vacuum heat treatment

Vacuum can refer to any gaseous space whose pressure is less than a normal atmospheric pressure (negative pressure). When the heat treatment process of metals is carried out in vacuum, it is called vacuum heat treatment. Vacuum heat treatment can achieve almost all heat treatment processes, such as quenching, annealing, tempering, carburizing, chromizing, etc. In quenching process, gas quenching, oil quenching, nitrate quenching, water quenching, degassing, etc. can also be used in chemical heat treatment after the appropriate medium is introduced.

The degree of negative pressure in a vacuum state is called vacuum degree. The most commonly used unit of vacuum is Pa and Torr (1 Torr = 133.3Pa). The lower the pressure is, the higher the vacuum is; the higher the pressure is, the lower the vacuum is. According to the degree of vacuum, vacuum is usually divided into four levels: low vacuum, medium vacuum, high vacuum and ultra-high vacuum. In addition, the dew point of water vapor in the vacuum state is often used to express the vacuum degree. The gas in vacuum furnace includes residual air, gas released from furnace body and workpiece, evaporating gas of lubricating oil and gas infiltrated from outside, etc. It is very complicated, so vacuum pump must be used to continuously exhaust to ensure the required vacuum.

The surface of metal workpiece obtained by vacuum heat treatment method can obtain special effect that general heat treatment does not have, showing certain superiority.

In the process of heat treatment of steel, vacuum atmosphere has the following beneficial or harmful effects.

(1) Degreasing

Before heat treatment, oil stains often stick to the surface of workpieces due to mechanical processing or pressure forming. The vapor pressure of grease and lubricant adhering to metal surface is high. When heated in vacuum, they can produce or decompose into gases such as water, hydrogen and carbon monoxide. They are pumped away by vacuum pump and react with different metal surfaces to obtain very smooth surface without oxidation and corrosion. However, pre-degreasing treatment is still needed in production to reduce the pollution of oil pollution to vacuum system.

(2) Degassing

When metals are smelted, liquid metals absorb H2, O2, N2, CO and other gases. Because the cooling rate is confirmed too fast, these gases remain in solid metals, forming various metallurgical defects such as pore and white spot, which affect the resistance, permeability, hardness, strength, plasticity and toughness of materials. According to the solubility of gases in metals, they are formed by the square root of partial pressure of the surrounding environment. In a proportional relationship, the smaller the partial pressure is, the higher the vacuum degree is, the less the solubility of the gas in the metal is, and the released gas is pumped away by the vacuum pump.

(3) Decomposition of oxides

The oxide film, rust, oxide and hydride on the metal surface disappear when heated in vacuum, which makes the metal surface smooth. When the vacuum degree of steel parts reaches 0.133-13.3 pa, the surface purification effect can be achieved. After the metal surface is purified, the activity is enhanced, which is conducive to the atomic absorption of C, N and B, and makes the chemical heat treatment faster and the composition uniform. When the vacuum is enough and the partial pressure of oxygen is lower than the decomposition pressure of oxide, the oxide formed on the surface can be decomposed and removed to obtain a bright surface.

(4) Surface protection

Vacuum heat treatment is essentially carried out in a very thin atmosphere. The residual trace gases in the furnace are insufficient to produce oxidative decarbonization and carburization of the treated metal materials. The chemical composition and original brightness of the metal surface remain unchanged.

Vacuum heat treatment also has the following disadvantages.

(1) The evaporation of alloying elements has different vapor pressures at different temperatures. When vacuum increases, the metals with high vapor pressures (Mn, Cr) are easy to evaporate, which damages the chemical composition of the material itself and pollutes other metal surfaces, causes bonding between parts or parts and baskets, and causes defects such as electrical short circuit and material modification.

For steel, the most easily evaporated alloying elements during vacuum heat treatment are Mn and Cr, which are common metal elements in steel. Usually the parts should be vacuum pumped and heated below 800 C, and inert gas should be used above 800 C to reduce the vacuum by 20-26.7Pa.

(2) Carburization of steel parts caused by oil quenching in vacuum heating

Oil quenching of steel by vacuum heating can cause carburization. For example, carburization can be found when 30CrMrSiN2A steel is OIL-QUENCHED at vacuum of 10-2 x 1333.3 3a and heating temperature of 900 C. This is because the material surface is in active state due to the purification of the surface during high temperature and vacuum heating. When the hot workpiece contacts with quenching oil, the carburizing process is caused by the surroundings of oil vapor.

Generally speaking, this kind of steel carburizing is harmful to the surface properties of materials. In order to reduce vacuum oil quenching carburizing, it is necessary to postpone the time when the workpiece enters the oil, to keep low pressure above the oil at the beginning of quenching, and to oxidize the workpiece slightly before it is immersed in the oil.