Principles of vacuum heat treatment process formulation

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Vacuum heat treatment equipment began in the 1920s, but its real development began in the 1960s and 1970s, mainly due to the market demand at that time and the research and development of graphite technology.

The working environment of vacuum heat treatment is actually
Below one standard atmosphere (1.013×105Pa),
Including low vacuum (105~102Pa),
Medium vacuum (102~ 10-1pa),
High vacuum (10-1~10-5Pa),
Ultra high vacuum (<10-5Pa).

Relatively speaking, vacuum heat treatment is also controllable atmosphere heat treatment, but the air in the working environment is extremely thin. Heating the workpiece in the vacuum state can avoid the oxidation and decarburization of conventional ordinary heat treatment, avoid hydrogen embrittement, and improve the comprehensive mechanical properties of the material parts. The service life of parts after vacuum heat treatment is usually dozens or even hundreds of times that of ordinary heat treatment.

The main content of making vacuum heat treatment process is to determine the heating system (temperature, time and way), determine the vacuum degree and air pressure regulation, and select the cooling way and medium.

1. Heating temperature
Vacuum heating has two characteristics. First, it is heated in a very thin atmosphere, avoiding oxidation, decarburization and erosion caused by heating in the air. Another characteristic is that the heat transfer in the vacuum state is a single radiation heat transfer, whose heat transfer capacity E is proportional to the fourth power of the absolute temperature T, that is, E=C (T/100) 4.

It can be seen that in the vacuum state, especially in the low temperature stage, the temperature rise is slow, so that the difference in temperature between the workpiece surface and the core reduces the thermal stress and the deformation of the workpiece is small. The selection of heating temperature is very important to the quality of the workpiece. In the process of making, the best heating temperature should be found out according to the technical requirements, service conditions and performance requirements of the workpiece. The lower temperature should be chosen as far as possible without affecting the performance and considering reducing deformation.

2. Holding time
The length of holding time depends on the size and shape of the workpiece and the amount of furnace. For the traditional heating and insulation time introduced in the general information, T is determined by the following formula:

T1 is equal to 30 plus (1.5-2) D
T2=30+ (1.0-1.5) D
T3=20+ (0.25-0.5) D

Where, D is the effective thickness of the workpiece (mm);
T1 is the first preheating time (min);
T2 is the second preheating time (min);
T3 is the final heat preservation time (min).

In fact, there are often several workpiece of different shapes and sizes at the same time in a furnace, which requires comprehensive consideration. We according to the size and shape of the workpiece, put way and furnace charging quantity, determine the time of heat preservation, but also consider that vacuum heating mainly rely on high temperature radiation, low temperature heating workpiece temperature (below 600 ℃) is very slow when the deformation of workpiece, no special requirement at this time, should make the first preheating and second preheating time shortened, as far as possible and improve preheating temperature, because of low temperature and long time of heat preservation, heat up after reaching the surface of workpiece is the core temperature still need some time.

According to the principle of vacuum heating, increasing the preheating temperature can reduce the temperature difference between inside and outside the workpiece, shorten the preheating time, and the final heat preservation time should be extended appropriately, so that the carbide in the steel is fully dissolved. In this way, the quality is guaranteed and the working efficiency is improved. The duration of insulation is also related to the following factors:

(1) Loading furnace quantity: workpiece size is the same fashion furnace quantity, then through burning time should be extended; On the contrary, it should be shortened.

(2) the form of the workpiece placement: because the vacuum furnace is radiant heating, generally speaking, if the workpiece shape is the same, should try to make the workpiece placement neat, avoid shielding thermal radiation, and set aside a certain space (<d), in=”” order=”” to=”” ensure=”” that=”” the=”” workpiece=”” can=”” be=”” subject=”” maximum=”” heat=”” radiation;<=”” span=””> For different parts of the same furnace, in addition to the maximum workpiece insulation time calculation, but also to increase the burning time. When the gap <d is=”” placed,=”” the=”” empirical=”” formula=”” as=”” follows:<=”” span=””></d></d),>
T1 = T2 = T3 = 0.4 G + D
Where, G is the furnace load (kg)
The meanings of the other symbols are the same as before.
In addition
For small workpieces (effective thickness D≤20mm)
Or the space between the workpiece is ≥D
Heat preservation time can be reduced:
T1 T2 = = 0.1 G + D
T3 = 0.3 G + D
For large work pieces (effective thickness D≥100mm)
The final holding time can be reduced
T1 = T2 = T3 = 0.4 G + 0.6 D

(3) Heating temperature: high heating temperature can shorten the heat preservation time.

3. Cooling time
(1) Precooling: for the small and medium-sized parts quenched at high temperature, also note that after entering the cold chamber from the hot chamber, whether to precool before quenching, will affect the quenching deformation. Its rule is: after the hot chamber enters the cold chamber, the direct oil cooling or air cooling will lead to the size change; If proper precooling is carried out, the dimensions before heat treatment can be kept unchanged; But if the precooling time is too long, the workpiece size will swell. The general rule is that for the workpiece with an effective thickness of 20~60mm, the precooling time is 0.5~3min.

According to the analysis, this is because when not directly precooling quenching, parts of the internal stress is given priority to with the thermal stress, so the volume contraction, and again in after a long time to precooling quenching, parts of the internal stress is given priority to with phase transformation stress, resulting in volume expansion, only after proper time of precooling, thermal stress and phase transformation stress the role of phase equilibrium, to reach the size of the workpiece is constant.

(2) air cooling: the vacuum furnace we use can be pressurized with nitrogen below 2bar for gas quenching, and cooled to below 100℃ from the oven. The empirical formula for calculating air cooling time is as follows:
T4 = 0.2 G + 0.3 D
Where, T4 is air cooling time (min).

(3) Oil cooling: quenching oil temperature is generally controlled at 60~80℃, and the oil output temperature of the working mold is usually controlled at 100~200℃. The empirical formula for calculating oil cooling time is as follows:
T5 = 0.02 G + 0.1 D
Where: T5 is the cooling time in the oil (min).
At this time the workpiece out of the oven temperature can generally be about 150℃.

4, the conclusion
Considering the amount of furnace loading and the space placed <d,< span=””></d,<>
The holding time shall be determined as T1=T2=T3= 0.4g +D;

For small workpieces (effective thickness D≤20mm, and space ≥D),
The holding time shall be determined as T1=T2= 0.1g +D T3= 0.3g +D.

For large work pieces (effective thickness D≥100mm),
The holding time shall be determined according to T1=T2=T3= 0.4g + 0.6d;

The air cooling time is determined according to T4= 0.2g + 0.3d;

Oil cooling time is determined by T5= 0.02g + 0.1d.

Learn More : Vacuum Heat Treatment Furnace