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Vacuum heat treatment of slender hollow parts

During the trial production stage of product development, key parts processing problems are often encountered. Especially for parts with complex shapes, the deformation law of heat treatment is difficult to control. After finishing and forming, the salt bath heat treatment method is used. After heat treatment, it will be found that the parts are distorted and deformed, the straightness is out of tolerance, and the residual salt stains in the holes are serious and cannot be cleaned. It is easy to be broken during manual correction, and it cannot meet the product design requirements after grinding. Such workpiece processing problems will directly affect the product development process.

Taking a thin-walled hollow part as an example, we can see that compared with the traditional salt bath heat treatment, vacuum heat treatment is more suitable for processing such complex and easily deformed parts.

This part is a thin-walled pipe with a slender shape, with an outer diameter of φ19°, an inner diameter of φ13, and a total length of 188. The cross-section of the pipe body is asymmetrical and oblate, and the center of the pipe has a stepped hole. The speech makes the thickness of the pipe wall change greatly; Porous configuration at both ends, if it is very difficult to process after fire, the inner hole must be finished and formed before fire. This kind of slender and thin-walled tubular parts with complex structure and asymmetric appearance will inevitably cause bending deformation during heat treatment, and the straightness is out of tolerance. , it is extremely difficult to correct, and the problem of correcting fragmentation is prominent.

In the past, this type of finishing moldings used a salt bath furnace heating and quenching heat treatment process scheme, which not only caused corrosion to the finishing moldings due to the residual salt in the pores and blind holes of the workpiece, but also caused difficulties in subsequent surface treatment. , the cost of auxiliary materials is too high; the working environment of the production site workers is polluted, and the labor intensity is high. Moreover, because the control cabinet adopts contactor temperature control, the furnace temperature fluctuates greatly, the heat preservation temperature of the workpiece fluctuates significantly, and the uniformity of quenching hardness is not good. When the combined effect of the instantaneous thermal stress and the structural stress in the heating and quenching is greater than the yield strength of the steel, distortion will occur; when the combined effect of the two stresses exceeds the tensile strength of the steel, the workpiece will be cracked.

Process design process

According to the above analysis, the machining process and heat treatment process design are carried out.

Divide the machining part of the parts into two parts for processing: because the inner hole is too deep and the hole is too small, and the front hole is only φ1.9, it is difficult to complete the machining after the part is quenched, so the inner hole of the part is machined in place first, and then the heat treatment is carried out. Post-processing the shape, which can minimize the heat treatment deformation of the parts caused by the irregular external shape.

The salt bath heat treatment is changed to vacuum heat treatment, and the double-chamber vacuum quenching furnace is used to replace the salt bath furnace, which can reduce the residual salt brought out with the workpiece and reduce the waste of molten salt, and also prevent the molten salt from adhering to the workpiece gap and blind holes, causing damage to the workpiece. corrosion. The workpiece is automatically transported in and out of the furnace, the labor intensity of workers is reduced, and the production efficiency is significantly improved; a process temperature control curve that can store multi-stage temperature control is installed on the control cabinet of the double-chamber vacuum furnace, the process consistency is good, and the artificial temperature error can be effectively eliminated. The heat treatment hardness of the workpiece is more uniform, and the quenching deformation of the workpiece due to human operation is significantly improved.

In order to prevent the heat treatment deformation of the workpiece, the final heat treatment is not directly carried out on the parts after the fire machine forming, but the stress relief annealing is carried out in a box-type heat treatment furnace, and then heated and quenched by a hanging method in a vacuum furnace. During the heating and quenching process of the workpiece , Design and process stepped mandrel-type clamping support aids, so as to control the heat treatment deformation of slender hollow parts.

Process flow:

The workpiece is first subjected to stress relief annealing at 550 ± 10 °C, which can further eliminate the processing stress before the fire. In the final heat treatment, for the manual operation of the conventional salt bath furnace, the large fluctuation of the temperature control of the contactor leads to the poor uniformity of the workpiece heating and quenching hardness. , The combined effect of thermal stress and tissue stress of the workpiece leads to the problem of large quenching deformation. Using a double-chamber vacuum quenching furnace, the workpiece is heated and radiated in vacuum for heat transfer, and multi-stage preheating can be used for long-term heat preservation.

The four-step multi-stage heating process curve is selected for the workpiece. When quenching and heating, it is set at 650 °C for heat preservation and preheating. After the workpiece is tempered, it has passed the test and inspection. The phase structure is tempered troostite, which effectively reduces heat treatment distortion and embrittlement tendency.

Using vacuum quenching oil to replace the original ordinary quenching oil as the cooling medium of the workpiece, the quenching hardness of the workpiece is more uniform, the surface is bright and non-oxidizing, and the heat treatment straightness of the slender hollow part is guaranteed.

The process curve is as follows:

Conclusion

After this series of slender hollow parts adopts a new process method, through several batches of cold and hot processing verification of this type of parts, the effect is obviously improved, and it can meet the precision requirements of the parts. It solves the processing problem of slender thin-walled asymmetric tubes and ensures the progress of product development.

The application of the new processing technology of this type of workpiece, the inner hole of the slender hollow tube workpiece is processed and formed before the fire, the heat treatment adopts the hanging high vacuum heat treatment of the auxiliary tool clamping support, and the process innovation of finishing the appearance after the fire is achieved. The workpiece does not need to be corrected, and the cylindricity after heat treatment can be guaranteed to be less than 0.5, and the cylindricity of post-fire finishing can be guaranteed to be less than 0.1.

The new technology of vacuum heating and quenching is adopted to replace the traditional method of heating and quenching in salt bath furnace, which solves the problem of large fluctuations in the temperature control and heating temperature of the contactor for heating and quenching in salt bath furnace. The temperature control of the salt bath furnace adopts a contactor, and its temperature control accuracy can only meet the process requirements of ±10 °C. The intelligent temperature controller of the double-chamber vacuum quenching furnace adopts PID temperature control, which can achieve a temperature control accuracy of ±5 °C, ensuring a slender hollow core. The quenching temperature of the parts is more uniform. In the process of the process test, the process parameters were adjusted through the trial, and the four-stage multi-step heating and heat preservation was set. The suitable heating temperature of 840 ° C, the heating temperature of the workpiece was more uniform, and the uniformity of the furnace temperature was better, so that the heat treatment deformation problem was solved. It is good and solves the problems of heat treatment distortion and correction of brittle fracture.

The double-chamber vacuum quenching furnace is used for the heat treatment of slender hollow parts instead of the salt bath heating furnace, which fundamentally solves the problem that the salt remains in the blind holes and crevices of the workpiece to corrode the workpiece; realizes high-vacuum non-oxidative quenching, and reduces the cost of subsequent surface treatment. The goal of improving quality and efficiency has been achieved.

Selection of vacuum heat treatment equipment:The double-chamber vacuum quenching furnace provides a guarantee for the bright quenching of the workpiece. The double-chamber vacuum quenching furnace has a vacuum heating chamber and a vacuum quenching chamber. The quenching ensures that the workpiece is continuously heated under high temperature and the quenching process is fully enclosed and high vacuum. The vacuum degree is 4×10-1Pa, and the vacuum quenching oil is used for quenching and cooling to ensure that the workpiece can meet the requirements of non-oxidative bright quenching.

The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.