Principle of 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 and the development of graphite technology.
The working environment of vacuum heat treatment is:
- It is lower than a 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 controlled atmosphere heat treatment, but its working environment air is extremely thin. The workpiece heating in vacuum state can avoid the oxidation and decarburization of conventional heat treatment, avoid hydrogen embrittlement, and improve the comprehensive mechanical properties of material parts. The service life of components after vacuum heat treatment is usually dozens or even hundreds of times longer than that of ordinary heat treatment.
The main contents of vacuum heat treatment process are: determine the heating system (temperature, time and mode), determine the vacuum degree and air pressure regulation, select cooling mode and medium, etc.
Vacuum heating has two major characteristics: one is heating in a very thin atmosphere to avoid oxidation, decarburization, erosion and other phenomena caused by heating in air; the other is that heat transfer in vacuum is single radiation heat transfer, and its heat transfer capacity e is positively proportional to the fourth power of 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 temperature difference between the surface and the center of the workpiece is reduced, the thermal stress is small, and the deformation of the workpiece is also small. The selection of heating temperature is very important to the quality of the workpiece. When making the process, the best heating temperature should be found according to the technical requirements, service conditions and performance requirements of the workpiece. The lower limit temperature should be selected as far as possible without affecting the performance and considering the reduction of deformation.
The length of holding time depends on the size and shape of the workpiece and the amount of furnace charging. T is determined by the following formula when traditional heating and heat preservation are introduced in general data:
In the formula:
- 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 holding time (min).
In fact, several workpieces with different shapes and sizes are often installed in one furnace at the same time, which requires comprehensive consideration. We determine the holding time according to the size, shape, placing mode and charging capacity of the workpiece. At the same time, we also consider that vacuum heating mainly depends on high temperature radiation. When the workpiece is heated at low temperature (below 600 ℃), the temperature rise of the workpiece is very slow. At this time, when there is no special deformation requirement for the workpiece, the time for the first preheating and the second preheating should be shortened as far as possible, and the preheating temperature should be increased because of the low temperature insulation If the time is longer, it will take a certain time for the center of the workpiece to reach the surface temperature after heating.
According to the principle of vacuum heating, increasing the preheating temperature can reduce the temperature difference between the inside and outside of the workpiece, shorten the preheating time, and the final holding time should be appropriately extended, so that the carbides in the steel can be fully dissolved. In this way, not only the quality is guaranteed, but also the work efficiency is improved. The holding time is also related to the following factors:
① Charging capacity: when the size of workpieces is the same, if the furnace capacity is large, the time of thorough burning should be prolonged; otherwise, it should be shortened.
② Workpiece placement form: since the vacuum furnace is radiant heating, generally speaking, if the shape of the workpiece is the same, the workpiece should be placed in order as far as possible to avoid shielding the heat radiation, and a certain space (< d) should be reserved to ensure that the workpiece can receive the maximum heat radiation; for different parts installed in the same furnace, in addition to calculating the holding time according to the maximum workpiece, the through burning time should be increased. When the space is less than D, the empirical formula is as follows:
In the formula: G is the charging capacity (kg)
The other symbols have the same meaning as before.
In addition, for small workpieces (effective thickness d ≤ 20mm) or the gap between workpieces ≥ D, the holding time can be reduced
For large workpieces (effective thickness d ≥ 100 mm), the final holding time can be reduced.
③ Heating temperature: high heating temperature can shorten the holding time.
① Precooling: for the small and medium-sized parts quenched at high temperature, it is also noted that whether the pre cooling is carried out before quenching after entering the cold chamber from the hot chamber will affect the quenching deformation. The rule is: after entering the cold chamber from the hot chamber, oil cooling or air cooling directly will lead to size change; if proper precooling is carried out, the size before heat treatment can be kept unchanged; however, if the precooling time is too long, the workpiece size will swell. The general rule is that the precooling time is 0.5 ~ 3 min for the workpiece with effective thickness of 20 ~ 60mm.
According to the analysis, this is because when the parts are directly quenched without precooling, the internal stress in the parts is mainly thermal stress, so the volume shrinkage occurs. However, when the parts are re quenched after a long time of precooling, the internal stress in the parts is mainly the phase transformation stress, resulting in volume expansion. Only after proper precooling, can the effect of thermal stress and phase change stress be balanced, can the work be achieved The size of the parts remains unchanged.
② Gas cooling: our vacuum furnace can be filled with nitrogen below 2bar for pressurized gas quenching and cooled to below 100 ℃. The empirical formula for calculating the air cooling time is as follows:
In the formula: T4 is the air cooling time (min).
③ Oil cooling: the quenching oil temperature is generally controlled at 60 ~ 80 ℃, and the oil outlet temperature of tools and dies is usually controlled at 100 ~ 200 ℃. The empirical formula for calculating oil cooling time is as follows:
In the formula: T5 is the cooling time in oil (min).
At this time, the temperature of the workpiece can be about 150 ℃.
- ① The holding time is determined by T1 = T2 = T3 = 0.4g + D, considering the charging capacity and the space < D;
- ② For small workpieces (effective thickness d ≤ 20 mm, and placement space ≥ d), the holding time is determined by T1 = T2 = 0.1g + D T3 = 0.3g + D;
- ③ For large workpiece (effective thickness d ≥ 100 mm), 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;
- ⑤ The oil cooling time is determined by T5 = 0.02g + 0.1D.
Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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