used salt bath furnace and gas furnace tempering. .

Equipment selection: The VOGQ 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 air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.