1. Vacuum heat treatment heating process
Heating curve type
In the vacuum, the workpiece is mainly heated by radiation, and the radiation heat transfer has its own special law. The characteristic of this law is the fourth-order law (Stephen Boltzmann's law) which accords with the radiation heat transfer. See the following formula:
Q-radiation in the formula—the heat transferred by radiation;
————radiation heat transfer coefficient;
T 1 ———the surface temperature of the radiating element;
T 2 — The surface temperature of the irradiated object.
The above formula shows that even a small temperature difference at high temperatures produces a high heat transfer rate. It is calculated that the heat transfer rate caused by the 1 °C temperature difference at 1200 °C is 5 times that of the 1 °C temperature difference at 540 °C. At the same time, there is also information telling us that under the same circumstances, the vacuum addition time is about 3 times that of the air furnace and 6 times that of the salt bath furnace. All of these indicate that the low-temperature heating in the vacuum furnace is slow and the heating rate is "lag". For this reason, when a vacuum furnace is used, the processes of the air furnace, the salt bath furnace, and the atmosphere furnace cannot be copied. The three types of heating processes shown in Figures 1, 2 and 3, only Figure 1 is correct, precisely because it embodies the characteristics of heating in a vacuum furnace.
2. Determination of heating time
(1) Approximate calculation method of heating time. The time at different temperatures in Figure 4 can be calculated as follows:
T 1 = 30 + ( 2.0 ~ 1.5) × D ( 1)
T 2 = 20 + ( 1.5 ～ 1.0) × D ( 2)
T 3 = 15 + ( 1.0 to 0.8) × D ( 3)
Where: T1, T2, and T3 are time (min). D is the effective thickness (mm) of the workpiece to be heated, and is considered according to the relevant regulations, that is, the cylindrical workpiece is calculated by diameter, and the tubular workpiece is calculated by height when the height/wall thickness is ≤1.5mm; when the height/thickness is ≥1.5mm Calculated with a wall thickness of 1.5 mm; when the outer diameter/inner diameter is >7, the hollow inner cylinder is calculated by the outer diameter multiplied by 0.8 according to the solid cylinder.
The data in the formula brackets is the heating coefficient. The shape of the workpiece in (1) and (2) is complex, or the binding is dense, and the lower limit is selected when the shielding is serious. The value is simple. The workpiece is simple in shape and the upper limit is selected when the pendulum is relaxed. small) . (3) The lower limit of the selection of high alloy steel in the formula (large value); the upper limit of high speed steel selection (small value). 30, 20, and 15 are preset heating time (min) according to the internal heating type vacuum furnace and heating characteristics of different temperature sections.
Practice has proved that when the material has no quality problem and the temperature and cooling method are selected correctly, using this calculation method to select the time can ensure that the deformation will be reduced and will not be quenched. However, the disadvantage of this method is that the upper and lower limits of the heating coefficient are likely to be biased when the selection is not correct, and the hardness is insufficient or the efficiency is lowered when the deviation is large.
(2) An empirical method for observing the color of the furnace in the heating chamber.
1 The first stage is preheated. As shown in Fig. 4, the preheating temperature is 650 ° C, the purpose is to slowly dry through at this temperature, to eliminate the original stress (forging or cold working stress) of the heated part, and to reduce the newly generated thermal stress. The temperature selection principle is lower than Ac1 of the material, but not too low, too low to observe the color of the furnace. Furnace color observation (from the rear door of the heating chamber) requires an experienced heat treatment technician or skilled worker. The furnace color is “dark red†at 650 °C. The main points of observation are: Since the temperature rises, the color of the heated part is not darker than the color of other parts of the furnace (the basket, thermocouple, inner surface lining of the furnace, etc.), until the dark red is basically the same, it is considered to be ventilated. This period of time (OB segment) is the time T1 required for the first stage of warm-up. Of course, this time is characterized by "slow, long time."
2 The second section is preheated. As shown in Fig. 4, 850 °C is a second stage preheating, and the purpose is to preheat the same as the first stage. The color of the furnace at 850 °C is "orange". Observing the essentials: Since the temperature of the furnace starts from 650 °C, the color of the heated part is not darker than the color of the other parts of the furnace (the basket, the thermocouple, the inner surface of the furnace, etc.), until the orange color is basically the same, that is, It is already ventilated. This period of time (BD segment) is the time T2 required for the second stage to warm up. The characteristics of this period of time are: Compared with the first preheating, the heating speed is increased and the diathermic time is shortened.
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