What are the principles of the vacuum heat treatment process?

Principles for making vacuum heat treatment process

Vacuum heat treatment equipment began in the 1920s, but its real development began in the 1960s and 1970s, mainly because of the market demand and the development of graphite technology.

The working environment of vacuum heat treatment is actually

Below one standard atmospheric pressure (1.013×105Pa),

Including low vacuum (105~102Pa),

Medium vacuum (102~ 10-1pa)

High vacuum (10-1~ 10-5pa)

Ultra-high vacuum (< 10-5pa).

Vacuum heat treatment is relatively controllable atmosphere heat treatment, but its working environment is very thin air, the workpiece in the vacuum heating can avoid the conventional heat treatment of oxidation, decarburization, avoid hydrogen embrittlement, deformation is relatively small, improve the comprehensive mechanical properties of material parts. The service life of the parts after vacuum heat treatment is usually dozens of times, even hundreds of times of the service life of ordinary heat treatment.

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

1. Heat the temperature

Vacuum heating has two major characteristics. One is heating in a very thin atmosphere, avoiding oxidation, decarbonization, erosion and other phenomena caused by heating in the air. Another characteristic is that the heat transfer in the vacuum state is the heat transfer of single radiation, and its heat transfer capacity E is proportional to the absolute temperature T to the fourth power, 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 temperature difference between the surface and the core of the workpiece reduces the thermal stress and the deformation of the workpiece is small. The selection of heating temperature is crucial to the quality of the workpiece, in the process, according to the workpiece's technical requirements, service conditions and performance requirements, find the best heating temperature, in the case that does not affect the performance and consider reducing deformation, as far as possible to choose the lower limit temperature.

2. Insulation time

The length of insulation time, depends on the size and shape of the workpiece and how much furnace load. For the traditional heat preservation described in the general information, T is determined by the following formula:

T1 is equal to 30 plus 1.5 minus 2 times D

T2 is equal to 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 insulation time (min).

In fact, a number of workpiece of different shapes and sizes are often installed in a furnace at the same time, 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 of the workpiece and shorten the preheating time. In this way, the quality is guaranteed and the work efficiency is improved. The length of insulation time is also related to the following factors:

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

(2) workpiece placement form: because the vacuum furnace is radiation heating, generally speaking, if the workpiece shape is the same, should try to make the workpiece neat, to avoid blocking the heat radiation, and leave a certain space (<d), in="" order="" to="" ensure="" that="" the="" workpiece="" can="" receive="" maximum="" heat="" radiation;<="" span="">For different workpiece with a furnace, in addition to the maximum workpiece insulation time, but also to increase the burning time. When the gap<d is="" placed,="" the="" empirical="" formula="" is:<="" span="">

T1 = T2 = T3 = 0.4 G + D

Where, G is furnace loading quantity (kg)

The rest of the symbols have the same meaning as before.

In addition

For small workpiece (effective thickness D≤20mm)

Or the placement gap between the workpiece ≥D

Insulation time can be reduced:

T1 T2 = = 0.1 G + D

T3 = 0.3 G + D

For large pieces (effective thickness D≥100mm)

The final holding time can be reduced

T1 = T2 = T3 = 0.4 G + 0.6 D

Heating temperature: high heating temperature, can shorten the holding time.

3. Cooling time

(1) pre-cooling: for small and medium-sized parts of high temperature quenching, also note that after the hot room into the cold room, before the quenching whether to pre-cooling, will affect the quenching deformation. The rule is: from the hot chamber into the cold chamber, direct oil cooling or air cooling, will lead to size changes; If proper precooling is carried out, the dimensions before heat treatment can be kept unchanged; However, if the precooling time is too long, it will cause the size of the workpiece to swell. The general rule is that for the workpiece with effective thickness of 20~60mm, the pre-cooling 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 used can be filled with nitrogen under 2bar for pressure gas quenching, cooling to 100℃ below the furnace. The empirical formula for calculating the air cooling time is as follows:

T4 = 0.2 G + 0.3 D

Where, T4 is the air cooling time (min).

Oil cooling: quenching oil temperature is generally controlled in 60~80℃, the mold oil temperature is usually controlled in 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 oil (min).

At this time the workpiece temperature can be about 150℃.

4, the conclusion

(1) considering the amount of furnace loading and placing the gap<d,< span="">

The insulation time shall be determined by T1=T2=T3= 0.4g +D;

For small workpiece (effective thickness D≤20mm, and space ≥D),

The insulation time is determined by T1=T2= 0.1g +D T3= 0.3g +D;

For large pieces (effective thickness D≥100mm),

The insulation time shall be determined by T1=T2=T3= 0.4g + 0.6d;

(4) air cooling time is determined by T4= 0.2g + 0.3d;

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

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