What is vacuum heat treatment processing technology? Principles for vacuum heat treatment process

- Apr 11, 2019-

What is vacuum heat treatment processing technology? Principles for vacuum heat treatment process


What is vacuum heat treatment processing technology?


It mainly refers to the new heat treatment technology combining the vacuum technology and heat treatment technology. Among them, the vacuum environment of the vacuum heat treatment refers to the atmospheric environment under one atmosphere pressure, including low vacuum, medium vacuum, high vacuum and ultra-high vacuum, etc. Therefore, the vacuum heat treatment actually belongs to the atmosphere controlled heat treatment.


Vacuum heat treatment refers to all and part of the heat treatment process in the vacuum state, vacuum heat treatment can achieve almost all conventional heat treatment can be involved in the heat treatment process, but the quality of heat treatment greatly improved.


Compared with conventional heat treatment, vacuum heat treatment processing technology can achieve no oxidation, no decarburization, no carburization at the same time, can remove the phosphorus chip on the workpiece surface, and have degreasing and degassing, so as to achieve the effect of bright surface purification.


1. Application of vacuum heat treatment processing technology


Actually, vacuum heat treatment processing technology abroad application earlier, vacuum of Hays companies in the United States and Japan in 1968, has developed the vacuum quenching oil and water-based quenching medium, thereby, vacuum quenching technology in heat treatment industry gets rapid development, from a single chamber furnace to combined fleet, from the general development to the high pressure gas quenching vacuum hardening, vacuum water quenching, vacuum carburizing and carbonitriding and multivariate altogether permeability, etc.


In China, after decades of efforts, vacuum furnace manufacturers have greatly improved their design, manufacturing level and quality, and gradually replaced imported vacuum equipment with domestic vacuum equipment, thus reducing the production cost per unit, and rapidly expanding the application range of vacuum heat treatment.



2. Process principle of vacuum heat treatment processing technology

By taking advantage of the characteristics of metal phase change in the vacuum state, the thermodynamics and kinetics of solid phase change do not change in the vacuum within the range of 0.1MPa from the atmospheric pressure. The principle of solid phase transition at atmospheric pressure and the data of microstructure transition of various types can be used as reference when making the technological regulations of vacuum heat treatment. At the same time, under the effect of vacuum degassing, the physical properties and mechanical properties of metal materials can be improved. Under vacuum heating, the elements on the surface of metal workpieces will evaporate. The vacuum degree that the metal realizes without oxidation heating needs, surface purify action, realize little without oxidation and little without take off.


3. Characteristics of vacuum heat treatment processing technology



(1) advantages of vacuum heat treatment processing

Vacuum heat treatment processing is a non-oxidation heat treatment technology with a wide range of applications and controllable atmosphere. Vacuum heat treatment can not only realize no oxidation and no decarbonization of steel parts, but also realize no pollution and less distortion of workpiece. At present, it has become an irreplaceable advanced technology in die production.

(2) The distortion of vacuum heat treatment is small

According to domestic and foreign experience, the distortion of vacuum heat treatment of workpiece is only one third of that of salt bath heating quenching. It is of great significance to popularize the vacuum heat treatment technology to study the vacuum heating modes of various materials and parts with different degrees of complexity and the distortion rules under various cooling conditions and to simulate them by computer. During vacuum heating, atmospheric or high-pressure air quenching, airflow uniformity has a great influence on the quenching effect and quality dispersion of parts. It is of great significance to improve the furnace structure to study the air circulation rule in the furnace by means of computer simulation.


(3) adopt vacuum heat treatment furnace


Modern vacuum heat treatment furnace refers to a cold wall furnace which can be heated by vacuum of components and then quenched in oil or in normal pressure and pressurized gas. Research and development of this type of equipment is a comprehensive, interdisciplinary work involving many fields of science and technology.


The application prospect of vacuum heat treatment of mold materials is very great. Most die steels are currently heated in vacuum and then cooled and quenched in gas. In order to obtain satisfactory mechanical properties on the surface and inside of the workpiece, vacuum high-pressure gas quenching technology must be adopted. At present, the international pressure of true air quenching has been increased from 0.2mpa, 0.6mpa to 1-2mpa or even 3MPa, so the gradual increase of cooling gas pressure of high-pressure air quenching vacuum furnace is an important development trend.



Principles for vacuum heat treatment process


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 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).


Vacuum heat treatment is also controlled atmosphere heat treatment, but the working environment air is extremely thin, the workpiece heated in vacuum state can avoid the oxidation and decarbonization of conventional and ordinary heat treatment, avoid hydrogen embrittlement, relatively small deformation, improve the comprehensive mechanical properties of material parts. The life of parts after vacuum heat treatment is usually dozens or even hundreds of times as long as that of ordinary heat treatment.


The main contents of formulating the vacuum heat treatment process are: determining the heating system (temperature, time and mode), determining the vacuum degree and air pressure regulation, and selecting the cooling mode and medium, etc.



1. Heating temperature
Vacuum heating has two major characteristics. First, it is heated in a very thin atmosphere to avoid oxidation, decarbonization and erosion. Another characteristic is that the heat transfer in vacuum is a single radiation heat transfer, and its 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 rises slowly, so that the temperature difference between the workpiece surface and the heart reduces the thermal stress and the workpiece deformation is small. The selection of heating temperature is crucial to the quality of the workpiece. In the process formulation, the optimal heating temperature shall be found according to the technical requirements, service conditions and performance requirements of the workpiece. The lower limit temperature shall be selected as far as possible without affecting the performance and considering reducing the deformation.



2. Holding time


The length of holding time depends on the size and shape of the workpiece and the amount of furnace. T is determined according to the following formula when traditional heating and heat preservation is introduced in general data:





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


In fact, in a furnace often contain several different shapes and sizes of workpieces, 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 the workpiece and the outside, and shorten the preheating time. In this way, the quality is guaranteed and the working efficiency is improved. The length of holding time is also related to the following factors:


(1) furnace capacity: the workpiece size of the same fashion furnace capacity, the burning time should be extended; On the contrary, should shorten.

(2) Workpiece layout: since the vacuum furnace is radiative heating, generally speaking, if the workpiece is of the same shape, the workpiece should be placed neatly as far as possible to avoid shielding the heat radiation, and leave some space (<D) to ensure that the workpiece can receive the maximum heat radiation; For different workpieces with a furnace, in addition to calculate the maximum workpiece insulation time, but also to increase through the burning time. When spacing <D, the empirical formula is:

T1 = T2 = T3 = 0.4 G + D
Where G is the charge quantity (kg)
The other symbols have the same meaning as before.

In addition
For small workpieces (effective thickness D 20mm)
Or the spacing between workpieces D
The holding time can be reduced:

T1 =T2 = = 0.1 G + D
T3 = 0.3 G + D
For large workpieces (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 holding time.


3. Cooling time

(1) precooling: for the small and medium parts of high temperature quenching, also note that from the hot chamber into the cold chamber, before quenching whether precooling, will affect the quenching deformation. Its rule is: after entering the cold room from the hot room, directly 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. But if the pre-cooling time is too long, it will cause the workpiece size to swell. As a general rule, for workpieces with 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 cooled: we adopted by the vacuum furnace can be pressurized ventilation with 2 bar under nitrogen gas quenching, cooled to below 100 . The empirical formula for calculating air cooling time is as follows:

T4 = 0.2 G + 0.3 D
Where, T4 is the air cooling time (min).


(3) oil cooling: general control in 60 ~ 80 quenching oil temperature, working mould oil temperature usually control 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 (min) in oil.
When the workpiece temperature oven can generally be at about 150 ℃.


4, the conclusion
(1) considering the furnace load and put gap <D,
The holding time was determined as T1=T2=T3= 0.4g +D;
For small workpieces (effective thickness D 20mm, and the spacing D),
The holding time was determined as T1=T2= 0.1g +D T3= 0.3g +D;
For large workpiece (effective thickness D 100mm),
The holding time was determined as T1=T2=T3= 0.4g + 0.6d.
(4) air cooling time is determined according to T4= 0.2g + 0.3d;
(5) oil cooling time according to T5=0.02G+ 0.1d to determine.

IKS PVD,vacuum coating machine,contact:iks.pvd@foxmail.com