vacuum heat treatment process principles for making
Vacuum heat treatment equipment began in the 1920s, but its real development began in the 1960s and 1970s, mainly due to the market demand and graphite technology research and development.
The working environment of vacuum heat treatment is actually lower than a standard atmospheric pressure (1.013 x 105Pa), including low vacuum (105-102 Pa), medium vacuum (102-10-1 Pa), high vacuum (10-1-10-5 Pa) and ultra-high vacuum (10-5 Pa).
Vacuum heat treatment is also relatively controllable atmosphere heat treatment, but the working environment air is extremely thin, workpiece heating in vacuum state can avoid conventional ordinary heat treatment oxidation, decarbonization, avoid hydrogen embrittlement, deformation is relatively small, improve the comprehensive mechanical properties of materials and components. The service life of parts after vacuum heat treatment is usually several tens or even hundreds of times that of ordinary heat treatment.
The main contents of vacuum heat treatment process are as follows: determining heating system (temperature, time and mode), deciding vacuum and pressure regulation, selecting cooling mode and medium, etc.
1, heating temperature
Vacuum heating has two main characteristics: one is heating in a very thin atmosphere, avoiding oxidation, decarbonization, erosion and other phenomena caused by heating in the air; the other is the heat transfer in vacuum state is a single radiative heat transfer, the heat transfer capacity E and the absolute temperature T is proportional to the fourth power, namely E = C (T / 100) 4.
It can be seen that in vacuum, especially in low temperature stage, the temperature rise is slow, so that the temperature difference between the surface and the center of the workpiece reduces the thermal stress and the workpiece deformation is small. The selection of heating temperature is very important to the quality of the workpiece. According to the technical requirements, service conditions and performance requirements of the workpiece, the optimum heating temperature is found. The lower limit temperature is selected as far as possible without affecting the performance and considering the reduction of deformation.
2. Holding time
The length of holding time depends on the size and shape of the workpiece and the amount of the furnace installed. The traditional heating and insulation system introduced in general information is determined by T press mode.
T1=30+ (1.5-2) D
T2=30+ (1.0-1.5) D
T3=20+ (0.25-0.5) D
Formula: D is the effective thickness of workpiece (mm).
T1 is the first warm-up time (min).
T2 is the second warm-up time (min).
T3 is the final holding time (min).
In fact, a furnace is often equipped with a number of different shapes and sizes of the workpiece, which requires comprehensive consideration. We determine the holding time according to the size, shape, placement and furnace load of the workpiece. At the same time, we also consider that the vacuum heating mainly relies on high temperature radiation. When the workpiece is heated at low temperature (below 600 degrees centigrade), the temperature of the workpiece rises very slowly. When the workpiece has no special deformation requirements, the time of first and second preheating should be shortened as far as possible. Short, and increase the preheating temperature, because the low temperature holding time is longer, after heating the workpiece center to reach the surface temperature still needs some time.
Increasing the preheating temperature according to the vacuum heating principle can reduce the temperature difference between the inside and outside of the workpiece and shorten the preheating time. The final holding time should be appropriately extended so that the carbide in the steel can be fully dissolved. In this way, both quality and efficiency are guaranteed. The duration of heat preservation is also related to the following factors:
(1) Loading capacity: If the workpiece size is the same, the time of percolation should be prolonged; otherwise, the time of percolation should be shortened.
(2) Workpiece placement: Since the vacuum furnace is heated by radiation, generally speaking, if the shape of the workpiece is the same, the workpiece should be placed as neatly as possible to avoid shielding thermal radiation, and leave a certain space (<D) to ensure that the workpiece can be subjected to the maximum thermal radiation; different workpieces are installed in the same furnace, except according to the maximum workpiece calculation. Besides heat preservation time, it is necessary to increase penetration time. When the gap <D is placed, the empirical formula is:
Type G is installed capacity (kg).
The meaning of the remaining symbols is the same as before.
For small workpiece (effective thickness D < 20mm)
Or the gap between the workpiece is more than D.
The holding time can be reduced.
For large workpiece (effective thickness D > 100mm)
The final holding time can be reduced.
Heating temperature: high heating temperature can shorten the holding time.
3, cooling time
(1) Precooling: For medium and small parts quenched at high temperature, it is also noticed that the quenching deformation will be affected if precooling is carried out before quenching after entering the cold chamber from the hot chamber. The law is: after entering the cold chamber from the hot chamber, directly oil cooling or gas cooling will lead to size changes; if the appropriate pre-cooling, can keep the size of heat treatment before the same; but if the pre-cooling time is too long, will lead to workpiece size expansion. The general rule is that for the workpiece with effective thickness of 20~60mm, the precooling time is 0.5~3min.
According to the analysis, this is because the internal stress in the part is mainly thermal stress when it is quenched without pre-cooling, so volume shrinkage occurs. However, when the part is pre-cooled for a long time and then quenched again, the internal stress in the part is mainly phase change stress, which leads to volume expansion. Only after pre-cooling for a suitable time, the thermal stress occurs. The size of the workpiece can not be changed until the effect of phase transformation stress is balanced.
(2) Gas cooling: The vacuum furnace we adopted can be quenched by pressurized gas quenching with nitrogen below 2 bar and cooled to 100 ~C. The empirical formula for calculating gas cooling time is as follows:
Medium: T4 is gas cooling time (min).
(3) Oil cooling: the temperature of quenching oil is generally controlled at 60-80, and the temperature of tool and mould is usually controlled at 100-200. The empirical formula for calculating oil cooling time is as follows:
Medium: T5 is oil cooling time (min).
At this time, the temperature of the workpiece can generally be around 150 C.
(1) considering the furnace load and the space <D,
The holding time is determined by T1=T2=T3=0.4G+D.
(2) for small workpiece (effective thickness D < 20mm, and placing gaps equal to D),
The holding time is determined by T1=T2=0.1G+D T3=0.3G+D.
For large workpiece (effective thickness D > 100mm),
The holding time is 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.