vacuum heat treatment study on heating lag time

1 introduction

Vacuum heat treatment is characterized by radiant heating, which is much faster than that of workpiece. On the one hand, because the heat insulation material in the furnace is mostly made of graphite felt and ceramic fiber, the heat capacity of this kind of material is small, the heat preservation performance is good, therefore, the thermal inertia of the furnace is small, the heating speed is fast. On the other hand, due to the very thin gas in the furnace, the heat transfer method of heating elements to the workpiece is mainly radiation heat transfer, convection heat transfer effect is very weak, so, the heating rate of the workpiece is very slow. Therefore, there exists the problem of heating lag time. The heat preservation time of vacuum heat treatment is actually composed of the following two parts, one is the workpiece through burning time (that is, the temperature homogenization time, that is, the heating lag time we discussed); Second, the time of organizational transformation. The heating and holding time of any kind of heat treatment is actually composed of these two parts, but because the heating method of vacuum heat treatment is very special, so the first part (that is, the heating lag time) becomes a more prominent problem.

2. Main factors affecting the heating lag time

The factors that affect the heating lag time of workpiece in vacuum furnace include the material, size, shape and surface brightness of the workpiece, the heating temperature and heating mode, the amount of furnace and the way of furnace loading, etc. The test was carried out on the wzc-30g vacuum furnace.

2.1 influence of heating temperature

40 crmnsimova ultra high strength steel in the heating temperature is 920 ℃, 900 ℃ and 870 ℃ respectively, and the sample size is 50 mmx150mm phi, only 1 piece of sample, and track the thermocouple inserted in the center of sample test conditions, due to the radiation energy to its absolute temperature heating element four direct ratio, thus, the higher the temperature, the higher the radiation efficiency, workpiece heating lag time is shorter, the workpiece speed is faster.

2.2 influence of heating mode

40CrMnSiMoVA ultra-high strength steel in the heating temperature of 920℃, sample size 35mmX105mm, divided into two layers, and the upper and lower two layers of sample no. 4 on the upper surface and the outer surface are fixed with a tracking thermocouple. Under the test condition of 7 pieces per layer with a total weight of 10kg, the heating curves obtained by different heating methods were obtained.

The heating rate of the samples lags behind that of the furnace whether preheating or not. The heating rate of the central sample lagged behind that of the outer sample. The preheating method can reduce the temperature difference on the section of the workpiece and the temperature difference between the workpiece and the furnace.

According to the characteristics of vacuum heating, materials such as stainless steel and high-temperature alloy with poor thermal conductivity, structural steel with carbon content higher than 0.4%, die steel, etc., workpiece with large section thickness change or sharp Angle and complex shape, and workpiece with hardness of >35HRC should be heated in the way of preheating in order to reduce the risk of deformation and cracking. If the heating temperature is low, it can be preheated once at 650-700℃. High heating temperature can be adopted 650-700℃ and 850-900℃ secondary preheating. The preheating time should ensure that the effective section of the workpiece reaches the heating temperature, which is generally 0.5-1 times of the holding time, or determined by the actual measurement.

2.3 influence of size

Individual heating curves of 40CrMnSiMoVA steel samples of different sizes (heating temperature 920℃). The larger the diameter, the longer the heating lag time. Therefore, when vacuum heat treatment, different sizes or shapes of the workpiece should not be mixed together for heating. When mixing is needed, the heating time shall be determined by the workpiece with the largest effective thickness to the temperature.

The heating lag time with large amount of furnace loading is longer than that with less amount of furnace loading. Therefore, the amount of furnace loading should be appropriate, as far as possible, in order to reduce the workpiece heating mutual shielding, heating more uniform.

3. Determination of heating lag time

Since the heating rate of workpiece in vacuum is slower than that in the box air furnace and slower than that in the salt bath furnace, the heating time of air furnace or salt bath furnace cannot be used as the heating time of vacuum heat treatment. The heating time of vacuum heat treatment can be determined only after the heating lag time is determined and the conventional tissue homogenization time is added. The method for determining the heating lag time is described below.

3.1 directly connect the tracking thermocouple on the workpiece (measured method)

This method can accurately indicate the workpiece to temperature time, can accurately determine the workpiece heating lag time. It is suitable for vacuum annealing, vacuum tempering and vacuum positive pressure gas quenching in a single chamber vacuum furnace at room temperature.

3.2 actual determination of heating time under specific conditions (simulation method)

In practice, the heating curves of several representative workpiece thickness, heating temperature, furnace loading quantity and furnace loading mode are measured, and the corresponding heating lag time is determined. Later in production, according to the workpiece's material, thickness, heating temperature, furnace loading method and furnace loading amount and other factors to choose the most similar (high is not low) measured heating curve, determine the heating lag time.

3.3 extend the holding time of air furnace by 50% (empirical method)

If there is no measured heating curve to choose from, then the insulation time of the air furnace is extended by 50% as compensation for the heating lag time of the workpiece. If the workpiece is heated at the inflatable pressure above 7.5X104, it is considered to be heated in air, and the time does not need to be compensated.

4 conclusion

(1) the main factors affecting the heating lag time of workpiece in vacuum furnace are heating temperature, heating mode, furnace loading quantity and furnace loading mode.

(2) the measurement method, simulation method and empirical method can be used to determine the heating lag time of vacuum heat treatment.

(3) only by accurately and reasonably determining the heating lag time of vacuum heat treatment, can the workpiece quality of vacuum heat treatment meet the requirements of relevant technical documents, and can improve the economic benefits.

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